java threading lock variable properly

Question

Thread A does:

class A{
    public String value;
    public void methodA(String value){ //lets say value="test"
        this.value=value;
        //some code
        // Thread B interrupts
        System.out.println(value); // prints "haha" but I want it to be "test"
    }
}

Thread B does:

class B{
    public void methodB(){
        a.setValue("haha");  
    }
}

methodB and methodA are some kinds of listener methods, which are executed in separate Threads.

How can I make sure that value does not change, as long as methodA has not finished? But I want also that "haha" is assigned to value afterwards. So I want B to wait till A has finished methodA and then assign "haha" to value.

my problem is that this is not a defined path. so Method B could be executed after Method A has finished(result I want). It could also be that MethodB is executed before MethodA. but sometimes, MethodB interrupts MethodA and I get this problem — user2071938
– user2071938, Commented Nov 30, 2015 at 12:06
A very basic solution would be to make methodA and setValue both synchronized. Then you would want to read the Oracle concurrency tutorial, so you understand what you've just done. — Kayaman
– Kayaman, Commented Nov 30, 2015 at 12:20

Mati · Accepted Answer · 2015-11-30 12:35:50Z

1

The easiest way is to use 'synchronized' keyword on method that change the value of a field.

For example we have class that stores the data:

public class Data {
   String value = "";
   public synchronized void setValue(String val) {
      this.value = val;
      System.out.println(val);
   }
}

And then Threads only use this method to update the value. Only one thread at the time can execute this method (no interrupts).

If you want to propagate this on two methods (like I suppose is the case). You can use two options. Or use synchronized on both methods or use external lock object.

If you want to be sure, that for example thread A has to be first to execute, you can use CountDownLatch object, which will stop other threads until thread A won't decrement the latch.

There are many ways to handle synchronization. You should be more precise to what you want to achieve and what kind of scenario you want to handle. Like for example - Is a.setValue("haha") a method from class A?

I would also recommend to look at documentation about concurrency https://docs.oracle.com/javase/tutorial/essential/concurrency/ .

answered Nov 30, 2015 at 12:35

Mati

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2 Comments

user2071938 Over a year ago

let's asume I have 2 methodes setValue1(String value) setValue2(String value) both sets the value;

Mati Over a year ago

if they are member of one class, then keyword synchronized should be enough. Otherwise you can do this for example ReentrantLock object which you have to provide to both classes. Then use flow like 1. lock, 2. change value, 3. unlock.

Solomon Slow · Accepted Answer · 2017-07-31 16:28:16Z

If you just want methodA() to complete before methodB() is called, then you should call both methods from the same thread. In general, if you want your program to do certain things in a certain order, the best way to accomplish it is to do all of the things in a single thread.

On the other hand, you might want both threads to work in parallel most of the time, but there might be one particular point that you don't want thread B to pass until thread A gets there. The Java standard library provides a variety of different synchronization objects that you could use. E.g., java.util.concurrent.CountDownLatch.

Initialization:

CountDownLatch countDownLatch=new CountDownLatch(1);
threadA.start();
threadB.start();

Thread A:

doSomeStuff();
methodA();
countDownLatch.countDown();
doSomeMoreStuff();

ThreadB:

doSomeOtherStuff();
countDownLatch.await();
methodB();
doSomeMoreOtherStuff();

The doSomeStuff() and doSomeOtherStuff() calls could happen concurrently, and the doSomeMoreStuff() and doSomeMoreOtherStuff() calls could happen concurrently, but the methodA() and methodB() would be serialized in this case.

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