ReentrantLock可重入锁的理解和源码简单分析

Posted 2020-10-21 指尖上的艺术

import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.ReentrantLock;

/**
 * @author admin
 * @date 2018/1/16 12:16
 * ReentrantLock 可重入锁：
 * 1、启动两个线程，线程A获取锁，然后执行；同时线程B进来后，一直阻塞，直到线程A释放锁之后，线程B才接着执行
 */
public class ReentrantLockTest {
    ReentrantLock lock = new ReentrantLock();

    public void reentrantLockRun1(String threadName) {
        System.out.println(threadName + "进入");
        lock.lock();
        System.out.println(threadName + "方法被锁");
        try {
            System.out.println(threadName + "方法执行");
            TimeUnit.SECONDS.sleep(3);
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
            System.out.println(threadName + "锁被释放");
        }
    }

    public static void main(String[] args) {
        ReentrantLockTest rltest = new ReentrantLockTest();
        Thread thread = new Thread() {
            public void run() {
                rltest.reentrantLockRun1("线程A");
            }
        };
        thread.start();

        Thread thread2 = new Thread() {
            public void run() {
                rltest.reentrantLockRun1("线程B");
            }
        };
        thread2.start();
    }
}

运行结果：
线程A进入
线程A方法被锁
线程A方法执行
线程B进入
线程A锁被释放
线程B方法被锁
线程B方法执行
线程B锁被释放

 

import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.ReentrantLock;

/**
 * @author admin
 * @date 2018/1/16 12:16
 * ReentrantLock 可重入锁：
 * 1、启动两个线程，线程A获取锁，然后执行,不释放锁，接着线程A再调用reentrantLockRun2，不需要阻塞，接着执行，最后释放锁；说明同一个线程对ReentrantLock可重复获取
 * 2、线程B在这个过程中一直阻塞，等到线程A把所有的锁释放完之后，再获取锁，执行方法，最后释放锁
 */
public class ReentrantLockTest2 {
    ReentrantLock lock = new ReentrantLock();

    public ReentrantLock reentrantLockRun1(String threadName) {
        System.out.println(threadName + "进入");
        lock.lock();
        System.out.println(threadName + "方法被锁");
        try {
            System.out.println(threadName + "方法执行");
            TimeUnit.SECONDS.sleep(3);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        return lock;
    }

    public ReentrantLock reentrantLockRun2(String threadName) {
        System.out.println(threadName + "进入");
        lock.lock();
        System.out.println(threadName + "方法被锁");
        try {
            System.out.println(threadName + "方法执行");
            TimeUnit.SECONDS.sleep(3);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        return lock;
    }

    public static void main(String[] args) {
        ReentrantLockTest2 rltest = new ReentrantLockTest2();
        Thread thread = new Thread() {
            public void run() {
                ReentrantLock lock1 = rltest.reentrantLockRun1("线程A");
                ReentrantLock lock2  = rltest.reentrantLockRun2("线程A2");
                lock1.unlock();
                System.out.println("线程A释放锁");
                lock2.unlock();
                System.out.println("线程A2释放锁");

            }
        };
        thread.start();

        Thread thread2 = new Thread() {
            public void run() {
                ReentrantLock lock = rltest.reentrantLockRun1("线程B");
                lock.unlock();
                System.out.println("线程B释放锁");

            }
        };
        thread2.start();
    }
}

运行结果：
线程A进入
线程A方法被锁
线程A方法执行
线程B进入
线程A2进入
线程A2方法被锁
线程A2方法执行
线程A释放锁
线程A2释放锁
线程B方法被锁
线程B方法执行
线程B释放锁

 

根据源码发现：维护了这个可见性变量state ；同一个线程对可重入锁体现用state标记作累加，int nextc = c + acquires;

private volatile int state;

 

public void lock() {
    sync.lock();
}

 

 

public final void acquire(int arg) {
    if (!tryAcquire(arg) &&
        acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
        selfInterrupt();
}

// 判断是不是第一次获取锁，如果是操作state=1；否则判断是不是同一个线程如果是state+1，如果不是同一个线程直接返回false

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;
    }
}
// 大概就是用一个链表来维护等待线程

final boolean acquireQueued(final Node node, int arg) {
    boolean failed = true;
    try {
        boolean interrupted = false;
        for (;;) {
            final Node p = node.predecessor();
            if (p == head && tryAcquire(arg)) {
                setHead(node);
                p.next = null; // help GC
                failed = false;
                return interrupted;
            }
            if (shouldParkAfterFailedAcquire(p, node) &&
                parkAndCheckInterrupt())
                interrupted = true;
        }
    } finally {
        if (failed)
            cancelAcquire(node);
    }
}