通过unsafe手写一个乐观锁

思路是通过unsafe类cas指令操作实现乐观锁

public class Test {
    
    
    //需要反射一个unsafe类,jdk不允许直接获取这个对象实例
    static Unsafe unsafe = null;
    //锁状态
    static volatile int state = 0;
    //state偏移量
    static long offset;

    static {
    
    
        try {
    
    
            Field field = Unsafe.class.getDeclaredFields()[0];
            field.setAccessible(true);
            try {
    
    
                //反射出对象
                unsafe = (Unsafe) field.get(null);
                //获取属性的偏移量
                offset = unsafe.staticFieldOffset(Test.class.getDeclaredField("state"));
            } catch (IllegalAccessException e) {
    
    
                e.printStackTrace();
            }
        } catch (NoSuchFieldException e) {
    
    
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
    
    
        Test test = new Test();
        new Thread(() -> {
    
    
            lock(test);
            System.out.println("线程1加锁================");
            try {
    
    
                Thread.sleep(3000);
            } catch (InterruptedException e) {
    
    
                e.printStackTrace();
            }finally {
    
    
                System.out.println("线程1解锁================");
                unlock(test);
            }
        },"1").start();

        new Thread(() -> {
    
    
            lock(test);
            System.out.println("线程2加锁================");
            try {
    
    
                Thread.sleep(3000);
            } catch (InterruptedException e) {
    
    
                e.printStackTrace();
            }finally {
    
    
                System.out.println("线程2解锁================");
                unlock(test);
            }
        },"2").start();
    }

    public static void lock(Test test){
    
    
        //如果cas成功,就代表加锁成功
        while (!unsafe.compareAndSwapInt(test,offset,0,1)){
    
    
            System.out.println("线程"+Thread.currentThread().getName()+"在循环");
            System.out.println(unsafe.getIntVolatile(test,offset));
            try {
    
    
                Thread.sleep(500);
            } catch (InterruptedException e) {
    
    
                e.printStackTrace();
            }
            //强锁失败让出cpu
            Thread.yield();
        }
        System.out.println("线程"+Thread.currentThread().getName()+"把state修改为"+unsafe.getIntVolatile(test,offset));
    }

    public static void unlock(Test test){
    
    
        //解锁 状态设置为0
        unsafe.getAndSetInt(test,offset,0);
        System.out.println("修改state = 0");
    }

}

运行后输出结果为: