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## 类图结构
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J.U.C 的锁组件中 类相对较少，从 JDK 相应的包中也能看出来，下图标记了其中最主要的几个接口和类，也是本文要分析的重点。
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下图 将这几个接口和类 以类图的方式展现出来，其中包含了它们所声明的主要方法。
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## Lock 组件
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Lock 组件的结构很简单，只有一个接口和一个实现类，源码如下。
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```java
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public interface Lock {
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/**
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* 获取锁
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*/
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void lock();
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/**
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* 获取锁，除非当前线程中断
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*/
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void lockInterruptibly() throws InterruptedException;
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/**
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* 只有当调用时 锁是空闲的情况下，才获取锁
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*/
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boolean tryLock();
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/**
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* 如果锁在给定的等待时间内空闲且当前线程未被中断，则获取该锁
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*/
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boolean tryLock(long time, TimeUnit unit) throws InterruptedException;
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/**
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* 释放锁
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*/
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void unlock();
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}
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public class ReentrantLock implements Lock, java.io.Serializable {
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/** 提供所有实现机制的同步器，ReentrantLock 的主要方法都依赖于该对象进行实现 */
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private final Sync sync;
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/**
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* ReentrantLock锁 的同步控制基础。它的两个子类分别实现了公平锁和非公平锁，如下。
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*/
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abstract static class Sync extends AbstractQueuedSynchronizer {
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private static final long serialVersionUID = -5179523762034025860L;
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abstract void lock();
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/**
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* Performs non-fair tryLock. tryAcquire is implemented in
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* subclasses, but both need nonfair try for trylock method.
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*/
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final boolean nonfairTryAcquire(int acquires) {
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final Thread current = Thread.currentThread();
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int c = getState();
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if (c == 0) {
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if (compareAndSetState(0, acquires)) {
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setExclusiveOwnerThread(current);
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return true;
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}
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}
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else if (current == getExclusiveOwnerThread()) {
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int nextc = c + acquires;
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if (nextc < 0) // overflow
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throw new Error("Maximum lock count exceeded");
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setState(nextc);
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return true;
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}
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return false;
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}
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protected final boolean tryRelease(int releases) {
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int c = getState() - releases;
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if (Thread.currentThread() != getExclusiveOwnerThread())
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throw new IllegalMonitorStateException();
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boolean free = false;
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if (c == 0) {
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free = true;
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setExclusiveOwnerThread(null);
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}
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setState(c);
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return free;
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}
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final boolean isLocked() {
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return getState() != 0;
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}
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}
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/**
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* 非公平锁，基于上面的 Sync类
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*/
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static final class NonfairSync extends Sync {
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private static final long serialVersionUID = 7316153563782823691L;
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final void lock() {
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if (compareAndSetState(0, 1))
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setExclusiveOwnerThread(Thread.currentThread());
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else
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acquire(1);
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}
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protected final boolean tryAcquire(int acquires) {
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return nonfairTryAcquire(acquires);
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}
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}
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/**
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* 公平锁，基于上面的 Sync类
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*/
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static final class FairSync extends Sync {
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private static final long serialVersionUID = -3000897897090466540L;
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final void lock() {
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acquire(1);
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}
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protected final boolean tryAcquire(int acquires) {
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final Thread current = Thread.currentThread();
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int c = getState();
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if (c == 0) {
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if (!hasQueuedPredecessors() &&
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compareAndSetState(0, acquires)) {
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setExclusiveOwnerThread(current);
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return true;
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}
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}
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else if (current == getExclusiveOwnerThread()) {
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int nextc = c + acquires;
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if (nextc < 0)
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throw new Error("Maximum lock count exceeded");
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setState(nextc);
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return true;
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}
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return false;
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}
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}
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/**
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* 无参初始化时，默认实例化 非公平锁
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*/
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public ReentrantLock() {
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sync = new NonfairSync();
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}
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/**
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* 可通过参数fair 控制实例化的是 公平锁还是非公平锁
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*/
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public ReentrantLock(boolean fair) {
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sync = fair ? new FairSync() : new NonfairSync();
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}
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public void lock() {
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sync.lock();
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}
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public boolean tryLock() {
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return sync.nonfairTryAcquire(1);
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}
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public boolean tryLock(long timeout, TimeUnit unit)
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throws InterruptedException {
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return sync.tryAcquireNanos(1, unit.toNanos(timeout));
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}
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public void unlock() {
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sync.release(1);
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}
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public boolean isLocked() {
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return sync.isLocked();
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}
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public final boolean isFair() {
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return sync instanceof FairSync;
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}
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}
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```
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## ReadWriteLock 组件
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ReadWriteLock 组件的结构也很简单，与上面的 Lock 组件 不同的是，它提供了 公平的读锁写锁，以及非公平的读锁写锁。
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```java
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public interface ReadWriteLock {
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/**
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* 获取一个 读锁
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*/
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Lock readLock();
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/**
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* 获取一个 写锁
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*/
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Lock writeLock();
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}
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public class ReentrantReadWriteLock implements ReadWriteLock, java.io.Serializable {
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/** 由内部类提供的读锁 */
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private final ReentrantReadWriteLock.ReadLock readerLock;
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/** 由内部类提供的写锁 */
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private final ReentrantReadWriteLock.WriteLock writerLock;
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/** 提供所有实现机制的同步器 */
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final Sync sync;
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/**
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* 默认创建 非公平的读锁写锁
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*/
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public ReentrantReadWriteLock() {
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this(false);
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}
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/**
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* 由参数 fair 指定读锁写锁是公平的还是非公平的
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*/
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public ReentrantReadWriteLock(boolean fair) {
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sync = fair ? new FairSync() : new NonfairSync();
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readerLock = new ReadLock(this);
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writerLock = new WriteLock(this);
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}
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/**
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* 获取写锁
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* 获取读锁
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*/
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public ReentrantReadWriteLock.WriteLock writeLock() { return writerLock; }
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public ReentrantReadWriteLock.ReadLock readLock() { return readerLock; }
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abstract static class Sync extends AbstractQueuedSynchronizer {
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protected final boolean tryRelease(int releases) {
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if (!isHeldExclusively())
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throw new IllegalMonitorStateException();
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int nextc = getState() - releases;
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boolean free = exclusiveCount(nextc) == 0;
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if (free)
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setExclusiveOwnerThread(null);
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setState(nextc);
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return free;
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}
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protected final boolean tryAcquire(int acquires) {
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/*
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* Walkthrough:
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* 1. If read count nonzero or write count nonzero
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* and owner is a different thread, fail.
