Hibernate

1) What is Hibernate? Hibernate is a powerful, high performance object/relational persistence and query service. This lets the users to develop persistent classes following object-oriented principles such as association, inheritance, polymorphism, composition, and collections. 2) What is ORM? ORM stands for Object/Relational mapping. It is the programmed and translucent perseverance of objects in a Java application in to the tables of a relational database using the metadata that describes the mapping between the objects and the database. It works by transforming the data from one representation to another. 3) What does an ORM solution comprises of? • • • •

It should have an API for performing basic CRUD (Create, Read, Update, Delete) operations on objects of persistent classes Should have a language or an API for specifying queries that refer to the classes and the properties of classes An ability for specifying mapping metadata It should have a technique for ORM implementation to interact with transactional objects to perform dirty checking, lazy association fetching, and other optimization functions

4) What are the different levels of ORM quality? There are four levels defined for ORM quality. i.

ii. iii. iv.

Pure relational Light object mapping Medium object mapping Full object mapping

5) What is a pure relational ORM? The entire application, including the user interface, is designed around the relational model and SQL-based relational operations. 6) What is a meant by light object mapping? The entities are represented as classes that are mapped manually to the relational tables. The code is hidden from the business logic using specific design patterns. This approach is successful for applications with a less number of entities, or applications with common, metadata-driven data models. This approach is most known to all. 7) What is a meant by medium object mapping? The application is designed around an object model. The SQL code is generated at build time. And the associations between objects are supported by the persistence mechanism, and queries are specified using an object-oriented expression language. This is best suited for medium-sized applications with some complex transactions. Used when the mapping exceeds 25 different database products at a time.

8) What is meant by full object mapping? Full object mapping supports sophisticated object modeling: composition, inheritance, polymorphism and persistence. The persistence layer implements transparent persistence; persistent classes do not inherit any special base class or have to implement a special interface. Efficient fetching strategies and caching strategies are implemented transparently to the application. 9) What are the benefits of ORM and Hibernate? There are many benefits from these. Out of which the following are the most important one.

i. ii. iii.

iv.

Productivity – Hibernate reduces the burden of developer by providing much of the functionality and let the developer to concentrate on business logic. Maintainability – As hibernate provides most of the functionality, the LOC for the application will be reduced and it is easy to maintain. By automated object/relational persistence it even reduces the LOC. Performance – Hand-coded persistence provided greater performance than automated one. But this is not true all the times. But in hibernate, it provides more optimization that works all the time there by increasing the performance. If it is automated persistence then it still increases the performance. Vendor independence – Irrespective of the different types of databases that are there, hibernate provides a much easier way to develop a cross platform application.

10) How does hibernate code looks like?

Session session = getSessionFactory().openSession(); Transaction tx = session.beginTransaction(); MyPersistanceClass mpc = new MyPersistanceClass ("Sample App"); session.save(mpc); tx.commit(); session.close(); The Session and Transaction are the interfaces provided by hibernate. There are many other interfaces besides this. 11) What is a hibernate xml mapping document and how does it look like? In order to make most of the things work in hibernate, usually the information is provided in an xml document. This document is called as xml mapping document. The document defines, among other things, how properties of the user defined persistence classes’ map to the columns of the relative tables in database.



<property name="text" column="Persistance_message"/> <many-to-one name="nxtPer" cascade="all" column="NxtPerId"/> Everything should be included under tag. This is the main tag for an xml mapping document. 12) Show Hibernate overview?

13) What the Core interfaces are of hibernate framework? There are many benefits from these. Out of which the following are the most important one.

i. ii. iii. iv. v.

Session Interface – This is the primary interface used by hibernate applications. The instances of this interface are lightweight and are inexpensive to create and destroy. Hibernate sessions are not thread safe. SessionFactory Interface – This is a factory that delivers the session objects to hibernate application. Generally there will be a single SessionFactory for the whole application and it will be shared among all the application threads. Configuration Interface – This interface is used to configure and bootstrap hibernate. The instance of this interface is used by the application in order to specify the location of hibernate specific mapping documents. Transaction Interface – This is an optional interface but the above three interfaces are mandatory in each and every application. This interface abstracts the code from any kind of transaction implementations such as JDBC transaction, JTA transaction. Query and Criteria Interface – This interface allows the user to perform queries and also control the flow of the query execution.

14) What are Callback interfaces? These interfaces are used in the application to receive a notification when some object events occur. Like when an object is loaded, saved or deleted. There is no need to implement callbacks in hibernate applications, but they’re useful for implementing certain kinds of generic functionality. 15) What are Extension interfaces? When the built-in functionalities provided by hibernate is not sufficient enough, it provides a way so that user can include other interfaces and implement those interfaces for user desire functionality. These interfaces are called as Extension interfaces. 16) What are the Extension interfaces that are there in hibernate? There are many extension interfaces provided by hibernate. • • • • •

ProxyFactory interface - used to create proxies ConnectionProvider interface – used for JDBC connection management TransactionFactory interface – Used for transaction management Transaction interface – Used for transaction management TransactionManagementLookup interface – Used in transaction management.

• • • • •

Cahce interface – provides caching techniques and strategies CacheProvider interface – same as Cache interface ClassPersister interface – provides ORM strategies IdentifierGenerator interface – used for primary key generation Dialect abstract class – provides SQL support

17) What are different environments to configure hibernate? There are mainly two types of environments in which the configuration of hibernate application differs.

i.

ii.

Managed environment – In this kind of environment everything from database connections, transaction boundaries, security levels and all are defined. An example of this kind of environment is environment provided by application servers such as JBoss, Weblogic and WebSphere. Non-managed environment – This kind of environment provides a basic configuration template. Tomcat is one of the best examples that provide this kind of environment.

18) What is the file extension you use for hibernate mapping file? The name of the file should be like this : filenam.hbm.xml The filename varies here. The extension of these files should be “.hbm.xml”. This is just a convention and it’s not mandatory. But this is the best practice to follow this extension. 19) What do you create a SessionFactory?

