How did this synchronized code break?

Question

I have the following snippet of code.

synchronized (mObject) {
    if (mObject.variable != -1) {
        doThis();
        doThisAsWell();
        andThis();
        insertObjectInDb(); // This is crashing because mObject.variable is -1
    }
}

To my knowledge, if I use synchronized on an object, and then run that block, the value of the variable can't be changed by any other thread, right? I don't understand how the value of the variable is -1, when I specifically check for that before entering the next block. And no, none of the functions in the block are changing the value. Have I completely misunderstood how these blocks work?

If it matters at all, this is all in a doInBackground() method of an async task in an android app.

Any ideas?

score 6 · Accepted Answer · answered Feb 12 '15 at 12:20

6

To my knowledge, if I use synchronized on an object, and then run that block, the value of the variable can't be changed by any other thread, right?

No your assumption is wrong. synchronized is a monitor. The only guarantee you have is that the monitor is accessed by one thread at time.

answered Feb 12 '15 at 12:20

Blackbelt

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Ohh, I see, so if mObject is a private object, I need to ensure that any other place it's modified within the class is inside a synchronized(mObject) block as well? – Akhil Cherian Verghese Feb 12 '15 at 12:23
1

exactly. You can think about the synchronization object like a queue upon the thread are somehow suspended waiting for their turn. The order is up to the scheduler – Blackbelt Feb 12 '15 at 12:25
Brilliant, exactly what I needed, thanks! (will accept it in 5 mins, give or take) – Akhil Cherian Verghese Feb 12 '15 at 12:28

score 1 · Answer 2 · edited May 23 '17 at 12:06

In this case, the object "mObject" doesn't become locked, rather the object "mObject" is used as the mutex and the body is prevented from executing concurrently with other code sections also synchronized on "mObject." It has no effect on other fields/methods of "mObject" that aren't synchronized.

Found some details here : Java synchronized method lock on object, or method? See the top comment on this.

score 0 · Answer 3 · answered Feb 12 '15 at 12:23

0

Afaik synchronized just says that any access to mObject is threadsafe means that following code would guaranteed set i to 3 if called in 3 threads

int i=0;
synchronized(i){
   i = i + 1;
}

What you want to use is a mutex

answered Feb 12 '15 at 12:23

Verim

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How did this synchronized code break?

3 Answers3