Is ThreadLocal thread safe?
java, multithreading, thread-safety
Solution
It's safe because `getMap` returns the map for the given (i.e. current) thread. No other thread is going to be messing with that. So it's really down to the implementation of `getMap` to make sure that's is okay for any thread - and as far as I can see, that just delegates to a field within the `Thread` object. It's not clear to me whether `getMap` is ever passing in any thread other than the current thread - if it were, that could be potentially tricky - but I suspect it's all been written carefully to make sure that's not a problem :)
Problem
For example, we have a static ThreadLocal field an a setter: ``` private static final ThreadLocal threadLocalField = new ThreadLocal; public static void getSXTransaction() { threadLocalField.set(new MyValue()); } ``` I wonder, what are the guarantees of thread safety here as there is no implicit synchronization in the java.lang.ThreadLocal#set method? I know it makes sense for the TreadLocal class to be fully thread safe by it's nature but I cannot understand how it's being accomplished. Here is the source code for it: ``` /** * Sets the current thread's copy of this thread-local variable * to the specified value. Most subclasses will have no need to * override this method, relying solely on the {@link #initialValue} * method to set the values of thread-locals. * * @param value the value to be stored in the current thread's copy of * this thread-local. */ public void set(T value) { Thread t = Thread.currentThread(); ThreadLocalMap map = getMap(t); if (map != null) map.set(this, value); else createMap(t, value); } ```