4、共享模型之管程

4.1共享带来的问题

以上的结果可能是正数、负数、零。
为什么呢？因为 Java 中对静态变量的自增，自减并不是原子操作，要彻底理解，必须从字节码来进行分析

面向对象改进: 把需要保护的共享变量放入一个类

class Room {
    int value = 0;

    public void increment() {
        synchronized (this) {
            value++;
        }
    }

    public void decrement() {
        synchronized (this) {
            value--;
        }
    }

    public int get() {
        synchronized (this) {
            return value;
        }
    }
}

@Slf4j
public class TestCounterOop {

    public static void main(String[] args) throws InterruptedException {
        Room room = new Room();
        
        Thread t1 = new Thread(() -> {
            for (int j = 0; j < 5000; j++) {
                room.increment();
            }
        }, "t1");

        Thread t2 = new Thread(() -> {
            for (int j = 0; j < 5000; j++) {
                room.decrement();
            }
        }, "t2");
        t1.start();
        t2.start();

        t1.join();
        t2.join();
        log.debug("count: {}" , room.get());
    }
}

线程8锁

成员变量例子

class ThreadUnsafe {
      ArrayList<String> list = new ArrayList<>();
      public void method1(int loopNumber) {
            for (int i = 0; i < loopNumber; i++) {
                  // { 临界区, 会产生竞态条件
                        method2();
                        method3();
                  // } 临界区
             }
      }
      private void method2() {
            list.add("1");
       }
      private void method3() {
            list.remove(0);
       }
}

static final int THREAD_NUMBER = 2;
static final int LOOP_NUMBER = 200;
public static void main(String[] args) {
ThreadUnsafe test = new ThreadUnsafe();
for (int i = 0; i < THREAD_NUMBER; i++) {
      new Thread(() -> {
                  test.method1(LOOP_NUMBER);
                   }, "Thread" + i).start();
             }
}

将list变量改为局部变量

class ThreadSafe {
      public final void method1(int loopNumber) {
            ArrayList<String> list = new ArrayList<>();
            for (int i = 0; i < loopNumber; i++) {
                  method2(list);
                  method3(list);
             }
       }
        private void method2(ArrayList<String> list) {
            list.add("1");
        }
        private void method3(ArrayList<String> list) {
            list.remove(0);
       }
}

实例

4.5 习题

4.6 Monitor概念

撤销40次后，变为不可偏向状态；20次的时候我是不是偏向错了，40次的时候说明类竞争激烈，不要偏向任何线程了

4.7 wait notify

4.8 wait,notify 的正确姿势