一、可重入（P121）

可重入是指同一个线程如果首次获得了这把锁，那么因为它是这把锁的拥有者，因此有权利再次获取这把锁。

如果是不可重入，那么第二次获得锁时，自己也会被锁挡住。

@Slf4j(topic = "c.Test22")
public class Test22 {

    private static ReentrantLock lock = new ReentrantLock();

    public static void main(String[] args) {
        lock.lock();
        try {
            log.debug("enter main");
            m1();
        } finally {
            lock.unlock();
        }
    }

    public static void m1(){
        lock.lock();
        try {
            log.debug("enter m1");
            m2();
        } finally {
            lock.unlock();
        }
    }

    public static void m2(){
        lock.lock();
        try {
            log.debug("enter m2");
        } finally {
            lock.unlock();
        }
    }
}

二、可打断

@Slf4j(topic = "c.Test22")
public class Test22 {

    private static ReentrantLock lock = new ReentrantLock();

    public static void main(String[] args) throws InterruptedException {

        Thread t1 = new Thread(() -> {
            try {
                //如果没有竞争那么此方法就会获取lock对象锁
                //如果有竞争就进入阻塞队列，可以被其它线程用interruput方法打断
                log.debug("尝试获取锁");
                lock.lockInterruptibly();
            } catch (InterruptedException e) {
                e.printStackTrace();
                log.debug("没有获得锁，返回");
                return;
            }
            try {
                log.debug("获取到锁");
            } finally {
                lock.unlock();
            }
        },"t1");

        lock.lock();
        t1.start();

        Thread.sleep(1000);
        log.debug("打断 t1");
        t1.interrupt();
    }
}

注意如果是 不可中断（lock()方法） 模式，那么即使使用了 interrupt 也不会让等待中断

三、锁超时

@Slf4j(topic = "c.Test22")
public class Test22 {

    private static ReentrantLock lock = new ReentrantLock();

    public static void main(String[] args) throws InterruptedException {

        Thread t1 = new Thread(() -> {
            log.debug("尝试获取锁");
            // 立刻尝试获取锁
            boolean tryLock = lock.tryLock();
            if (!tryLock){
                log.debug("获取不到锁");
                return;
            }
            try {
                log.debug("获得到锁");
            } finally {
                lock.unlock();
            }
        },"t1");

        log.debug("获取锁");
        lock.lock();
        t1.start();
    }
}

@Slf4j(topic = "c.Test22")
public class Test22 {

    private static ReentrantLock lock = new ReentrantLock();

    public static void main(String[] args) throws InterruptedException {

        Thread t1 = new Thread(() -> {
            log.debug("尝试获取锁");
            try {
                // 2s后尝试获取锁
                boolean tryLock = lock.tryLock(2, TimeUnit.SECONDS);
                if (!tryLock){
                    log.debug("获取不到锁");
                    return;
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
                log.debug("获取不到锁");
                return;
            }
            try {
                log.debug("获得到锁");
            } finally {
                lock.unlock();
            }
        },"t1");

        log.debug("获取锁");
        lock.lock();
        t1.start();
    }
}

使用 tryLock 解决哲学家就餐问题

@Slf4j(topic = "c.Test23")
public class Test23 {public static void main(String[] args) {
    Chopstick c1 = new Chopstick("1");
    Chopstick c2 = new Chopstick("2");
    Chopstick c3 = new Chopstick("3");
    Chopstick c4 = new Chopstick("4");
    Chopstick c5 = new Chopstick("5");
    new Philosopher("苏格拉底", c1, c2).start();
    new Philosopher("柏拉图", c2, c3).start();
    new Philosopher("亚里士多德", c3, c4).start();
    new Philosopher("赫拉克利特", c4, c5).start();
    new Philosopher("阿基米德", c5, c1).start();
}
}

@Slf4j(topic = "c.Philosopher")
class Philosopher extends Thread {
    Chopstick left;
    Chopstick right;

    public Philosopher(String name, Chopstick left, Chopstick right) {
        super(name);
        this.left = left;
        this.right = right;
    }

