Java高并发实战——同步控制工具

作者: XHHP | 来源:发表于2019-08-06 00:03 被阅读0次

（1）、重入锁（ReentrantLock）

重入锁使用java.util.concurrent.locks.ReentrantLock，下面是简单的使用案例

public class ReenterLock implements Runnable {

    public static ReentrantLock lock = new ReentrantLock();
    public static int i = 0;

    @Override
    public void run() {
        for(int j = 0; j < 100000; j++) {
            lock.lock();
            try {
                i++;
            }finally {
                lock.unlock();
            }
        }
    }

    public static void main(String[] args) throws InterruptedException {
        ReenterLock t1 = new ReenterLock();
        Thread thread1 = new Thread(t1);
        Thread thread2 = new Thread(t1);
        thread1.start();
        thread2.start();
        thread1.join();
        thread2.join();
        System.out.println(i);
    }
}

之所以叫重入锁，是因为这种锁是可以反复进入的。这里的反复进入只局限于同一个线程。

lock.lock();
lock.lock();
try{
    i++;
}finally {
    lock.unlock();
    lock.unlock();
}

中断响应：重入锁提供可以使用中断响应来取消对锁的请求（lockInterruptibly()可以中断申请）

public class IntLock implements Runnable {

    public static ReentrantLock lock1 = new ReentrantLock();
    public static ReentrantLock lock2 = new ReentrantLock();
    public int lock;

    public IntLock(int lock) {
        this.lock = lock;
    }

    @Override
    public void run() {
        try {
            if (lock == 1) {
                lock1.lockInterruptibly();              //请求lock1
                try {
                    Thread.sleep(500);
                } catch (InterruptedException e) {
                }
                lock2.lockInterruptibly();              //请求lock2
            }else {
                lock2.lockInterruptibly();              //请求lock2
                try {
                    Thread.sleep(500);
                } catch (InterruptedException e) {}
                lock1.lockInterruptibly();              //请求lock1
            }
        }catch (InterruptedException e) {
            e.printStackTrace();
        }finally {
            if(lock1.isHeldByCurrentThread()) {         //查看当前线程是否被锁
                lock1.unlock();
            }
            if(lock2.isHeldByCurrentThread()) {         //查看当前线程是否被锁
                lock2.unlock();
            }
            System.out.println(Thread.currentThread().getName() + "线程结束");
        }
    }

    public static void main(String[] args) throws InterruptedException {
        IntLock intLock1 = new IntLock(1);
        IntLock intLock2 = new IntLock(2);
        Thread thread1 = new Thread(intLock1,"线程1");
        Thread thread2 = new Thread(intLock2,"线程2");
        thread1.start();
        thread2.start();
        Thread.sleep(1000);
        thread2.interrupt();        //中断线程2，取消线程2对lock1的请求
    }
}

在这里插入图片描述

锁申请等待限时
避免死锁其中一个方法就是限时等待。使用trylock（）方法。该方法接收两个参数，一个表示等待时长，另外一个代表计时单位。

public class TimeLock implements Runnable {

    public static ReentrantLock lock = new ReentrantLock();

    @Override
    public void run() {
        try {
            if(lock.tryLock(5, TimeUnit.SECONDS)) {        //尝试5秒获得锁
                Thread.sleep(6000);
            }else {
                System.out.println(Thread.currentThread().getName() + "get lock failed");
            }
        }catch (InterruptedException e) {
            e.printStackTrace();
        }finally {
            if(lock.isHeldByCurrentThread()) {
                lock.unlock();
            }
        }
    }

    public static void main(String[] args) {
        TimeLock time = new TimeLock();
        Thread thread1 = new Thread(time);
        Thread thread2 = new Thread(time);
        thread1.start();
        thread2.start();
    }
}

公平锁：设置为公平锁，它会按照时间的先后顺序，保证先到者先得，后到者后得。不会产生饥饿现象，只要你排队就可以获得资源
一般锁的申请都是非公平的，没有特殊必要就不要设置为公平锁，因为需要产生有序队列，性能较低。

public class FairLock implements Runnable {
    public static ReentrantLock fairLock = new ReentrantLock(true);         //设置为公平锁

    @Override
    public void run() {
        while(true) {
            try {
                fairLock.lock();
                System.out.println(Thread.currentThread().getName() + "获得锁");
            }finally {
                fairLock.unlock();
            }
        }
    }

    public static void main(String[] args) {
        FairLock f1 = new FairLock();
        Thread t1 = new Thread(f1, "线程1");
        Thread t2 = new Thread(f1, "线程2");
        t1.start();
        t2.start();
    }
}

（2）、重入锁的好搭档：Condition

Condition接口的基本方法如下

//使线程进入等待，释放当前锁，等待其他线程调用signal（）方法唤醒，线程被中断也能唤醒
void await();           
//与await()方法基本相似，但是不会被线程中断唤醒
void awaitUninterruptibly()
boolean await(long time, TimeUnit unit);
boolean await(Date deadline);
//唤醒线程
void signal();
void signalAll();

public class ReenterLockCondition implements Runnable {
    public static ReentrantLock lock = new ReentrantLock();
    public static Condition condition = lock.newCondition();//lock生成一个与之绑定的Condition对象