Configuration cfg = new Configuration(); cfg.addResource("myinstance/MyConfig.hbm.xml"); cfg.setProperties( System.getProperties() ); SessionFactory sessions = cfg.buildSessionFactory(); First, we need to create an instance of Configuration and use that instance to refer to the location of the configuration file. After configuring this instance is used to create the SessionFactory by calling the method buildSessionFactory(). 20) What is meant by Method chaining? Method chaining is a programming technique that is supported by many hibernate interfaces. This is less readable when compared to actual java code. And it is not mandatory to use this format. Look how a SessionFactory is created when we use method chaining.

SessionFactory sessions = new Configuration() .addResource("myinstance/MyConfig.hbm.xml") .setProperties( System.getProperties() ) .buildSessionFactory();

21) What does hibernate.properties file consist of? This is a property file that should be placed in application class path. So when the Configuration object is created, hibernate is first initialized. At this moment the application will automatically detect and read this hibernate.properties file.

hibernate.connection.datasource = java:/comp/env/jdbc/AuctionDB hibernate.transaction.factory_class = net.sf.hibernate.transaction.JTATransactionFactory hibernate.transaction.manager_lookup_class = net.sf.hibernate.transaction.JBossTransactionManagerLookup hibernate.dialect = net.sf.hibernate.dialect.PostgreSQLDialect 22) What should SessionFactory be placed so that it can be easily accessed? As far as it is compared to J2EE environment, if the SessionFactory is placed in JNDI then it can be easily accessed and shared between different threads and various components that are hibernate aware. You can set the SessionFactory to a JNDI by configuring a property hibernate.session_factory_name in the hibernate.properties file. 23) What are POJOs? POJO stands for plain old java objects. These are just basic JavaBeans that have defined setter and getter methods for all the properties that are there in that bean. Besides they can also have some business logic related to that property. Hibernate applications works efficiently with POJOs rather then simple java classes. 24) What is object/relational mapping metadata? ORM tools require a metadata format for the application to specify the mapping between classes and tables, properties and columns, associations and foreign keys, Java types and SQL types. This information is called the object/relational mapping metadata. It defines the transformation between the different data type systems and relationship representations. 25) What is HQL? HQL stands for Hibernate Query Language. Hibernate allows the user to express queries in its own portable SQL extension and this is called as HQL. It also allows the user to express in native SQL. 26) What are the different types of property and class mappings? • • • • • • • • • •

Typical and most common property mapping <property name="description" column="DESCRIPTION" type="string"/> Or <property name="description" type="string"> Derived properties <property name="averageBidAmount" formula="( select AVG(b.AMOUNT) from BID b where b.ITEM_ID = ITEM_ID )" type="big_decimal"/>

• • • • • •

Typical and most common property mapping <property name="description" column="DESCRIPTION" type="string"/> Controlling inserts and updates <property name="name" column="NAME" type="string" insert="false" update="false"/>

27) What is Attribute Oriented Programming? XDoclet has brought the concept of attribute-oriented programming to Java. Until JDK 1.5, the Java language had no support for annotations; now XDoclet uses the Javadoc tag format (@attribute) to specify class-, field-, or method-level metadata attributes. These attributes are used to generate hibernate mapping file automatically when the application is built. This kind of programming that works on attributes is called as Attribute Oriented Programming. 28) What are the different methods of identifying an object? There are three methods by which an object can be identified.

i. ii. iii.

Object identity – Objects are identical if they reside in the same memory location in the JVM. This can be checked by using the = = operator. Object equality – Objects are equal if they have the same value, as defined by the equals( ) method. Classes that don’t explicitly override this method inherit the implementation defined by java.lang.Object, which compares object identity. Database identity – Objects stored in a relational database are identical if they represent the same row or, equivalently, share the same table and primary key value.

29) What are the different approaches to represent an inheritance hierarchy? i. ii. iii.

Table per concrete class. Table per class hierarchy. Table per subclass.

30) What are managed associations and hibernate associations? Associations that are related to container management persistence are called managed associations. These are bi-directional associations. Coming to hibernate associations, these are unidirectional. 1) Introduction While working with Hibernate web applications we will face so many problems in its performance due to database traffic. That to when the database traffic is very heavy . Actually hibernate is well used just because of its high performance only. So some techniques are necessary to maintain its performance. Caching is the best technique to solve this problem. In this article we will discuss about, how we can improve the performance of Hibernate web applications using caching. The performance of Hibernate web applications is improved using caching by optimizing the database applications. The cache actually stores the data already loaded from the database, so that the traffic between our application and the database will be reduced when the application want to access that data again. Maximum the application will works with the data in the cache

only. Whenever some another data is needed, the database will be accessed. Because the time needed to access the database is more when compared with the time needed to access the cache. So obviously the access time and traffic will be reduced between the application and the database. Here the cache stores only the data related to current running application. In order to do that, the cache must be cleared time to time whenever the applications are changing. Here are the contents. •

•

•

• • • •

•

•

Introduction. o First-level cache. o Second-level cache. Cache Implementations. o EHCache. o OSCache. o SwarmCache. o JBoss TreeCache. Caching Stringategies. o Read-only. o Read-Write. o Nonstriict read-write. o Transactional. Configuration. element. Caching the queries. Custom Cache. o Configuration. o Implementation :: ExampleCustomCache. Something about Caching. o Performance. o About Caching. Conclusion.

Hibernate uses two different caches for objects: first-level cache and second-level cache.. 1.1) First-level cache First-level cache always Associates with the Session object. Hibernate uses this cache by default. Here, it processes one transaction after another one, means wont process one transaction many times. Mainly it reduces the number of SQL queries it needs to generate within a given transaction. That is instead of updating after every modification done in the transaction, it updates the transaction only at the end of the transaction. 1.2) Second-level cache Second-level cache always associates with the Session Factory object. While running the transactions, in between it loads the objects at the Session Factory level, so that those objects will available to the entire application, don’t bounds to single user. Since the objects are already loaded in the cache, whenever an object is returned by the query, at that time no need to go for a database transaction. In this way the second level cache works. Here we can use query level cache also. Later we will discuss about it. 2) Cache Implementations

Hibernate supports four open-source cache implementations named EHCache (Easy Hibernate Cache), OSCache (Open Symphony Cache), Swarm Cache, and JBoss Tree Cache. Each cache has different performance, memory use, and configuration possibilities. 2.1) 2.1 EHCache (Easy Hibernate Cache) (org.hibernate.cache.EhCacheProvider) • • • • • •

It is fast. lightweight. Easy-to-use. Supports read-only and read/write caching. Supports memory-based and disk-based caching. Does not support clustering.

