1. Creative Commons Attribution- ShareAlike 2.5 License Sakai Programmer's Café Sakai Oxford Tetra ELF Workshop Sakai Persistence and Intro to Hibernate Aaron Zeckoski aaronz@vt.edu

2. 2 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)

3. 3 How do we persist data in Sakai? Sakai has a single central database configured –Virtually every tool/component uses this Sakai in production is typically a clustered environment, and the clustering implementation is largely based around the database –Tools that will not cluster are very unlikely to be adopted into the core You should use the database in pretty much every case –File system storage can be appropriate for “bulky” resources (Files or Blobs) so data is accessible via WebDav

4. 4 Databases and Sakai Settings to control the database that is used are stored in sakai.properties –This is a standard format Java.properties file The sakai.properties file is located in your sakai home in the subdirectory named sakai –this is configurable but is normally your Tomcat home Look for the following line in the file # DATABASE CONFIGURATION

5. 5 Supported databases Only 3 databases are supported for use in Sakai 1.HSQLDB (*very* lightweight, pure-Java database, largely for testing and dev. Terrible performance characteristics – storage either in memory, or a flat text file) 2.MySQL (Open source, pretty good performance, used widely in production, some very unfortunate locking issues) 3.Oracle (Costs money, pretty rock solid, works well for those that can/are prepared to afford it) Since the majority of Sakai code uses raw SQL in some form, adding a new database is a major effort

6. 6 Some DB config tips Always leave auto.ddl=true –You might want to turn this off for production environments HSQLDB is turned on by default, it only stores data in memory by default HSQLDB works well for development and for demos –Caveat: You cannot look at the HSQLDB database without some serious trickery

7. 7 More DB config 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 If all else fails, switch to HSQLDB file storage

8. 8 HSQLDB file storage To use HSQLDB in file mode (where it stores data on the filesystem), comment out this line: url@javax.sql.BaseDataSource=jdbc:hsqldb:. and uncomment this one url@javax.sql.BaseDataSource= jdbc:hsqldb:${sakai.home}/db/sakai.db

9. 9 MySQL config To use MySQL, uncomment the six lines under this line: ## MySQL settings Comment out the 7 lines under this one: ## HSQLDB settings Update the username and password lines to match your MySQL database

10. 10 One last DB tip You can turn on verbose Hibernate logging in the sakai.properties file Change the following from false to true # enable hibernate SQL debugging output hibernate.show_sql=false –Note: You do NOT want to leave this on in a production environment

11. 11 3 ways to persist data to the DB JDBC http://java.sun.com/products/jdbc/ Spring JDBC http://www.springframework.org/docs/refere nce/jdbc.html Hibernate http://www.hibernate.org/

12. 12 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/http://java.sun.com/products/jdbc/

13. 13 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.htmlhttp://www.springframework.org/docs/reference/jdbc.html

14. 14 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/http://www.hibernate.org/

15. 15 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 http://www.hibernate.org/hib_docs/v3/reference/en/html/architecture.html

16. 16 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.htmlhttp://www.hibernate.org/354.html

17. 17 Hibernate Commentary Beyond the hype: –Hibernate *is* the best ORM persistence framework out there (probably in any language) –Not to say it is without numerous issues (ORM is a *very* tricky problem and cannot actually be solved “in general”) –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 rock-hard and cannot be changed, poor cross-ClassLoader behaviour) Advice –Use it cautiously! –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 (extreme) cases you may still actually want to fall back to SQL (this is actually recommended by the Hibernate team)

18. 18 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 ToolsHibernate Tools Bottom up (good for existing database or code conversion) –start with an existing data model –use the Hibernate Tools to generate the mapping documentsHibernate Tools –use the Hibernate Tools to generate skeletal Java codeHibernate Tools –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 codeHibernate Tools –fill in the business logic by hand –export the database tables using the Hibernate ToolsHibernate 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.htmlhttp://www.hibernate.org/355.html

19. 19 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 findByProperties If you do decide to use primitives, you cannot leave them null/unset when doing a findByProperties 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)

20. 20 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

21. 21 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

22. 22 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 http://www.hibernate.org/hib_docs/reference/en/html/best-practices.html

23. 23 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.htmlhttp://www.hibernate.org/255.html

24. 24 Hibernate in Sakai 3 ways of using Hibernate in Sakai 1.Create a SessionFactory using settings inside your tool 2.Create a SessionFactory from the global Sakai sessionFactoryBase 3.Add our HBMs to the global Sakai sessionFactory Sakai 2.2+ uses Hibernate 3  Previous versions used Hibernate 2 From: http://bugs.sakaiproject.org/confluence/display/BOOT/Hibernate+in+Sakai http://bugs.sakaiproject.org/confluence/display/BOOT/Hibernate+in+Sakai

