How to prevent memory leak in JTextPane.setCaretPosition(int)?

Question

I'm working on a Java application with Swing-based GUI. The application uses JTextPane to output log messages as follows: 1) truncate existing text to keep total text size under the limit; 2) append new text; 3) scroll to the end (actual logic is slightly different, but it's irrelevant here).

I was using Eclipse with JVM Monitor to determine reasonable text size limit and found significant memory leak. I tried to remove UndoableEditListeners from the underlying document and disable automatic caret position updates (by changing the position explicitly with DefaultCaret.NEVER_UPDATE and JTextPane.setCaretPosition(int)), but without success. Finally, I decided to disable changing caret position completely, and this fixed the leak.

I have two questions:

1. Is there an issue with my code? If yes, how can I change it to accomplish the task?

2. Is it a Swing/JVM bug? If yes, how can I report it?

Details:

Here is SSCCE: GUI with textPane and two buttons, for small and stress tests. FIX and FIXXX flags correspond to my attempts to fix memory leak.

package memleak;

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.UndoableEditListener;
import javax.swing.text.*;

class TestMain
{
  private JTextPane textPane;
  // try to fix memory leak
  private static final boolean FIX = false;
  // disable caret updates completely
  private static final boolean FIXXX = false;
  // number of strings to append
  private static final int ITER_SMALL = 20;
  private static final int ITER_HUGE = 1000000;
  // limit textPane content
  private static final int TEXT_SIZE_MAX = 100;

  TestMain()
  {
    JFrame frame = new JFrame();
    JPanel panel = new JPanel();
    textPane = new JTextPane();
    textPane.setEditable(false);
    if (FIX)
    {
      tryToFixMemory();
    } // end if FIX
    JScrollPane scrollPane = new JScrollPane(textPane);
    scrollPane.setPreferredSize(new Dimension(100, 100) );
    panel.add(scrollPane);
    JButton buttonSmall = new JButton("small test");
    buttonSmall.addActionListener(new ButtonHandler(ITER_SMALL) );
    panel.add(buttonSmall);
    JButton buttonStress = new JButton("stress test");
    buttonStress.addActionListener(new ButtonHandler(ITER_HUGE) );
    panel.add(buttonStress);
    frame.add(panel);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.pack();
    frame.setVisible(true);
  } // end constructor

  public static void main(String[] args)
  {
    @SuppressWarnings("unused")
    TestMain testMain = new TestMain();
  } // end main

  private void append(String s)
  {
    Document doc = textPane.getDocument();
    try
    {
      int extraLength = doc.getLength() + s.length() - TEXT_SIZE_MAX;
      if (extraLength > 0)
      {
        doc.remove(0, extraLength);
      } // end if extraLength
      doc.insertString(doc.getLength(), s, null);
      if (FIX && !FIXXX)
      {  // MEMORY LEAK HERE
        textPane.setCaretPosition(doc.getLength() );
      } // end if FIX
    }
    catch (Exception e)
    {
      e.printStackTrace();
      System.exit(1);
    } // end try
  } // end method append

  private void tryToFixMemory()
  {    
    // disable caret updates
    Caret caret = textPane.getCaret();
    if (caret instanceof DefaultCaret)
    {
      ( (DefaultCaret) caret).setUpdatePolicy(
          DefaultCaret.NEVER_UPDATE);
    } // end if DefaultCaret

    // remove registered UndoableEditListeners if any
    Document doc = textPane.getDocument();
    if (doc instanceof AbstractDocument)
    {
      UndoableEditListener[] undoListeners = 
          ( (AbstractDocument) doc).getUndoableEditListeners();
      if (undoListeners.length > 0)
      {
        for (UndoableEditListener undoListener : undoListeners)
        {
          doc.removeUndoableEditListener(undoListener);
        } // end for undoListener
      } // end if undoListeners
    } // end if AbstractDocument
  } // end method tryToFixMemory

  private class ButtonHandler implements ActionListener
  {
    private final int iter;

    ButtonHandler(int iter)
    {
      this.iter = iter;
    } // end constructor

    @Override
    public void actionPerformed(ActionEvent e)
    {
      for (int i = 0; i < iter; i++)
      {
        append(String.format("%10d\n", i) );
      } // end for i
    } // end method actionPerformed

  } // end class ButtonHandler

} // end class TestMain

JVM was from official Oracle Java SE Development Kit 8u45 for Linux x64. All tests were done with -Xmx100m limit.

