ThreadLocal value access across different threads

Question

Given that a ThreadLocal variable holds different values for different threads, is it possible to access the value of one ThreadLocal variable from another thread?

Answer 1

Based on the answer of Andrzej Doyle here a full working solution:

ThreadLocal threadLocal = new ThreadLocal();
threadLocal.set("Test"); // do this in otherThread

Thread otherThread = Thread.currentThread(); // get a reference to the otherThread somehow (this is just for demo)

Field field = Thread.class.getDeclaredField("threadLocals");
field.setAccessible(true);
Object map = field.get(otherThread);

Method method = Class.forName("java.lang.ThreadLocal$ThreadLocalMap").getDeclaredMethod("getEntry", ThreadLocal.class);
method.setAccessible(true);
WeakReference entry = (WeakReference) method.invoke(map, threadLocal);

Field valueField = Class.forName("java.lang.ThreadLocal$ThreadLocalMap$Entry").getDeclaredField("value");
valueField.setAccessible(true);
Object value = valueField.get(entry);

System.out.println("value: " + value); // prints: "value: Test"

All the previous comments still apply of course - it's not safe!

But for debugging purposes it might be just what you need - I use it that way.