我想通过java代码执行ant构建。我使用以下代码，出现错误。
Ant helper = new Ant();
helper.setTarget("build_new");
helper.setAntfile("build_new.xml");
helper.execute();
错误：
线程"AWT-EventQueue-0"java.lang.NullPointerException中出现异常网址：org.apache.tools.ant.taskdefs.ant.init（ant.java:125）网址：org.apache.tools.ant.taskdefs.ant.reinit（ant.java:139）网址：org.apache.tools.ant.taskdefs.ant.execute（ant.java:292）网址：com.demo.test.JavaSamp$4.actionPerformed（JavaSamp.java:216）位于javax.swing.AbstractButton.fireActionPerformed（未知源）位于javax.swing.AbstractButton$Handler.actionPerformed（未知源）位于javax.swing.DefaultButtonModel.fireActionPerformed（未知源）位于javax.swing.DefaultButtonModel.setPressed（未知源）位于javax.swing.AbstractButton.doClick（未知源代码）位于javax.swing.plaf.basic.BasicMenuItemUI.doClick（未知源）位于javax.swing.plaf.basic.BasicMenuItemUI$Handler.mouseReleased（未知源）位于java.awt.Component.processMouseEvent（未知源）位于javax.swing.JComponent.processMouseEvent（未知源）位于java.awt.Component.processEvent（未知源）位于java.awt.Container.processEvent（未知源）位于java.awt.Component.dispatchEventImpl（未知源）位于java.awt.Container.dispatchEventImpl（未知源）位于java.awt.Component.dispatchEvent（未知源）位于java.awt.LightweightDispatcher.retargetMouseEvent（未知源）位于java.awt.LightweightDispatcher.processMouseEvent（未知源）位于java.awt.LightweightDispatcher.dispatchEvent（未知源）位于java.awt.Container.dispatchEventImpl（未知源）位于java.awt.Window.dispatchEventImpl（未知源）位于java.awt.Component.dispatchEvent（未知源）位于java.awt.EventQueue.dispatchEventImpl（未知源）位于java.awt.EventQueue.access$000（未知来源）位于java.awt.EventQueue$1.run（未知源）位于java.awt.EventQueue$1.run（未知源）位于java.security.AccessController.doPrivileged（本机方法）在java.security.AccessControlContext$1.doIntersectionPrivilege（未知源）在java.security.AccessControlContext$1.doIntersectionPrivilege（未知源）位于java.awt.EventQueue$2.run（未知源）位于java.awt.EventQueue$2.run（未知源）位于java.security.AccessController.doPrivileged（本机方法）在java.security.AccessControlContext$1.doIntersectionPrivilege（未知源）位于java.awt.EventQueue.dispatchEvent（未知源）位于java.awt.EventDispatchThread.pumpOneEventForFilters（未知源）位于java.awt.EventDispatchThread.pumpEventsForFilter（未知源）位于java.awt.EventDispatchThread.pumpEventsForHierarchy（未知源）位于java.awt.EventDispatchThread.pumpEvents（未知源）位于java.awt.EventDispatchThread.pumpEvents（未知源）位于java.awt.EventDispatchThread.run（未知源）
任何建议请
它也用于执行ant构建。
File buildFile = new File("build.xml");
Project p = new Project();
p.setUserProperty("ant.file", buildFile.getAbsolutePath());
p.init();
ProjectHelper helper = ProjectHelper.getProjectHelper();
p.addReference("ant.projectHelper", helper);
helper.parse(p, buildFile);
p.executeTarget(p.getDefaultTarget());