This tutorial explains a setup of Hibernate 3.0 with the Apache Tomcat servlet container for a web-based application. Hibernate works well in a managed environment with all major J2EE application servers, or even in standalone Java applications. The database system used in this tutorial is PostgreSQL 7.4, support for other database is only a matter of changing the Hibernate SQL dialect configuration and connection properties. First, we have to copy all required libraries to the Tomcat installation. We use a separate web context (webapps/quickstart) for this tutorial, so we've to consider both the global library search path (TOMCAT/common/lib) and the classloader at the context level in webapps/quickstart/WEB-INF/lib (for JAR files) and webapps/quickstart/WEB-INF/classes. We refer to both classloader levels as the global classpath and the context classpath. Now, copy the libraries to the two classpaths: Copy the JDBC driver for the database to the global classpath. This is required for the DBCP connection pool software which comes bundled with Tomcat. Hibernate uses JDBC connections to execute SQL on the database, so you either have to provide pooled JDBC connections or configure Hibernate to use one of the directly supported pools (C3P0, Proxool). For this tutorial, copy the pg74jdbc3.jar library (for PostgreSQL 7.4 and JDK 1.4) to the global classloaders path. If you'd like to use a different database, simply copy its appropriate JDBC driver. Never copy anything else into the global classloader path in Tomcat, or you will get problems with various tools, including Log4j, commons-logging and others. Always use the context classpath for each web application, that is, copy libraries to WEB-INF/lib and your own classes and configuration/property files to WEB-INF/classes. Both directories are in the context level classpath by default. Hibernate is packaged as a JAR library. The hibernate3.jar file should be copied in the context classpath together with other classes of the application. Hibernate requires some 3rd party libraries at runtime, these come bundled with the Hibernate distribution in the lib/ directory; see . Copy the required 3rd party libraries to the context classpath. Library Description antlr (required) Hibernate uses ANTLR to produce query parsers, this library is also needed at runtime. dom4j (required) Hibernate uses dom4j to parse XML configuration and XML mapping metadata files. CGLIB (required) Hibernate uses the code generation library to enhance classes at runtime (in combination with Java reflection). Commons Collections, Commons Logging (required) Hibernate uses various utility libraries from the Apache Jakarta Commons project. EHCache (required) Hibernate can use various cache providers for the second-level cache. EHCache is the default cache provider if not changed in the configuration. Log4j (optional) Hibernate uses the Commons Logging API, which in turn can use Log4j as the underlying logging mechanism. If the Log4j library is available in the context library directory, Commons Logging will use Log4j and the log4j.properties configuration in the context classpath. An example properties file for Log4j is bundled with the Hibernate distribution. So, copy log4j.jar and the configuration file (from src/) to your context classpath if you want to see whats going on behind the scenes. Required or not? Have a look at the file lib/README.txt in the Hibernate distribution. This is an up-to-date list of 3rd party libraries distributed with Hibernate. You will find all required and optional libraries listed there. We now set up the database connection pooling and sharing in both Tomcat and Hibernate. This means Tomcat will provide pooled JDBC connections (using its builtin DBCP pooling feature), Hibernate requests theses connections through JNDI. Tomcat binds the connection pool to JNDI, we add a resource declaration to Tomcats main configuration file, TOMCAT/conf/server.xml: factory org.apache.commons.dbcp.BasicDataSourceFactory url jdbc:postgresql://localhost/quickstart driverClassNameorg.postgresql.Driver username quickstart password secret maxWait 3000 maxIdle 100 maxActive 10 ]]> The context we configure in this example is named quickstart, its base is the TOMCAT/webapp/quickstart directory. To access any servlets, call the path http://localhost:8080/quickstart in your browser (of course, adding the name of the servlet as mapped in your web.xml). You may also go ahead and create a simple servlet now that has an empty process() method. Tomcat provides connections now through JNDI at java:comp/env/jdbc/quickstart. If you have trouble getting the connection pool running, refer to the Tomcat documentation. If you get JDBC driver exception messages, try to setup JDBC connection pool without Hibernate first. Tomcat & JDBC tutorials are available on the Web. Your next step is to configure Hibernate. Hibernate has to know how it should obtain JDBC connections We use Hibernates XML-based configuration. The other approach, using a properties file, is equivalent in features, but doesn't offer any advantages. We use the XML configuration because it is usually more convenient. The XML configuration file is placed in the context classpath (WEB-INF/classes), as hibernate.cfg.xml: java:comp/env/jdbc/quickstart false org.hibernate.dialect.PostgreSQLDialect ]]> We turn logging of SQL commands off and tell Hibernate what database SQL dialect is used and where to get the JDBC connections (by declaring the JNDI address of the Tomcat bound pool). The dialect is a required setting, databases differ in their interpretation of the SQL "standard". Hibernate will take care of the differences and comes bundled with dialects for all major commercial and open source databases. A SessionFactory is Hibernate's concept of a single datastore, multiple databases can be used by creating multiple XML configuration files and creating multiple Configuration and SessionFactory objects in your application. The last element of the hibernate.cfg.xml declares Cat.hbm.xml as the name of a Hibernate XML mapping file for the persistent class Cat. This file contains the metadata for the mapping of the POJO class Cat to a datbase table (or tables). We'll come back to that file soon. Let's write the POJO class first and then declare the mapping metadata for it. Hibernate works best with the Plain Old Java Objects (POJOs, sometimes called Plain Ordinary Java Objects) programming model for persistent classes. A POJO is much like a JavaBean, with properties of the class accessible via getter and setter methods, shielding the internal representation from the publicly