CODEWITHSUNDEEP
javaconcurrencysynchronized
Remco
Remco

Reputation: 21

What should be synchronized with Java objects

Say you have 2 or more threads which share an object A.

object A has a list inside of it, which contains another object B

Object A List Object B

Now the threads modify/read Object B

Question is, what should be synchronized? Object A as they both access it, or Object B since that's what they read/modify

I cannot seem to find the answer to my question anywhere.

Thanks in advance

Upvotes: 2

Views: 115

Answers (2)

Victor Sorokin
Victor Sorokin

Reputation: 12006

Going a simplest and safest route, you need to synchronize on the outmost object in your object hierarchy, which should be operated in a atomic manner.

In your example, it seems like you need to synchronize on a list (unfortunately, I can't be sure what's really you going to do without actual code).

Here's sample code:

class B {
}

class A {
    private final List<B> listOfBs = new ArrayList<B>();

    void add(B b) {
        synchronized (listOfBs) {
            listOfBs.add(b);
        }
    }

    List<B> getSnapshot() {
        List<B> copy = new ArrayList<B>();
        synchronized (listOfBs) {
            copy.addAll(listOfBs);
        }
        return copy;
    }
}

Please, note that you need to declare list as final to eliminate possibility that some code may change reference to a list which violate atomicity:

    // will make add() and getSnapshot() not mutually exclusive, thus violating atomicity of operations 
    ...
    listOfBs = new ArrayList<B>();
    ...

Reading Java concurrent programming tutorial will help.

Upvotes: 2

Ștefan
Ștefan

Reputation: 758

If you modify object B through one of its methods, then you should make that method (or the part that does the writing) synchronized.

Upvotes: 0

Related Questions