java“锁事“ 之不同的锁讲解

• • • 公平锁和非公平锁
    • • 源码上的分析
      • • 公平锁源码:
        • 非公平锁源码:
      • 为什么默认非公平
      • 使用公平锁的问题 怎么选择
    • 可重入锁(递归锁)
    • 死锁

按序排队公平锁,就是判断同步队列是否还有先驱节点的存在,如果没有先驱节点才能获取锁;
先占先得非公平锁不会管这些,直接去抢夺占用到同步状态
公平锁多了一个 !hasQueuedPredecessors() 这个方法的判断

public final boolean hasQueuedPredecessors() {
        // The correctness of this depends on head being initialized
        // before tail and on head.next being accurate if the current
        // thread is first in queue.
        Node t = tail; // Read fields in reverse initialization order
        Node h = head;
        Node s;
        return h != t &&
            ((s = h.next) == null || s.thread != Thread.currentThread());
    }

protected final boolean tryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                if (!hasQueuedPredecessors() &&
                    compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0)
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }

final boolean nonfairTryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                if (compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0) // overflow
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }

首先是公平锁多了一个判断方法 会占用更多的时间.非公平锁能更充分的利用cpu的时间片
非公平锁下刚释放锁的线程在此获取同步状态的概率会非常大,这样就减少了线程上下文切换的次数,减少开销,效率就比较高

会导致锁饥饿的现象,因为排在后面的线程大概率持有不到锁
如果要求吞吐量的话就选择非公平锁 因为开销较低

同一个线程在外层方法获取锁的时候,在进入该线程的内层方法会自动获取锁,不会因为还没释放锁就阻塞住.比如说用synchronized修饰的递归方法. 显式锁一定要记得加锁几次就要解锁几次 否则会导致其他的线程永远无法获取锁

死锁代码示例:

public class DeadLockDemo {

    static Object lockA = new Object();

    static Object lockB = new Object();

    public static void main(String[] args) {
        new Thread(()->{
            synchronized (lockA){
                System.out.println(Thread.currentThread().getName()+"自己持有a,想去获取b");
                try {
                    TimeUnit.SECONDS.sleep(2L);
                } catch (InterruptedException e) {
                    throw new RuntimeException(e);
                }
                synchronized (lockB){
                    System.out.println(Thread.currentThread().getName()+"尝试获取b");
                }
            }
        },"a").start();

        new Thread(()->{
            synchronized (lockB){
                try {
                    TimeUnit.SECONDS.sleep(2L);
                } catch (InterruptedException e) {
                    throw new RuntimeException(e);
                }
                System.out.println(Thread.currentThread().getName()+"自己持有b,想去获取a");
                synchronized (lockA){
                    System.out.println(Thread.currentThread().getName()+"尝试获取a");
                }
            }
        },"b").start();
    }

}