Java在原子操作上同步

Question

Is this Java class thread safe or reset method needs to be synchronized too? If yes can someone tell me the reason why?

public class NamedCounter {
   private int count;
   public synchronized void increment() { count++; }
   public synchronized int getCount() { return count; }
   public void reset() { count = 0; }
}

Answer 1

Not without synchronizing rest() and adding more methods. You will run into cases where you will need more methods. For example

NamedCounter counter = new NamedCounter();
counter.increment();
// at this exact time (before reaching the below line) another thread might change changed the value of counter!!!!
if(counter.getCount() == 1) {
    //do something....this is not thread safe since you depeneded on a value that might have been changed by another thread
}

To fix the above you need something like

NamedCounter counter = new NamedCounter();
if(counter.incrementAndGet()== 1) { //incrementAndGet() must be a synchronized method
    //do something....now it is thread safe
}

Instead, use Java's bulit-in class AtomicInteger which covers all cases. Or if you are trying to learn thread safety then use AtomicInteger as a standard (to learn from).

For production code, go with AtomicInteger without even thinking twice! Please note that using AtomicInteger does not automatically guarantee thread safety in your code. You MUST make use of the methods that are provided by the api. They are there for a reason.

Answer 2

Note that synchronized is not just about mutual exclusion, it is fundamentally about the proper ordering of operations in terms of the visibility of their actions. Therefore reset must be synchronized as well, otherwise the writes it makes may occur concurrently to other two methods, and have no guarantee to be visible.

To conclude, your class is not thread-safe as it stands, but will be as soon as you synchronize the reset method.

Answer 3

You have to synchronize your reset() method also.

To make a class thread safe you have to synchronize all paths that access a variable else you will have undesired results with the unsynchronized paths.

Answer 4

You need to add synchronized to reset method too and then it will be synchronized. But in this way you achieve syncronization through locks, that is, each thread accesing the method will lock on the NamedCounter object instace.

However, if you use AtomicInteger as your count variable, you don't need to syncronize anymore because it uses the CAS cpu operation to achieve atomicity without the need to synchronize.

Answer 5

Not an answer, but too long for a comment:

If reset() is synch'ed, then the 0 become visible to any thread that reads or increments the counter later. Without synchronization, there is no visibility guarantee. Looking at the interaction of concurrent increment and the unsychronized reset, it may be that 0 becomes visible to the incrementing thread before entering the method, then the result will be 1. If counter is set to 0 between increment's read and write, the reset will be forgotten. If it is set after the write, the end result will be 0. So, if you want to assert that for every reading thread, the counter is 0 after reset, that method must be synchronized, too. But David Schwartz is correct that those low-level synchronizations make little sense whithout higher-level semantics of those interactions.