    @Override
    public void run() {
        try {
            lock.lock();
            condition.await();              //释放锁，进入等待状态
            System.out.println("Thread is going on");
        }catch (InterruptedException e) {
            e.printStackTrace();
        }finally {
            lock.unlock();
        }
    }

    public static void main(String[] args) throws InterruptedException {
        ReenterLockCondition t = new ReenterLockCondition();
        Thread t1 = new Thread(t);
        t1.start();
        Thread.sleep(2000);
        lock.lock();
        condition.signal();             //唤醒线程
        lock.unlock();
    }
}

（3）、允许多个线程同时访问：信号量（Samephore）

信号量主要提供了以下构造函数：

public Semaphore(int permits);
public Semaphore(int permits,boolean fair)  //第二个参数指定是否为公平

在构造信号量对象时，必须要指定信号量的准入数，即同时能申请多少个许可。使用acquire（）方法申请许可证，一个线程可以申请一个或者多个许可证
信号量的主要逻辑方法有：

//获取一个准时许可和获取多个准入许可
public void acquire() throws InterruptedException
public void acquire(int permits)
//与前两个方法相同，不过不接受中断
public void acquireUninterruptibly()
//尝试获得许可证
public boolean tryAcquire()
public boolean tryAcquire(long timeout, TimeUnit unit)
//释放资源
public void release()

public class SemapDemp implements Runnable {

    final Semaphore semp = new Semaphore(5);        //初始化许可证个数

    @Override
    public void run() {
        try {
            semp.acquire();                         //尝试获得一个许可证
            Thread.sleep(2000);
            System.out.println(Thread.currentThread().getId()+":done");
        }catch (InterruptedException e) {
            e.printStackTrace();
        }finally {
            semp.release();         //释放资源
        }
    }

    public static void main(String[] args) {
        ExecutorService exec = Executors.newFixedThreadPool(20);    //初始化线程池
        final SemapDemp demo = new SemapDemp();
        for(int i = 0; i < 20 ; i++) {
            exec.submit(demo);          //执行线程
        }
    }
}

线程获取许可证之后必须记得释放，否则会使许可证个数越来越少。

（4）、ReadWriteLock读写锁

ReadWriteLock是JDK5中提供的读写分离锁。读写分离锁可以有效的减少锁竞争，提升性能。
读-读不互斥：读读之间不阻塞
读-写互斥：读阻塞写，写也会阻塞读
写-写互斥：写写阻塞

public class ReadWriteLockDemo {
    private static Lock lock = new ReentrantLock();
    private static ReentrantReadWriteLock readWriteLock = new ReentrantReadWriteLock();
    private static Lock readLock = readWriteLock.readLock();
    private static Lock writeLock = readWriteLock.writeLock();
    private int value;

    public Object handleRead(Lock lock) throws InterruptedException {
        try {
            lock.lock();        //模拟读操作
            Thread.sleep(1000);
            return value;
        }finally {
            lock.unlock();
        }
    }

    public void handleWrite(Lock lock, int index) throws InterruptedException {
        try {
            lock.lock();
            Thread.sleep(1000);
            value=index;            //模拟写操作
        }finally {
            lock.unlock();
        }
    }

    public static void main(String[] args) {
        final ReadWriteLockDemo demo = new ReadWriteLockDemo();
        Runnable readRunnable = new Runnable() {
            @Override
            public void run() {
                try {
                    demo.handleRead(readLock);          //读，传入读锁
                }catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        };
        Runnable writeRunnable = new Runnable() {
            @Override
            public void run() {
                try {
                    demo.handleWrite(writeLock,new Random().nextInt());  //写，传入写锁
                }catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        };

        for(int i = 0; i < 18; i++) {
            new Thread(readRunnable).start();       //读线程之间不互相干扰
        }
        for(int i = 18; i < 20; i++) {
            new Thread(writeRunnable).start();      //写线程会相互阻塞
        }
    }
}

（5）、倒计数器：CountDownLatch

CountDownLatch相当于设置了一个计数器，awit（）方法用于等待CountDownLatch的值减为0，才能继续向下执行。countDown（）方法用于减少CountDownLatch的值

在这里插入图片描述

public class CountDownLatchDemo implements Runnable {
    static final CountDownLatch end =new CountDownLatch(10);            //初始化CountDownLatch
    static final CountDownLatchDemo demo = new CountDownLatchDemo();

    @Override
    public void run() {
        try {
            Thread.sleep(1000);                 //模拟程序
            System.out.println("check complete");
            end.countDown();                        //countDownLatch减1
        }catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) throws InterruptedException {
        ExecutorService service = Executors.newFixedThreadPool(10);     //初始化线程池
        for(int i = 0; i < 10; i++) {
            service.submit(demo);           //执行线程
        }
        end.await();                    //等待CountDownLatch减为0，才能继续工作
        System.out.println("finish");
        service.shutdown();         //关闭线程池
    }
}

