CODEWITHSUNDEEP
javathread-safety
codeisee
codeisee

Reputation: 1742

Is incrementing an integer thread safe in java?

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;

Upvotes: 8

Views: 18056

Answers (4)

Avner Levy
Avner Levy

Reputation: 6741

You should probably use atomicvars

Upvotes: 1

Mark Rotteveel
Mark Rotteveel

Reputation: 109079

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++;
    }
}

Upvotes: 26

niculare
niculare

Reputation: 3687

I do not think this is thread safe since the static variable is public and can be accessed by other threads in a non-thread safe manner. In order to be thread safe you must declare the variable as follows:

public static volatile int value;

Now value being volatile, will be accessed in a synchronized block.

Upvotes: -1

SteveP
SteveP

Reputation: 19103

If you are using this method in two threads then you do need the volatile keyword. Without it, another thread may not get the most up to date value. (C#)

Upvotes: -1

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