Java并发教程–锁定：显式锁定

1.简介

在许多情况下，使用隐式锁定就足够了。有时，我们将需要更复杂的功能。在这种情况下， java.util.concurrent.locks包为我们提供了锁定对象。当涉及到内存同步时，这些锁的内部机制与隐式锁相同。区别在于显式锁提供了其他功能。

与隐式同步相比，主要优点或改进是：

通过读取或写入来分离锁。
一些锁允许并发访问共享资源（ ReadWriteLock ）。
获取锁的不同方式：
- 阻塞：lock（）

2.锁对象的分类

锁定对象实现以下两个接口之一：

Lock ：定义锁对象必须实现的基本功能。基本上，这意味着获取和释放锁。与隐式锁相反，此锁允许以非阻塞或可中断的方式（除阻塞方式外）获取锁。主要实现：
- 重入锁

ReadWriteLock ：它保留一对锁，一个锁用于只读操作，另一个锁用于写操作。读锁可以由不同的读取器线程同时获取（只要写锁尚未获取资源），而写锁是互斥的。这样，只要没有写操作，我们就可以让多个线程同时读取资源。主要实现：
- 重入ReadWriteLock

下面的类图显示了不同锁类之间的关系：

显式锁定

3.重入锁

此锁的工作方式与同步块相同。只要一个线程尚未被另一个线程获取，它就会获取该锁，并且直到调用unlock之前，它才会释放该锁。如果另一个线程已经获取了该锁，则尝试获取该锁的线程将被阻塞，直到另一个线程释放它为止。

我们将从一个没有锁的简单示例开始，然后我们将添加一个可重入锁以查看其工作方式。

public class NoLocking {public static void main(String[] args) {Worker worker = new Worker();Thread t1 = new Thread(worker, "Thread-1");Thread t2 = new Thread(worker, "Thread-2");t1.start();t2.start();}private static class Worker implements Runnable {@Overridepublic void run() {System.out.println(Thread.currentThread().getName() + " - 1");System.out.println(Thread.currentThread().getName() + " - 2");System.out.println(Thread.currentThread().getName() + " - 3");}}
}

由于上面的代码未同步，因此线程将被交错。让我们看一下输出：

Thread-2 - 1
Thread-1 - 1
Thread-1 - 2
Thread-1 - 3
Thread-2 - 2
Thread-2 - 3

现在，我们将添加一个可重入锁，以序列化对run方法的访问：

public class ReentrantLockExample {public static void main(String[] args) {Worker worker = new Worker();Thread t1 = new Thread(worker, "Thread-1");Thread t2 = new Thread(worker, "Thread-2");t1.start();t2.start();}private static class Worker implements Runnable {private final ReentrantLock lock = new ReentrantLock();@Overridepublic void run() {lock.lock();try {System.out.println(Thread.currentThread().getName() + " - 1");System.out.println(Thread.currentThread().getName() + " - 2");System.out.println(Thread.currentThread().getName() + " - 3");} finally {lock.unlock();}}}
}

上面的代码将安全地执行，而不会交错线程。您可能意识到我们可以使用同步块，并且效果是相同的。现在出现的问题是可重入锁提供给我们什么好处？

下面介绍了使用这种类型的锁的主要优点：

通过实现Lock接口提供了获取锁的其他方式：
- lockInterruptible ：如果另一个线程拥有锁，则当前线程将尝试获取解除锁定并被阻塞，例如使用lock（）方法。
ReentrantLock类提供的其他方法，主要用于监视或测试。例如， getHoldCount或isHeldByCurrentThread方法。

让我们看一个使用tryLock的示例，然后再继续下一个锁类。

3.1尝试获取锁

在下面的示例中，我们有两个线程，试图获取相同的两个锁。

一个线程获取lock2 ，然后阻止尝试获取lock1 ：

public void lockBlocking() {LOGGER.info("{}|Trying to acquire lock2...", Thread.currentThread().getName());lock2.lock();try {LOGGER.info("{}|Lock2 acquired. Trying to acquire lock1...", Thread.currentThread().getName());lock1.lock();LOGGER.info("{}|Both locks acquired", Thread.currentThread().getName());} finally {lock1.unlock();lock2.unlock();}
}

另一个线程获取lock1 ，然后尝试获取lock2 。

public void lockWithTry() {LOGGER.info("{}|Trying to acquire lock1...", Thread.currentThread().getName());lock1.lock();try {LOGGER.info("{}|Lock1 acquired. Trying to acquire lock2...", Thread.currentThread().getName());boolean acquired = lock2.tryLock(4, TimeUnit.SECONDS);if (acquired) {try {LOGGER.info("{}|Both locks acquired", Thread.currentThread().getName());} finally {lock2.unlock();}}else {LOGGER.info("{}|Failed acquiring lock2. Releasing lock1", Thread.currentThread().getName());}} catch (InterruptedException e) {//handle interrupted exception} finally {lock1.unlock();}
}

使用标准锁定方法，这将导致死锁，因为每个线程将永远等待另一个线程释放该锁。但是，这次我们尝试使用tryLock指定超时来获取它。如果四秒钟后仍未成功，它将取消操作并释放第一把锁。这将允许另一个线程解锁并获得两个锁。

