Troubleshooting OS Freezes During SWT Drag And Drop Debugging In Eclipse

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Hey everyone,

I wanted to share a heads-up about a rather serious issue I've encountered while debugging SWT (Standard Widget Toolkit) drag-and-drop functionality in Eclipse. This isn't your run-of-the-mill IDE crash; it can potentially freeze your entire operating system. Yep, you heard that right – a full-on system freeze! The only way I've found to recover is by switching to a command line interface (like tty3 on Linux) and manually killing the Java process. So, buckle up, and let's dive into the details.

The Dreaded System Freeze

System freezes can be incredibly frustrating, especially when you're in the middle of debugging a complex application. Imagine the scenario: you're stepping through your code, carefully examining variables and execution flow, when suddenly your system becomes unresponsive. The mouse cursor might still move, but clicking does nothing. Keyboard input is ignored. Panic sets in as you realize your unsaved work is potentially at risk. This is the reality I've been facing when debugging SWT drag-and-drop widgets in Eclipse, and it's not a pretty picture.

What makes this issue particularly concerning is that it's not just the Eclipse IDE that freezes; it's the entire operating system. This suggests a deep-seated problem, likely involving resource starvation or a deadlock situation. My suspicion is that Eclipse, while running in debug mode and handling drag-and-drop events, is performing so much work on the UI thread that it effectively blocks everything else. The operating system's UI components become starved for resources, leading to the freeze. While the UI is blocked, the underlying system is still somewhat functional, as evidenced by the ability to switch to a text-based terminal. This glimmer of hope allows for a somewhat graceful recovery by killing the offending Java process, but it's hardly an ideal solution.

My Setup

Before we get into reproducing the issue, let me give you a quick rundown of my system:

  • Operating System: Kubuntu 24.04 (This is my daily driver, so I'm pretty familiar with its quirks.)
  • Desktop Environment: KDE Plasma 5.27.12 (Plasma is my DE of choice for its flexibility and features.)
  • Windowing System: X11 (Yes, I'm still rocking X11. Maybe it's time to switch to Wayland?)
  • Eclipse Version: 20250707-0159 (2025-09 M1) (I'm using a relatively recent Eclipse build, so this isn't some ancient bug.)

I've encountered this freezing issue across multiple Eclipse releases, so it's not a recent regression. Unfortunately, I can't pinpoint exactly when it started, but it's been a recurring problem for a while now. The fact that it persists across different versions suggests a more fundamental issue within Eclipse's SWT implementation or its interaction with the underlying operating system.

How to Reproduce the Freeze

Alright, let's get down to the nitty-gritty. If you're feeling adventurous (and have saved your work!), you can try reproducing this issue yourself. Here's how:

  1. Grab the Code Snippet: The key to triggering the freeze lies in a specific piece of code. I've been using Snippet79 from the SWT repository as my test case. You can find this snippet online or within the Eclipse SWT examples.

  2. Set Up Your Debug Environment: Open Eclipse and create a new Java project. Then, create a new class and paste the code from Snippet79 into it.

  3. Set Breakpoints: This is crucial. Set breakpoints in the following methods within the DropTargetAdapter:

    • dragEnter()
    • dragOperationChanged()
    • drop()

    These breakpoints will allow you to observe the drag-and-drop events as they occur and potentially catch the moment when the freeze happens.

  4. Run in Debug Mode: Launch the snippet in debug mode. This is essential for triggering the issue.

  5. Initiate the Drag: The snippet creates two labels: a drag source on the left and a drop target on the right. Click and drag the data from the left label to the right label.

  6. Experience the Freeze (Hopefully Not!): As you drag, the breakpoints should be hit. Step through the code carefully. In my experience, the freeze often occurs during the drag operation, particularly when the debugger is actively inspecting variables or stepping through code. If you're unlucky (or lucky, depending on your perspective), your system will freeze solid.

The Code Snippet (Snippet79)

For those who want to dive deeper, here's the code snippet I've been using:

/*******************************************************************************
 * Copyright (c) 2000, 2017 IBM Corporation and others.
 *
 * This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License 2.0
 * which accompanies this distribution, and is available at
 * https://www.eclipse.org/legal/epl-2.0/
 *
 * SPDX-License-Identifier: EPL-2.0
 *
 * Contributors:
 *     IBM Corporation - initial API and implementation
 ******************************************************************************/
package org.eclipse.swt.snippets;

