Persistence hibernate
- 1. 1 What is persistence? • The storage of an object on a disk or other permanent storage device or data that exists from session to session – as opposed to transient data • Persistent data typically implies that it is durable (i.e. will survive a crash or shutdown of the process) usually with some guarantee of integrity • Persistence generally implies use of a database – One could use the file system (with suitably careful procedures)
- 2. 2 3 ways to persist data to the DB JDBC http://java.sun.com/products/jdbc/ Spring JDBC http://www.springframework.org/docs/reference/ Hibernate http://www.hibernate.org/
- 3. 3 JDBC Info • Java Database Connectivity • Industry standard but has some issues: – The developer needs to deal with lot of plumbing and infrastructure, such as endless try-catch-finally-try-catch blocks. – Applications need complex error handling to ensure that connections are properly closed after they're used, which makes the code verbose, bloated, and repetitive. – JDBC uses the rather uninformative SQLException. – JDBC has no exception hierarchy • Bottom Line: Don’t use this! From: http://java.sun.com/products/jdbc/
- 4. 4 Spring JDBC Info • Abstraction framework for JDBC – i.e. It does lots of stuff for you! • Some features of Spring JDBC – JdbcDaoSupport – superclass, provides JdbcTemplate access – Spring provides an abstract exception layer, moving verbose and error-prone exception handling out of application code into the framework. The framework takes care of all exception handling; application code can concentrate on using appropriate SQL. – Spring provides a significant exception hierarchy for your application code to work with in place of SQLException. – For creating instances of oracle.sql.BLOB (binary large object) and oracle.sql.CLOB(character large object), Spring provides the class org.springframework.jdbc.support.lob.OracleLobHandler. • Bottom Line: If you love writing SQL, use this! From: http://www.springframework.org/docs/reference/jdbc.html
- 5. 5 Hibernate Info • Object / Relational mapping (ORM) and persistence / query framework – i.e. It does even more stuff for you! • Some features of Hibernate – HibernateDaoSupport – superclass, easy HibernateTemplate access – Database independence - sits between the database and your java code, easy database switch without changing any code – Object / Relational Mapping (ORM) - Allows a developer to treat a database like a collection of Java objects – Object oriented query language (HQL) - *Portable* query language, supports polymorphic queries etc. – You can also still issue native SQL, and also queries by “Criteria” (specified using “parse tree” of Java objects) – Hibernate Mapping - Uses HBM XML files to map value objects (POJOs) to database tables – Transparent persistence - Allows easy saves/delete/retrieve for simple value objects – Very high performance “in general” due to intelligent (2-level) caching, although in a few cases hand-written SQL might beat it From: http://www.hibernate.org/
- 6. 6 More Hibernate Info • Hibernate basically sits between the DB and your code • Can map persistent objects to tables • In Sakai, the Hibernate configuration is set for you already From: http://www.hibernate.org/hib_docs/v3/reference/en/html/architecture.html
- 7. 7 Even more Hibernate Info • Hibernate 2-tier web architecture • Can send data to JDBC or XML files • Best to just use it the way Sakai does (JDBC) • Bottom Line: Use this! From: http://www.hibernate.org/354.html
- 8. 8 Hibernate Commentary • Beyond the hype: – Hibernate *is* the best ORM persistence framework • probably in any language – Not to say it is without numerous issues • ORM is a tricky problem and general solutions are very difficult – Many aspects of the Hibernate framework are “over-eager” • lazy Collections, cascade options, long transactions – Many aspects of Hibernate are overly rigid • proxy behaviour, initial configuration sets cannot be changed, poor cross-ClassLoader behaviour • Advice – Use it cautiously! (pay attention to tips) – Avoid lazy Collections, be conservative with cascade options – In general just use it on one entity at a time, with explicit save/load on for each database operation – In some cases you may still actually want to fall back to SQL • recommended by the Hibernate team for certain situations
- 9. 9 Some database tips • Always turn on hbm2ddl.auto <prop key="hibernate.hbm2ddl.auto">update</prop> • You may want to turn this off for production environments • HSQLDB works well for development and for demos – Caveat: You cannot look at the HSQLDB database without some serious trickery • If all else fails, switch to HSQLDB file storage
- 10. 10 More database tips • MySQL despite being a “production” option is actually really easy to set up for development – Allows you to look at the database through its console to see if things are working – Works well on most platforms and tends to get into a lock state somewhat easily which helps you find transaction problems
- 11. 11 One last database tip • You can turn on verbose Hibernate logging to see every SQL statement that it runs – Change the following from false to true <prop key="hibernate.show_sql">false</prop> • Note: You do NOT want to leave this on in a production environment