2.2)OSCache (Open Symphony Cache) (org.hibernate.cache.OSCacheProvider) • • • • •

It is a powerful . flexible package supports read-only and read/write caching. Supports memory- based and disk-based caching. Provides basic support for clustering via either JavaGroups or JMS.

2.3)SwarmCache (org.hibernate.cache.SwarmCacheProvider) • • •

is a cluster-based caching. supports read-only or nonstrict read/write caching . appropriate for applications those have more read operations than write operations.

2.4)JBoss TreeCache (org.hibernate.cache.TreeCacheProvider) • •

is a powerful replicated and transactional cache. useful when we need a true transaction-capable caching architecture .

3) Caching Stringategies Important thing to remembered while studying this one is none of the cache providers support all of the cache concurrency strategies. 3.1) Read-only • • •

Useful for data that is read frequently but never updated. It is Simple . Best performer among the all.

Advantage if this one is, It is safe for using in a cluster. Here is an example for using the readonly cache strategy.

....

3.2) Read-Write • • • • • •

Used when our data needs to be updated. It’s having more overhead than read-only caches. When Session.close() or Session.disconnect() is called the transaction should be completed in an environment where JTA is no used. It is never used if serializable transaction isolation level is required. In a JTA environment, for obtaining the JTA TransactionManager we must specify the property hibernate.transaction.manager_lookup_class. To use it in a cluster the cache implementation must support locking.

Here is an example for using the read-write cache stringategy.

…. <set name="yuv" ... > …. 3.3) Nonstrict read-write • • •

Needed if the application needs to update data rarely. we must specify hibernate.transaction.manager_lookup_class to use this in a JTA environment . The transaction is completed when Session.close() or Session.disconnect() is called In other environments (except JTA) .

Here is an example for using the nonstrict read-write cache stringategy.

…. 3.4) Transactional • •

It supports only transactional cache providers such as JBoss TreeCache. only used in JTA environment.

4) Configuration For configuring cache the hibernate.cfg.xml file is used. A typical configuration file is shown below.

<session-factory> ... <property name="hibernate.cache.provider_class">

org.hibernate.cache.EHCacheProvider ... The name in <property> tag must be hibernate.cache.provider_class for activating secondlevel cache. We can use hibernate.cache.use_second_level_cache property, which allows you to activate and deactivate the second-level cache. By default, the second-level cache is activated and uses the EHCache. 5) element The element of a class has the following form:

• • •

usage (mandatory) specifies the caching stringategy: transactional, read-write, nonstringict-read-write or read-only. region (optional) specifies the name of the second level cache region . include (optional) non-lazy specifies that properties of the entity mapped with lazy="true" may not be cached when attribute-level lazy fetching is enabled.

The element of a class is also called as the collection mapping. 6) Caching the queries Until now we saw only caching the transactions. Now we are going to study about the caching the queries.Suppose some queries are running frequently with same set of parameters, those queries can be cached. We have to set hibernate.cache.use_query_cache to true by calling Query.setCacheable(true) for enabling the query cache. Actually updates in the queries occur very often. So, for query caching, two cache regions are necessary. • •

For storing the results.( cache identifier values and results of value type only). For storing the most recent updates.

Query cache always used second-level cache only. Queries wont cached by default. Here is an example implementation of query cache.

List xyz = abc.createQuery("Query") .setEntity("…",….) .setMaxResults(some integer) .setCacheable(true) .setCacheRegion("region name") .list(); We can cache the exact results of a query by setting the hibernate.cache.use_query_cache property in the hibernate.cfg.xml file to true as follows:

<property name="hibernate.cache.use_query_cache">true Then, we can use the setCacheable() method on any query we wish to cache. 7) Custom Cache To understand the relation between cache and the application the cache implementation must generate statistics of cache usage. 7.1) Custom Cache Configuration In the hibernate.properties file set the property hibernate.cache.provider_class = examples.customCache.customCacheProvider. 7.2) Implementation :: ExampleCustomCache Here is the implementation of ExampleCustomCache. Here it uses Hashtable for storing the cache statistics.

package examples.ExampleCustomCache; import net.sf.hibernate.cache; import java.util; import org.apache.commons.logging; public class ExampleCustomCache implements Cache { public Log log = LogFactory.getLog(ExapleCustomCache.class); public Map table = new Hashtable(100); int hits, misses, newhits, newmisses, locks, unlocks, remhits, remmisses, clears, destroys; public void statCount(StringBuffer input, String string1, int value) { input.append(string1 + " " + value); } public String lStats() { StringBuffer res = new StringBuffer(); statCount(res, statCount(res, statCount(res, statCount(res, statCount(res, statCount(res, statCount(res, statCount(res, statCount(res, statCount(res, }

"hits", hits); "misses", misses); "new hits", newhits); "new misses", newmisses); "locks", lock); "unlocks", unlock); "rem hits ", remhits); "rem misses", remmisses); "clear", clears); "destroy", destroys);

return res.toString();

public Object get(Object key) { if (table.get(key) == null) { log.info("get " + key.toString () + " missed"); misses++; } else { log.info("get " + key.toString () + " hit"); hits++; } return table.get(key); } public void put(Object key, Object value) { log.info("put " + key.toString ()); if (table.containsKey(key)) { newhits++; } else { newmisses++; } table.put(key, value); } public void remove(Object key) { log.info("remove " + key.toString ()); if (table.containsKey(key)) { remhits++; } else { remmisses++; } table.remove(key); } public void clear() { log.info("clear"); clears++; table.clear(); } public void destroy() { log.info("destringoy "); destroys++; } public void lock(Object key) { log.info("lock " + key.toStringing()); locks++;

} public void unlock(Object key) { log.info("unlock " + key.toStringing()); unlocks++; } Here is the example of Custom Cache.

