Annotations

Annotation Jdk1.6 Pawan Modi Senior R&D Engineer

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Agenda • • • • • •

Introduction First Example Defining Annotations Annotations Types & Usage Example Summary

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Introduction  Annotations are generally a way to add meta-data information to an element.  An annotation type definition takes an "at" (@) sign, followed by the interface keyword plus the annotation name.  An element can be a class, method, field, constructor, variables etc.  This meta-data are processed by the tools i.e. compilers, javadoc, etc. 6/25/2009

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First Example

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Defining Annotation Some rules to define an annotation type

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 Annotation declaration should start with an 'at' sign like @, following with an interface keyword, following with the annotation name.  Method declarations should not have any parameters.  Method declarations should not have any throws clauses.  Return types of the method should be one of the following:     

primitives String Class enum array of the above types

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Annotation Types 

There are two types of annotations available in JDK5 ◦ Simple annotations:  Just to annotate your code.  Developer cannot use them to create a custom annotation type.  There are only three types of simple annotations provided by JDK5  Override  Deprecated  Suppresswarnings

◦ Meta annotations:  These are the annotation types designed for annotating annotationtype declarations.  Simply speaking, these are called the annotations-of-annotations.

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Simple Annotations Override ◦ This annotation tells compiler that method must be an overridden method.

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Simple Annotations Conti… 

Deprecated ◦ This annotation informs client not to use this method / class anymore because in the future versions this old method / class may not be supported. ◦ Clients may prepare themselves not to depend on the old method anymore.

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Simple Annotations Conti…

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Simple Annotations Conti… 

SuppressWarnings ◦ From Java 5.0, references to any of the Collection types should be parameterized. Else the compiler will issue a warning message. The solution for this problem is to make use of @SuppressWarnings annotation.

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Simple Annotations Conti…

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Annotation Types Conti… 

Meta annotations: ◦ These are the annotation types designed for annotating annotation-type declarations. Simply speaking, these are called the annotations-of-annotations. ◦ There are four types of meta-annotations.    

Target Retention Documented Inherited 6/25/2009

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Meta Annotations 

Target  tells for which element type this annotation is applicable  It contains the following enumerated types as its value:  @Target(ElementType.TYPE)—can be applied to any element of a class  @Target(ElementType.FIELD)—can be applied to a field or property  @Target(ElementType.METHOD)—can be applied to a method level annotation  @Target(ElementType.PARAMETER)—can be applied to the parameters of a method  @Target(ElementType.CONSTRUCTOR)—can be applied to constructors  @Target(ElementType.LOCAL_VARIABLE)—can be applied to local variables  @Target(ElementType.ANNOTATION_TYPE)—indicates that the declared type itself is an annotation type 6/25/2009

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Meta Annotations Conti . . .

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Meta Annotations Conti… Retention  Tells what extent the Annotations should be retained.  The three possible ways of telling this are,  RetentionPolicy.SOURCE—Annotations with this type will be by retained only at the source level and will be ignored by the compiler  RetentionPolicy.CLASS—Annotations with this type will be by retained by the compiler at compile time, but will be ignored by the VM  RetentionPolicy.RUNTIME—Annotations with this type will be retained by the VM so they can be read only at run-time

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Meta Annotations Conti…

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Meta Annotations Conti… Document ◦ indicates that an annotation with this type should be documented by the javadoc tool ◦ By default, annotations are not included in javadoc ◦ But if @Documented is used, it then will be processed by javadoc-like tools and the annotation type information will also be included in the generated document.

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Meta Annotations Conti…

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Meta Annotations Conti… Inherited ◦ It indicates that the annotated class with this type is automatically inherited. ◦ if you define an annotation with the @Inherited tag, then annotate a class with your annotation, and finally extend the class in a subclass. ◦ All properties of the parent class will be inherited into its subclass

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Meta Annotations Conti…

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Example

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Example

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Example Conti…

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Summary  Annotation is a special form of syntactic metadata that can be added to Java source code.  They provide information about various elements of a Java class.  Classes, methods, variables, parameters and packages may be annotated.  There are seven annotations provided in the J2SE 5.0 release.  @Deprecated, @Override, @SuppressWarnings, @Documented, @Inherited, @Retention and @Target.

 These are not data about data but data about classes, methods, instance and static variables etc 6/25/2009

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References Online Tutorial  http://en.wikipedia.org/wiki/Java_annotation  http://www.javabeat.net/articles/30-annotationsin-java-50-1.html  http://www.developer.com/java/other/article.php/ 10936_3556176_1  http://java.sun.com/j2se/1.5.0/docs/guide/language/ annotations.html

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Question ?? Kindly send all you queries to the following id. [email protected]

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Thank you!

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