visible interface (Hibernate can also access fields directly, if needed): Hibernate is not restricted in its usage of property types, all Java JDK types and primitives (like String, char and Date) can be mapped, including classes from the Java collections framework. You can map them as values, collections of values, or associations to other entities. The id is a special property that represents the database identifer (primary key) of that class, it is highly recommended for entities like a Cat. Hibernate can use identifiers only internally, but we would lose some of the flexibility in our application architecture. No special interface has to be implemented for persistent classes nor do you have to subclass from a special root persistent class. Hibernate also doesn't require any build time processing, such as byte-code manipulation, it relies solely on Java reflection and runtime class enhancement (through CGLIB). So, without any dependency of the POJO class on Hibernate, we can map it to a database table. The Cat.hbm.xml mapping file contains the metadata required for the object/relational mapping. The metadata includes declaration of persistent classes and the mapping of properties (to columns and foreign key relationships to other entities) to database tables. ]]> Every persistent class should have an identifer attribute (actually, only classes representing entities, not dependent value-typed classes, which are mapped as components of an entity). This property is used to distinguish persistent objects: Two cats are equal if catA.getId().equals(catB.getId()) is true, this concept is called database identity. Hibernate comes bundled with various identifer generators for different scenarios (including native generators for database sequences, hi/lo identifier tables, and application assigned identifiers). We use the UUID generator (only recommended for testing, as integer surrogate keys generated by the database should be prefered) and also specify the column CAT_ID of the table CAT for the Hibernate generated identifier value (as a primary key of the table). All other properties of Cat are mapped to the same table. In the case of the name property, we mapped it with an explicit database column declaration. This is especially useful when the database schema is automatically generated (as SQL DDL statements) from the mapping declaration with Hibernate's SchemaExport tool. All other properties are mapped using Hibernate's default settings, which is what you need most of the time. The table CAT in the database looks like this: You should now create this table in your database manually, and later read if you want to automate this step with the hbm2ddl tool. This tool can create a full SQL DDL, including table definition, custom column type constraints, unique constraints and indexes. We're now ready to start Hibernate's Session. It is the persistence manager, we use it to store and retrieve Cats to and from the database. But first, we've to get a Session (Hibernate's unit-of-work) from the SessionFactory: The call to configure() loads the hibernate.cfg.xml configuration file and initializes the Configuration instance. You can set other properties (and even change the mapping metadata) by accessing the Configuration before you build the SessionFactory (it is immutable). Where do we create the SessionFactory and how can we access it in our application? A SessionFactory is usually only build once, e.g. at startup with a load-on-startup servlet. This also means you should not keep it in an instance variable in your servlets, but in some other location. Furthermore, we need some kind of Singleton, so we can access the SessionFactory easily in application code. The approach shown next solves both problems: startup configuration and easy access to a SessionFactory. We implement a HibernateUtil helper class: This class does not only take care of the SessionFactory with its static initializer, but also has a ThreadLocal variable which holds the Session for the current thread. Make sure you understand the Java concept of a thread-local variable before you try to use this helper. A more complex and powerful HibernateUtil class can be found in CaveatEmptor, http://caveatemptor.hibernate.org/ A SessionFactory is threadsafe, many threads can access it concurrently and request Sessions. A Session is a non-threadsafe object that represents a single unit-of-work with the database. Sessions are opened from a SessionFactory and are closed when all work is completed. An example in your servlet's process() method might look like this (sans exception handling): In a Session, every database operation occurs inside a transaction that isolates the database operations (even read-only operations). We use Hibernates Transaction API to abstract from the underlying transaction strategy (in our case, JDBC transactions). This allows our code to be deployed with container-managed transactions (using JTA) without any changes. Note that you may call HibernateUtil.currentSession(); as many times as you like, you will always get the current Session of this thread. You have to make sure the Session is closed after your unit-of-work completes, either in your servlet code or in a servlet filter before the HTTP response is send. The nice side effect of the second option is easy lazy initialization: the Session is still open when the view is rendered, so Hibernate can load unitialized objects while you navigate the current object graph. Hibernate has various methods that can be used to retrieve objects from the database. The most flexible way is using the Hibernate Query Language (HQL), which is an easy to learn and powerful object-oriented extension to SQL: Hibernate also offers an object-oriented query by criteria API that can be used to formulate type-safe queries. Hibernate of course uses PreparedStatements and parameter binding for all SQL communication with the database. You may also use Hibernate's direct SQL query feature or get a plain JDBC connection from a Session in rare cases. We only scratched the surface of Hibernate in this small tutorial. Please note that we don't include any servlet specific code in our examples. You have to create a servlet yourself and insert the Hibernate code as you see fit. Keep in mind that Hibernate, as a data access layer, is tightly integrated into your application. Usually, all other layers depent on the persistence mechanism. Make sure you understand the implications of this design. For a more complex application example, see http://caveatemptor.hibernate.org/ and have a look at other tutorials linked on http://www.hibernate.org/Documentation