Both flags are false:

Small test

Works as expected:

Stress test

GUI freezes at an intermediate point:

Memory is leaking:

At some point there is no memory left and I got the following error:

Exception in thread "AWT-EventQueue-0" java.lang.OutOfMemoryError: GC overhead limit exceeded
  at java.util.Formatter.parse(Formatter.java:2560)
  at java.util.Formatter.format(Formatter.java:2501)
  at java.util.Formatter.format(Formatter.java:2455)
  at java.lang.String.format(String.java:2928)
  at memleak.TestMain$ButtonHandler.actionPerformed(TestMain.java:117)
  at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
  at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2346)
  at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
  at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
  at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
  at java.awt.Component.processMouseEvent(Component.java:6525)
  at javax.swing.JComponent.processMouseEvent(JComponent.java:3324)
  at java.awt.Component.processEvent(Component.java:6290)
  at java.awt.Container.processEvent(Container.java:2234)
  at java.awt.Component.dispatchEventImpl(Component.java:4881)
  at java.awt.Container.dispatchEventImpl(Container.java:2292)
  at java.awt.Component.dispatchEvent(Component.java:4703)
  at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4898)
  at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4533)
  at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4462)
  at java.awt.Container.dispatchEventImpl(Container.java:2278)
  at java.awt.Window.dispatchEventImpl(Window.java:2750)
  at java.awt.Component.dispatchEvent(Component.java:4703)
  at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758)
  at java.awt.EventQueue.access$500(EventQueue.java:97)
  at java.awt.EventQueue$3.run(EventQueue.java:709)
  at java.awt.EventQueue$3.run(EventQueue.java:703)
  at java.security.AccessController.doPrivileged(Native Method)
  at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:75)
  at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:86)
  at java.awt.EventQueue$4.run(EventQueue.java:731)
  at java.awt.EventQueue$4.run(EventQueue.java:729)
Exception in thread "AWT-EventQueue-0" java.lang.OutOfMemoryError: GC overhead limit exceeded
  at javax.swing.text.GlyphPainter1.modelToView(GlyphPainter1.java:147)
  at javax.swing.text.GlyphView.modelToView(GlyphView.java:653)
  at javax.swing.text.CompositeView.modelToView(CompositeView.java:265)
  at javax.swing.text.BoxView.modelToView(BoxView.java:484)
  at javax.swing.text.ParagraphView$Row.modelToView(ParagraphView.java:900)
  at javax.swing.text.CompositeView.modelToView(CompositeView.java:265)
  at javax.swing.text.BoxView.modelToView(BoxView.java:484)
  at javax.swing.text.CompositeView.modelToView(CompositeView.java:265)
  at javax.swing.text.BoxView.modelToView(BoxView.java:484)
  at javax.swing.plaf.basic.BasicTextUI$RootView.modelToView(BasicTextUI.java:1509)
  at javax.swing.plaf.basic.BasicTextUI.modelToView(BasicTextUI.java:1047)
  at javax.swing.text.DefaultCaret.repaintNewCaret(DefaultCaret.java:1308)
  at javax.swing.text.DefaultCaret$1.run(DefaultCaret.java:1287)
  at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:311)
  at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:756)
  at java.awt.EventQueue.access$500(EventQueue.java:97)
  at java.awt.EventQueue$3.run(EventQueue.java:709)
  at java.awt.EventQueue$3.run(EventQueue.java:703)
  at java.security.AccessController.doPrivileged(Native Method)
  at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:75)
  at java.awt.EventQueue.dispatchEvent(EventQueue.java:726)
  at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
  at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
  at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
  at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
  at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
  at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)
Exception in thread "RMI TCP Connection(idle)" java.lang.OutOfMemoryError: GC overhead limit exceeded
  at java.lang.Thread.getName(Thread.java:1135)
  at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(TCPTransport.java:677)
  at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
  at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
  at java.lang.Thread.run(Thread.java:745)

