Is the following incrementation code thread safe in java?

Question

Java Code:

public class IncreaseTest {
    public static int value = 0;

    public synchronized int increment() {
        return value++;
    }
}

Is method increment() thread-safe? Do I have to add the modifier keyword volatile as follows：

  public static volatile int value = 0;

Answer 1

This code is not thread-safe. The instance method will synchronize on an instance, if you have multiple instances they will not use the same monitor and therefor the updates can interleave.

You either need to remove the static from the value field or add static to the increment() method.

Also, as you have made value public, there is the additional problem that value can be changed or read outside of this method without using synchronisation which could result in reading old values.

So changing your code to the following will make it thread-safe:

public class IncreaseTest {
    private int value = 0;

    public synchronized int increment() {
        return value++;
    }
}

Answer 2

You should probably use atomicvars