前面提到ReentrantLock，是一种“排它锁”。也就是说，这些锁在同一时刻只允许一个线程进行访问。而读写锁可以再同一时刻允许多个读线程访问。Java提供了ReentrantReadWriteLock类作为读写锁的默认实现，内部维护了两个锁：一个读锁，一个写锁。
ReadWriteLock接口
这里面有两把锁，一把读锁，用来处理读相关的操作；一把写锁，用来处理写相关的操作。
public interface ReadWriteLock {
/**
* Returns the lock used for reading.
*
* @return the lock used for reading
*/
Lock readLock();
/**
* Returns the lock used for writing.
*
* @return the lock used for writing
*/
Lock writeLock();
}
ReentrantReadWriteLock类
这个类也是一个非抽象类，它是ReadWriteLock接口的JDK默认实现。它与ReentrantLock的功能类似，同样是可重入的，支持非公平锁和公平锁。不同的是，它还支持”读写锁“。
// 内部结构
private final ReentrantReadWriteLock.ReadLock readerLock;
private final ReentrantReadWriteLock.WriteLock writerLock;
final Sync sync;
abstract static class Sync extends AbstractQueuedSynchronizer {
// 具体实现
}
static final class NonfairSync extends Sync {
// 具体实现
}
static final class FairSync extends Sync {
// 具体实现
}
public static class ReadLock implements Lock, java.io.Serializable {
private final Sync sync;
protected ReadLock(ReentrantReadWriteLock lock) {
sync = lock.sync;
}
// 具体实现
}
public static class WriteLock implements Lock, java.io.Serializable {
private final Sync sync;
protected WriteLock(ReentrantReadWriteLock lock) {
sync = lock.sync;
}
// 具体实现
}
// 构造方法，初始化两个锁
public ReentrantReadWriteLock(boolean fair) {
sync = fair ? new FairSync() : new NonfairSync();
readerLock = new ReadLock(this);
writerLock = new WriteLock(this);
}
// 获取读锁和写锁的方法
public ReentrantReadWriteLock.WriteLock writeLock() { return writerLock; }
public ReentrantReadWriteLock.ReadLock readLock() { return readerLock; }
读读共享
public class MyTask {
private ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
public void read() {
try {
lock.readLock().lock();
System.out.println(Thread.currentThread().getName() + " start");
Thread.sleep(10000);
System.out.println(Thread.currentThread().getName() + " end");
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
lock.readLock().unlock();
}
}
}
public class ReadReadTest {
public static void main(String[] args) {
final MyTask myTask = new MyTask();
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
myTask.read();
}
}, "t1");
Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
myTask.read();
}
}, "t2");
t1.start();
t2.start();
}
}
输出结果：
t2 start
t1 start
t1 end
t2 end
写写互斥
在同一时间只允许一个线程执行lock()方法后面的代码。
public class MyTask {
private ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
public void write() {
try {
lock.writeLock().lock();
System.out.println(Thread.currentThread().getName() + " start");
Thread.sleep(10000);
System.out.println(Thread.currentThread().getName() + " end");
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
lock.writeLock().unlock();
}
}
}
public class ReadReadTest {
public static void main(String[] args) {
final MyTask myTask = new MyTask();
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
myTask.write();
}
}, "t1");
Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
myTask.write();
}
}, "t2");
t1.start();
t2.start();
}
}
输出结果：
t1 start
t1 end
t2 start
t2 end
读写互斥
public class MyTask {
private ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
public void read() {
try {
lock.readLock().lock();
System.out.println(Thread.currentThread().getName() + " start");
Thread.sleep(10000);
System.out.println(Thread.currentThread().getName() + " end");
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
lock.readLock().unlock();
}
}
public void write() {
try {
lock.writeLock().lock();
System.out.println(Thread.currentThread().getName() + " start");
Thread.sleep(10000);
System.out.println(Thread.currentThread().getName() + " end");
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
lock.writeLock().unlock();
}
}
}
public class ReadReadTest {
public static void main(String[] args) {
final MyTask myTask = new MyTask();
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
myTask.read();
}
}, "t1");
Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
myTask.write();
}
}, "t2");
t1.start();
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
t2.start();
}
}
输出结果：
t1 start
t1 end
t2 start
t2 end
写读互斥
public class ReadReadTest {
public static void main(String[] args) {
final MyTask myTask = new MyTask();
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
myTask.write();
}
}, "t1");
Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
myTask.read();
}
}, "t2");
t1.start();
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
t2.start();
}
}
输出结果：
t1 start
t1 end
t2 start
t2 end
总结
读操作的锁叫共享锁，写操作的锁叫排他锁。就是遇见写锁就需互斥。那么以此可得出读读共享，写写互斥，读写互斥，写读互斥。
