介绍

ReentrantLock是互斥排他锁，同一时间只能有一个线程在执行任务，ReentrantLock支持锁的重入功能，虽然保证了线程的安全性，但是效率不高，实际上应该是写操作互斥，读操作共享。而jdk提供了读写锁ReentrantReadWriteLock。

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.setName("t1");
        
        Thread t2 = new Thread(new Runnable() {

            @Override
            public void run() {
                myTask.read();
            }
        });
        t2.setName("t2");
        
        t1.start();
        t2.start();
    }
}

结果：

t2 start
t1 start
t1 end
t2 end

写写互斥

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.setName("t1");
        
        Thread t2 = new Thread(new Runnable() {

            @Override
            public void run() {
                myTask.write();
            }
        });
        t2.setName("t2");
        
        t1.start();
        t2.start();
    }
}

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.setName("t1");
        
        Thread t2 = new Thread(new Runnable() {

            @Override
            public void run() {
                myTask.write();
            }
        });
        t2.setName("t2");
        
        t1.start();
        
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        
        t2.start();
    }
}

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.setName("t1");
        
        Thread t2 = new Thread(new Runnable() {

            @Override
            public void run() {
                myTask.read();
            }
        });
        t2.setName("t2");
        
        t1.start();
        
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        
        t2.start();
    }
}

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。