Lock锁的使用- 校招VIP

转载声明：文章来源https://blog.csdn.net/qq_40409115/article/details/80229188

在Java多线程中，可以使用synchronized关键字实现线程之间的同步互斥，在jdk1.5后新增的ReentrantLock类同样可达到此效果，且在使用上比synchronized更加灵活。

观察ReentrantLock类可以发现其实现了Lock接口

public class ReentrantLock implements Lock,java.io.Serializable

1、使用ReentrantLock实现同步

lock()方法:上锁
unlock()方法：释放锁

/*
 * 使用ReentrantLock类实现同步
 * */
class MyReenrantLock implements Runnable{
	//向上转型
	private Lock lock = new ReentrantLock();
	public void run() {
		//上锁
		lock.lock();
		for(int i = 0; i < 5; i++) {
			System.out.println("当前线程名： "+ Thread.currentThread().getName()+" ,i = "+i);
		}
		//释放锁
		lock.unlock();
	}
}
public class MyLock {
	public static void main(String[] args) {
		MyReenrantLock myReenrantLock =  new MyReenrantLock();
		Thread thread1 = new Thread(myReenrantLock);
		Thread thread2 = new Thread(myReenrantLock);
		Thread thread3 = new Thread(myReenrantLock);
		thread1.start();
		thread2.start();
		thread3.start();
	}
}

由此我们可以看出，只有当当前线程打印完毕后，其他的线程才可继续打印，线程打印的数据是分组打印，因为当前线程持有锁，但线程之间的打印顺序是随机的。

即调用lock.lock()代码的线程就持有了“对象监视器”，其他线程只有等待锁被释放再次争抢。

2、使用Condition实现等待/通知

synchronized关键字结合wait()和notify()及notifyAll()方法的使用可以实现线程的等待与通知模式。在使用notify()、notifyAll()方法进行通知时，被通知的线程是JVM随机选择的。

类ReentrantLock类同样可以实现该功能，需要借助Condition对象，可实现“选择性通知”。Condition类是jdk1.5提供的，且在一个Lock对象中可以创建多个Condition(对象监视器)实例。

Condition类的await():是当前执行任务的线程处于等待状态

/*
 * 错误的使用Condition实现等待、通知
 * */
class MyCondition implements Runnable{
	private Lock lock = new ReentrantLock();
	public Condition condition = lock.newCondition();
	public void run() {
		try {
			System.out.println("当前线程名:"+Thread.currentThread().getName()+" 开始等待时间："+System.currentTimeMillis());
			//线程等待
			condition.await();
			System.out.println("我陷入了等待...");
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}
}
public class MyLock{
	public static void main(String[] args) {
		MyCondition myCondition = new MyCondition();
		Thread thread1 = new Thread(myCondition,"线程1");
		thread1.start();
	}
}

观察运行结果可以发现，报出监视器出错的异常，解决的办法是我们必须在condition.await()方法调用前用lock.lock()代码获得同步监视器。对上述代码做出如下修改：

/*
 * 使用Condition实现等待
 * */
class MyCondition implements Runnable{
	private Lock lock = new ReentrantLock();
	public Condition condition = lock.newCondition();
	public void run() {
		try {
			//上锁
			lock.lock();
			System.out.println("当前线程名:"+Thread.currentThread().getName()+" 开始等待时间："+System.currentTimeMillis());
			//线程等待
			condition.await();
			System.out.println("我陷入了等待...");
		} catch (InterruptedException e) {
			e.printStackTrace();
		}finally {
			//释放锁
			lock.unlock();
			System.out.println("锁释放了!");
		}
	}
}
public class MyLock{
	public static void main(String[] args) {
		MyCondition myCondition = new MyCondition();
		Thread thread1 = new Thread(myCondition,"线程1");
		thread1.start();
	}
}