Package examples.ExapleCustomCache; import java.util; import net.sf.hibernate.cache; public class ExampleCustomCacheProvider implements CacheProvider { public Hashtable cacheList = new Hashtable(); public Hashtable getCacheList() { return cacheList; } public Stringing cacheInfo () { StringingBuffer aa = new StringingBuffer(); Enumeration cList = cacheList.keys(); while (cList.hasMoreElements()) { Stringing cName = cList.nextElement().toStringing(); aa.append(cName); ExapleCustomCache myCache = (ExapleCustomCache)cacheList.get(cName); }

aa.append(myCache.lStats());

return aa.toStringing(); } public ExampleCustomCacheProvider() { } public Cache bCache(String string2, Properties properties) { ExampleCustomCache nC = new ExapleCustomCache(); cacheList.put(string2, nC); return nC; } } 8) Something about Caching

8.1) Performance Hibernate provides some metrics for measuring the performance of caching, which are all described in the Statistics interface API, in three categories: • • •

Metrics related to the general Session usage. Metrics related to the entities, collections, queries, and cache as a whole. Detailed metrics related to a particular entity, collection, query or cache region.

8.2) About Caching • • • • •

All objects those are passed to methods save(), update() or saveOrUpdate() or those you get from load(), get(), list(), iterate() or scroll() will be saved into cache. flush() is used to synchronize the object with database and evict() is used to delete it from cache. contains() used to find whether the object belongs to the cache or not. Session.clear() used to delete all objects from the cache . Suppose the query wants to force a refresh of its query cache region, we should call Query.setCacheMode(CacheMode.REFRESH).

9) Conclusion Caching is good one and hibernate found a good way to implement it for improving its performance in web applications especially when more database traffic occurs. If we implement it very correctly, we will get our applications to be running at their maximum capacities. I will cover more about the caching implementations in my coming articles. Try to get full coding guidelines before going to implement this. 1) Introduction This article deals with Hibernate Interceptors. Hibernate is an open-source project that provides ORM solution. For more information about Hibernate, novice readers are encouraged to read the article An Introduction to Hibernate on javabeat before reading this article. Situations may demand to perform some set of pre-requisite/post-requisite operations before/after the core functional logic. In such a case, an interceptor can be used to intercept the existing business functionality to provide extensible or add-on features. They provide pluggable architecture and are generally callback methods that will be called by the framework in response to a particular set of events/actions if properly registered and configured. They follow the standard Interceptor pattern and they have various advantages in an application design. They can be used to monitor the various parts of the input that are passed to an application to validate them. They even have the capability to overwrite the core functional logic of the module. For example, consider an online shopping system that ships goods to the customer's shipping address upon placing a request. Suppose, there is an enhancement to this application telling that the request has to be validated because of the increasing number of spams and the customer should be notified through e-mail (or mobile) upon successful delivery of the goods. These two enhancements have to be projected into the application's core logic. Having a general overview of the core logic will look something like the following, • •

Validate the User Request Ship the Goods to the customer

•

Notify the customer about its successful delivery

As we can see above, the two enhancements have to be projected within the application's core logic which requires code changes. But, if the application has to be properly designed with the notion of Interceptors, then the code change can be eliminated to the maximum. 2) Interceptors in Hibernate Hibernate provides an ORM solution for persisting and querying data in the database. A Hibernate application can be structured in a way such that certain methods can be make to be invoked when a particular life-cycle event occurs. Not always the API in a software/product will completely satisfy the application needs and requirements. Hibernate is no more away from this. Therefore, Hibernate API is designed in such a way to provide pluggable framework through the notion of Interceptors. In a multi-tiered application, the situation for the inclusion of Interceptors can happen at any level. It can happen at the Client level, Server level and even at the persistence level. Imagine an application is saving employee records in a database and now the application mandates to display to the Database admin about the history of inserts and updates. A simple general overview of the logic looks like the following, • •

Insert/Update the records in the Database During Insert/Update, maintain the log information in a file

As we can see, the maintenance of this logging information should happen whenever when an insert/update goes to the Database. Such a logger interceptor can be easily plugged into the application with minimal code change because of the flexible design of Hibernate. 2.1) Types of Interceptors Based on their scope, Interceptors in Hibernate can fall under two categories. They are, • •

Application-scoped Interceptors Session-scoped Interceptors

2.1.1) Application-scoped Interceptor An application can contain one or more database sessions represented by the Session interface. If an application is configured to use Global Interceptors, then it will affect the persistent objects in all the sessions. The following code configures a global interceptor, Configuration configuration = new Configuration(); configuration.setInterceptor(new MyInterceptor()); SessionFactory sessionFactory = configuration.buildSessionFactory(); Session session1 = sessionFactory.openSession(); Employee e1, e2 = null;

// Assume e1 and e2 objects are associated with session1. Session session2 = sessionFactory.openSession(); User u1, u2 = null //Assume u1 and u2 objects are associated with session1. A global-scoped interceptor can be set to an application by calling the Configuration.setInterceptor(Interceptor) method. In the above code, we have two different session objects 'session1' and 'session2'. Let us assume that e1 and e2 Employee objects are associated with session 'session1' and u1 and u2 User objects are associated with session 'session2'. The applied application-scoped interceptor would have affected all the objects (e1, e2, u1 and u2), even though they are in different sessions. 2.1.2) Session-scoped Interceptor A session-scoped interceptor will affect all the persistent objects that are associated with that particular session only. The following code shows how to configure a session-scoped interceptor, Configuration configuration = new Configuration(); SessionFactory sessionFactory = configuration.buildSessionFactory(); MyInterceptor myInterceptor = new MyInterceptor(); Session session1 = sessionFactory.openSession(myInterceptor); Employee e1, e2 = null; // Assume e1 and e2 objects are associated with session 'session1'. MyAnotherInterceptor myAnotherInterceptor = new MyAnotherInterceptor (); Session session2 = sessionFactory.openSession(myAnotherInterceptor); User u1, u2 = null; // Assume u1 and u2 objects are associated with session 'session2'. From the above code, we can infer that a session-scoped interceptor can be set by calling the method SessionFactory.openSession(Interceptor). In the above code, we have two different session objects 'session1' and 'session2' being configured with interceptors MyInterceptor and MyAnotherInterceptor respectively. So, e1 and e2 objects will be affected by MyInterceptor, whereas u1 and u2 objects will be affected by MyAnotherInterceptor. 3) Interceptor API Three interfaces related to Interceptors are available in Hibernate, out of which 2 are the classical interfaces. Lifecycle and Validatable are the classic interfaces and whereas Interceptor is available in org.hibernate package.

