多线程的问题

• 正确性
  • 安全:竞争条件/死锁

//竞争条件示例:
    private static int counter = 0;

    public static void main(String[] args) throws InterruptedException {

        for (int i = 0; i < 100; i++) {
            new Thread(() -> {
                try {
                    Thread.sleep(1);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                counter++;
            }).start();
        }
        Thread.sleep(100);
        System.out.println(counter);
    }

控制台结果:
95

- 协同:同时、随机执行的线程,如何协同工作?
      - 解决问题的办法:
      1. java原生实现
       1.1 同步: synchronized
       1.2 协同: wait() / notify() / notifyAll()

• 效率与易用性
  • 执行地越快越好
  • 用起来越不容易出错越好

CAS(compare and swap)

预期值1 -> 新值2 内存中的旧值0
当且仅当更新瞬间, 内存的值 = 预期值 时,才能操作成功;否则,更新失败;

• 乐观锁:
  我现在追不到你,过一会再来问; = 自旋锁(spin lock);

private static AtomicInteger counter = new AtomicInteger();

    public static void main(String[] args) throws InterruptedException {

        for (int i = 0; i < 100; i++) {
            new Thread(() -> {
                try {
                    Thread.sleep(1);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                increment();
            }).start();
        }
        Thread.sleep(100);
        System.out.println(counter);
    }

    private static void increment(){
        counter.getAndIncrement();
    }

• 悲观锁:
  我现在追不到你,就阻塞了;

private static int counter = 0;

    public static void main(String[] args) throws InterruptedException {

        for (int i = 0; i < 100; i++) {
            new Thread(() -> {
                try {
                    Thread.sleep(1);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                increment();
            }).start();
        }
        Thread.sleep(100);
        System.out.println(counter);
    }

    private synchronized static void increment(){ //synchronized 悲观锁
        counter++;
    }

• 潜在问题:
  ABA问题;
  解决方案:时间戳/版本号;

• 乐观锁 VS 悲观锁实例
  ConcurrentHashMap VS Collections.synchronizedMap / HashTable
  (Collections.synchronizedMap与hashTable基本上在所有的方法上粗暴的加了synchronized)

附:
mutex = lock
java.util.Collections.SynchronizedMap#mutex
来源于: mutual(共享) + exclusive(排他)