How To Generate DDL Scripts from Hibernate

One of the nice things about using Hibernate in your persistence layer is that it can automatically make updates to your database schema for you. This is nice in development, but oftentimes you need to have the ddl script file. Lucky for us, Hibernate ships with the SchemaExport class. This is what hibernate uses to make updates to your database. I'll show you how we can hijack it and use it for our own purposes as well.

Here we go. As it seems a lot of developers are migrating towards Hibernate annotations and EJB JPA, the following code assumes your classes are configured with annotations. This simple class takes the name of the package where you have your domain objects stored and generates ddl for mysql, oracle and hsql.

package com.jandrewthompson;

import java.io.File;
import java.net.URL;
import java.util.ArrayList;
import java.util.List;
import org.hibernate.cfg.AnnotationConfiguration;
import org.hibernate.tool.hbm2ddl.SchemaExport;

/**
 * @author john.thompson
 *
 */
public class SchemaGenerator
{
  private AnnotationConfiguration cfg;
 
  public SchemaGenerator(String packageName) throws Exception
  {
    cfg = new AnnotationConfiguration();
    cfg.setProperty("hibernate.hbm2ddl.auto","create");
  
    for(Class<Object> clazz : getClasses(packageName))
    {
      cfg.addAnnotatedClass(clazz);
    }
  }
 
  /**
   * Method that actually creates the file.  
   * @param dbDialect to use
   */
  private void generate(Dialect dialect)
  {
    cfg.setProperty("hibernate.dialect", dialect.getDialectClass());
  
    SchemaExport export = new SchemaExport(cfg);
    export.setDelimiter(";");
    export.setOutputFile("ddl_" + dialect.name().toLowerCase() + ".sql"); 
    export.execute(true, false, false, false);
  }
 
  /**
   * @param args
   */
  public static void main(String[] args) throws Exception
  {
    SchemaGenerator gen = new SchemaGenerator("org.jthompson.myapp.domain");
    gen.generate(Dialect.MYSQL);
    gen.generate(Dialect.ORACLE);
    gen.generate(Dialect.HSQL);
  }

  /**
   * Utility method used to fetch Class list based on a package name.
   * @param packageName (should be the package containing your annotated beans.
   */
  private List getClasses(String packageName) throws Exception
  {
    List classes = new ArrayList();
    File directory = null;
    try 
    {
      ClassLoader cld = Thread.currentThread().getContextClassLoader();
      if (cld == null) {
        throw new ClassNotFoundException("Can't get class loader.");
      }
      String path = packageName.replace('.', '/');
      URL resource = cld.getResource(path);
      if (resource == null) {
        throw new ClassNotFoundException("No resource for " + path);
      }
      directory = new File(resource.getFile());
    } catch (NullPointerException x) {
      throw new ClassNotFoundException(packageName + " (" + directory
          + ") does not appear to be a valid package");
    }
    if (directory.exists()) {
      String[] files = directory.list();
      for (int i = 0; i < files.length; i++) {
        if (files[i].endsWith(".class")) {
          // removes the .class extension
          classes.add(Class.forName(packageName + '.'
              + files[i].substring(0, files[i].length() - 6)));
        }
      }
    } else {
      throw new ClassNotFoundException(packageName
          + " is not a valid package");
    }
  
    return classes;
  }
 
  /**
   * Holds the classnames of hibernate dialects for easy reference.
   */
  private static enum Dialect 
  {
    ORACLE("org.hibernate.dialect.Oracle10gDialect"),
    MYSQL("org.hibernate.dialect.MySQLDialect"),
    HSQL("org.hibernate.dialect.HSQLDialect");
  
    private String dialectClass;
    private Dialect(String dialectClass)
    {
      this.dialectClass = dialectClass;
    }
    public String getDialectClass()
    {
      return dialectClass;
    }
  }
}

It should be pretty easy to modify this code to suit your needs. If you are using hibernate xml configuration, just swap out the AnnotationConfiguration class for org.hibernate.cfg.Configuration.