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lock锁, 基于队列同步器AQS, 实现公平锁、非公平锁
队列同步器AQS可以阅读我这篇文章#xff1a; 点击传送
实现了Lock接口:
public class ReentrantLock implements Lock// 加锁 获取不到锁一直等待
void lock();
// 加锁 获取不到锁一直等待 等待过程可以被中断…1. 概述
lock锁, 基于队列同步器AQS, 实现公平锁、非公平锁
队列同步器AQS可以阅读我这篇文章 点击传送
实现了Lock接口:
public class ReentrantLock implements Lock// 加锁 获取不到锁一直等待
void lock();
// 加锁 获取不到锁一直等待 等待过程可以被中断
void lockInterruptibly() throws InterruptedException;
// 尝试获取锁 获取成功返回 true
boolean tryLock();
// 尝试获取锁 等待指定的时间 获取失败返回false
boolean tryLock(long time, TimeUnit unit) throws InterruptedException;
// 解锁
void unlock();2. 源码阅读
2.1 属性
// 队列同步器继承了AbstractQueuedSynchronizer
private final Sync sync;
abstract static class Sync extends AbstractQueuedSynchronizer2.2 构造函数
构造函数: // 默认非公平锁public ReentrantLock() {sync new NonfairSync();}// true:公平锁 false:非公平锁public ReentrantLock(boolean fair) {sync fair ? new FairSync() : new NonfairSync();}2.3 Sync队列同步器
2.3.1 非公平尝试获取同步状态 nonfairTryAcquire
state为0 ● 直接CAS 尝试获取同步状态, compareAndSetState(0, acquires) ● 成功设置当前线程占有锁 setExclusiveOwnerThread(current); 当前线程已经获取锁: ● state下一个值 nextc c acquires; ● setState(nextc) 设置值 final boolean nonfairTryAcquire(int acquires) {final Thread current Thread.currentThread();int c getState();if (c 0) {// 直接尝试获取同步状态if (compareAndSetState(0, acquires)) {// 设置当前线程占有锁setExclusiveOwnerThread(current);return true;}}else if (current getExclusiveOwnerThread()) {// 当前线程持有锁 可重入锁 int nextc c acquires;if (nextc 0) // overflowthrow new Error(Maximum lock count exceeded);setState(nextc);return true;}return false;}当前线程是否持有锁
protected final boolean isHeldExclusively() {return getExclusiveOwnerThread() Thread.currentThread();
}获取持有锁的线程
final Thread getOwner() {return getState() 0 ? null : getExclusiveOwnerThread();
}2.3.2 tryRelease
尝试释放同步状态: protected final boolean tryRelease(int releases) {int c getState() - releases;// 持有锁的才可以释放同步状态if (Thread.currentThread() ! getExclusiveOwnerThread())throw new IllegalMonitorStateException();boolean free false;// 同步状态为0 即要释放锁了if (c 0) {free true;setExclusiveOwnerThread(null);}setState(c);return free;}2.3.3 NonfairSync、FairSync
NonfairSync 非公平的 Sync , FairSync 公平的 Sync 公平和非公平锁是在 ReentrantLock 重写 AQS tryAcquire 中实现的 ● 非公平锁 lock 的时候直接尝试一下获取锁 成功就成功了 失败了才进队列排队获取 ● 公平锁tryAcquire AQS同步队列中没有数据, 即没人排队才可以获取锁 static final class NonfairSync extends Sync {private static final long serialVersionUID 7316153563782823691L;final void lock() {// 非公平锁 lock 的时候直接尝试一下获取锁 成功就成功了 失败了才进队列排队获取if (compareAndSetState(0, 1))setExclusiveOwnerThread(Thread.currentThread());elseacquire(1);}// 非公平尝试获取同步状态 protected final boolean tryAcquire(int acquires) {return nonfairTryAcquire(acquires);}}static final class FairSync extends Sync {private static final long serialVersionUID -3000897897090466540L;final void lock() {acquire(1);}// 尝试获取同步状态protected final boolean tryAcquire(int acquires) {final Thread current Thread.currentThread();int c getState();if (c 0) {// !hasQueuedPredecessors() AQS 同步队列中没有数据, 即没人排队才可以获取锁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;}}2.4 ReentrantLock 调用 Sync public void lock() {sync.lock();}public void lockInterruptibly() throws InterruptedException {sync.acquireInterruptibly(1);}public boolean tryLock(long timeout, TimeUnit unit)throws InterruptedException {return sync.tryAcquireNanos(1, unit.toNanos(timeout));}public void unlock() {sync.release(1);}3. 测试代码
import java.util.concurrent.locks.ReentrantLock;public class ReentrantLockStudy {static int COUNT 0;public static void main(String[] args) throws InterruptedException {ReentrantLock lock new ReentrantLock();// 加锁lock.lock();System.out.println(1);// 可重入lock.lock();System.out.println(2);// 解锁lock.unlock();System.out.println(3);lock.unlock();System.out.println(4);new Thread(() - {for (int i0; i10000; i) {lock.lock();COUNT;lock.unlock();}System.out.println(end);}).start();new Thread(() - {for (int i0; i10000; i) {lock.lock();COUNT;lock.unlock();}System.out.println(end);}).start();;while (COUNT ! 20000);System.out.println(COUNT);}
}
