Propagating ThreadLocal to a new Thread fetched from a ExecutorService

The set of ThreadLocal instances associated with a thread are held in private members of each Thread. Your only chance to enumerate these is to do some reflection on the Thread; this way, you can override the access restrictions on the thread's fields.

Once you can get the set of ThreadLocal, you could copy in the background threads using the beforeExecute() and afterExecute() hooks of ThreadPoolExecutor, or by creating a Runnable wrapper for your tasks that intercepts the run() call to set an unset the necessary ThreadLocal instances. Actually, the latter technique might work better, since it would give you a convenient place to store the ThreadLocal values at the time the task is queued.

Update: Here's a more concrete illustration of the second approach. Contrary to my original description, all that is stored in the wrapper is the calling thread, which is interrogated when the task is executed.

static Runnable wrap(Runnable task){  Thread caller = Thread.currentThread();  return () -> {    Iterable<ThreadLocal<?>> vars = copy(caller);    try {      task.run();    }    finally {      for (ThreadLocal<?> var : vars)        var.remove();    }  };}/** * For each {@code ThreadLocal} in the specified thread, copy the thread's  * value to the current thread.   *  * @param caller the calling thread * @return all of the {@code ThreadLocal} instances that are set on current thread */private static Collection<ThreadLocal<?>> copy(Thread caller){  /* Use a nasty bunch of reflection to do this. */  throw new UnsupportedOperationException();}

Based on @erickson answer I wrote this code. It is working for inheritableThreadLocals. It builds list of inheritableThreadLocals using same method as is used in Thread contructor. Of course I use reflection to do this. Also I override the executor class.

public class MyThreadPoolExecutor extends ThreadPoolExecutor{   @Override   public void execute(Runnable command)   {      super.execute(new Wrapped(command, Thread.currentThread()));   }}

Wrapper:

   private class Wrapped implements Runnable   {      private final Runnable task;      private final Thread caller;      public Wrapped(Runnable task, Thread caller)      {         this.task = task;         this.caller = caller;      }      public void run()      {         Iterable<ThreadLocal<?>> vars = null;         try         {            vars = copy(caller);         }         catch (Exception e)         {            throw new RuntimeException("error when coping Threads", e);         }         try {            task.run();         }         finally {            for (ThreadLocal<?> var : vars)               var.remove();         }      }   }

copy method:

public static Iterable<ThreadLocal<?>> copy(Thread caller) throws Exception   {      List<ThreadLocal<?>> threadLocals = new ArrayList<>();      Field field = Thread.class.getDeclaredField("inheritableThreadLocals");      field.setAccessible(true);      Object map = field.get(caller);      Field table = Class.forName("java.lang.ThreadLocal$ThreadLocalMap").getDeclaredField("table");      table.setAccessible(true);      Method method = ThreadLocal.class              .getDeclaredMethod("createInheritedMap", Class.forName("java.lang.ThreadLocal$ThreadLocalMap"));      method.setAccessible(true);      Object o = method.invoke(null, map);      Field field2 = Thread.class.getDeclaredField("inheritableThreadLocals");      field2.setAccessible(true);      field2.set(Thread.currentThread(), o);      Object tbl = table.get(o);      int length = Array.getLength(tbl);      for (int i = 0; i < length; i++)      {         Object entry = Array.get(tbl, i);         Object value = null;         if (entry != null)         {            Method referentField = Class.forName("java.lang.ThreadLocal$ThreadLocalMap$Entry").getMethod(                    "get");            referentField.setAccessible(true);            value = referentField.invoke(entry);            threadLocals.add((ThreadLocal<?>) value);         }      }      return threadLocals;   }

As I understand your problem, you can have a look at InheritableThreadLocal which is meant to pass ThreadLocal variables from Parent Thread context to Child Thread Context