ThreadLocal Access (Java)

ThreadLocal.java

ThreadLocal<String> threadLocal = new ThreadLocal<String>();
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"

public static void injectThreadExitCallback(final Runnable callback) {
    final curr = Thread.currentThread();
    new Thread() {
        @Override
        public void run() {
            try {
                curr.join();
            } catch (InterruptedException ex) {
               
            } finally {
               callback.run();
            }
        }
    }.start();
}