servlet线程安全问题

Question

我知道servlet不是线程安全的，如果在servlet中定义了成员变量，在多线程环境中，可能就会出现问题。比如我想做个页面访问量的计数器，定义了一个成员变量count，每次访问就在doGet方法中加1，那么该如何做才能保证它的计数是准确的呢？除了加锁之外。上次面试就被问了这个问题。

Answer 1

把count加在application上面

Answer 2

如果count是Servlet成员变量，那么需要用synchronized来同步。

synchronized(this) {
  count++;
}

线程安全问题可以参考我写的一篇博客：http://xxgblog.com/2013/05/16/java-thread-safe/

Answer 3

使用原子类，比如AtomicInteger或者AtomicLong

import java.util.concurrent.atomic.AtomicInteger;

public class AtomicIntegerExample {

    private static AtomicInteger at = new AtomicInteger(0);

    static class MyRunnable implements Runnable {

        private int myCounter;
        private int myPrevCounter;
        private int myCounterPlusFive;
        private boolean isNine;

        public void run() {
            myCounter = at.incrementAndGet();
            System.out.println("Thread " + Thread.currentThread().getId() + "  / Counter : " + myCounter);
            myPrevCounter = at.getAndIncrement();
            System.out.println("Thread " + Thread.currentThread().getId() + " / Previous : " + myPrevCounter); 
            myCounterPlusFive = at.addAndGet(5);        
            System.out.println("Thread " + Thread.currentThread().getId() + " / plus five : " + myCounterPlusFive);
            isNine = at.compareAndSet(9, 3);
            if (isNine) {
                System.out.println("Thread " + Thread.currentThread().getId() 
                        + " / Value was equal to 9, so it was updated to " + at.intValue());
            }

        }
    }

    public static void main(String[] args) {
        Thread t1 = new Thread(new MyRunnable());
        Thread t2 = new Thread(new MyRunnable());
        t1.start();
        t2.start();
    }
}

参考：Java AtomicInteger Example