Generate JPA 2 Entities from existing Database

You can use a plugin like Eclipse Dali to do the trick for you. You can refer to the documentation, section 3.11 Generating Entities from Tables.

I do not know of any specific vendor independent tool to do this, though.

You should have a look at minuteproject on this track reverse-engineering for JPA2. It is a vendor independent tool. In release 0.5.5 it generates:

• JPA2 entities (for table and views) and associated metamodel
• persistence.xml (only hibernate configuration is available in 0.5.5) but you can overwrite it.
• maven pom with querydsl integration
• Enum classes (if specified at enrichment i.e. in config file).

I have not tried it with H2 but to do this you need to

• add the H2 jdbc driver in $MP_HOME/application/lib/extra
• add the driver class and your connection parameters in MinuteProject configuration.

Try using OPENJPA Reverse mapping tools. They offer lot more facility and are easy to configure. This example would clarify.

If you are using maven as your build tool, add this entry to your pom.xml

    <plugin>
<groupId>org.codehaus.mojo</groupId>
<artifactId>exec-maven-plugin</artifactId>
<version>1.2</version>
<configuration>
<mainClass>org.apache.openjpa.jdbc.meta.ReverseMappingTool</mainClass>
<commandlineArgs>
    -directory src/main/java -accessType fields
    -useGenericCollections true -package org.yourproject.model
    -metadata none -annotations true
    -innerIdentityClasses false -useBuiltinIdentityClass false
    -primaryKeyOnJoin false
    </commandlineArgs>
<includePluginDependencies>true</includePluginDependencies>
</configuration>
<dependencies>
    <dependency>
        <groupId>javax.validation</groupId>
        <artifactId>validation-api</artifactId>
        <version>1.0.CR3</version>
    </dependency>
    <dependency>
        <groupId>org.apache.openjpa</groupId>
        <artifactId>openjpa-all</artifactId>
        <version>2.0.1</version>
    </dependency>
</dependencies>
    </plugin>

Also add following properties in the persistence.xml which lies in your META-INF folder of your resources. These would be harnessed by openjpa tool to establish connection to the database.

    <properties>
<property name="openjpa.ConnectionUserName" value="${db.username}"/>
<property name="openjpa.ConnectionPassword" value="${db.password}"/>
<property name="openjpa.ConnectionURL" value="${db.url}"/>  
<property name="openjpa.ConnectionDriverName"  value="${db.driver.class}"/>     
   </properties>

To generate the Entity files simply launch the maven goal in the project directory using mvn org.codehaus.mojo:exec-maven-plugin:java and it will generate the files at the desired location.