Question

也许你们中的一些人可以帮助我解决java和JComboboxes的一个非常奇怪的问题。我试了几个小时来追查问题，但我找不到解决办法。我不想在这里粘贴一个巨大的代码，但这个简单的循环演示了它：

    JComboBox cb;
    for(int i=0;i<1000;i++)
    {
        cb=new JComboBox();
    }

我可以在任何我想要的地方运行这个代码，1000个ComboBoxes从不GCed我不明白，为什么???

Answer 1

JComboBox创建一个带有组合框侦听器的DefaultListModel。因此推迟了这种对象集群上的垃圾收集。然而，在我第四次运行之后，收集了垃圾或者JIT发现它不需要创建这些对象。

也许你的问题是明确调用System.gc()没有清理它？我可以想象。

* 如何追踪问题

我尝试了以下操作来排除组件。

private static class ReducedJComboBox<T> extends JComboBox<T> {

    @Override
    public void setEditor(ComboBoxEditor anEditor) {
    }

}

public static void main( String[] args )
{
    System.out.println("; " + Runtime.getRuntime().freeMemory());
    long t0 = System.currentTimeMillis();
    ComboBoxModel model = new DefaultComboBoxModel();
    ComboBoxEditor editor = new ComboBoxEditor() {

        public Component getEditorComponent() {
            throw new UnsupportedOperationException("Not supported yet.");
        }

        public void setItem(Object anObject) {
        }

        public Object getItem() {
            throw new UnsupportedOperationException("Not supported yet.");
        }

        public void selectAll() {
        }

        public void addActionListener(ActionListener l) {
        }

        public void removeActionListener(ActionListener l) {
        }
    };
    JComboBox cb;
    for (int i = 0; i < 1000; i++) {
        cb = new JComboBox();
        cb.setModel(model);
        cb.setEditable(false);
        cb.setEditor(editor);
    }
    long dt = t0 - System.currentTimeMillis();
    System.out.println("dt=" + dt + " ms; " + Runtime.getRuntime().freeMemory());
    System.gc();
    System.out.println("finally " + Runtime.getRuntime().freeMemory());
}

Answer 2

我使用NBTestCase#assertGC方法尝试使用简单的测试用例。这种方法的好处是打印出对内存泄漏时正在检查的对象的强引用。它还通过用一个哑字节数组填充堆来触发GC，从而强制GC启动。

我使用的非常简单的测试用例

public class ComboBoxMemoryLeak {

  public static void main( String[] args ) {
    EventQueue.invokeLater( new Runnable() {
      @Override
      public void run() {
        List<WeakReference<JComboBox>> references = new ArrayList<WeakReference<JComboBox>>(  );
        JComboBox comboBox;
        for (int i = 0; i < 1000; i++ ){
          comboBox = new JComboBox(  );
          references.add( new WeakReference<JComboBox>( comboBox ) );
        }
        comboBox = null;
        for ( int i = 0, referencesSize = references.size(); i < referencesSize; i++ ) {
          System.out.println( "i = " + i );
          WeakReference<JComboBox> weakReference = references.get( i );
          NbTestCase.assertGC( "Combobox", weakReference );
        }
        System.out.println("No memory leak found");
      }
    } );
  }
}

在使用JDK1.6运行的Mac上导致以下跟踪

i = 0
Exception in thread "AWT-EventQueue-0" junit.framework.AssertionFailedError: Combobox:
private static sun.awt.AppContext sun.awt.AppContext.mainAppContext->
sun.awt.AppContext@30f7f540-table->
java.util.HashMap@c324b85-table->
[Ljava.util.HashMap$Entry;@770fba26-[8]->
java.util.HashMap$Entry@63adf08f-value->
java.beans.PropertyChangeSupport@4f1b8540-children->
java.util.Hashtable@2305454a-table->
[Ljava.util.Hashtable$Entry;@4a9a4ba3-[0]->
java.util.Hashtable$Entry@6aed0f19-value->
java.beans.PropertyChangeSupport@23597cac-listeners->
sun.awt.EventListenerAggregate@2f39c244-listenerList->
[Ljava.beans.PropertyChangeListener;@2e2e06bd-[0]->
javax.swing.JViewport$1@2a72cf60-this$0->
javax.swing.JViewport@2b9c1dc4-parent->
javax.swing.JScrollPane@b99f7c6-parent->
com.apple.laf.AquaComboBoxPopup@6699166f-comboBox->
javax.swing.JComboBox@3bc634b9
    at junit.framework.Assert.fail(Assert.java:50)
    at org.netbeans.junit.NbTestCase$4.run(NbTestCase.java:1351)
    at org.netbeans.junit.internal.NbModuleLogHandler.whileIgnoringOOME(NbModuleLogHandler.java:143)
    at org.netbeans.junit.NbTestCase.assertGC(NbTestCase.java:1309)
    at org.netbeans.junit.NbTestCase.assertGC(NbTestCase.java:1285)
    at ComboBoxMemoryLeak$1.run(ComboBoxMemoryLeak.java:32)
    at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:209)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:677)
    at java.awt.EventQueue.access$000(EventQueue.java:85)
    at java.awt.EventQueue$1.run(EventQueue.java:638)
    at java.awt.EventQueue$1.run(EventQueue.java:636)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:647)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:296)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:211)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:201)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:196)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:188)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)

所以是的，在我的机器上，我会得出结论，JComboBox仍然通过AppContext中的引用存在于内存中。搜索到这导致我们this SO question。我尝试将JConsole连接到带有语句的while循环的main方法。不幸的是，我的主程序在JConsole可以连接之前抛出了一个OutOfMemoryException，所以我无法在他的回答中生成像trashgod那样的漂亮图片。

Answer 3

i can do this code where ever i wont the 1000 ComboBoxes are never GCed 
and i do not get it, why

答案可能非常简单如果不存在对此Object的另一个引用，则对象可能是GC'ed，或者另一个问题可能是您从静态对象引用/从未引用的任何问题（从不GC引用只能取空值），或你的对象是静态的

即使没有引用JCombobox内存泄漏？

3 个答案: