multithreading - Java synchronizing based on a parameter (named mutex/lock)

Question

I'm looking for a way to synchronize a method based on the parameter it receives, something like this:

public synchronized void doSomething(name){
//some code
}

I want the method doSomething to be synchronized based on the name parameter like this:

Thread 1: doSomething("a");

Thread 2: doSomething("b");

Thread 3: doSomething("c");

Thread 4: doSomething("a");

Thread 1 , Thread 2 and Thread 3 will execute the code without being synchronized , but Thread 4 will wait until Thread 1 has finished the code because it has the same "a" value.

Thanks

UPDATE

Based on Tudor explanation I think I'm facing another problem: here is a sample of the new code:

private HashMap locks=new HashMap();
public void doSomething(String name){
    locks.put(name,new Object());
    synchronized(locks.get(name)) {
        // ...
    }
    locks.remove(name);
}

The reason why I don't populate the locks map is because name can have any value.

Based on the sample above , the problem can appear when adding / deleting values from the hashmap by multiple threads in the same time, since HashMap is not thread-safe.

So my question is if I make the HashMap a ConcurrentHashMap which is thread safe, will the synchronized block stop other threads from accessing locks.get(name) ??

Answer 1

Use a map to associate strings with lock objects:

Map<String, Object> locks = new HashMap<String, Object>();
locks.put("a", new Object());
locks.put("b", new Object());
// etc.

then:

public void doSomething(String name){
    synchronized(locks.get(name)) {
        // ...
    }
}