Synchronize two methods in different classes (and

Question

I'm facing an issue regarding Java method synchronization. Let's hope I can explain it briefly:

I have two different methods in two different classes, in two different packages. Something like:

Package1
     |_Class1
            \MethodA

Package2
     |_Class2
            \MethodB

Ok, so now I need to synchronize these two methods, which are not threads. So far, I have two different approaches:

Shared semaphore.

Create a shared static semaphore outside Package1 and Package2, something like:

 package Package3;

 public class SemaphoreClass {

    public static Semaphore;

}

I don't really know if JVM would consider this a shared semaphore, anyway.

Synchronized (SharedClass.class).

Using a shared class to synchronize those two methods, something like:

 public void MethodA() {
        synchronized (SharedClass.class) {
           //...
        }

and

public void MethodB() {
        synchronized (SharedClass.class) {
           //...
        }

Anyway, these are just approaches. I would like to hear what's the best way to implement what I'm trying to achieve. Having a shared semaphore for JVM would make things easier, but well, I know there must be a solution for this. Thanks in advance.

Answer 1

Both of your approaches should work, but I don't think locking on class is a recommended practices. Prefer locking on instances of Object, or use proper locks from java.util. Also, do not lock on String.

Alternatively, you could let instances of classes Class1 and Class2 can take a lock as parameter during instantiation:

   class Class1 {
        private final Object lock;
        public Class1( Object lock ) { this.lock = lock }
        public method() { synchronize( lock ) { } );
   }

Then make sure that you create one lock (with new Object()) and pass it to the two instances of Class1 and Class2.

So basically, you've broken down the original problem in two: 1) the two classes do no share anything statically global, they just receive a parameter. 2) the clients of Class1 and Class2 must pass the correct lock. The client of Class1 and Class2 acts as the "orchestrator".