How to clean up ThreadLocals

Question

Does any one have an example how to do this? Are they handled by the garbage collector? I\'m using Tomcat 6.

Answer 1

Here is some code to clean all thread local variables from the current thread when you do not have a reference to the actual thread local variable. You can also generalize it to cleanup thread local variables for other threads:

    private void cleanThreadLocals() {
        try {
            // Get a reference to the thread locals table of the current thread
            Thread thread = Thread.currentThread();
            Field threadLocalsField = Thread.class.getDeclaredField("threadLocals");
            threadLocalsField.setAccessible(true);
            Object threadLocalTable = threadLocalsField.get(thread);

            // Get a reference to the array holding the thread local variables inside the
            // ThreadLocalMap of the current thread
            Class threadLocalMapClass = Class.forName("java.lang.ThreadLocal$ThreadLocalMap");
            Field tableField = threadLocalMapClass.getDeclaredField("table");
            tableField.setAccessible(true);
            Object table = tableField.get(threadLocalTable);

            // The key to the ThreadLocalMap is a WeakReference object. The referent field of this object
            // is a reference to the actual ThreadLocal variable
            Field referentField = Reference.class.getDeclaredField("referent");
            referentField.setAccessible(true);

            for (int i=0; i < Array.getLength(table); i++) {
                // Each entry in the table array of ThreadLocalMap is an Entry object
                // representing the thread local reference and its value
                Object entry = Array.get(table, i);
                if (entry != null) {
                    // Get a reference to the thread local object and remove it from the table
                    ThreadLocal threadLocal = (ThreadLocal)referentField.get(entry);
                    threadLocal.remove();
                }
            }
        } catch(Exception e) {
            // We will tolerate an exception here and just log it
            throw new IllegalStateException(e);
        }
    }