1.3.1 Lock接口及其实现

1.锁的本质

2.Lock接口使用ReentrenLock

3.读写锁使用

4.读写锁实现

Lock接口方法

有点意思的是lockInterruptibly(), 只要没有获取到锁就会一直等待，直到某一地方对当前线程执行interrupt()方法后，

lockInterruptibly()处会抛出异常，可以在catch中对此异常情况进行处理

 

synchronized+wait+notify  对比  reentrantLock+condition+await+signal:

两种方式大相径庭，wait和await都会释放锁，最明显的不同是condition有多个等待队列，wait/notify只有一个等待队列

ReentrantLock可重入锁

 ReentrantLock基本原理

线程通过ReentrantLock.lock()加锁时：

判断count是否是0，若是则代表锁未被占用，开始抢锁，若抢到锁则CAS修改count值，并将owner设置为自身线程的引用；若否，则判断当前锁的占用者owner是不是自己，若是自己则count+1，若不是则进入等待队列waiters

手写ReentrantLock

package com.study.lock.locks1;

import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.LockSupport;

public class JamesReentrantLock implements Lock {

	// 锁的拥有者
	AtomicReference<Thread> owner = new AtomicReference<>();

	// 等待队列
	private LinkedBlockingQueue<Thread> waiters = new LinkedBlockingQueue<>();

	// 标记重入次数的count值
	AtomicInteger count = new AtomicInteger(0);

	@Override
	public boolean tryLock() {// 浅尝辄止
		// 判断count是否为0，若count！=0，说明锁被占用
		int ct = count.get();
		if (ct != 0) {
			// 判断锁是否被当前线程占用，若被当前线程占用，做重入操作，count+=1
			if (owner.get() == Thread.currentThread()) {
				count.set(ct + 1);
				return true;
			} else {
				// 若不是当前线程占用，互斥，抢锁失败，return false
				return false;
			}
		} else {
			// 若count=0， 说明锁未被占用，通过CAS（0，1） 来抢锁
			if (count.compareAndSet(ct, ct + 1)) {
				// 若抢锁成功，设置owner为当前线程的引用
				owner.set(Thread.currentThread());
				return true;
			} else {
				// CAS操作失败，说明情锁失败 返回false
				return false;
			}
		}
	}

	@Override
	public void lock() {// 不死不休
		// 尝试抢锁
		if (!tryLock()) {
			// 如果失败，进入等待队列
			waiters.offer(Thread.currentThread());

			// 自旋
			for (;;) {
				// 判断是否是队列头部，如果是
				Thread head = waiters.peek();
				if (head == Thread.currentThread()) {
					// 再次尝试抢锁
					if (!tryLock()) {
						// 若抢锁失败，挂起线程，继续等待
						LockSupport.park();
					} else {
						// 若成功，就出队列
						waiters.poll();
						return;
					}
				} else {
					// 如果不是，就挂起线程
					LockSupport.park();
				}
			}
		}
	}

	@Override
	public void unlock() {
		if (tryUnlock()) {
			Thread th = waiters.peek();
			if (th != null) {
				LockSupport.unpark(th);
			}
		}
	}

	public boolean tryUnlock() {
		// 判断，是否是当前线程占有锁，若不是，抛异常
		if (owner.get() != Thread.currentThread()) {
			throw new IllegalMonitorStateException();
		} else {
			// 如果是，就将count-1 若count变为0 ，则解锁成功
			int ct = count.get();
			int nextc = ct - 1;
			count.set(nextc);

			// 判断count值是否为0
			if (nextc == 0) {
				owner.compareAndSet(Thread.currentThread(), null);
				return true;
			} else {
				return false;
			}
		}
	}

	@Override
	public void lockInterruptibly() throws InterruptedException {

	}

	@Override
	public boolean tryLock(long time, TimeUnit unit) throws InterruptedException {
		int ct = count.get();
		if (ct == 0) {
			// 未被占用，CAS修改count
			long end = System.currentTimeMillis() + unit.toMillis(time);
			for (;;) {
				long now = System.currentTimeMillis();
				if (now > end) {
					return false;
				} else {
					if (count.compareAndSet(0, 1)) {
						owner.set(Thread.currentThread());
						return true;
					}
				}
			}
		} else {
			long end = System.currentTimeMillis() + unit.toMillis(time);
			for (;;) {
				long now = System.currentTimeMillis();
				if (now > end) {
					return false;
				} else {
					if (count.compareAndSet(0, 1)) {
						owner.set(Thread.currentThread());
						return true;
					}
				}
			}
		}
	}

	@Override
	public Condition newCondition() {
		return null;
	}
}

 Lock与Synchronized对比