Reverse engineer DDL from JPA entities

问题:

I'm playing around with some JPA stuff, changing the mappings to see how they're supposed to be etc. It's basic experimentation. However I can't find a tool that will simply read my entities and then generate the table schema for me. I tried to find something like this in JBoss tools but nada. Eclipse integration will be a huge plus but i'll take a command line tool or an ant task.

Any ideas?

回答1:

I don't think there is an universal way of doing this with JPA, you have to directly use the underlying JPA implementation to achieve this.

For Hibernate, there are several possibilities:

• Use the method duffymo posted earlier, that makes Hibernate update the database schema automatically.
• If you do not want that, you can use the hbm2ddl tool from Hibernate Tools (note: link sucks, but apparently their home page is a bit broken right now). You can use that to automatically generate database creation scripts from your JPA entities; there are also plugins for Maven and Ant that invoke hbm2ddl automatically.

For EclipseLink (formerly Oracle TopLink, the JPA 2.0 RI) see Using EclipseLink JPA Extensions for Schema Generation. In principle it is very similar to Hibernate, although at first glance I don't see anything that could be used as a stand-alone utility for creating a DB script.

Other JPA implementations (BEA/Oracle Kodo, Apache OpenJPA) probably have their own specific methods of achieving this.

回答2:

Try adding the following to your persistence.xml

For Hibernate:

To create:

<property name="hibernate.hbm2ddl.auto" value="update"/>

To drop and create:

<property name="hibernate.hbm2ddl.auto" value="create-drop"/>

For Toplink:

To create:

<property name="toplink.ddl-generation" value="create-tables"/>

To drop and create:

<property name="toplink.ddl-generation" value="drop-and-create-tables"/>

For EclipseLink:

To create:

<property name="eclipselink.ddl-generation" value="create-tables"/>

To drop and create:

<property name="eclipselink.ddl-generation" value="drop-and-create-tables"/>

回答3:

When I use Hibernate I simply add this to my config file:

      <property name="hbm2ddl.auto">update</property>

Takes care of everything. I'm not sure what the equivalent is for JPA, but it's so heavily influenced by Hibernate that I'd be surprised if you couldn't find something similar.

No tool needed. I usually just run a simple JUnit test and the database is created for me.

回答4:

I use Hibernate's org.hibernate.tool.hbm2ddl.SchemaExport class and this small method to

generate the schema in the database:

 public static void rebuildSchema()
 {
     configuration = new Configuration();
     configuration.configure();

     new SchemaExport(configuration)
         .setHaltOnError(true)
         .execute(false, true, false, false);
 }

to create the DDL in an external file, use this call to execute.

     new SchemaExport(configuration)
         .setHaltOnError(true)
         .setOutputFile(outputFile)
         .setImportFile("")
         .setDelimiter(";")
         .setFormat(true)
         .execute(false, false, false, true);

It's considered bad form to set "hibernate.hbm2ddl.auto" to "update", since automatically changing the production database may break it. For an explanation, see Hibernate hbm2ddl.auto possible values and what they do?.

回答5:

By maven plugin:

        <plugin>
            <!-- run "mvn hibernate3:hbm2ddl" to generate a schema -->
            <groupId>org.codehaus.mojo</groupId>
            <artifactId>hibernate3-maven-plugin</artifactId>
            <version>3.0</version>
            <configuration>
                <hibernatetool>
                    <classpath>
                        <path location="${project.build.directory}/classes" />
                        <path location="${project.basedir}/src/main/resources/META-INF/" />                                               
                    </classpath>   

                    <jpaconfiguration persistenceunit="galleryPersistenceUnit" />                     
                    <hbm2ddl create="true" export="false" destdir="${project.basedir}/target" drop="true" outputfilename="mysql.sql" format="true" console="true"/>
                </hibernatetool>
            </configuration>
        </plugin>

回答6:

DataNucleus has its own SchemaTool capable of generating the schema for you, or generating the required DDL statements for you to adapt and apply yourself.

--Andy (DataNucleus)

回答7:

Adding to the list of James McMahon:

For OpenJPA:

<property name="openjpa.jdbc.SynchronizeMappings" value="buildSchema(ForeignKeys=true)"/>

回答8:

Antonio Goncalves says in his blog about API to generate the schema.
In JPA 2.1 generateSchema method was introduced for this purpose.

Example from blog:

public class Main {
    public static void main(String[] args) {
        Persistence.generateSchema("samplePU", null);
    }
}