8、并发基础-concurrent.locks锁机制

目录：

1. ReentrantLock（重入锁）
2. 重入锁，使用Condition类，实现锁于等待/通知
3. ReentrantReadWriteLock（读写锁）

在java多线程种，我们知道可以使用synchronized关键字来实现线程间的同步互斥工作，那么其实还有一个更优秀的机制完成这个“同步互斥”工作，他就是Lock对象，它主要有两种锁，重入锁和读写锁。她们具有比synchronized更为强大的功能，并且有嗅探锁定、多路分支等功能。

1、ReentrantLock（重入锁）

重入锁，在需要进行同步的代码部分加上锁，但不要忘记最后一定要释放锁定，不然会造成锁永远无法释放，其他线程永远进不来的结果。官方推荐的做法就是在finally里面释放锁。

 class X {
   private final ReentrantLock lock = new ReentrantLock();
   // ...
   public void m() {
     lock.lock();  // block until condition holds
     try {
       // ... method body
     } finally {
       lock.unlock()
     }
   }
}

测试代码：

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class UseReentrantLock {

    private Lock lock = new ReentrantLock();

    public void method1(){
        try {
            //获取锁
            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..");
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            //释放锁,如果不释放锁T1或者T2会有其中一个线程进不来
            lock.unlock();
        }
    }

    public void method2(){
        try {
            //获取锁
            lock.lock();
            System.out.println("当前线程:" + Thread.currentThread().getName() + "进入method2..");
            Thread.sleep(2000);
            System.out.println("当前线程:" + Thread.currentThread().getName() + "退出method2..");
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            //释放锁,如果不释放锁T1或者T2会有其中一个线程进不来
            lock.unlock();
        }
    }

    public static void main(String[] args) {

        final UseReentrantLock ur = new UseReentrantLock();
        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                ur.method1();
                ur.method2();
            }
        }, "t1");

        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                ur.method1();
                ur.method2();
            }
        }, "t2");

        //T1线程启动
        t1.start();
        t2.start();

        try {
            Thread.sleep(10);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }


}

2、重入锁，使用Condition类，实现锁于等待/通知

在使用synchronized的时候，如果需要多线程间进行协作工作，则需要Object的wait()和notify()、notifyAll()方法进行配合工作。

那么同样，我们在使用Lock的时候，可以使用一个新的等待/通知的类，它就和Condition.这个Condition一定是针对具体某一把锁的。也就是在只有锁的基础之上才会产生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..");

            //使当前线程等待它暗示或 interrupted，等待被其他线程唤醒
            condition.await();  // Object wait
            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..");

            //唤醒一个等待线程。
            condition.signal();//Object notify
        } 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();
    }

Condition还有个一个比Object wait/Object notify更强大的地方就是它支持一个Lock对象产生多个Condition进行多线程间的交互，非常的灵活。可以使用部分需要唤醒的线程唤醒，其他线程则继续等待通知。实现这个功能主要是靠signalAll()唤醒所有等待线程。

测试示例

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class UseManyCondition {

    private ReentrantLock lock = new ReentrantLock();

    //一个lock产生多个Condition对象
    private Condition c1 = lock.newCondition();
    private Condition c2 = lock.newCondition();

    public void m1(){
        try {
            lock.lock();
            System.out.println("当前线程：" +Thread.currentThread().getName() + "进入方法m1等待..");

            //使当前线程等待它暗示或 interrupted，等待被其他线程唤醒
            c1.await();
            System.out.println("当前线程：" +Thread.currentThread().getName() + "方法m1继续..");
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

    public void m2(){
        try {
            lock.lock();
            System.out.println("当前线程：" +Thread.currentThread().getName() + "进入方法m2等待..");

            //使当前线程等待它暗示或 interrupted，等待被其他线程唤醒
            c1.await();
            System.out.println("当前线程：" +Thread.currentThread().getName() + "方法m2继续..");
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

    public void m3(){
        try {
            lock.lock();
            System.out.println("当前线程：" +Thread.currentThread().getName() + "进入方法m3等待..");

            //使当前线程等待它暗示或 interrupted，等待被其他线程唤醒
            c2.await();
            System.out.println("当前线程：" +Thread.currentThread().getName() + "方法m3继续..");
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

    public void m4(){
        try {
            lock.lock();
            System.out.println("当前线程：" +Thread.currentThread().getName() + "唤醒..");

            //唤醒所有等待线程。
            c1.signalAll();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

    public void m5(){
        try {
            lock.lock();
            System.out.println("当前线程：" +Thread.currentThread().getName() + "唤醒..");
            c2.signal();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

    public static void main(String[] args) {

        final UseManyCondition umc = new UseManyCondition();

        Thread t1 = new Thread(() -> umc.m1(),"t1");
        Thread t2 = new Thread(() -> umc.m2(),"t2");
        Thread t3 = new Thread(() -> umc.m3(),"t3");
        Thread t4 = new Thread(() -> umc.m4(),"t4");
        Thread t5 = new Thread(() -> umc.m5(),"t5");

        t1.start(); // c1
        t2.start(); // c1
        t3.start(); // c2


        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        t4.start(); // c1
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        t5.start(); // c2

    }



}

3、ReentrantReadWriteLock（读写锁）

读写锁ReenTrantReadWriteLock,其核心就是实现读写分离的锁。在高并发访问下，尤其是读多写少的情况下，性能要远高于重入锁。

synchronized、ReentrantLock同一时间内，它们只能支持一个线程进行访问被锁定的代码。读写锁则不同，其本质是分成两个锁，即读锁、写锁。在读锁下，多个线程可以并发的进行访问，但是在写锁的时候，只能一个一个的顺序访问。

写入锁提供了一个Condition实施的行为，以同样的方式，相对于写锁，由ReentrantLock.newCondition()的Condition实现对ReentrantLock。当然这Condition可以，只能用写锁。

读锁不支持Condition和readLock().newCondition()抛出UnsupportedOperationException。

口诀：读读共享，写写互斥，读写互斥。

import java.util.concurrent.locks.ReentrantReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock.ReadLock;
import java.util.concurrent.locks.ReentrantReadWriteLock.WriteLock;

public class UseReentrantReadWriteLock {

    //读写锁对象
    private ReentrantReadWriteLock rwLock = new ReentrantReadWriteLock();
    //读锁
    private ReadLock readLock = rwLock.readLock();
    //写锁
    private 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(() -> urrw.read(), "t1");
        Thread t2 = new Thread(() -> urrw.read(), "t2");
        Thread t3 = new Thread(() -> urrw.write(), "t3");
        Thread t4 = new Thread(() -> urrw.write(), "t4");

        //读读共享
        t1.start();
        t2.start();

        //读写互斥
//      t1.start(); // R
//      t3.start(); // W

        //写写互斥
//      t3.start();
//      t4.start();

    }
}