    @Override
    public void run() {
        while (true) {
            //　尝试获得左手筷子
            if(left.tryLock()) {
                try {
                    // 尝试获得右手筷子
                    if(right.tryLock()) {
                        try {
                            eat();
                        } finally {
                            right.unlock();
                        }
                    }
                } finally {
                    left.unlock(); // 释放自己手里的筷子
                }
            }
        }
    }

    Random random = new Random();
    private void eat() {
        log.debug("eating...");
        Sleeper.sleep(0.5);
    }
}

class Chopstick extends ReentrantLock {
    String name;

    public Chopstick(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "筷子{" + name + '}';
    }
}

四、公平锁

ReentrantLock 默认是不公平的

// true：公平
// false（默认）：不公平
ReentrantLock lock = new ReentrantLock(true);
公平锁一般没有必要，会降低并发度，后面分析原理时会讲解。

五、条件变量

synchronized 中也有条件变量，就是我们讲原理的 waitSet 休息室，当条件不满足时进入等待。

ReentrantLock 的条件变量比 synchronized 强大之处在于，它是支持多个条件变量的，这就好比：

（1）synchronized 是那些不满足条件的线程都在一间休息室等消息

（2）而 ReentrantLock 支持多间休息室，唤醒时也是按休息室来唤醒的

使用要点：
（1）await 前需要获得锁

（2）await 执行后，会释放锁，进入 conditionObject 等待

（3）await 的线程被唤醒（或打断、或超时）去重新竞争 lock 锁

（4）竞争 lock 锁成功后，从 await 后继续执行
@Slf4j(topic = "c.Test22")
public class Test22 {

    private static ReentrantLock lock = new ReentrantLock();

    public static void main(String[] args) throws InterruptedException {

        // 创建一个新的条件变量（休息室）
        Condition condition1 = lock.newCondition();
        Condition condition2 = lock.newCondition();

        lock.lock();
        // 进入休息室等待
        condition1.await();

        condition1.signal();
        condition1.signalAll();

    }
}

@Slf4j(topic = "c.Test24")
public class Test24 {
    static final Object room = new Object();
    static boolean hasCigarette = false;
    static boolean hasTakeout = false;
    static ReentrantLock ROOM = new ReentrantLock();
    // 等待烟的休息室
    static Condition waitCigaretteSet = ROOM.newCondition();
    // 等外卖的休息室
    static Condition waitTakeoutSet = ROOM.newCondition();

    public static void main(String[] args) {


        new Thread(() -> {
            ROOM.lock();
            try {
                log.debug("有烟没？[{}]", hasCigarette);
                while (!hasCigarette) {
                    log.debug("没烟，先歇会！");
                    try {
                        waitCigaretteSet.await();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                log.debug("可以开始干活了");
            } finally {
                ROOM.unlock();
            }
        }, "小南").start();

        new Thread(() -> {
            ROOM.lock();
            try {
                log.debug("外卖送到没？[{}]", hasTakeout);
                while (!hasTakeout) {
                    log.debug("没外卖，先歇会！");
                    try {
                        waitTakeoutSet.await();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                log.debug("可以开始干活了");
            } finally {
                ROOM.unlock();
            }
        }, "小女").start();

        sleep(1);
        new Thread(() -> {
            ROOM.lock();
            try {
                hasTakeout = true;
                waitTakeoutSet.signal();
            } finally {
                ROOM.unlock();
            }
        }, "送外卖的").start();

        sleep(1);

        new Thread(() -> {
            ROOM.lock();
            try {
                hasCigarette = true;
                waitCigaretteSet.signal();
            } finally {
                ROOM.unlock();
            }
        }, "送烟的").start();
    }

}

六、同步模式之顺序控制

1. 固定运行顺序

1.1 wait notify 版

@Slf4j(topic = "c.Test25")
public class Test25 {
    static final Object lock = new Object();
    // 表示 t2 是否运行过
    static boolean t2runned = false;

    public static void main(String[] args) {
        Thread t1 = new Thread(() -> {
            synchronized (lock) {
                while (!t2runned) {
                    try {
                        lock.wait();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                log.debug("1");
            }
        }, "t1");


        Thread t2 = new Thread(() -> {
            synchronized (lock) {
                log.debug("2");
                t2runned = true;
                lock.notify();
            }
        }, "t2");