Following sections discusses more about the Interceptor interfaces in detail: 3.1) The 'Validatable' Interface This classic interface can be implemented by Persistent Java Class to validate the state of the persistent object. This interface has a single method called Validatable.validate(), which can be given implementation to check the validity of the state of the object. Consider the following code, import java.util.Date; import org.hibernate.classic.Validatable; import org.hibernate.classic.ValidationFailure; public class ProjectDuration implements Validatable{ private Date startDate; private Date endDate; /// Other Code here. public void validate(){ if (startDate.after(endDate)){ throw new ValidationFailure( "Start Date cannot be greater than the End Date.") } } } The above persistent class ProjectDuration implements the Validatable interface, and has a simple validation rule in the validate method, stating that the project start date cannot come after the end date. This Validatable.validate() method will be called by the framework during the save operation. A save opeation can happen whenever Session.save(), Session.update(), Session.saveOrUpdate() or Session.flush() methods are invoked. 3.2) The 'Lifecycle' Interface A persistent object goes through the various phases in its life-cycle. It can be newly created, persisted in the database, can be loaded at a later-time, will undergo modifications if needed and finally deleted. The various phases that happen in the life of a persistent object are encapsulated in the Lifecycle interface. Following are the available methods in the Lifecycle Interface.

onLoad()

This method will be called by the framework before the loading of the persistent object, i.e, when the Session.load() is called.

onSave()

This method will be called by the framework before the save operation, when the Session.save() or Session.saveorUpdate() method is called.

onUpdate() This method will be called by the framework, before updating any properties on the persistent object, i.e,

when a call to Session.update() is made. onDelete()

This method is called before the delete operation, i.e. a call to Session.delete() is made.

All the four methods are passed a Session object, which represents the session in which the persistent objects are associated with . A persistent class can implement the above interceptor for providing any customization, like the following, import org.hibernate.Session; import org.hibernate.classic.Lifecycle; class MyCustomizedPeristentClass implements Lifecycle{ … public boolean onDelete(Session s) throws CallbackException { return false; } public void onLoad(Session s, Serializable id) { System.out.println("Loading"); } public boolean onSave(Session s) throws CallbackException { return false; } public boolean onUpdate(Session s) throws CallbackException { return false; } } 3.3) The 'Interceptor' Interface This interface allows application to provide greater customization for persisting the objects. It even allows the code to modify the state of the persistent object. It has more than 15 different methods and so the designers of Hibernate provide the concrete EmptyInterceptor class which implements the Interceptor interface to provide default/empty method implementations. Applications can use EmptyInterceptor class instead of depending on the Interceptor interface. Following are the most common operations that are available in the Interceptor interface,

afterTransactionBegin()

Called by the framework before the start of a transaction, i.e. when the Session.startTransaction() method is called.

Called by the framework, before the transaction is about to end (either committed or rolled-back) beforeTransactionCompletion() , i.e. when a call is made to Transaction.commit() or Transaction.rollback() afterTransactionCompletion()

Called by the framework after the transaction has ended (committed or rolled-back)

onSave()

This revised onSave() method is passed with various information like the property names of the entities, their values, their states etc., as arguments and will be called by the framework during the save operation. A save operation may happen during Session.save(), Session.saveOrUpdate(), Session.flush() or Transaction.commit() method calls.

onUpdate()

This revised onUpdate() method is passed with various information like the property names of the entities, their values, their states etc., as arguments and will be called by the framework when the properties of a persistent object is about to be updated. An update operation may happen during a call to Session.update(), Session.saveOrUpdate(), Session.flush() or Transaction.commit() is made.

onLoad()

This revised onLoad() method is passed with various information like the property names of the entities, their values, their states etc., as arguments and will be called by the framework when a persistent object is about to be loaded, i.e. when Session.load() is called.

onDelete()

This revised onDelete() method which is passed with various information like the property names of the entities, their values, their states etc., as arguments will be called by the framework when a persistent

object is about to be deleted, i.e., when a call to Session.delete() is made.

4) Test Application Following section provides a sample application, which uses the interceptors that was discussed in the preceding few sections. 4.1) Pre-requisites The following are the pre-requisite softwares/products needed to run the sample application. • • • •

Java Development Kit (http://java.sun.com/javase/downloads/index.jsp) Hibernate 3.2 (http://www.hibernate.org/6.html) MySQL Database Server (http://dev.mysql.com/downloads/mysql/6.0.html) MySQL Database Driver (http://dev.mysql.com/downloads/connector/j/3.1.html)

Let us have a simple scenario which makes use of the two Interceptors. The first interceptor called the CustomSaveInterceptor populates the persistent object with additional values apart from the original values that are given by the user. A database table called "PlayersName" is created with column names "fName", "mName", "lName", "completeName" representing the first name, middle name, last name and the complete name of the player. Let the user give values only for fName, mName, lName and not for the completeName. The completeName which is just the concatenation of the fName, mName and lName along with white-spaces between them will be taken care by the CustomSaveInterceptor. The second is the LoggerInterceptor which keeps tracks of all the Insertions that are made to the Database table. PlayerName.java: package interceptor; import java.io.Serializable; import import import import import

org.hibernate.CallbackException; org.hibernate.Session; org.hibernate.classic.Lifecycle; org.hibernate.classic.Validatable; org.hibernate.classic.ValidationFailure;

public class PlayerName implements Validatable, Lifecycle { private private private private

