Does wait() need synchronization on local variable

Question

I had this code (which was working fine):

public static void runOnUiThread(Activity c, final Runnable action) {
    // Check if we are on the UI Thread
    if (Looper.getMainLooper() == Looper.myLooper()) {
        // If we are, execute immediately
        action.run();
        return;
    } // Else run the runnable on the UI Thread and wait
    Runnable r = new Runnable() {
        @Override
        public void run() {
            action.run();
            synchronized (this) {
                this.notify();
            }
        }
    };
    synchronized (r) {
        try {
            c.runOnUiThread(r);
            r.wait();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

And I was getting Synchronization on local variable warning. As suggested, I removed the synchronization on the local variable, in order to fix the warning:

public static void runOnUiThread(Activity c, final Runnable action) {
    // Check if we are on the UI Thread
    if (Looper.getMainLooper() == Looper.myLooper()) {
        // If we are, execute immediately
        action.run();
        return;
    } // Else run the runnable on the UI Thread and wait
    Runnable r = new Runnable() {
        @Override
        public void run() {
            action.run();
            this.notify();
        }
    }
    try {
        c.runOnUiThread(r);
        r.wait();
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
}

And now I am getting an exception when calling this method:

06-15 09:18:13.252 27282 27282 E   AndroidRuntime                               FATAL EXCEPTION: main
06-15 09:18:13.252 27282 27282 E   AndroidRuntime                               java.lang.IllegalMonitorStateException: object not locked by thread before notify()
06-15 09:18:13.252 27282 27282 E   AndroidRuntime                               at java.lang.Object.notify(Native Method)
06-15 09:18:13.252 27282 27282 E   AndroidRuntime                               at org.matapps.android.simpleappcreator.Utils$100000007.run(Utils.java:673)
06-15 09:18:13.252 27282 27282 E   AndroidRuntime                               at android.os.Handler.handleCallback(Handler.java:615)
06-15 09:18:13.252 27282 27282 E   AndroidRuntime                               at android.os.Handler.dispatchMessage(Handler.java:92)
06-15 09:18:13.252 27282 27282 E   AndroidRuntime                               at android.os.Looper.loop(Looper.java:137)
06-15 09:18:13.252 27282 27282 E   AndroidRuntime                               at android.app.ActivityThread.main(ActivityThread.java:4867)
06-15 09:18:13.252 27282 27282 E   AndroidRuntime                               at java.lang.reflect.Method.invokeNative(Native Method)
06-15 09:18:13.252 27282 27282 E   AndroidRuntime                               at java.lang.reflect.Method.invoke(Method.java:511)
06-15 09:18:13.252 27282 27282 E   AndroidRuntime                               at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1007)
06-15 09:18:13.252 27282 27282 E   AndroidRuntime                               at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:774)

I'm pretty sure I need the synchronization (as the runnable runs on a different thread), but I was told local variables shouldn't have synchronization. Any tips? Thanks!

Both wait and notify must be invoked in a synchronized block — Maurice Perry
– Maurice Perry, Commented Jun 15, 2015 at 6:33
Have you searched for java.lang.IllegalMonitorStateException? — assylias
– assylias, Commented Jun 15, 2015 at 6:34
First you must understand why there is a "synchronization on local variable" warning in the first place. Which is that it's usually a mistake. But in this case, it's not a mistake. — Stack Exchange Broke The Law
– Stack Exchange Broke The Law, Commented Jun 15, 2015 at 6:44

Solomon Slow · Accepted Answer · 2015-06-15 13:55:42Z

1

I don't know Android development tools, but the warning sounds over-zealous. The compiler is trying to help you to avoid synchronizing on an instance that is only visible to a single thread. It's smart enough to know that the local variable, r, can only be seen by one thread, but apparently it is not smart enough to know that r in the original thread and this in the new thread both refer to the same instance.

I would try to work around the problem by making r an instance variable.

answered Jun 15, 2015 at 13:55

Solomon Slow

27.6k5 gold badges39 silver badges61 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Chaoz Over a year ago

All I needed to know is that the code is OK, and simply the compiler doesn't know that runOnUiThread actually runs the runnable on a different thread... Anyway, I fixed it with a '@SuppressWarnings'. Thanks to anybody who helped!

Collectives™ on Stack Overflow

Does wait() need synchronization on local variable

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related