Count number of instances of a class in multi-threading environment?

问题

I am trying to count how many instances of a class generated during the run time of a process under multi-threading environment. The way how I do it is to increase a static counter in the constructor by looking at this post: How to Count Number of Instances of a Class

So in multi-threading environment, here is how i define the class:

class Television {
      private static volatile int counter = 0;
      public Television(){
             counter ++;
      }
}

However, I am not sure whether there is a potential bug with the code above since I think constructor in java does not imply synchronization and counter++ is not atomic so if two threads are creating instances simultaneously, is the code a bug somehow? but I am not quite sure yet.

回答1:

There is a bug in this code (specifically, a race condition), because the read of counter and write to counter aren't atomically executed.

In other words, two threads can read the same value of counter, increment that value, and then write the same value back to the variable.

Thread 1    Thread 2
========    ========

Read 0
            Read 0
Increment
            Increment
Write 1
            Write 1

So the value would be 1, not 2, afterwards.

Use AtomicInteger and AtomicInteger.incrementAndGet() instead.

回答2:

As counter++ is NOT atomic, you can replace it with JDK's AtomicInteger which is threadsafe.

You can AtomicInteger's use getAndIncrement() method as shown below:

class Television {
      private static final AtomicInteger counter = new AtomicInteger();
      public Television(){
            counter.getAndIncrement();
      }
}

An AtomicInteger is used in applications such as atomically incremented counters, and cannot be used as a replacement for an Integer.

You can look here

回答3:

There are two ways here to bypass the underlying "++ on int" not being an atomic operation:

A) as others suggested, use AtomicInteger

B) introduce a common LOCK that all ctors can be using to sync on; like:

private final static Object LOCK = new Object();

public Television() {
  synchronized (LOCK) { 
    counter++; 
  }

来源：https://stackoverflow.com/questions/40597894/count-number-of-instances-of-a-class-in-multi-threading-environment

标签

java

multithreading