java多写多读锁

Question

我正在尝试实现一个遵循以下约束的系统：

• 我有一个共享资源，例如原子数组
• 我想同时支持从阵列中进行多次读取。
• 我想同时支持对数组的多次写入
• 我不希望读取和写入操作同时发生。

我发现 [this][1] stackoverflow post 关于类似的目标，但我认为建议的解决方案是允许同时读取和写入：

Class ReadAndWrite {
 private ReentrantLock readLock;
 private ReentrantLock writeLock;
 private AtomicInteger readers;
 private AtomicInteger writers;
 private File file;

 public void write() {
   if (!writeLock.isLocked()) {
    readLock.tryLock();
    writers.incrementAndGet(); // Increment the number of current writers
    // ***** Write your stuff *****
    writers.decrementAndGet(); // Decrement the number of current writers
    if (readLock.isHeldByCurrentThread()) {
     while(writers != 0); // Wait until all writers are finished to release the lock
     readLock.unlock();
    }
   } else {
     writeLock.lock();
     write();
   }
  }

 public void read() {
   if (!readLock.isLocked()) {
    writeLock.tryLock();
    readers.incrementAndGet(); 
    // ***** read your stuff *****
    readers.decrementAndGet(); // Decrement the number of current read
    if (writeLock.isHeldByCurrentThread()) {
     while(readers != 0); // Wait until all writers are finished to release the lock
     writeLock.unlock();
    }
   } else {
     readLock.lock();
     read();
   }
  }

在我看来，这段代码允许同时读取和写入，例如：两个线程将尝试同时读取/写入。如果在写/读中，他们每个人都将进入第一个。如何确保写入阻止读取和读取阻止写入？ [1]：Multiple readers and multiple writers(i mean multiple) synchronization

Answer 1

与其反复检查锁，不如尝试使用它：

private void writeInternal() {
    //thread-unsafe writing code
}

public void write() {
    if (!writeLock.tryLock()) {
        writeLock.lock();
    }
    try {
        this.writeInternal(); //in try-block to ensure unlock is called
    } finally {
        writeLock.unlock();
    }
}

使用 readLock 将是一种类似的方法。您还想确保您真正使用的是读/写锁，而不仅仅是两个单独的锁：

private final ReadWriteLock lock;

public ReadAndWrite() {
    this.lock = new ReentrantReadWriteLock();
}

然后您将通过this.lock.readLock() 等访问读/写锁。