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For some time Tomcat has had some means of protection against memory leaks when stopping or redeploying applications. This page tries to list them, and shows the situations where leaks can be detected and fixed.

Diagnose a classloader leak upon request

Starting with tomcat 6.0.25, the manager webapp has a new "Find Leaks" button. When triggered, it displays a list of webapps (their context path) that have been stopped (this includes undeployed and redeployed ones) but whose classloader failed to be GCed.

If a leaking webapp is redeployed several times, it will appear as many times as it actually leaked.

Caution: This diagnosis calls System.gc() which may not be desirable in production environments.

Different types of leaks that Tomcat can detect (or not)

When a webapp execution is stopped (this encompassed redeploy and undeploy), tomcat tries to detect and fix leaks.

Starting with tomcat 6.0.24, messages are logged to indicate the kind of leak that was detected.

ThreadLocal leaks

Classloader leaks because of uncleaned ThreadLocal variables are quite common. Depending on the use cases, they can be detected or not.

Custom ThreadLocal class

Suppose we have the following 3 classes in our webapp :

public class MyCounter {
	private int count = 0;

	public void increment() {
		count++;
	}

	public int getCount() {
		return count;
	}
}

public class MyThreadLocal extends ThreadLocal<MyCounter> {
}

public class LeakingServlet extends HttpServlet {
	private static MyThreadLocal myThreadLocal = new MyThreadLocal();

	protected void doGet(HttpServletRequest request,
			HttpServletResponse response) throws ServletException, IOException {

		MyCounter counter = myThreadLocal.get();
		if (counter == null) {
			counter = new MyCounter();
			myThreadLocal.set(counter);
		}

		response.getWriter().println(
				"The current thread served this servlet " + counter.getCount()
						+ " times");
		counter.increment();
	}
}

If the LeakingServlet is invoked at least once and the Thread that served it is not stopped, then we created a classloader leak !

The leak is caused because we have a custom class for the ThreadLocal instance, and also a custom class for the value bound to the Thread. Actually the important thing is that both classes were loaded by the webapp classloader.

Hopefully tomcat 6.0.24 can detect the leak when the application is stopped: each Thread in the JVM is examined, and the internal structures of the Thread and ThreadLocal classes are introspected to see if either the ThreadLocal instance of the value bound to it were loaded by the WebAppClassLoader of the application being stopped.

In this particular case, the leak is detected, a message is logged and internal structures of the JDK (ThreadLocalMap) are modified to remove the reference to the ThreadLocal instance.

TODO: add an example of log

Note: this particular leak was actually already cured by previous versions of tomcat, because static references of classes loaded by the webappclassloader are nullified (see later).

Webapp class instance as ThreadLocal value

Webapp class instance indirectly held through a ThreadLocal value

ThreadLocal pseudo-leak

Threads ContextClassLoader

Threads spawned by JRE classes

Threads spawned by classes loaded by the common classloader

Threads spawned by webapps

Child classloaders

static class variables

LogFactory

JavaBean Introspector cache

JDBC driver registration

RMI target

Summary matrix

References

Mark Thomas interview on DZone

Related issues

• 48837 - Memory leaks protection does not cure leaks triggered by JSP pages code