/*
 * Drag and Drop example snippet: define my own data transfer type
 *
 * For a list of all SWT example snippets see
 * http://www.eclipse.org/swt/snippets/
 *
 * @since 3.1
 */
import java.io.*;

import org.eclipse.swt.*;
import org.eclipse.swt.dnd.*;
import org.eclipse.swt.layout.*;
import org.eclipse.swt.widgets.*;

public class Snippet79 {

/* The data being transferred is an **array of type MyType** where MyType is define as: */
static class MyType {
	String fileName;
	long fileLength;
	long lastModified;
}

static class MyTransfer extends ByteArrayTransfer {

	private static final String MYTYPENAME = "name_for_my_type";
	private static final int MYTYPEID = registerType (MYTYPENAME);
	private static MyTransfer _instance = new MyTransfer ();

public static MyTransfer getInstance () {
	return _instance;
}

@Override
public void javaToNative (Object object, TransferData transferData) {
	if (!checkMyType(object) || !isSupportedType (transferData)) {
		DND.error(DND.ERROR_INVALID_DATA);
	}
	MyType [] myTypes = (MyType []) object;
	try (ByteArrayOutputStream out = new ByteArrayOutputStream ();
		DataOutputStream writeOut = new DataOutputStream (out)) {
		// write data to a byte array and then ask super to convert to pMedium
		for (MyType myType : myTypes) {
			byte [] buffer = myType.fileName.getBytes ();
			writeOut.writeInt (buffer.length);
			writeOut.write (buffer);
			writeOut.writeLong (myType.fileLength);
			writeOut.writeLong (myType.lastModified);
		}
		byte [] buffer = out.toByteArray ();
		writeOut.close ();
		super.javaToNative (buffer, transferData);
	} catch (IOException e) {}
}

@Override
public Object nativeToJava (TransferData transferData) {
	if (isSupportedType (transferData)) {
		byte [] buffer = (byte []) super.nativeToJava (transferData);
		if (buffer == null) return null;

		MyType [] myData = new MyType [0];
		try (ByteArrayInputStream in = new ByteArrayInputStream (buffer);
			DataInputStream readIn = new DataInputStream (in)) {
			while (readIn.available () > 20) {
				MyType datum = new MyType ();
				int size = readIn.readInt ();
				byte [] name = new byte [size];
				readIn.read (name);
				datum.fileName = new String (name);
				datum.fileLength = readIn.readLong ();
				datum.lastModified = readIn.readLong ();
				MyType [] newMyData = new MyType [myData.length + 1];
				System.arraycopy (myData, 0, newMyData, 0, myData.length);
				newMyData [myData.length] = datum;
				myData = newMyData;
			}
			readIn.close ();
		}
		catch (IOException ex) {
			return null;
		}
		return myData;
	}

	return null;
}

@Override
protected String [] getTypeNames () {
	return new String [] {MYTYPENAME};
}

@Override
protected int [] getTypeIds () {
	return new int [] {MYTYPEID};
}

boolean checkMyType(Object object) {
	if (object == null ||
		!(object instanceof MyType[]) ||
		((MyType[])object).length == 0) {
		return false;
	}
	MyType[] myTypes = (MyType[])object;
	for (MyType myType : myTypes) {
		if (myType == null ||
			myType.fileName == null ||
			myType.fileName.length() == 0) {
			return false;
		}
	}
	return true;
}

@Override
protected boolean validate(Object object) {
	return checkMyType(object);
}
}

public static void main (String [] args) {
	Display display = new Display ();
	Shell shell = new Shell (display);
	shell.setText("Snippet 79");
	shell.setLayout (new FillLayout ());
	final Label label1 = new Label (shell, SWT.BORDER | SWT.WRAP);
	label1.setText ("Drag Source for MyData[]");
	final Label label2 = new Label (shell, SWT.BORDER | SWT.WRAP);
	label2.setText ("Drop Target for MyData[]");

	DragSource source = new DragSource (label1, DND.DROP_COPY);
	source.setTransfer (MyTransfer.getInstance ());
	source.addDragListener (new DragSourceAdapter () {
		@Override
		public void dragSetData (DragSourceEvent event) {
			MyType myType1 = new MyType ();
			myType1.fileName = "C:\\abc.txt";
			myType1.fileLength = 1000;
			myType1.lastModified = 12312313;
			MyType myType2 = new MyType ();
			myType2.fileName = "C:\\xyz.txt";
			myType2.fileLength = 500;
			myType2.lastModified = 12312323;
			event.data = new MyType [] {myType1, myType2};
		}
	});
	DropTarget target = new DropTarget (label2, DND.DROP_COPY | DND.DROP_DEFAULT);
	target.setTransfer (MyTransfer.getInstance ());
	target.addDropListener (new DropTargetAdapter () {
		@Override
		public void dragEnter (DropTargetEvent event) {
			if (event.detail == DND.DROP_DEFAULT) {
				event.detail = DND.DROP_COPY;
			}
		}