Detailed memory statistics show very high counts for java.awt.event.InvocationEvent, sun.awt.EventQueueItem, and javax.swing.text.DefaultCaret$1 (absent in the fixed version):

Setting FIX = true did not improve the situation.

Both flags are true:

Small test

Now shows that the caret position is not updated:

Stress test

Works and has no indication of memory leak:

Merrimerriam

See also Initial Threads. – Hardpressed

@Merrimerriam did you ever find out whats causing it? I have the same issue in one of my apps with HTMLEditorKit and I cant find out how to fix the memory leak. Here is an example I never got an answer to: #43712473 – Hourly

@M.H. yes, check the answer below. In brief, appending the string in the event handler and then updating the caret position fires another event that is queued until all strings are processed. If this queue is small, then there is no issues, otherwise JVM is stuck since there is no memory left to process the events. There is a big difference between appending 10000 strings of 1 character and one string of 10000 characters. – Merrimerriam

@andrey I dont really know what you mean with this actually, I have looked at your code, but I dont see how to fix it. Did you look at my example? There I juse actually a thread so how could there be a queue? A queue of what? I also have the text pane set to noteditable, and I never change a caret. I also dont appending a string I use insertBeforeEnd via the htmleditorkit. – Hourly

@M.H. the link you provided gives me 404, so I cannot comment on that. The solution in my case is to build a single long String and then append it at once, not in the loop, so I avoid calling setCaretPosition() multiple times. – Merrimerriam

@Merrimerriam #44935634 I dont set a caret there so I think its a bit of a different problem, but still related. Like I said I am not appending strings to the textpane, I am using the htmleditorkit. I also cant control how fast inputs come because it is a chat program, adding a new line everytime someones speaks. – Hourly

Answer 1

The reason for this is that you are running the for loop in the event dispatch thread (see Event Dispatch Thread). This is the thread in which all user interface interactions happens.

If you are running a long task, then you should run it in a different thread, so that the user interface keeps responsive. If you need to make a change on the user interface, like changing text in your JTextPane and setting the caret position out of a different thread than the event dispatch thread, you need to invoke either EventQueue.invokeLater() or EventQueue.invokeAndWait() (see EventQueue).

I think the triggered events from setting the caret position are queued in your case and can only be processed, when the loop is finished (cause both are processed in event dispatch thread). So you should try something like this:

@Override
public void actionPerformed(ActionEvent e)
{
  new Thread(new Runnable() {
    @Override
    public void run() {
      for (int i = 0; i < iter; i++)
      {
        final String display = String.format("%10d\n", i);
        try {
          EventQueue.invokeAndWait(new Runnable() {
            @Override
            public void run() {
              append(display);
            }
          });
        } catch (Exception e) {
          e.printStackTrace();
        }
      } // end for i
    }
  }).start();
} 

Might be even better if you call EventQueue.invokeAndWait only after x iterations and cache the previous results, which needs to be displayed.

Paganini

I'm not sure if this would work. Have you tried this code? It should be noted that I used loop with String.format() only to simulate real data, and optimization of this part is worthless. Anyway, the problem is with textPane.setCaretPosition(doc.getLength() );, not with the strings. – Merrimerriam

I was going to accept your answer, but your code cannot be compiled. I don't have enough reputation to edit it, only to suggest the edit. However, my edit was rejected. Could you please update the code yourself? – Merrimerriam

@Paganini I dont think so. I have the same issue in one of my apps with HTMLEditorKit and I cant find out how to fix the memory leak. Here is an example I never got an answer to: #43712473 – Hourly