java 内存可见性问题

深碍√TFBOYSˉ_ 2022-05-24 06:26 250阅读 0赞

问题描述：同一个对象，在多线程中进行操作，会出现内存可见性问题。

![2018052215071593][]

![20180522150700493][]

针对这种问题，我们做了一些应对措施

1. 加volatile， 结果发现没有效果

2. 加lock

修改代码如下：

package feedback.common.lock;
    
    import java.util.concurrent.TimeUnit;
    import java.util.concurrent.TimeoutException;
    import java.util.concurrent.atomic.AtomicBoolean;
    import java.util.concurrent.locks.Condition;
    import java.util.concurrent.locks.Lock;
    import java.util.concurrent.locks.ReentrantLock;
    
    /**
     * 所对象
     * Created by sff on 2018/5/8.
     */
    public class FeedBackLock {
        private Lock lock = new ReentrantLock();
        private Condition done = lock.newCondition();
        private volatile AtomicBoolean finished = new AtomicBoolean(false);
        private volatile boolean success;
        private volatile Exception cause;
    
        public FeedBackLock(){
            success = false;
        }
    
        public void lock(long timeout) throws InterruptedException{
            lock.lock();
            try{
                if(!finished.get()){
                    // 时间一到直接就过去了，不会抛异常的。
                    done.await(timeout, TimeUnit.MILLISECONDS);
                }
            } catch (InterruptedException e) {
                throw e;
            } finally {
                lock.unlock();
            }
        }
    
        public void unLock() {
            lock.lock();
            try{
                this.success = cause == null;
                // 时间一到直接就过去了，不会抛异常的。
                done.signal();
                finished.set(true);
            } finally {
                lock.unlock();
            }
        }
    
        public boolean isSuccess() {
            lock.lock();
            try {
                return success;
            } finally {
                lock.unlock();
            }
        }
    
        public Exception getCause() {
            lock.lock();
            try {
                return cause != null ? cause : new TimeoutException("feedback timeout");
            } finally {
                lock.unlock();
            }
        }
    
        public void setCause(Exception cause) {
            this.cause = cause;
        }
    }

发现成功了。不会出现之前的那个问题。

[2018052215071593]: /images/20220524/705094be6ca848d9961bc11f1c3e8cd5.png
[20180522150700493]: /images/20220524/b85d1e4e32ec48018a3ac47fbd3cc050.png