在控制台只打印出一句，原因是调用了Condition对象的await()方法，是的当前执行任务的线程进入等待状态。

Condition类的signal():是当前执行任务的线程处于等待状态

/*
 * 使用Condition实现等待、通知
 * */
class MyCondition implements Runnable{
	private Lock lock = new ReentrantLock();
	public Condition condition = lock.newCondition();
	public void run() {
		try {
			//上锁
			lock.lock();
			System.out.println(" 开始等待时间："+System.currentTimeMillis());
			System.out.println("我陷入了等待...");
			//线程等待
			condition.await();
			//释放锁
			lock.unlock();
			System.out.println("锁释放了!");
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}
	//通知方法
	public void signal(){
		try {
			lock.lock();
			System.out.println("结束等待时间："+System.currentTimeMillis());
	     	//通知等待线程
			condition.signal();
		} finally {
			lock.unlock();
		}
	}
}
public class MyLock{
	public static void main(String[] args) throws InterruptedException {
		MyCondition myCondition = new MyCondition();
		Thread thread1 = new Thread(myCondition,"线程1");
		thread1.start();
		Thread.sleep(3000);
		myCondition.signal();
	}
}

观察结果我们成功地实现了等待通知。

可以得知：
1）Object类中的wait()方法等同于Condition类中的await()方法。
2）Object类中的wait(long timeout)方法等同于Condition类中的await(long time,TimeUnit unit)方法。
3）Object类中的notify()方法等同于Condition类中的singal()方法。
4）Object类中的notifyAll()方法等同于Condition类中的singalAll()方法。

3、生产者消费者模式

/*
 * 生产者、消费者模式
 * 一对一交替打印
 * */
class MyServer{
	private ReentrantLock lock = new ReentrantLock();
	public Condition condition = lock.newCondition();
	public Boolean flag = false;
	public void set() {
		try {
			lock.lock();
			while(flag == true) {
				condition.await();
			}
			System.out.println("当前线程名："+Thread.currentThread().getName()+" hello");
			flag = true;
			condition.signal();
			} catch (InterruptedException e) {
			e.printStackTrace();
		}finally {
			lock.unlock();
		}
	}
	public void get() {
		try {
			lock.lock();
			while(flag == false) {
				condition.await();
			}
			System.out.println("当前线程名："+Thread.currentThread().getName()+" lemon");
			flag = false;
			condition.signal();
			} catch (InterruptedException e) {
			e.printStackTrace();
		}finally {
			lock.unlock();
		}
	}
}
class MyCondition1 extends Thread{
	private MyServer myServer;
	public MyCondition1(MyServer myServer) {
		super();
		this.myServer = myServer;
	}
	public void run() {
		for(int i = 0 ;i < Integer.MAX_VALUE;i++) {
			myServer.set();
		}
	}	
}
class MyCondition2 extends Thread{
	private MyServer myServer;
	public MyCondition2(MyServer myServer) {
		super();
		this.myServer = myServer;
	}
	public void run() {
		for(int i = 0 ;i < Integer.MAX_VALUE;i++) {
			myServer.get();
		}
	}	
}
public class MyLock{
	public static void main(String[] args) throws InterruptedException {
		MyServer myServer = new MyServer();
		MyCondition1  myCondition1 = new MyCondition1(myServer);
		MyCondition2  myCondition2 = new MyCondition2(myServer);
		myCondition1.start();
		myCondition2.start();
	}
}

/*
 * 生产者、消费者模式
 * 多对多交替打印
 * */
class MyServer{
	private ReentrantLock lock = new ReentrantLock();
	public Condition condition = lock.newCondition();
	public Boolean flag = false;
	public void set() {
		try {
			lock.lock();
			while(flag == true) {
				System.out.println("可能会有连续的hello进行打印");
				condition.await();
			}
			System.out.println("当前线程名："+Thread.currentThread().getName()+" hello");
			flag = true;
			condition.signal();
			} catch (InterruptedException e) {
			e.printStackTrace();
		}finally {
			lock.unlock();
		}
	}
	public void get() {
		try {
			lock.lock();
			while(flag == false) {
				System.out.println("可能会有连续的lemon进行打印");
				condition.await();
			}
			System.out.println("当前线程名："+Thread.currentThread().getName()+" lemon");
			flag = false;
			condition.signal();
			} catch (InterruptedException e) {
			e.printStackTrace();
		}finally {
			lock.unlock();
		}
	}
}
class MyCondition1 extends Thread{
	private MyServer myServer;
	public MyCondition1(MyServer myServer) {
		super();
		this.myServer = myServer;
	}
	public void run() {
		for(int i = 0 ;i < Integer.MAX_VALUE;i++) {
			myServer.set();
		}
	}	
}
class MyCondition2 extends Thread{
	private MyServer myServer;
	public MyCondition2(MyServer myServer) {
		super();
		this.myServer = myServer;
	}
	public void run() {
		for(int i = 0 ;i < Integer.MAX_VALUE;i++) {
			myServer.get();
		}
	}	
}
public class MyLock{
	public static void main(String[] args) throws InterruptedException {
		MyServer myServer = new MyServer();
		MyCondition1[]  myCondition1 = new MyCondition1[10];
		MyCondition2[]  myCondition2 = new MyCondition2[10];
		for(int i = 0; i < 10; i++) {
			myCondition1[i] = new MyCondition1(myServer);
			myCondition2[i] = new MyCondition2(myServer);
			myCondition1[i].start();
			myCondition2[i].start();
		}
	}
}