25. 25 Method 1 Create a Hibernate SessionFactory using config settings in your toolSessionFactory –You should use this when connecting to an external database –Do not use this method to connect to the internal Sakai database! –More info on session configuration: http://www.hibernate.org/hib_docs/reference/en/html/session-configuration.html

26. 26 Method 2 Create a SessionFactory from the global Sakai SessionFactoryBaseSessionFactory –This is not the recommended method but if you are using a single Maven project for your app then you have to use it This method works well for simple tools –Demonstrated in tasklist-simple –More complex tools should use method 3 From: http://bugs.sakaiproject.org/confluence/display/BOOT/Creating+sessions+from+the+Sakai+SessionFactoryBase http://bugs.sakaiproject.org/confluence/display/BOOT/Creating+sessions+from+the+Sakai+SessionFactoryBase

27. 27 Method 3 Add our HBMs to the global Sakai SessionFactory using AddableSessionFactoryBean AddableSessionFactoryBean –This is the preferred method –Works best for all but the simplest apps –Requires the tool to deploy portions to shared and components so it cannot be used for simple tools Demonstrated in tasklist From: http://bugs.sakaiproject.org/confluence/display/BOOT/Using+the+Sakai+global+sessionFactory http://bugs.sakaiproject.org/confluence/display/BOOT/Using+the+Sakai+global+sessionFactory

28. 28 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)

29. 29 More on GenericDao Get the code and Javadocs from the VT Maven repository: –http://source.edtech.vt.edu/maven/generic-dao/http://source.edtech.vt.edu/maven/generic-dao/ Usage is demonstrated in the tasklist code here: –https://source.sakaiproject.org/contrib/programmer scafe/trunk/tasklist/https://source.sakaiproject.org/contrib/programmer scafe/trunk/tasklist/

30. 30 Let’s look at some code! Use the Sakai RAD tool in eclipse to create a simple CRUD app You can also look at the tasklist code here if you like: –https://source.sakaiproject.org/contrib/prog rammerscafe/trunk/tasklist/https://source.sakaiproject.org/contrib/prog rammerscafe/trunk/tasklist/ Let’s see what it takes to use Hibernate in Sakai…

31. 31 Update project.xml Add the Hibernate dependency to the tool/project.xml file Note that we use 3 property variables from master/project.properties ${sakai.hibernate.groupId} ${sakai.hibernate.artifactId} ${sakai.hibernate.version}

32. 32 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

33. 33 Package for the HBM Create a new Java package for the HBM (mapping file) –org.sakaiproject.toolname.impl.hbm Create a new file in this package –MyObject.hbm.xml

34. 34 Basic HBM template MYOBJECT_ID_SEQ

35. 35 Template customization Change the class name and table name –org.sakaiproject.toolname.model.MyObject Change the id sequence name Copy and paste the property block to add the properties from your persistent object –owner –siteId –creationDate Etc…

36. 36 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 HibernateDaoSupportHibernateDaoSupport –part of Spring-Hibernate Add import for HibernateDaoSupportHibernateDaoSupport –Make sure you use the one for hibernate 3 Or use Generic DAO package!

37. 37 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

38. 38 Adding our HBMs to the Sakai Global SessionFactory <bean id="org.sakaiproject.yourapp.hibernate. AdditionalHibernateMappings" class="org.sakaiproject.springframework.orm.hibernate. impl.AdditionalHibernateMappingsImpl"> org/sakaiproject/yourapp/impl/hbm/Item1.hbm.xml This allows us to use our persistent objects with the Sakai Global SessionFactory

39. 39 Inject the Global SessionFactory into the DAO This connects the new DAO to Hibernate The DAO implementation should extend HibernateDaoSupport <bean id="org.sakaiproject.yourapp.dao.MyToolDaoTarget" class="org.sakaiproject.yourapp.dao.impl.MyToolDaoImpl"> <ref bean="org.sakaiproject.springframework.orm. hibernate.GlobalSessionFactory" />

40. 40 Define a declarative transaction interceptor If your operations are not in a transaction you will see many problems, especially in MySQL This involves much less work than opening and closing transactions in code, and is more reliable <bean id="org.sakaiproject.yourapp.dao.MyToolDao” class="org.springframework.transaction.interceptor.TransactionProxyFactoryBean"> <ref bean= "org.sakaiproject.springframework.orm.hibernate.GlobalTransactionManager" /> PROPAGATION_REQUIRED

41. 41 Any questions? Check out the section on persistence in the Programmers’ Café for more information http://bugs.sakaiproject.org/confluence/display/BOOT/Persistence Hibernate: http://www.hibernate.org/http://www.hibernate.org/ Spring ORM http://www.springframework.org/docs/reference/orm.html