Java Tutorials - Herong's Tutorial Examples
Dr. Herong Yang, Version 6.20

JDK 1.3 Bug - Memory Leak With Unstarted Threads

This section describes a reported bug for JDK 1.3 that unstarted thread objects cause memory leak.

There was a bug opened in 2001 for JDK 1.3 titled "Unstarted Thread causes memory leak" See for details.

The bug report included the following program to show the memory leak:

public class ThreadTest {
    public static void main(String args[]) {
	while (true) {
	  try {
        Thread t = new Thread();
	  // start thread for memory leak workaround
//        t.start();
	  t = null;
	} catch (InterruptedException ie) {

I ran this program on my J2SDK 1.4.1 system. As predicted, the memory usage of the JVM went up slowly at a rate of about 4K per 15 seconds.

As I mentioned in previous sections, Thread object created without a specific thread group will be added into the "main" group. So in the ThreadTest program included in the bug report, all the threads created will be added in the "main" group, and will stay there until they are started and ended. If we keep creating new threads, the number of threads in the "main" group will grow, and memory usage will also grow.

But in my opinion, this is not really a bug. JDK is designed to organize all Thread and ThreadGroup objects in a single thread group tree. Application programs are forced to use this structure, and must follow the rules.

However, this designed feature is dangerous if not used properly, as demonstrated by the bug report. It could be a risk for any server type application, if new threads are created for new connection requests and the start() calls are not executed for some reason.

One option to solve this problem is to:

  • At the Thread object creation time, add it to the thread group with a WeakReference.
  • At start time (when start() is called), change the WeakReference to a regular reference.

A couple of notes on this option:

  • The weak reference for an unstarted thread will allow the garbage collector to remove the thread, if there is no more regular references to this thread in the application.
  • Replacing the weak reference by a regular reference when a thread is started will prevent the garbage collector to remove this running thread, even if there is no more regular reference to this thread in the application.
  • The implementation of this option could be two separate collections of threads in a thread group: one for unstarted threads with weak references, and the other for started threads with regular references.

Without any help from JDK, finding a simple workaround to prevent this from the application side is not easy.

However, you can put a logic in your application to watch the number of unstarted threads. If this number is growing, you know that somewhere in your application threads are created without the start() calls. Here is the code to calculate the number of unstarted threads:

   ThreadGroup g = Thread.currentThread().getThreadGroup();
   while (g.getParent()!=null) g = g.getParent();
   Thread[] l = new Thread[g.activeCount()]];
   int unstartedThreadCount = g.activeCount() - g.enumerate(l,true):

Last update: 2012.

Table of Contents

 About This Book

 Installing JDK 1.7 on Windows

 Execution Process, Entry Point, Input and Output

 Primitive Data Types and Literals

 Bits, Bytes, Bitwise and Shift Operations

 Managing Bit Strings in Byte Arrays

 Reference Data Types and Variables

 StringBuffer - The String Buffer Class

 System Properties and Runtime Object Methods

 Generic Classes and Parameterized Types

 Generic Methods and Type Inference

 Execution Threads and Multi-Threading Java Programs

ThreadGroup Class and "system" ThreadGroup Tree

 "ThreadGroup" Class - Container of Threads and Thread Groups

 Displaying the "system" ThreadGroup Tree

 Adding Threads and Thread Groups

JDK 1.3 Bug - Memory Leak With Unstarted Threads

 Synchronization Technique and Synchronized Code Blocks

 Deadlock Condition Example Programs

 Garbage Collection and the gc() Method

 Outdated Tutorials


 PDF Printing Version

Dr. Herong Yang, updated in 2012
JDK 1.3 Bug - Memory Leak With Unstarted Threads