哈哈!我自己解决了!告诉大家一下阿!
  <target 
      name="middlegen" 
      description="Run Middlegen" 
      unless="middlegen.skip"
      depends="init,fail-if-no-xdoclet-1.2,check-driver-present,panic-if-driver-not-present"
   >
      <mkdir dir="${build.gen-src.dir}"/>      <taskdef
         name="middlegen"
         classname="middlegen.MiddlegenTask"
         classpathref="lib.class.path"
      />      <middlegen
         appname="${name}"
         prefsdir="${src.dir}"
         gui="true"   《----------原来的值是${gui}改了之后就可以出现界面了!
         databaseurl="${database.url}"
         initialContextFactory="${java.naming.factory.initial}"
         providerURL="${java.naming.provider.url}"
         datasourceJNDIName="${datasource.jndi.name}"
         driver="${database.driver}"
         username="${database.userid}"
         password="${database.password}"
         schema="${database.schema}"
         catalog="${database.catalog}"
         includeViews="false"
      >别笑我无知阿!自己解决的!原来官方网站说明很详细!呵呵!用这种东西!要加强E文阿!呵呵!挺高兴!