鎖（五）：讀寫鎖

• 讀寫鎖特性

特性：寫寫互斥、讀寫互斥、讀讀共享
鎖降級：寫線程擷取寫入鎖後可以擷取讀取鎖，然後釋放寫入鎖，這樣就從寫入鎖變成了讀取鎖，進而實作鎖降級的特性      

• 案例1

public class ReentrantReadWriteLockDemo {

    private int i = 0;
    private int j = 0;

    // 擷取鎖
    private ReadWriteLock lock = new ReentrantReadWriteLock();
    Lock readLock = lock.readLock();
    Lock writeLock = lock.writeLock();

    // 使用讀鎖
    public void out(){
        readLock.lock();
        try {
            System.out.println(Thread.currentThread().getName()+"i的值====》"+i + "j的值====》"+j);
        }finally {
            readLock.unlock();
        }
    }

    // 使用寫鎖
    public void inCreate() {
        writeLock.lock();
        try {
            i++;
            Thread.sleep(500L);
            j++;
        } catch (InterruptedException e) {
            e.printStackTrace();
        }finally {
            writeLock.unlock();
        }
    }

    // 測試
    public static void main(String[] args) {
        ReentrantReadWriteLockDemo reentrantReadWriteLockDemo = new ReentrantReadWriteLockDemo();
        // 測試1，3個線程，先寫後讀
        for (int i = 0; i < 3; i++) {
            new Thread(()->{
                reentrantReadWriteLockDemo.inCreate();
                reentrantReadWriteLockDemo.out();
            }).start();
        }
    }

}

# 控制台結果：
Thread-2i的值====》3j的值====》3
Thread-1i的值====》3j的值====》3
Thread-0i的值====》3j的值====》3      

• debug調試



• 線程0拿到寫鎖，釋放寫鎖



• 執行完畢



• 之後執行線程1和線程2
• 案例2

public class ReentrantReadWriteLockDemo {

    private int i = 0;
    private int j = 0;

    // 擷取鎖
    private ReadWriteLock lock = new ReentrantReadWriteLock();
    Lock readLock = lock.readLock();
    Lock writeLock = lock.writeLock();

    // 使用讀鎖
    public void out(){
        readLock.lock();
        try {
            System.out.println(Thread.currentThread().getName()+"i的值====》"+i + "j的值====》"+j);
        }finally {
            readLock.unlock();
        }
    }

    // 使用寫鎖
    public void inCreate() {
        writeLock.lock();
        try {
            i++;
            Thread.sleep(500L);
            j++;
        } catch (InterruptedException e) {
            e.printStackTrace();
        }finally {
            writeLock.unlock();
        }
    }

    // 測試
    public static void main(String[] args) {
        ReentrantReadWriteLockDemo reentrantReadWriteLockDemo = new ReentrantReadWriteLockDemo();
        // 測試2，2個線程，先讀後寫
        new Thread(()->{
            reentrantReadWriteLockDemo.out();
        },"讀線程").start();
        new Thread(()->{
            reentrantReadWriteLockDemo.inCreate();
        },"寫線程").start();
    }

}      

• debug調試



• 如下：讀線程擷取到鎖，但還沒有釋放



• 切換到寫線程，下一步時報錯如下，說明讀寫是互斥的



• 案例3

public class ReentrantReadWriteLockDemo {

    private int i = 0;
    private int j = 0;

    // 擷取鎖
    private ReadWriteLock lock = new ReentrantReadWriteLock();
    Lock readLock = lock.readLock();
    Lock writeLock = lock.writeLock();

    // 使用讀鎖
    public void out(){
        readLock.lock();
        try {
            System.out.println(Thread.currentThread().getName()+"i的值====》"+i + "j的值====》"+j);
        }finally {
            readLock.unlock();
        }
    }

    // 使用寫鎖
    public void inCreate() {
        writeLock.lock();
        try {
            i++;
            Thread.sleep(500L);
            j++;
        } catch (InterruptedException e) {
            e.printStackTrace();
        }finally {
            writeLock.unlock();
        }
    }

    // 測試
    public static void main(String[] args) {
        ReentrantReadWriteLockDemo reentrantReadWriteLockDemo = new ReentrantReadWriteLockDemo();
        // 測試3，2個線程，讀讀操作
        new Thread(()->{
            reentrantReadWriteLockDemo.out();
        },"讀線程1").start();
        new Thread(()->{
            reentrantReadWriteLockDemo.out();
        },"讀線程2").start();
    }

}      

• debug調試
• 首先讀線程1擷取到鎖，但還沒有釋放



• 切換到讀線程2，下一步，可以擷取到鎖，說明讀讀是共享的