Hibernate In Practice
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Hibernate in Practice Rob Harrop
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Introductions • • • •
Rob Harrop VP, Interface21 Core developer on Spring Frequent presenter (JUGs, JavaOne, JavaPolis, OSCON...) • Consultant with Fortune 500 clients (mainly banking) and government • Author of best-selling Pro Spring
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Agenda • What you need to know to be effective – Mapping object graphs – Session management – Fetching strategies
• Hibernate favourites – Criteria API – Testing
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
What you need to know to be effective • Getting started with Hibernate is easy • Unfortunately, so is getting it wrong! • Mastering some central concepts makes success much more likely – Domain mapping – Session handling – Fetching
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Mapping object graphs • Aims – Loose coupling – High cohesion – Accurate mapping of the domain
• Avoid – Total navigability – Relationships to simplify queries
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Comparing Mapping Styles
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Comparing Mapping Styles
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Mapping object graphs • Encapsulate structural logic – Support code evolution – Provide a cleaner API
• Avoid mapping every relationship – Reduce coupling – Ease maintenance
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Session Management • Session handling can make or break an application • Too fine-grained and you lose any performance benefits and real transparent persistence • Too coarse-grained and you risk memory usage problems and slow flushes.
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Session per Operation • Do not do this!
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Options • Session per transaction – Session lifecycle is synchronized with transaction boundary – Do not use transaction per operation :)
• Long running session – Sessions live across multiple transactions
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Issues with Long Running Sessions • Memory usage – Keep Sessions around in memory
• Unexpected flushes • Manual management patterns – Filter/ThreadLocal
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Session per transaction • Sessions live for the duration of a transaction • DAOs share the same session and transaction • Sessions and transactions are managed transparently – Do not pass them around in method signatures
• This gives a clean unit of work within which persistence logic can be handled
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Session per transaction
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Open Session in View • • • •
Steer clear wherever possible Unclear transactional boundaries Potential for incorrect data Real potential for poor performance – n+1 selects
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Fetch Joins • Fetch joins allows eager loading to be selected on a per association, per query basis – No need to make everything eager – No need to use OSiV
• Typically associations are lazy by default and then eagerly fetched as needed
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Nuances with Fetch Joins • Unexpected number of results • The exact number of queries may surprise you
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Criteria API • Programmatic construction of queries • Support for all common operators – =, <>, >, <, IN, EXISTS, BETWEEN, LIKE
• Full support for projections • Support for custom SQL – Projection and selection
• Additional features – Paging, sorting, grouping
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Criteria API • Simple query Criteria c = session.createCriteria(Product.class); c.add(Restrictions.eq("name", value)); return c.list();
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Criteria API • Query and projection Criteria c = session.createCriteria(Product.class); c.add(eq("name", value)); c.setProjection(distinct(property("name")));
Static imports from Projections Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Criteria API • Using SQL Criteria c = session.createCriteria(Product.class); c.add(sqlRestriction("{alias}.name = ?", value, Hibernate.STRING)); return c.list();
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
When to use the Criteria API • Ad-hoc querying • Saved queries • Conditional queries
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Testing • Testing tools: – DBUnit – Spring Mock
• Testing strategy – Test with Hibernate – Verify with JDBC – Work against known sets of data
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Useful Resources • http://ww.hibernate.org/hib_docs/v3/reference/en/html/ • Java Persistence with Hibernate (King, Bauer) • DBUnit - http://dbunit.sourceforge.net/
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Introductions • • • •
Rob Harrop VP, Interface21 Core developer on Spring Frequent presenter (JUGs, JavaOne, JavaPolis, OSCON...) • Consultant with Fortune 500 clients (mainly banking) and government • Author of best-selling Pro Spring
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Agenda • What you need to know to be effective – Mapping object graphs – Session management – Fetching strategies
• Hibernate favourites – Criteria API – Testing
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
What you need to know to be effective • Getting started with Hibernate is easy • Unfortunately, so is getting it wrong! • Mastering some central concepts makes success much more likely – Domain mapping – Session handling – Fetching
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Mapping object graphs • Aims – Loose coupling – High cohesion – Accurate mapping of the domain
• Avoid – Total navigability – Relationships to simplify queries
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Comparing Mapping Styles
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Comparing Mapping Styles
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Mapping object graphs • Encapsulate structural logic – Support code evolution – Provide a cleaner API
• Avoid mapping every relationship – Reduce coupling – Ease maintenance
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Session Management • Session handling can make or break an application • Too fine-grained and you lose any performance benefits and real transparent persistence • Too coarse-grained and you risk memory usage problems and slow flushes.
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Session per Operation • Do not do this!
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Options • Session per transaction – Session lifecycle is synchronized with transaction boundary – Do not use transaction per operation :)
• Long running session – Sessions live across multiple transactions
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Issues with Long Running Sessions • Memory usage – Keep Sessions around in memory
• Unexpected flushes • Manual management patterns – Filter/ThreadLocal
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Session per transaction • Sessions live for the duration of a transaction • DAOs share the same session and transaction • Sessions and transactions are managed transparently – Do not pass them around in method signatures
• This gives a clean unit of work within which persistence logic can be handled
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Session per transaction
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Open Session in View • • • •
Steer clear wherever possible Unclear transactional boundaries Potential for incorrect data Real potential for poor performance – n+1 selects
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Fetch Joins • Fetch joins allows eager loading to be selected on a per association, per query basis – No need to make everything eager – No need to use OSiV
• Typically associations are lazy by default and then eagerly fetched as needed
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Nuances with Fetch Joins • Unexpected number of results • The exact number of queries may surprise you
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Criteria API • Programmatic construction of queries • Support for all common operators – =, <>, >, <, IN, EXISTS, BETWEEN, LIKE
• Full support for projections • Support for custom SQL – Projection and selection
• Additional features – Paging, sorting, grouping
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Criteria API • Simple query Criteria c = session.createCriteria(Product.class); c.add(Restrictions.eq("name", value)); return c.list();
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Criteria API • Query and projection Criteria c = session.createCriteria(Product.class); c.add(eq("name", value)); c.setProjection(distinct(property("name")));
Static imports from Projections Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Criteria API • Using SQL Criteria c = session.createCriteria(Product.class); c.add(sqlRestriction("{alias}.name = ?", value, Hibernate.STRING)); return c.list();
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
When to use the Criteria API • Ad-hoc querying • Saved queries • Conditional queries
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Testing • Testing tools: – DBUnit – Spring Mock
• Testing strategy – Test with Hibernate – Verify with JDBC – Work against known sets of data
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
Useful Resources • http://ww.hibernate.org/hib_docs/v3/reference/en/html/ • Java Persistence with Hibernate (King, Bauer) • DBUnit - http://dbunit.sourceforge.net/
Copyright 2004-2006, Interface21 Ltd. Copying, publishing, or distributing without expressed written permission is prohibited.
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