4、公平锁与非公平锁

锁Lock分为“公平锁”和“非公平锁”。
1）公平锁：表示线程获取锁的顺序是按照线程加锁的顺序来的进行分配的，即先来先得FIFO先进先出顺序。
2）非公平锁：一种获取锁的抢占机制，是随机拿到锁的，和公平锁不一样的是先来的不一定先拿到锁，这个方式可能造成某些线程一直拿不到锁，结果就是不公平的·。

/*
 * 公平锁
 * */
class MyService{
	private ReentrantLock lock;
	public MyService(boolean isFair) {
		super();
		lock = new ReentrantLock(isFair);
	}
	public void serviceMethod() {
		try {
			lock.lock();
			System.out.println("线程名："+Thread.currentThread().getName()+"获得锁定");
		} finally {
			lock.unlock();
		}
	}
}
public class MyLock{
	public static void main(String[] args) {
		//设置当前为true公平锁
		final MyService myService = new MyService(true);
		Runnable runnable = new Runnable() {
			public void run() {
				System.out.println("线程名："+Thread.currentThread().getName()+"运行了");
				myService.serviceMethod();
			}
		};
		Thread[] threads = new Thread[10];
		for(int i = 0;i < 10; i++) {
			threads[i] = new Thread(runnable);
		}
		for(int i = 0;i < 10; i++) {
			threads[i].start();
		}
	}
}

由打印结果可以看出，基本呈现有序的状态，这就是公平锁的特点。

/*
 * 非公平锁
 * */
class MyService{
	private ReentrantLock lock;
	public MyService(boolean isFair) {
		super();
		lock = new ReentrantLock(isFair);
	}
	public void serviceMethod() {
		try {
			lock.lock();
			System.out.println("线程名："+Thread.currentThread().getName()+"获得锁定");
		} finally {
			lock.unlock();
		}
	}
}
public class MyLock{
	public static void main(String[] args) {
		//设置当前为true公平锁
		final MyService myService = new MyService(false);
		Runnable runnable = new Runnable() {
			public void run() {
				System.out.println("线程名："+Thread.currentThread().getName()+"运行了");
				myService.serviceMethod();
			}
		};
		Thread[] threads = new Thread[10];
		for(int i = 0;i < 10; i++) {
			threads[i] = new Thread(runnable);
		}
		for(int i = 0;i < 10; i++) {
			threads[i].start();
		}
	}
}

非公平锁的运行结果基本都是无须的，则可以表明先start()启动的线程并不一定先获得锁。

5、使用ReentrantReadWriteLock类

类ReentrantLock具有完全互斥排他的效果，即同一时间只有一个线程在执行ReentrantLock.lock()方法后的任务。这样虽然保证了实例变量的线程安全性，但是效率低下。所以在Java中提供有读写锁ReentrantReadWriteLock类，使其效率可以加快。在某些不需要操作实例变量的方法中，完全可以使用ReentrantReadWriteLock来提升该方法代码运行速度。