- 12. 12 Hibernate Development • 4 methods of development using Hibernate • Top down (good for existing code) – implement a Java (JavaBeans) object model – write a mapping document by hand, or generate it from XDoclet tags – export the database tables using the Hibernate Tools • Bottom up (good for existing database or code conversion) – start with an existing data model – use the Hibernate Tools to generate the mapping documents – use the Hibernate Tools to generate skeletal Java code – fill in the business logic by hand • Middle out (good for new development) – express your conceptual object model directly as a mapping document – use the Hibernate Tools to generate skeletal Java code – fill in the business logic by hand – export the database tables using the Hibernate Tools • Meet in the middle (good for existing JDBC to Hibernate switch) – start with an existing data model and existing Java classes – write a mapping document to adapt between the two models From: http://www.hibernate.org/355.html
- 13. 13 Hibernate Tips - Avoid primitives • Don’t use primitives for properties on persistent objects – This works fine in general but it does not work if you are doing a findByExample • If you do decide to use primitives, you cannot leave them null/unset when doing a findByExample or they will be set to the default value for that primitive – Things seem to work better when not using primitives sometimes (e.g. Boolean)
- 14. 14 Hibernate Tips - don’t preset values • Don’t set the values of persistent objects in the POJO – This can cause problems with frameworks that expect to be able to instantiate the POJO with all properties unset – It may be more work to set the properties for all non-null attributes but it is worth it
- 15. 15 Hibernate Tips - save dependent objects first • If you have any dependent entities as properties of a persistent object you *must* save them before saving the parent class – Hibernate has numerous “cascade” options that claim to do this automatically, but it is best to start simple – The same thing goes for deleting
- 16. 16 Hibernate Tips - non-primitive generated ids • Use non-primitive generated ids for the primary key of persistent objects – It is more efficient and is a good idea in most databases anyway – Use java.lang.Long or java.lang.String for best results • More best practices here: http://www.hibernate.org/hib_docs/reference/en/html/best-practices.html
- 17. 17 Hibernate Tools • Hibernate provides a set of Eclipse tools http://www.hibernate.org/255.html – Mapping Editor: An editor for Hibernate XML mapping files, supporting auto-completion and syntax highlighting – Console: a view in Eclipse. Provides a tree overview of console configurations and interactive view of persistent classes and relationships. Also allows the execution of HQL queries against your database and browsing of results in Eclipse. – Development Wizards: Includes the Hibernate configuration (cfg.xml) files wizard and reverse engineering wizard for turning an existing database schema into POJO source files and HBM files. From: http://www.hibernate.org/255.html
- 18. 18 Using hibernate in your app • Create a Hibernate SessionFactory using config settings in your app – You should only create one Session Factory per database • You can create another one when connecting to an external database – More info on session configuration: http://www.hibernate.org/hib_docs/reference/en/html/session-configuration.html
- 19. 19 Use the Generic Dao package • The GenericDao is an abstraction layer that will allow you to use Hibernate with your persistent objects without needing to write a DAO at all • It has usage information in the Javadocs • Highly configurable and extendable • Has no Hibernate dependencies in the interfaces (*any* DAO should be like this) URL: http://bugs.sakaiproject.org/confluence/display/BOOT/Generic+DAO+package
- 20. 20 More on GenericDao • Get the code and javadocs from the VT Maven repository: – http://source.edtech.vt.edu/maven/generic-dao/ • Usage (Sakai related) is demonstrated in the tasklist code here: – https://source.sakaiproject.org/contrib/programmerscafe/
- 21. 21 Let’s look at some code! • Let’s see what it takes to use Hibernate – Hibernate and Spring packages – Hibernate mapping file(s) – Hibernate properties file – Hibernate related Spring beans – DAO beans
- 22. 22 Hibernate and Spring packages • Download the Hibernate Core from: http://www.hibernate.org/6.html – Get at least version 3.1.3 • Download the Spring framework here: http://www.springframework.org/download – Get version 1.2.8 for now – Version 2.0 is risky, wait for patches
- 23. 23 Hibernate Mapping Files • Hibernate uses an XML file to map Java objects onto database columns • We will create our mapping file from a simple template attached to the persistence page • For applications with many tables, use a tool to help generate the HBM files
- 24. 24 Basic HBM template <?xml version="1.0"?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> <hibernate-mapping> <class name="org.sakaiproject.toolname.model.MyObject" table="TOOLNAME_MYOBJECT"> <id name="id" type="long"> <generator class="native"> <param name="sequence">MYOBJECT_ID_SEQ</param> </generator> </id> <property name="myProperty" type="string" length="255" not-null="true”/> </class> </hibernate-mapping>
- 25. 25 Template customization • Change the class name and table name – edu.vt.group.toolname.model.MyObject • Change the id sequence name • Copy and paste the property block to add the properties from your persistent object – owner – title – creationDate • Etc…
- 26. 26 Creating a DAO for Hibernate • Create a new class which implements your DAO interface – Write a DAO interface if you do not have one • Extend HibernateDaoSupport – part of Spring-Hibernate • Add import for HibernateDaoSupport – Make sure you use the one for hibernate 3 • Or use Generic DAO package!
