如何销毁SelectionKey附件？attach（null）不起作用

我的Java应用程序存在内存泄漏-当我的资源清除代码正在执行任务管理器时，表明内存使用情况未更改。我的密码

while (isRunning) {
      try 
      {

             selector.select(); 

             long sum=0;

             Set keys =  selector.selectedKeys();
             Iterator it = keys.iterator(); 
             while(it.hasNext())
             {
                 SelectionKey key = (SelectionKey)it.next();
                 if (key.isReadable())
                 { 
                      SocketChannel sc = (SocketChannel) key.channel();

                      ByteBuffer bb;
                      if(key.attachment()==null)
                      {
                          bb = ByteBuffer.allocate(1024*1024);
                          key.attach(bb);
                      }
                      else
                      {
                          bb = (ByteBuffer)key.attachment();
                          bb.clear();
                      }

                      int x =  sc.read(bb); 

                       System.out.println(x +" bytes were read");
                       if(x==-1)
                       {
                           key.attach(null); //doesn't work
                           sc.close();
                           //bb = null; // also doesn't work
                       }
                 } 
             }
           keys.clear();
      } 
      catch (Exception ex) 
      {     
          ex.printStackTrace(new PrintStream(System.out));
      }
      finally
      { 
            //stopServer();
      }
    }

测试逻辑-我编写了简单的TCP客户端Java程序，向服务器发送了100条消息。我特意分配了大缓冲区-每个连接1MB。客户完成工作后，int x = sc.read(bb);返回-1并执行以下代码：

if(x==-1)
                           {
                               key.attach(null); //doesn't work
                               sc.close();
                               //bb = null; // also doesn't work
                           }

我检查了调试输出，确实执行了此代码，但任务管理器仍然显示大量内存使用情况。问题出在哪儿？