定義

可重入鎖，也叫做遞歸鎖，指的是在同一線程內(nèi)，外層函數(shù)獲得鎖之后，內(nèi)層遞歸函數(shù)仍然可以獲取到該鎖。換一種說(shuō)法：同一個(gè)線程再次進(jìn)入同步代碼時(shí)，可以使用自己已獲取到的鎖。

作用

防止在同一線程中多次獲取鎖而導(dǎo)致死鎖發(fā)生。

如下，我們通過(guò)自旋鎖來(lái)判斷是否會(huì)發(fā)生死鎖：

public class SpinLock {
    private AtomicReference<Thread> sign = new AtomicReference<>();
    public void lock(){
        Thread current = Thread.currentThread();
        while (!sign.compareAndSet(null,current)){
        }
    }
    public void unlock(){
        Thread cur = Thread.currentThread();
        sign.compareAndSet(cur,null);
    }
}

public class SpinLockDemo {
    private SpinLock lock =  new SpinLock();
    class Widget{
        public void doSomething(){
            lock.lock();
            System.out.println("Widget calling doSomething");
            lock.unlock();
        }
    }

    class LoggingWidget extends Widget {
        @Override
        public void doSomething() {
            lock.lock();
            System.out.println("LoggingWidget calling doSomething");
            super.doSomething();
            lock.unlock();
        }
    }

    public static void main(String[] args){
        SpinLockDemo spinLockDemo = new SpinLockDemo();
        SpinLockDemo.Widget widget = spinLockDemo.new LoggingWidget();
        widget.doSomething();
    }
}

輸出結(jié)果：
LoggingWidget calling doSomething

我們可以看到在LoggingWidget類中doSomething方法時(shí)，通過(guò)鎖進(jìn)入臨界區(qū)，并在臨界區(qū)中調(diào)用了父類的該方法，而父類的方法要獲取到同一個(gè)鎖，被阻塞，導(dǎo)致死鎖發(fā)生。

常見的可重入鎖

• Synchronized關(guān)鍵字：

public class SynchronizedDemo {
     class Widget{
        public synchronized void doSomething(){
            System.out.println("Widget calling doSomething...");
        }
    }

     class LoggingWidget extends Widget{
        @Override
        public synchronized void doSomething() {
            System.out.println("LoggingWidget calling doSomething");
            super.doSomething();
        }
    }

    public static void main(String[] args){
         SynchronizedDemo synchronizedDemo = new SynchronizedDemo();
        SynchronizedDemo.Widget widget = synchronizedDemo.new LoggingWidget();
        widget.doSomething();
    }
}

輸出結(jié)果：
LoggingWidget calling doSomething
Widget calling doSomething...

根據(jù)結(jié)果，我們可以看到Synchronized關(guān)鍵字是可重入鎖。

• ReetrantLock：

public class ReentrantLockDemo {
    private Lock lock =  new ReentrantLock();
    class Widget{
        public void doSomething(){
            lock.lock();
            System.out.println("Widget calling doSomething");
            lock.unlock();
        }
    }

    class LoggingWidget extends Widget {
        @Override
        public void doSomething() {
            lock.lock();
            System.out.println("LoggingWidget calling doSomething");
            super.doSomething();
            lock.unlock();
        }
    }

    public static void main(String[] args){
        ReentrantLockDemo reentrantLockDemo = new ReentrantLockDemo();
        Widget widget = reentrantLockDemo.new LoggingWidget();
        widget.doSomething();
    }

}

輸出結(jié)果：
LoggingWidget calling doSomething
Widget calling doSomething

根據(jù)結(jié)果，我們可以看出ReetrantLock鎖時(shí)可重入的。

實(shí)現(xiàn)可重入鎖

為每個(gè)鎖關(guān)聯(lián)一個(gè)獲取計(jì)數(shù)器和一個(gè)所有者線程，當(dāng)計(jì)數(shù)值為0時(shí)，這個(gè)鎖就被認(rèn)為是沒(méi)有被任何線程所占有的。當(dāng)線程請(qǐng)求一個(gè)未被持有的鎖時(shí)，計(jì)數(shù)值將會(huì)遞增。而當(dāng)線程退出同步代碼時(shí)，計(jì)數(shù)器會(huì)相應(yīng)地遞減。當(dāng)計(jì)數(shù)值為0時(shí)，則釋放該鎖。

如下：我們通過(guò)修改自旋鎖來(lái)實(shí)現(xiàn)一個(gè)可重入的自旋鎖

public class SpinLockDemo {
    private MySpinLock lock =  new MySpinLock();
    class Widget{
        public void doSomething(){
            lock.lock();
            System.out.println("Widget calling doSomething");
            lock.unlock();
        }
    }

    class MySpinLock{
        private AtomicReference<Thread> owner = new AtomicReference<>();
        private int count = 0;
        public void lock(){
            Thread cur = Thread.currentThread();
            if (cur == owner.get()){
                count ++;
                return;
            }
            while (! owner.compareAndSet(null,cur)){

            }
        }

        public void unlock(){
            Thread cur = Thread.currentThread();
            if (cur == owner.get()){
                if (count != 0){
                    count --;
                } else {
                    owner.compareAndSet(cur,null);
                }
            }
        }
    }

    class LoggingWidget extends Widget {
        @Override
        public void doSomething() {
            lock.lock();
            System.out.println("LoggingWidget calling doSomething");
            super.doSomething();
            lock.unlock();
        }
    }

    public static void main(String[] args){
        SpinLockDemo spinLockDemo = new SpinLockDemo();
        SpinLockDemo.Widget widget = spinLockDemo.new LoggingWidget();
        widget.doSomething();
    }
}

輸出結(jié)果：
LoggingWidget calling doSomething
Widget calling doSomething