关与JTree的个问题！请高手帮忙！先谢谢了哈。
为什么用JTree生成的数中单击子节点不会响应事件？
我的程序如下：
import   java.awt.*;
import   java.awt.event.*;
import   javax.swing.*;
import   javax.swing.tree.*;
import   javax.swing.event.*;

  class   Mtree   extends   JFrame   implements   TreeSelectionListener
{         JTree   tree=null;
  JPanel   p=null;
  JButton   button;
Mtree(String   s)
{   super(s);
Container   con=this.getContentPane();
DefaultMutableTreeNode   root=new   DefaultMutableTreeNode( "主业务功能 ");
DefaultMutableTreeNode   t1=new   DefaultMutableTreeNode( "账户管理 ");
DefaultMutableTreeNode   t2=new   DefaultMutableTreeNode( "事物查询 ");
DefaultMutableTreeNode   t3=new   DefaultMutableTreeNode( "用户存款 ");
DefaultMutableTreeNode   t4=new   DefaultMutableTreeNode( "用户取款 ");
DefaultMutableTreeNode   t5=new   DefaultMutableTreeNode( "数据维护 ");
DefaultMutableTreeNode   t6=new   DefaultMutableTreeNode( "系统表查看 ");
DefaultMutableTreeNode   t7=new   DefaultMutableTreeNode( "系统账户管理 ");
DefaultMutableTreeNode   t8=new   DefaultMutableTreeNode( "退出系统 ");

DefaultMutableTreeNode   t1_1=new   DefaultMutableTreeNode( "新用户开户 ");
DefaultMutableTreeNode   t1_2=new   DefaultMutableTreeNode( "用户资料明细 ");
DefaultMutableTreeNode   t1_3=new   DefaultMutableTreeNode( "账户密码修改 ");
DefaultMutableTreeNode   t2_1=new   DefaultMutableTreeNode( "账户金额查询 ");
DefaultMutableTreeNode   t2_2=new   DefaultMutableTreeNode( "交易记录查询 ");
DefaultMutableTreeNode   t2_3=new   DefaultMutableTreeNode( "账户信息查询 ");
DefaultMutableTreeNode   t5_1=new   DefaultMutableTreeNode( "数据导出 ");
DefaultMutableTreeNode   t5_2=new   DefaultMutableTreeNode( "数据导入 ");
DefaultMutableTreeNode   t7_1=new   DefaultMutableTreeNode( "添加管理员账户 ");
DefaultMutableTreeNode   t7_2=new   DefaultMutableTreeNode( "管理员密码修改 ");
root.add(t1);root.add(t2);root.add(t3);root.add(t4);
root.add(t5);root.add(t6);root.add(t7);root.add(t8);
t1.add(t1_1);t1.add(t1_2);t1.add(t1_3);
t2.add(t2_1);t2.add(t2_2);t2.add(t2_3);
t5.add(t5_1);t5.add(t5_2);t7.add(t7_1);t7.add(t7_2);
JTree   tree=   new   JTree(root);
JScrollPane   panel   =   new   JScrollPane(p=new   JPanel());
JSplitPane   splitpane=new   JSplitPane(JSplitPane.HORIZONTAL_SPLIT,true,tree,panel);

                con.add(splitpane);
                tree.addTreeSelectionListener(this);
                System.out.println(tree);
                this.validate();    

addWindowListener(new   WindowAdapter(){    
public   void   windowClosing(WindowEvent   e){
System.exit(0);
}
});
this.setBounds(80,   60,   800,   600);    
this.setVisible(true);  
System.out.println( "333 ");
}


public   void   valueChanged(TreeSelectionEvent   e)
{ //   TODO   自动生成方法存根
System.out.println( "444 ");
if(e.getSource()==tree)//这里似乎无法获得事件源