读写锁表示两个锁：
1）读操作相关的锁，也成为共享锁。
2）写操作相关的锁，也叫排他锁。
多个读锁之间不互斥，读锁与写锁互斥，多个写锁互斥。

在没有线程Thread进行写入操作时，进行读操作的多个Thread可以获取读锁，但是进行写入操作时的Thread只有获取写锁后才能进行写入操作。

（1）多个读锁共享

/*
 * 多个读锁共享
 * */
class MyService{
	private ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
	public void read() {
		try {
			//读锁
			lock.readLock().lock();
			System.out.println("线程名: "+Thread.currentThread().getName()+"获取读锁" );
			Thread.sleep(1000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}finally {
			//释放读锁
			lock.readLock().unlock();
		}
	}
}
//线程1
class Thread1 extends Thread{
	private MyService myService;
	public Thread1(MyService myService) {
		super();
		this.myService = myService;
	}
	public void run() {
		myService.read();
	}
}
//线程2
class Thread2 extends Thread{
	private MyService myService;
	public Thread2(MyService myService) {
		super();
		this.myService = myService;
	}
	public void run() {
		myService.read();
	}
}
public class MyLock{
	public static void main(String[] args) {
		MyService myService = new MyService();
		Thread1 thread1 = new Thread1(myService);
		Thread2 thread2 = new Thread2(myService);
		thread1.start();
		thread2.start();
		
	}
}

从打印结果可以看出，两个线程几乎同时进入lock()方法后面的代码。
说明在此时使用lock.readLock()读锁可以提高程序运行效率，允许多个线程同时执行lock()方法后的代码。

（2）多个写锁互斥

/*
 * 多个写锁互斥
 * */
class MyService{
	private ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
	public void write() {
		try {
			//写锁
			lock.writeLock().lock();
			System.out.println("线程名: "+Thread.currentThread().getName()+"获取写锁，获得时间："+System.currentTimeMillis() );
			Thread.sleep(1000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}finally {
			//释放写锁
			lock.writeLock().unlock();
		}
	}
}
//线程1
class Thread1 extends Thread{
	private MyService myService;
	public Thread1(MyService myService) {
		super();
		this.myService = myService;
	}
	public void run() {
		myService.write();
	}
}
//线程2
class Thread2 extends Thread{
	private MyService myService;
	public Thread2(MyService myService) {
		super();
		this.myService = myService;
	}
	public void run() {
		myService.write();
	}
}
public class MyLock{
	public static void main(String[] args) {
		MyService myService = new MyService();
		Thread1 thread1 = new Thread1(myService);
		Thread2 thread2 = new Thread2(myService);
		thread1.start();
		thread2.start();
		
	}
}

使用写锁代码writeLock.lock()的效果就是同一时间只允许一个线程执行lock()方法后的代码。

（3）读写/写读互斥

/*
 * 读写/写读互斥,
 * */
class MyService{
	private ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
	public void read() {
		try {
			//读锁
			lock.readLock().lock();
			System.out.println("线程名: "+Thread.currentThread().getName()+"获取读锁，获得时间:"+System.currentTimeMillis() );
			Thread.sleep(1000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}finally {
			//释放读锁
			lock.readLock().unlock();
		}
	}
	public void write() {
		try {
			//写锁
			lock.writeLock().lock();
			System.out.println("线程名: "+Thread.currentThread().getName()+"获取写锁，获得时间："+System.currentTimeMillis() );
			Thread.sleep(1000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}finally {
			//释放写锁
			lock.writeLock().unlock();
		}
	}
}
//线程1
class Thread1 extends Thread{
	private MyService myService;
	public Thread1(MyService myService) {
		super();
		this.myService = myService;
	}
	public void run() {
		myService.read();
	}
}
//线程2
class Thread2 extends Thread{
	private MyService myService;
	public Thread2(MyService myService) {
		super();
		this.myService = myService;
	}
	public void run() {
		myService.write();
	}
}
public class MyLock{
	public static void main(String[] args) {
		MyService myService = new MyService();
		Thread1 thread1 = new Thread1(myService);
		Thread2 thread2 = new Thread2(myService);
		thread1.start();
		thread2.start();	
	}
}

此运行结果说明“读写/写读”操作是互斥的。
由此可表明：只要出现“写”操作，就是互斥的。