Java 显式锁

Java线程教程 - Java显式锁

显式锁定机制可以用于协调对多线程环境中的共享资源的访问。

在java.util.concurrent.locks包中声明的Lock接口定义了显式锁定操作。

ReentrantLock类在同一个包中，是Lock接口的具体实现。

Lock接口声明如下：

public interface Lock {
  void lock();

  Condition newCondition();

  void lockInterruptibly() throws InterruptedException;

  boolean tryLock();

  boolean tryLock(long time, TimeUnit unit) throws InterruptedException;

  void unlock();
}

lock()方法获取一个锁的行为与使用synchronized关键字相同。

我们必须在完成锁定后通过调用Lock接口的unlock()方法释放锁定。

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class Main {
  // Instantiate the lock object
  private Lock myLock = new ReentrantLock();

  public void updateResource() {

    try {
      // Acquire the lock
      myLock.lock();
    } finally {
      // Release the lock
      myLock.unlock();
    }
  }
}

例子

下面的代码模拟哲学家，假设ReentrantLock类的一个对象表示一个fork。

import java.util.concurrent.locks.Lock;

class Philosopher {
  private Lock leftFork;
  private Lock rightFork;
  private String name; // Philosopher"s name

  public Philosopher(Lock leftFork, Lock rightFork, String name) {
    this.leftFork = leftFork;
    this.rightFork = rightFork;
    this.name = name;
  }

  public void think() {
    System.out.println(name + "  is thinking...");
  }

  public void eat() {
    if (leftFork.tryLock()) {
      try {
        if (rightFork.tryLock()) {
          try {
            System.out.println(name + "  is eating...");
          } finally {
            rightFork.unlock();
          }
        }
      } finally {
        leftFork.unlock();
      }
    }
  }
}

例2

ReentrantReadWriteLock类是ReadWriteLock接口的实现。只有一个线程可以保持ReentrantReadWriteLock的写锁，而多个线程可以保持其读锁。

下面的代码演示了ReentrantReadWriteLock的用法。

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

public class Main {
  private int value;
  private ReentrantReadWriteLock rwLock = new ReentrantReadWriteLock();
  private Lock rLock = rwLock.readLock();
  private Lock wLock = rwLock.writeLock();

  public Main(int value) {
    this.value = value;
  }

  public int getValue() {
    rLock.lock();
    try {
      return this.value;
    } finally {
      rLock.unlock();
    }
  }

  public void setValue(int value) {
    wLock.lock();
    try {
      this.value = value;
    } finally {
      wLock.unlock();
    }
  }
}