并发编程之重入锁和读写锁

Java多线程中，除了sychronized关键字实现多线程之间同步互斥操作，其实还有另外一种高效的机制去完成“同步互斥”操作。即Lock对象，比synchronized关键字更为强大功能，并且有嗅探锁定，多路分支等功能。

重入锁

分为公平锁和非公平锁，默认非公平锁

public ReentrantLock() {
    sync = new NonfairSync();
}

public ReentrantLock(boolean fair) {
    sync = fair ? new FairSync() : new NonfairSync();
}

private Lock lock = new ReentrantLock();
 public void method1(){
    try {
         //加锁
        lock.lock();
        System.out.println("当前线程:" + Thread.currentThread().getName() + "进入method1..");
        Thread.sleep(1000);
        System.out.println("当前线程:" + Thread.currentThread().getName() + "退出method1..");
    } catch (InterruptedException e) {
        e.printStackTrace();
    } finally {
        //一定要释放锁，否则其他线程拥有无法获取
        lock.unlock();
    }
}

以前使用synchronized 关键字进行多线程协同工作时，需要使用Object的wait()阻塞、释放锁，notify()唤醒、不释放锁等进行配合使用。

使用Lock时可以使用新的等待/通知类，Condition一定是针对某一把固定的锁，也就是说，只有在有锁的基础上才会产生Condition。

public class UseCondition {

    private Lock lock = new ReentrantLock();
    private Condition condition = lock.newCondition();

    public void method1(){
        try {
            lock.lock();
            System.out.println("当前线程：" + Thread.currentThread().getName() + "进入等待状态..");
            Thread.sleep(3000);
            System.out.println("当前线程：" + Thread.currentThread().getName() + "释放锁..");
            // 释放锁，类似于Object.wait()，阻塞于此
            condition.await();
            System.out.println("当前线程：" + Thread.currentThread().getName() +"继续执行...");
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

    public void method2(){
        try {
            lock.lock();
            System.out.println("当前线程：" + Thread.currentThread().getName() + "进入..");
            Thread.sleep(3000);
            System.out.println("当前线程：" + Thread.currentThread().getName() + "发出唤醒..");
            // 不释放锁类，似于Object.notify()
            condition.signal();
            System.out.println("当前线程：" + Thread.currentThread().getName() + "没有释放锁");
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

    public static void main(String[] args) {

        final UseCondition uc = new UseCondition();

        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                uc.method1();
            }
        }, "t1");

        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                uc.method2();
            }
        }, "t2");

        t1.start();
        t2.start();
    }
}

结果：

读写锁

核心思想实现读写分离，高并发下特别适合读多写少的场景。

之前的synchronized关键字和ReentrantLock同一时间只能有一个线程进行访问被锁定的代码，读写锁的机制则不是，本质上分为两把锁，读锁和写锁，在读锁情况下，多个线程可以并发访问资源，只有当是写锁时，只能一个一个的顺序执行。

口诀：读读共享，写写互斥，读写互斥。

public class UseReentrantReadWriteLock {

    private ReentrantReadWriteLock rwLock = new ReentrantReadWriteLock();
    private ReentrantReadWriteLock.ReadLock readLock = rwLock.readLock();
    private ReentrantReadWriteLock.WriteLock writeLock = rwLock.writeLock();

    public void read() {
        try {
            readLock.lock();
            System.out.println("当前线程:" + Thread.currentThread().getName() + "进入...");
            Thread.sleep(3000);
            System.out.println("当前线程:" + Thread.currentThread().getName() + "退出...");
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            readLock.unlock();
        }
    }

    public void write() {
        try {
            writeLock.lock();
            System.out.println("当前线程:" + Thread.currentThread().getName() + "进入...");
            Thread.sleep(3000);
            System.out.println("当前线程:" + Thread.currentThread().getName() + "退出...");
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            writeLock.unlock();
        }
    }

    public static void main(String[] args) {

        final UseReentrantReadWriteLock urrw = new UseReentrantReadWriteLock();

        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                urrw.read();
            }
        }, "t1");
        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                urrw.read();
            }
        }, "t2");
        Thread t3 = new Thread(new Runnable() {
            @Override
            public void run() {
                urrw.write();
            }
        }, "t3");
        Thread t4 = new Thread(new Runnable() {
            @Override
            public void run() {
                urrw.write();
            }
        }, "t4");

        t1.start(); // R
        t2.start(); // R

        //      t1.start(); // R
        //      t3.start(); // W

        t3.start(); // W
        t4.start(); // W
    }
}

结果：