String String String String

firstName; middleName; lastName; completeName;

private String primaryKey;

public PlayerName(){ } public String getFirstName() { return firstName; } public void setFirstName(String firstName) { this.firstName = firstName; } public String getLastName() { return lastName; } {

public void setLastName(String lastName) }

this.lastName = lastName;

public String getMiddleName() { return middleName; } public void setMiddleName(String middleName) { this.middleName = middleName; } public String getCompleteName() { return completeName; } public void setCompleteName(String completeName) { this.completeName = completeName; } public String getPrimaryKey() { return primaryKey; } public void setPrimaryKey(String primaryKey) { this.primaryKey = primaryKey; } public void validate() throws ValidationFailure { if ((firstName.equals(middleName)) && (middleName.equals(lastName))){ throw new ValidationFailure("First Name, Middle Name

cannot be the same"); }

and Last Name

} public boolean onDelete(Session s) throws CallbackException { return false; } public void onLoad(Session s, Serializable id) { System.out.println("Loading"); } public boolean onSave(Session s) throws CallbackException { return false; } public boolean onUpdate(Session s) throws CallbackException { return false; } } The above class 'PlayerName' represents the persistent class that we wish to save to the database. This class has various properties like firstName, middleName, lastName, completeName which represents the first name, middle name, last name and the complete name of the player. It also has a field called primaryKey for storing the primary key values which will be set manually by the application. This class implements the validation Interceptor called Validatable for doing a simple validate on the value of the name. It will just ensure whether the first name, middle name and the last name are unique by giving simple implementation in the validate() method. If they are not unique, then a ValidationFailureException is thrown by the program. PlayerName class also implements the Lifecycle interface and the methods onLoad(), onSave(), onUpdate(), onDelete() are given default implementation. CustomSaveInterceptor.java: Following is the code for cutom save interceptor which extends the EmptyInterceptor class and provides a simple logic of updating the persistent object with the complete name value. package interceptor; import java.io.Serializable; import org.hibernate.EmptyInterceptor; import org.hibernate.type.Type;

public class CustomSaveInterceptor extends EmptyInterceptor { public boolean onSave(Object entity, Serializable id, Object[] state, String[] propertyNames, Type[] types) { if (entity instanceof PlayerName){ PlayerName playerName = (PlayerName)entity; String completeName = playerName.getFirstName() + " " + playerName.getMiddleName() + " " + playerName.getLastName(); playerName.setCompleteName(completeName); } return super.onSave(entity, id, state, propertyNames, types); } } The onSave() method is the method of interest here, and we can see that this method is passed with various parameters. The entity represents the persistent entity object that is about to be saved. The id represents the Serializable primary key (which in our case will be a simple string object). The state array represents the values of the properties of the persistent object, The propertyNames array holds a list of string values, which are firstName, middleName, lastName, completeName. Since all the types in the PlayerName class are strings, the Type array points to the string type. The code initially does a pre-conditionary check to ensure whether the entity is of the right PlayerName type, and then updates the completeName property which is simply the concatenation of the firstName, middleName and the lastName values. LoggerInterceptor.java: package interceptor; import java.io.Serializable; import org.hibernate.EmptyInterceptor; import org.hibernate.type.Type; public class LoggerInterceptor extends EmptyInterceptor{ public boolean onSave(Object entity, Serializable id, Object[] state, String[] propertyNames,

{

Type[] types)

System.out.println("Saving the persistent Object " + entity.getClass() + " with Id " + id); return super.onSave(entity, id, state, propertyNames, types); } } The implementation of the LoggerInterceptor class is relatively simple, as this class does nothing apart from overriding the onSave() method and writing the log information to the console. InterceptorTest.java: package interceptor; import java.util.List; import org.hibernate.*; import org.hibernate.cfg.Configuration; import org.hibernate.classic.Validatable; public class InterceptorTest { public static void main(String[] args) { Configuration configuration = new Configuration().configure(); configuration.setInterceptor(new CustomSaveInterceptor()); SessionFactory sessionFactory = configuration.buildSessionFactory(); Session session = sessionFactory.openSession(new LoggerInterceptor()); createPlayerNames(session); listPlayerNames(session); } private static void createPlayerNames(Session session){ PlayerName rahul = createPlayerName("Rahul", "Sharad", "Dravid", "RSD"); PlayerName dhoni = createPlayerName("Mahendra", "Singh", "Dhoni", "MSD"); PlayerName karthik = createPlayerName("Krishnakumar", "Dinesh", "Karthik",

"KDK");

PlayerName same = createPlayerName("Same", "Same", "Same", "SME"); Transaction transaction = session.beginTransaction(); try{ session.save(rahul); session.save(dhoni); session.save(karthik); Transaction innerTransaction = null; try{ innerTransaction = session.beginTransaction(); session.save(same); }catch(Exception exception){ System.out.println("\n" + exception.getMessage()); }finally{ if (innerTransaction.isActive()){ innerTransaction.commit(); } } }catch(Exception exception){ System.out.println(exception.getMessage()); transaction.rollback(); session.clear(); }finally{ if (transaction.isActive()){ transaction.commit(); } } session.flush(); } private static PlayerName createPlayerName(String fName, String mName,String lName, String id){ PlayerName playerName = new PlayerName(); playerName.setFirstName(fName); playerName.setMiddleName(mName); playerName.setLastName(lName); playerName.setPrimaryKey(id); return playerName; } private static void listPlayerNames(Session

session){

Query query = session.createQuery("From PlayerName"); List allPlayers = query.list(); System.out.println("\n"); for(PlayerName player : allPlayers){ listPlayerName(player); } } private static void listPlayerName(PlayerName player){ StringBuilder result = new StringBuilder(); result.append("First Name = ").append(player.getFirstName()) append(" , Middle Name = ")

. .

append(player.getMiddleName()). append(" , Last Name = "). append(player.getLastName()). append(" , Full Name = ").append(player.getCompleteName()); System.out.println(result.toString()); } } The above code sets the CustomSaveInterceptor globally by calling the Configuration.setInterceptor(new CustomSaveInterceptor()). LoggerInterceptor is configured on the session-basis by calling the SessionFactory.openSession(new LoggerInterceptor()). The createPlayerNames() method creates some test player objects. Note that the player object 'same' is created with first-name, middle-name and last-name pointing out to 'Same', which is expected to be captured by the Validator Interceptor. All the objects are saved by calling the Session.save() method, within a transactional context, but the save operation for the 'same' object has been done within a separate transaction, so that even if the persistence operation of the 'same' object fails, the rest of the operation wont gets affected. This also tells the support for nested transaction. The code then lists out the player objects fetched from the database by executing a simple query "FROM PlayerName". The output of the above program will look like this, Saving the persistent Object class interceptor.PlayerName with Id RSD Saving the persistent Object class interceptor.PlayerName with Id MSD Saving the persistent Object class interceptor.PlayerName with Id KDK