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

1.2 Park Unpark 版

@Slf4j(topic = "c.Test26")
public class Test26 {
    public static void main(String[] args) {

        Thread t1 = new Thread(() -> {
            LockSupport.park();
            log.debug("1");
        }, "t1");
        t1.start();

        new Thread(() -> {
            log.debug("2");
            LockSupport.unpark(t1);
        },"t2").start();
    }
}
park 和 unpark 方法比较灵活，他俩谁先调用，谁后调用无所谓。并且是以线程为单位进行『暂停』和『恢复』，不需要『同步对象』和『运行标记』

2. 交替输出

线程 1 输出 a 5 次，线程 2 输出 b 5 次，线程 3 输出 c 5 次。现在要求输出 abcabcabcabcabc 怎么实现

2.1 wait notify 版

@Slf4j(topic = "c.Test27")
public class Test27 {
    public static void main(String[] args) {
        WaitNotify wn = new WaitNotify(1, 5);
        new Thread(() -> {
            wn.print("a", 1, 2);
        }).start();
        new Thread(() -> {
            wn.print("b", 2, 3);
        }).start();
        new Thread(() -> {
            wn.print("c", 3, 1);
        }).start();
    }
}

/*
输出内容       等待标记     下一个标记
   a           1             2
   b           2             3
   c           3             1
 */
class WaitNotify {
    // 打印               a           1             2
    public void print(String str, int waitFlag, int nextFlag) {
        for (int i = 0; i < loopNumber; i++) {
            synchronized (this) {
                while(flag != waitFlag) {
                    try {
                        this.wait();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                System.out.print(str);
                flag = nextFlag;
                this.notifyAll();
            }
        }
    }

    // 等待标记
    private int flag; // 2
    // 循环次数
    private int loopNumber;

    public WaitNotify(int flag, int loopNumber) {
        this.flag = flag;
        this.loopNumber = loopNumber;
    }
}

2.2 Lock 条件变量版

public class Test30 {
    public static void main(String[] args) throws InterruptedException {
        AwaitSignal awaitSignal = new AwaitSignal(5);
        Condition a = awaitSignal.newCondition();
        Condition b = awaitSignal.newCondition();
        Condition c = awaitSignal.newCondition();
        new Thread(() -> {
            awaitSignal.print("a", a, b);
        }).start();
        new Thread(() -> {
            awaitSignal.print("b", b, c);
        }).start();
        new Thread(() -> {
            awaitSignal.print("c", c, a);
        }).start();

        Thread.sleep(1000);
        awaitSignal.lock();
        try {
            System.out.println("开始...");
            a.signal();
        } finally {
            awaitSignal.unlock();
        }

    }
}

class AwaitSignal extends ReentrantLock{
    private int loopNumber;

    public AwaitSignal(int loopNumber) {
        this.loopNumber = loopNumber;
    }
    // 参数1 打印内容， 参数2 进入哪一间休息室, 参数3 下一间休息室
    public void print(String str, Condition current, Condition next) {
        for (int i = 0; i < loopNumber; i++) {
            lock();
            try {
                current.await();
                System.out.print(str);
                next.signal();
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                unlock();
            }
        }
    }
}

2.3 Park Unpark 版

@Slf4j(topic = "c.Test31")
public class Test31 {

    static Thread t1;
    static Thread t2;
    static Thread t3;
    public static void main(String[] args) {
        ParkUnpark pu = new ParkUnpark(5);
        t1 = new Thread(() -> {
            pu.print("a", t2);
        });
        t2 = new Thread(() -> {
            pu.print("b", t3);
        });
        t3 = new Thread(() -> {
            pu.print("c", t1);
        });
        t1.start();
        t2.start();
        t3.start();

        LockSupport.unpark(t1);
    }
}

class ParkUnpark {
    public void print(String str, Thread next) {
        for (int i = 0; i < loopNumber; i++) {
            LockSupport.park();
            System.out.print(str);
            LockSupport.unpark(next);
        }
    }

    private int loopNumber;

    public ParkUnpark(int loopNumber) {
        this.loopNumber = loopNumber;
    }
}