让我们看完整的例子：

public class TryLock {private static final Logger LOGGER = LoggerFactory.getLogger(TryLock.class);private final ReentrantLock lock1 = new ReentrantLock();private final ReentrantLock lock2 = new ReentrantLock();public static void main(String[] args) {TryLock app = new TryLock();Thread t1 = new Thread(new Worker1(app), "Thread-1");Thread t2 = new Thread(new Worker2(app), "Thread-2");t1.start();t2.start();}public void lockWithTry() {LOGGER.info("{}|Trying to acquire lock1...", Thread.currentThread().getName());lock1.lock();try {LOGGER.info("{}|Lock1 acquired. Trying to acquire lock2...", Thread.currentThread().getName());boolean acquired = lock2.tryLock(4, TimeUnit.SECONDS);if (acquired) {try {LOGGER.info("{}|Both locks acquired", Thread.currentThread().getName());} finally {lock2.unlock();}}else {LOGGER.info("{}|Failed acquiring lock2. Releasing lock1", Thread.currentThread().getName());}} catch (InterruptedException e) {//handle interrupted exception} finally {lock1.unlock();}}public void lockBlocking() {LOGGER.info("{}|Trying to acquire lock2...", Thread.currentThread().getName());lock2.lock();try {LOGGER.info("{}|Lock2 acquired. Trying to acquire lock1...", Thread.currentThread().getName());lock1.lock();LOGGER.info("{}|Both locks acquired", Thread.currentThread().getName());} finally {lock1.unlock();lock2.unlock();}}private static class Worker1 implements Runnable {private final TryLock app;public Worker1(TryLock app) {this.app = app;}@Overridepublic void run() {app.lockWithTry();}}private static class Worker2 implements Runnable {private final TryLock app;public Worker2(TryLock app) {this.app = app;}@Overridepublic void run() {app.lockBlocking();}}
}

如果执行代码，将产生以下输出：

13:06:38,654|Thread-2|Trying to acquire lock2...
13:06:38,654|Thread-1|Trying to acquire lock1...
13:06:38,655|Thread-2|Lock2 acquired. Trying to acquire lock1...
13:06:38,655|Thread-1|Lock1 acquired. Trying to acquire lock2...
13:06:42,658|Thread-1|Failed acquiring lock2. Releasing lock1
13:06:42,658|Thread-2|Both locks acquired

在第四行之后，每个线程都已获取一个锁，并且在尝试获取另一个锁时被阻塞。在下一行，您会注意到四秒钟的间隔。自超时以来，第一个线程无法获取锁并释放它已经获取的锁，从而允许第二个线程继续。

4. ReentrantReadWriteLock

这种类型的锁保留一对内部锁（ ReadLock和WriteLock ）。如接口所述，此锁允许多个线程同时从资源读取。当资源具有频繁读取但很少写入的资源时，这特别方便。只要没有需要写的线程，资源就将被并发访问。

以下示例显示了三个线程同时从共享资源读取。当第四个线程需要写入时，它将排他地锁定资源，从而防止读取线程在写入时访问该资源。一旦写入完成并释放了锁定，所有读取器线程将继续并发访问资源：

public class ReadWriteLockExample {private static final Logger LOGGER = LoggerFactory.getLogger(ReadWriteLockExample.class);final ReadWriteLock readWriteLock = new ReentrantReadWriteLock();private Data data = new Data("default value");public static void main(String[] args) {ReadWriteLockExample example = new ReadWriteLockExample();example.start();}private void start() {ExecutorService service = Executors.newFixedThreadPool(4);for (int i=0; i<3; i++) service.execute(new ReadWorker());service.execute(new WriteWorker());service.shutdown();}class ReadWorker implements Runnable {@Overridepublic void run() {for (int i = 0; i < 2; i++) {readWriteLock.readLock().lock();try {LOGGER.info("{}|Read lock acquired", Thread.currentThread().getName());Thread.sleep(3000);LOGGER.info("{}|Reading data: {}", Thread.currentThread().getName(), data.getValue());} catch (InterruptedException e) {//handle interrupted} finally {readWriteLock.readLock().unlock();}}}}class WriteWorker implements Runnable {@Overridepublic void run() {readWriteLock.writeLock().lock();try {LOGGER.info("{}|Write lock acquired", Thread.currentThread().getName());Thread.sleep(3000);data.setValue("changed value");LOGGER.info("{}|Writing data: changed value", Thread.currentThread().getName());} catch (InterruptedException e) {//handle interrupted} finally {readWriteLock.writeLock().unlock();}}}
}

控制台输出显示结果：

11:55:01,632|pool-1-thread-1|Read lock acquired
11:55:01,632|pool-1-thread-2|Read lock acquired
11:55:01,632|pool-1-thread-3|Read lock acquired
11:55:04,633|pool-1-thread-3|Reading data: default value
11:55:04,633|pool-1-thread-1|Reading data: default value
11:55:04,633|pool-1-thread-2|Reading data: default value
11:55:04,634|pool-1-thread-4|Write lock acquired
11:55:07,634|pool-1-thread-4|Writing data: changed value
11:55:07,634|pool-1-thread-3|Read lock acquired
11:55:07,635|pool-1-thread-1|Read lock acquired
11:55:07,635|pool-1-thread-2|Read lock acquired
11:55:10,636|pool-1-thread-3|Reading data: changed value
11:55:10,636|pool-1-thread-1|Reading data: changed value
11:55:10,636|pool-1-thread-2|Reading data: changed value

如您所见，当编写器线程获得写锁定（线程4）时，其他任何线程都无法访问该资源。