- 27. 27 DAO sample code public interface MyAppDAO { } Make an interface for your DAO public class MyAppDAOImpl extends HibernateDaoSupport implements MyAppDAO { } Make an implementation of the DAO interface Note that it Extends HibernateDaoSupport
- 28. 28 Spring configuration • Now we need to tie everything together with Spring • First we will tell hibernate about our MyObject.hbm.xml mapping file • Next we will give the hibernate stuff to our DAO implementation • Finally we will tie the new DAO to the rest of the webapp
- 29. 29 Create a Data Source <bean id=“myLocalDataSource" class="org.springframework.jdbc.datasource.DriverManagerDataSource"> <property name="driverClassName"> <value>oracle.jdbc.driver.OracleDriver</value> </property> <property name="url"> <value>jdbc:oracle:thin:@myDB.host.com:1521:SCHEMA</value> </property> <property name="username"> <value>USERNAME</value> </property> <property name="password"> <value>PASSWORD</value> </property> </bean> • Setup the connection settings for the database
- 30. 30 Create a SessionFactory (part 1) <bean id=“myAppSessionFactory" class="org.springframework.orm.hibernate3.LocalSessionFactoryBean"> <property name="dataSource"> <ref local=“myLocalDataSource" /> </property> <property name="mappingResources"> <list> <value> com/group/myapp/impl/hbm/MyObject.hbm.xml </value> </list> </property> ... • This ties our persistent objects with the newly created SessionFactory bean
- 31. 31 Create a SessionFactory (part 2) <property name="hibernateProperties"> <props> <prop key="hibernate.dialect"> org.hibernate.dialect.Oracle9Dialect</prop> <prop key="hibernate.show_sql">false</prop> <prop key="hibernate.cache.provider_class"> org.hibernate.cache.EhCacheProvider</prop> <prop key="hibernate.query.substitutions">true 1, false 0</prop> <prop key="hibernate.hbm2ddl.auto">update</prop> <!-- create, update, create-drop (wipe and create), or blank --> </props> </property> </bean> • This sets up the various properties (could also come from a props file)
- 32. 32 Create a transaction manager <bean id=“myAppTransactionManager" class="org.springframework.orm.hibernate3.HibernateTransactionManager"> <property name="sessionFactory"> <ref local=“myAppSessionFactory" /> </property> </bean> • Creates a spring transaction manager – We need this in order to manage transactions in a reasonable way later – You can manage them manually, but why?
- 33. 33 Create a DAO bean <bean id=“com.group.myapp.dao.target.MyAppDAO" class=“com.group.myapp.dao.impl.MyAppDAOImpl" init-method="init"> <property name="sessionFactory"> <ref local=“myAppSessionFactory" /> </property> </bean> • Create a DAO bean using the data access object class that we have created • This injects the SessionFactory into that class bean
- 34. 34 Define a declarative transaction interceptor • This involves much less work than opening and closing transactions in code, and is more reliable – Note that this is what we will access, not the actual DAO bean (the use of the name of the interface is a convention, not a requirement) <bean id=“com.group.myapp.dao.MyAppDAO" class="org.springframework.transaction.interceptor.TransactionProxyFactoryBean"> <property name="target"> <ref local=“com.group.myapp.dao.target.MyAppDAO"/> </property> <property name="transactionManager"> <ref bean=“myAppTransactionManager" /> </property> <property name="transactionAttributes"> <props> <prop key="*">PROPAGATION_REQUIRED</prop> </props> </property> </bean>
- 35. 35 Use Hibernate in code • Access the persistent objects just like you would any normal java POJO • Use the dao operations (save, delete, etc.) to control the lifetimes of objects • Take advantage of the Hibernate tools
- 36. 36 Example App revisit • Same basic structure – Alpha is the main class – Bravo handles user interaction – Charlie handles application logic – Delta handles data access • New implementation of the Delta interface – UserString model class and hbm Alpha Charlie Bravo Delta A B = A depends on B DeltaHibernate UserString (hbm and class)
- 37. 37 Changes to Example App • Implemented Delta interface using Spring HibernateDaoSupport • Adjusted bean definitions to point to the new implementation • Created hbm file and model class • Added bean definitions for Hibernate Programmers Cafe - Example App Spring Hibernate
- 38. 38 Any questions? • Hibernate: http://www.hibernate.org/ • Spring ORM http://www.springframework.org/docs/reference/orm.html