		@Override
		public void dragOperationChanged (DropTargetEvent event) {
			if (event.detail == DND.DROP_DEFAULT) {
				event.detail = DND.DROP_COPY;
			}
		}

		@Override
		public void drop (DropTargetEvent event) {
			if (event.data != null) {
				MyType [] myTypes = (MyType []) event.data;
				if (myTypes != null) {
					String string = "";
					for (MyType myType : myTypes) {
						string += myType.fileName + " ";
					}
					label2.setText (string);
				}
			}
		}

	});
	shell.setSize (200, 200);
	shell.open ();
	while (!shell.isDisposed ()) {
		if (!display.readAndDispatch ()) display.sleep ();
	}
	display.dispose ();
}
}

This snippet demonstrates a basic drag-and-drop operation using a custom data transfer type (MyType). It creates two labels, one acting as the drag source and the other as the drop target. The MyTransfer class handles the conversion of MyType objects to and from a byte array for transfer. The DragSourceListener and DropTargetListener handle the drag and drop events, respectively.

A Closer Look at the Code

Let's break down some key parts of the code to understand what might be contributing to the freeze:

  • MyType and MyTransfer: The snippet defines a custom data type MyType and a corresponding MyTransfer class. This is a common pattern in SWT drag-and-drop, allowing you to transfer complex data structures. The MyTransfer class is responsible for converting MyType objects into a byte array for transfer (javaToNative) and back (nativeToJava).

  • Drag and Drop Listeners: The DragSourceListener and DropTargetListener handle the drag-and-drop events. The dragSetData method in the DragSourceListener is where the data to be transferred is set. The dragEnter, dragOperationChanged, and drop methods in the DropTargetListener handle the events that occur as the data is dragged over the target.

  • UI Thread Interactions: SWT is a single-threaded UI toolkit, meaning that all UI operations must be performed on the main UI thread. This includes handling drag-and-drop events. If a long-running operation is performed on the UI thread, it can block the UI and lead to unresponsiveness.

Possible Causes of the Freeze

So, what's going on here? Why is debugging this particular scenario causing a system freeze? Here are a few potential culprits:

  1. Excessive UI Thread Activity: As I mentioned earlier, my primary suspicion is that the combination of debugging and drag-and-drop events is overwhelming the UI thread. The debugger adds overhead by inspecting variables and stepping through code, while drag-and-drop events can trigger a flurry of UI updates. This can lead to a situation where the UI thread is constantly busy, unable to process other events, and eventually causing the system to freeze.

  2. Deadlocks: It's possible that a deadlock situation is occurring within SWT or the underlying operating system. A deadlock happens when two or more threads are blocked indefinitely, waiting for each other to release a resource. In the context of drag-and-drop, this could involve locks related to UI updates, data transfer, or event handling.

  3. Resource Starvation: Another possibility is that the debugging process is consuming so many system resources (CPU, memory, etc.) that other critical system processes are being starved. This could lead to instability and eventually a system freeze.

  4. Underlying Bug in SWT or the OS: It's also conceivable that there's a bug in SWT's drag-and-drop implementation or in the operating system's event handling mechanisms. These kinds of bugs can be notoriously difficult to track down.

Workarounds and Next Steps

While we haven't pinpointed the exact cause of the freeze, there are a few things you can try to mitigate the issue:

  • Reduce Debugging Overhead: Try disabling some debugger features, such as variable inspection or expression evaluation, to reduce the load on the UI thread.

  • Simplify the Code: If possible, try simplifying the drag-and-drop logic in your application to see if that resolves the issue. This might involve reducing the amount of data being transferred or simplifying the event handling code.

  • Increase System Resources: Make sure your system has enough CPU and memory to handle the debugging process. Closing unnecessary applications can free up resources.

  • Test on Different Platforms: Try reproducing the issue on different operating systems and Eclipse versions to see if it's platform-specific.

  • Report the Bug: If you're consistently able to reproduce the freeze, consider reporting a bug to the Eclipse or SWT project. The more information you can provide, the better the chances of the issue being resolved.

Conclusion

Debugging SWT drag-and-drop widgets can be a tricky business, and the potential for a system freeze adds a significant level of complexity. While the exact cause of this issue remains elusive, understanding the possible culprits and trying the workarounds mentioned above can help you navigate this challenging situation. Remember, save your work frequently, and be prepared to switch to the command line if things go south. Happy debugging, and may your systems remain unfrozen!