（6）、循环栅栏：CyclicBarrier

CyclicBarrier可以理解为循环栅栏。栅栏就是一种障碍物。
CyclicBarrier可以用来阻止线程继续执行，要求线程在栅栏外等待。
前面Cyclic意为循环，开始，计数器设置为10，那么凑齐第一批10个线程等待了。计数器会归零，接着凑下一批10个线程。
Cyclic常用的构造函数如下：

public CyclicBarrier(int parties, Runnable barrierAction)

parties代表计数器的值，barrierAction是当一次计数完成后，系统会执行的动作。

public class CyclicBarrierDemo {
    public static class Solder implements Runnable {

        private String solider;
        private final CyclicBarrier cyclicBarrier;

        Solder(CyclicBarrier cyclicBarrier, String soliderName) {       //构造函数
            this.cyclicBarrier = cyclicBarrier;
            this.solider = soliderName;
        }

        @Override
        public void run() {
            try {
                cyclicBarrier.await();          //栅栏外等候,第一次计数
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(solider + "任务完成");
                cyclicBarrier.await();           //栅栏外等候,第二次计数
            }catch (InterruptedException e) {
                e.printStackTrace();
            }catch (BrokenBarrierException e) {
                e.printStackTrace();
            }
        }
    }

    public static class BarrierRun implements Runnable {
        boolean flag;
        int N;
        public BarrierRun(boolean flag, int N) {
            this.flag = flag;
            this.N = N;
        }

        @Override
        public void run() {
            if(flag) {
                System.out.println("司令:[士兵" + N +"个，任务完成");
            }else {
                System.out.println("司令:[士兵" + N +"个，集合完毕");
                flag = true;
            }
        }
    }

    public static void main(String[] args) {
        final int N = 10;         //计数器的值，线程总数
        Thread[] allSolider = new Thread[N];
        boolean flag = false;     //标志位
        CyclicBarrier cyclicBarrier = new CyclicBarrier(N, new BarrierRun(flag,N));     //初始化CyclicBarrier
        System.out.println("集合队伍");
        for(int i = 0; i < N; i++) {
            System.out.println("士兵"+ i + "报道");
            allSolider[i] = new Thread(new Solder(cyclicBarrier,"士兵"+i));   //创建线程
            allSolider[i].start();          //运行
        }
    }
}

InterruptedException:线程等待过程中，线程被中断
BrokenBarrierException:有线程被中断了，无法达到计数器的值，无论如何无法达到预期值，避免无谓的等待。

在这里插入图片描述

（7）、线程阻塞工具类：LockSupport

LockSupport是一个非常方便的实用的线程阻塞工具，它可以在线程内任意位置让线程阻塞。
注意：即使unpark（）方法发生在park（）方法之前，也不会导致线程永远挂起
因为LockSupport使用类似于信号量的机制。它为线程准备了一个许可，如果许可可用，那么park（）方法立即返回，并且将许可变为不可用。如果许可不可用，线程就会阻塞。而unpark（）方法则会将许可变为可用。
LockSupport.park（）方法还能支持中断影响。但是不会抛出InterruptedException。但是可以通过Thread.interrupted()等方法获得中断标记

public class LockSupportIntDemo {
    public static Object u = new Object();
    static ChangeObjectThread t1 = new ChangeObjectThread("t1");
    static ChangeObjectThread t2 = new ChangeObjectThread("t2");

    public static class ChangeObjectThread extends Thread {
        public ChangeObjectThread(String name) {
            super.setName(name);
        }

        @Override
        public void run() {
            synchronized (u) {
                System.out.println("in " + getName());
                LockSupport.park();
                if(Thread.interrupted()) {
                    System.out.println(getName() + "被中断了");
                }
            }
            System.out.println(getName() + "执行结束");
        }
    }

    public static void main(String[] args) throws InterruptedException {
        t1.start();
        Thread.sleep(100);
        t2.start();
        t1.interrupt();
        LockSupport.unpark(t2);
    }
}

（8）、Guava和RateLimiter限流

Guava是Google下的一个核心库，提供了一大批设计精良、使用方便的工具类。Guava是JDK标准库的重要补充
RateLimiter采用了令牌桶算法在这里插入图片描述

public class RateLimiterDemo {
    static RateLimiter limiter = RateLimiter.create(2);

    public static class Task implements Runnable {

        @Override
        public void run() {
            System.out.println(System.currentTimeMillis());
        }
    }

    public static void main(String[] args) {
        for(int i = 0; i < 50; i++) {
            limiter.acquire();
            new Thread(new Task()).start();
        }
    }
}

Java高并发实战——同步控制工具

（1）、重入锁（ReentrantLock）

（2）、重入锁的好搭档：Condition

（3）、允许多个线程同时访问：信号量（Samephore）

（4）、ReadWriteLock读写锁

（5）、倒计数器：CountDownLatch

（6）、循环栅栏：CyclicBarrier

（7）、线程阻塞工具类：LockSupport

（8）、Guava和RateLimiter限流

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