Static versus non-static lock object in synchronized block

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The difference is simple: if the locked-on object is in a static field, then all instances of MyClass* will share that lock (i.e. no two objects will be able to lock on that object at the same time).

If the field is non-static, then each instance will have its own lock, so only calls of the method on the same object will lock each other.

When you use a static lock object:

  • thread 1 calls o1.foo()
  • thread 2 calls o1.foo(), will have to wait for thread 1 to finish
  • thread 3 calls o2.foo(), will also have to wait for thread 1 (and probably 2) to finish

When you use a non-static lock object:

  • thread 1 calls o1.foo()
  • thread 2 calls o1.foo(), will have to wait for thread 1 to finish
  • thread 3 calls o2.foo(), it can just continue, not minding thread 1 and 2

Which one of those you’ll need depends on what kind of data you try to protect with your synchronized block.

As a rule of thumb, you want the lock-object to have the same static-ness than the operated-on value. So if you manipulate non-static values only, you’ll want a non-static lock object. If you manipulate static values only, you’ll want a static lock object.

When you manipulate static and non-static values, then it’ll become complicated. The easy way would be to just use a static lock object, but that might increase the size of the synchronized-block more than absolutely necessary and might need to more lock contention than desired. In those cases you might need a combination of static and non-static lock objects.

In your particular case you use the lock in the constructor, which will only ever be executed once per instance, so a non-static lock-object doesn’t make any sense here.

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