CAS和Unsafe

Posted 2022-05-12 z啵唧啵唧

五、CAS

CAS实现锁的原理

@Slf4j(topic = "c.LockCas")
public class LockCas 

    /**
     * 0表示没有加锁
     * 1表示加锁
     */
    private AtomicInteger state = new AtomicInteger(0);

    public void lock() 
        //实际上在这个地方会存在空运转的问题
        while (true) 
            //cas操作，采用了cas的原理实现
            if (state.compareAndSet(0, 1)) 
                break;
            
        
    

    public void unlock() 
        log.debug("unlock...");
        //解锁将state的值设置为0即可
        state.set(0);
    

    public static void main(String[] args) 
        LockCas lock;
        lock = new LockCas();
        new Thread(() -> 
            log.debug("begin...");
            lock.lock();
            try 
                log.debug("lock...");
                Thread.sleep(1000);
             catch (InterruptedException e) 
                e.printStackTrace();
             finally 
                lock.unlock();
            
        ).start();
        new Thread(() -> 
            log.debug("begin...");
            //线程二去调用lock方法的时候发现修改不了，因为线程1先一步调用了lock方法并且将state的值改成了1
            //此时线程2只能等线程1睡眠时间过后调用了unlock方法将state的值改为了0，才能此时线程2才能调用lock方法进行加锁
            lock.lock();
            try 
                log.debug("lock...");

             finally 
                lock.unlock();
            
        ).start();
    

Unsafe类

• Unsafe对象提供了非常底层，操作内存，线程的方法，Unsafe对象不能直接调用，只能通过反射获得
• 如AtomicInteger底层的一些方法实际上就是调用的就是unsafe类的一些方法
• LockSupport类底层的park、unpark调用的也是unsafe对象的方法

unsafe对象的获取

public class TestUnsafe 
    public static void main(String[] args) throws NoSuchFieldException, IllegalAccessException 
        //获得unsafe对象
        //获得unsafe成员变量
        Field theUnsafe = Unsafe.class.getDeclaredField("theUnsafe");
        //设置允许访问私有变量
        theUnsafe.setAccessible(true);
        Unsafe unsafe = (Unsafe)theUnsafe.get(null);
        //打印查看是否成功获得
        System.out.println(unsafe);
    

unsafe CAS操作

public class TestUnsafe 
    public static void main(String[] args) throws NoSuchFieldException, IllegalAccessException 
        //获得unsafe对象
        //获得unsafe成员变量
        Field theUnsafe = Unsafe.class.getDeclaredField("theUnsafe");
        //设置允许访问私有变量
        theUnsafe.setAccessible(true);
        Unsafe unsafe = (Unsafe)theUnsafe.get(null);

        //1、获取域的偏移地址
         long idOffset = unsafe.objectFieldOffset(Teacher.class.getDeclaredField("id"));
         long nameOffset = unsafe.objectFieldOffset(Teacher.class.getDeclaredField("name"));

         Teacher t = new Teacher();
        //2、执行cas操作

        unsafe.compareAndSwapInt(t,idOffset,0,1);
        unsafe.compareAndSwapObject(t,nameOffset,null,"张三");

        //3、验证
        System.out.println(t);
    

@Data
class Teacher
    volatile int id;
    volatile String name;