First Name, Middle Name and Last Name cannot be the same First Name = Rahul , Middle Name = Sharad , Last Name = Dravid , Full Name = Rahul Sharad Dravid First Name = Mahendra , Middle Name = Singh , Last Name = Dhoni , Full Name = Mahendra Singh Dhoni First Name = Krishnakumar , Middle Name = Dinesh , Last Name = Karthik , Full Name = Krishnakumar Dinesh Karthik

4.2) Hibernate Configuration and Mapping Files Following are the hibernate configuration and the mapping files that have to be placed in the sample application's run-time class path. Hibernate.cfg.xml: The Hibernate Configuration File (hibernate.cfg.xml) provides configuration parameters to the application like the database URL, the username/password in the Database server etc. Given below is the Configuration file for the sample application. <session-factory> <property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver <property name="hibernate.connection.url">jdbc:mysql://localhost/dbforhibernate <property name="hibernate.connection.username">root <property name="hibernate.connection.password">root <property name="dialect">org.hibernate.dialect.MySQLDialect <mapping resource="playername.hbm.xml" /> playername.hmb.xml: Mapping Files provides mapping information like how a Java class is mapped to the relational database table. Any number of mapping files can be referenced from an application. Given below is the playername mapping file used in the sample application.

<property name="firstName"> <property name="middleName"> <property name="lastName"> <property name="completeName"> 5) Summary This article started with the definition of Interceptors, then defined where Interceptors can fit into the Hibernate Technology. Then are explained the differences between a global interceptor and a session-scoped interceptor. The various API's related to Hibernate interceptor are given detailed discussion. Finally, the article concluded with a simple sample application making use of the Interceptors. Integrating Spring Framework with Hibernate ORM Framework 1) Introduction Hibernate is a powerful technology for persisting data in any kind of Application. Spring, on the other hand is a dependency injection framework that supports IOC. The beauty of Spring is that it can integrates well with most of the prevailing popular technologies. In this article, we will discuss on how it is possible to integrate Spring with Hibernate. This article assumes that the reader has a basic understanding in both Spring and Hibernate Frameworks. If you are new to Spring and Hibernate frameworks, please read the introduction articles on Spring and Hibernate before start reading this article. Shunmuga Raja has explained in Introduction to Spring Framework. This article will help you to understand the fundamentals of the Spring framework. In another article Introduction to Hibernate published on 12/05/2007 by Shunmuga Raja explains what is ORM framework and how to start writing the simple hibernate application. 2) Spring and Hibernate

As a pre-requisite, let us understand the need for such integration before we actually get into the integration between these two technologies. It is well known that Hibernate is a powerful ORM tool that lies between Application and Database. It enables Application to access data from any database in a platform-independent manner. There is no need for the Application to depend on the low-level JDBC details like managing connection, dealing with statements and result sets. All the necessary details for accessing a particular data source is easily configurable in Xml files. Another good thing is that Hibernate can be coupled well with both J2SE and J2EE Applications. One of the problem with using Hibernate is that the client Application that accesses the database using Hibernate Framework has to depend on the Hibernate APIs like Configuration, SessionFactory and Session. These objects will continue to get scattered across the code throughout the Application. Moreover, the Application code has to manually maintain and manage these objects. In the case of Spring, the business objects can be highly configurable with the help of IOC Container. In simple words, the state of an object can be externalized from the Application code. It means that now it is possible to use the Hibernate objects as Spring Beans and they can enjoy all the facilities that Spring provides. 3) Integration Sample Instead of looking into the various Integration APIs that are available in the Spring Bundle, let us study and understand these APIs as we go through the sample code. The following sections cover the various steps involved in the Spring-Hiberante integration along with a detailed explanation. 3.1) Creating Database The following sample application uses the MySql database for dealing with data. MySql database can be downloaded from http://dev.mysql.com/downloads/mysql/5.0.html#downloads. After installing the database, start the MySql client and create a test database by issuing the following command,

Create database samples;

Note that the character ';' is the statement terminator for every command. Once the 'samples' database is created, use the database for creating tables by using the command,

Use samples;

This uses the 'samples' database for the current database session. It means that whatever operation we do, such as creating tables, will eventually affect the 'samples' database. Now, let us create a sample table called 'employee' which is having four fields namely id, name, age and salary. The following command creates the 'employee' table in the 'samples' database,

create table employee(id varchar(10), name varchar(20), age int(3), salary int(10));

Now an empty table (table with no records within it) is created.

3.2) The Employee class Now let us create a class called Employee for storing the data that are fetched from the employee table. The class design is such that the column names for the table 'employee' will be mapped as the variable names in the Java class with the appropriate data type. The complete code listing for the Employee class is as follows, Employee.java

package javabeat.spring.hibernate; public class Employee { private private private private

String id; String name; int age; double salary;

public Employee() { } public String getId(){ return id; } public void setId(String id){ this.id = id; } public String getName(){ return name; } public void setName(String name){ this.name = name; } public int getAge(){ return age; } public void setAge(int age){ this.age = age; } public double getSalary(){ return salary; } public void setSalary(double salary){ this.salary = salary; } public String toString(){ return "Id = " + id + ", Name = " + name + ", Age = " + age + ", Salary = " + salary; }

}

Note that the toString() method is overridden to give a meaningful display for the employee object. 3.3) Creating the Hibernate Mapping file We have created 'employee' table in the database and a corresponding Java class in the Application layer. However, we haven't specified that the 'employee' table should map to the Java class and the column names in the 'employee' table should map to the Java variables in the Employee class. This is where the Hibernate Mapping files comes into picture. Let us have a look at the Hibernate Mapping file, employee.hbm.xml

<property name="name"> <property name="age"> <property name="salary">

Note that the Mapping file is an Xml file and its name is employee.hbm.xml. The portion of the string 'hbm' in the mapping file stands for hibernate Mapping File. Although it is not necessary to follow this convention, it will be easy to figure what type of xml file is this, just by looking at the extension. Xml conforms to a well-defined DTD, the hibernate-mappings-3.0.dtd. The root element for the mapping file is the hibernate-mapping tag which can define one or more mappings, following which we have the class tag which defines a mapping between the database table name and the Java class. The 'name' attribute must point to a fully qualified Java class name whereas the table attribte must point to the database table. The next series of tags define the mapping definition of the column names against its Java variables counterparts. The 'id' tag defines an identifier for a row and it is commonly used as a primary key column. The property tag has an attribute called 'name' which points to the Java variable name, following which is the name of the column in the database table to which it maps to.

3.4) Creating the Spring Configuration File This section deals with configuring the various information needed for the Spring Framework. In Spring, all the business objects are configured in Xml file and the configured business objects are called Spring Beans. These Spring Beans are maintained by the IOC which is given to the Client Application upon request. Let us define a data source as follows, spring-hibernate.xml

<property name="driverClassName" value="com.mysql.jdbc.Driver"/> <property name="url" value="jdbc:mysql://localhost/samples"/> <property name="username" value="root"/> <property name="password" value="pwForRoot"/> …

The above bean defines a data-source of type 'org.apache.commons.dbcp.BasicDataSource'. More importantly, it defines the various connection properties that are needed for accessing the database. For accessing the MySql database, we need MySql database driver which can be downloaded from http://dev.mysql.com/downloads/connector/j/5.1.html. The first property called driverClassName should point to the class name of the MySql Database Driver. The second property url represents the URL string which is needed to connect to the MySql Database. The third and the fourth properties represent the database username and the password needed to open up a database session. Now, let us define the second Spring Bean which is the SessionFactoryBean. If you would have programmed in Hibernate, you will realize that SessionFactoryBean is responsible for creating Session objects through which Transaction and Data accessing is done. Now the same SessionFactoryBean has to be configured in Spring's way as follows,

<property name="dataSource" ref="myDataSource"/> <property name="mappingResources"> <list> ./resources/employee.hbm.xml <property name="hibernateProperties"> hibernate.dialect=org.hibernate.dialect.HSQLDialect

To make the SessionFactoryBean to get properly configured, we have given two mandatory information. One is the data-source information which contains the details for accessing the database. This we have configured already in the previous step and have referred it here using the 'ref' attribute in the 'property' tag. The second one is a list of Mapping files which contains the mapping information between the database tables and the Java class names. We have defined one such mapping file in section 2 and have referenced the same here with the 'list' tag. The 3rd important Spring Bean is the Hibernate Template. It provides a wrapper for low-level data accessing and manipulation. Precisely, it contains methods for inserting/delting/updating/finding data in the database. For the Hibernate Template to get configured, the only argument is the SessionFactoryBean object as represented in the following section,

<property name="sessionFactory">

The final Bean definition is the Dao class which is the client facing class. Since this class has to be defined in the Application level, it can contain any number of methods for wrapping data access to the Client. Since we know that it is the Hibernate Template class that interacts with the database, it will be ideal a refer an instance of Hibernate Template to the Dao class.

<property name="hibernateTemplate">

Note that a reference is made to EmployeeDao class which is discussed in the forthcoming section. 3.5) Defining the EmployeeDao class As described earlier, this EmployeeDao class can contain any number of methods that can be accessed by the clients. The design of this class can fall under two choices. One is this class can directly depend on the Hibernate Template object which is injected by the IOC for accessing the data. The second one is that it can make use of the Hibernate API for data accessing. The declaration of the class is as follows, EmployeeDao.java

package javabeat.spring.hibernate; import import import import

java.sql.SQLException; org.hibernate.HibernateException; org.hibernate.Session; org.springframework.orm.hibernate3.HibernateCallback;

import org.springframework.orm.hibernate3.HibernateTemplate; public class EmployeeDao { private HibernateTemplate hibernateTemplate; public void setHibernateTemplate(HibernateTemplate hibernateTemplate){ this.hibernateTemplate = hibernateTemplate; } public HibernateTemplate getHibernateTemplate(){ return hibernateTemplate; } public Employee getEmployee(final String id){ HibernateCallback callback = new HibernateCallback() { public Object doInHibernate(Session session) throws HibernateException,SQLException { return session.load(Employee.class, id); } }; return (Employee)hibernateTemplate.execute(callback); }

}

public void saveOrUpdate(final Employee employee){ HibernateCallback callback = new HibernateCallback() { public Object doInHibernate(Session session) throws HibernateException,SQLException { session.saveOrUpdate(employee); return null; } }; hibernateTemplate.execute(callback); }

This class makes use of Hibernate API (particularly the Session object) for data accessing. To instruct Spring to access the Hibernate API, we have the put the piece of logic that makes use of the Hibernate API into a particular well defined method in a well known interface that Spring knows. It happens to be the HibernateCallback interface with the method doInHibernate() with an instance of Hibernate Session being passed. Note that we have defined two methods; getEmployee() and saveOrUpdate in the EmployeeDao class. And to make use of the Hibernate APIs, we have defined the code in the HibernateCallback.doInHibernate() method and have informed Spring to execute this code by passing the interface reference to the HibernateTemplate.execute() method. 3.6) The Client Application SpringHibernateTest.java

package javabeat.spring.hibernate; import org.springframework.beans.factory.BeanFactory;

import import import import

org.springframework.beans.factory.xml.XmlBeanFactory; org.springframework.core.io.FileSystemResource; org.springframework.core.io.Resource; org.springframework.orm.hibernate3.LocalSessionFactoryBean;

public class SpringHibernateTest { public static void main(String[] args) { Resource resource = new FileSystemResource( "./src/resources/spring-hibernate.xml"); BeanFactory factory = new XmlBeanFactory(resource); Employee employee = new Employee(); employee.setId("123"); employee.setName("ABC"); employee.setAge(20); employee.setSalary(15000.00d); EmployeeDao employeeDao = (EmployeeDao)factory.getBean( "employeeDao"); employeeDao.saveOrUpdate(employee);

}

Employee empResult = employeeDao.getEmployee("123"); System.out.println(empResult);

}

Finally, we come to the sample client Application for accessing the test data. The control goes like this. When the method BeanFactory.getBean("employeeDao") is called, Spring finds the references made in the Bean definition of Employee Dao Bean. It happens to be the Hibernate Template object. Then an attempt will be made to initialize the Hibernate Template object where it will see that a Session Factory Bean object is referenced. Then, while constructing the Session Factory Bean object, the data-source information will get resolved along with the database tables and the Java classes. 4) Conclusion This article was aimed at discussing about Integration of Spring with Hibernate. It discussed the need for such an integration and also briefed about the benefits that it offers. Then, a very detailed step-by-step sample was given to clearly illustrate how the integration works.