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* 2. If count would saturate, fail. (This can only
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* happen if count is already nonzero.)
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* 3. Otherwise, this thread is eligible for lock if
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* it is either a reentrant acquire or
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* queue policy allows it. If so, update state
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* and set owner.
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*/
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Thread current = Thread.currentThread();
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int c = getState();
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int w = exclusiveCount(c);
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if (c != 0) {
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// (Note: if c != 0 and w == 0 then shared count != 0)
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if (w == 0 || current != getExclusiveOwnerThread())
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return false;
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if (w + exclusiveCount(acquires) > MAX_COUNT)
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throw new Error("Maximum lock count exceeded");
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// Reentrant acquire
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setState(c + acquires);
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return true;
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}
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if (writerShouldBlock() ||
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!compareAndSetState(c, c + acquires))
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return false;
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setExclusiveOwnerThread(current);
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return true;
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}
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protected final boolean tryReleaseShared(int unused) {
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Thread current = Thread.currentThread();
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if (firstReader == current) {
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// assert firstReaderHoldCount > 0;
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if (firstReaderHoldCount == 1)
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firstReader = null;
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else
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firstReaderHoldCount--;
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} else {
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HoldCounter rh = cachedHoldCounter;
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if (rh == null || rh.tid != getThreadId(current))
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rh = readHolds.get();
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int count = rh.count;
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if (count <= 1) {
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readHolds.remove();
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if (count <= 0)
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throw unmatchedUnlockException();
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}
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--rh.count;
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}
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for (;;) {
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int c = getState();
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int nextc = c - SHARED_UNIT;
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if (compareAndSetState(c, nextc))
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// Releasing the read lock has no effect on readers,
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// but it may allow waiting writers to proceed if
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// both read and write locks are now free.
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return nextc == 0;
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}
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}
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protected final int tryAcquireShared(int unused) {
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/*
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* Walkthrough:
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* 1. If write lock held by another thread, fail.
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* 2. Otherwise, this thread is eligible for
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* lock wrt state, so ask if it should block
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* because of queue policy. If not, try
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* to grant by CASing state and updating count.
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* Note that step does not check for reentrant
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* acquires, which is postponed to full version
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* to avoid having to check hold count in
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* the more typical non-reentrant case.
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* 3. If step 2 fails either because thread
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* apparently not eligible or CAS fails or count
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* saturated, chain to version with full retry loop.
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*/
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Thread current = Thread.currentThread();
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int c = getState();
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if (exclusiveCount(c) != 0 &&
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getExclusiveOwnerThread() != current)
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return -1;
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int r = sharedCount(c);
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if (!readerShouldBlock() &&
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r < MAX_COUNT &&
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compareAndSetState(c, c + SHARED_UNIT)) {
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if (r == 0) {
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firstReader = current;
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firstReaderHoldCount = 1;
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} else if (firstReader == current) {
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firstReaderHoldCount++;
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} else {
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HoldCounter rh = cachedHoldCounter;
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if (rh == null || rh.tid != getThreadId(current))
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cachedHoldCounter = rh = readHolds.get();
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else if (rh.count == 0)
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readHolds.set(rh);
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rh.count++;
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}
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return 1;
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}
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return fullTryAcquireShared(current);
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}
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/**
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* Performs tryLock for write, enabling barging in both modes.
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* This is identical in effect to tryAcquire except for lack
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* of calls to writerShouldBlock.
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*/
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final boolean tryWriteLock() {
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Thread current = Thread.currentThread();
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int c = getState();
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if (c != 0) {
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int w = exclusiveCount(c);
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if (w == 0 || current != getExclusiveOwnerThread())
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return false;
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if (w == MAX_COUNT)
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throw new Error("Maximum lock count exceeded");
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}
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if (!compareAndSetState(c, c + 1))
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return false;
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setExclusiveOwnerThread(current);
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return true;
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}
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/**
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* Performs tryLock for read, enabling barging in both modes.
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* This is identical in effect to tryAcquireShared except for
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* lack of calls to readerShouldBlock.
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*/
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final boolean tryReadLock() {
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Thread current = Thread.currentThread();
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for (;;) {
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int c = getState();
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if (exclusiveCount(c) != 0 &&
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getExclusiveOwnerThread() != current)
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return false;
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int r = sharedCount(c);
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if (r == MAX_COUNT)
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throw new Error("Maximum lock count exceeded");
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if (compareAndSetState(c, c + SHARED_UNIT)) {
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if (r == 0) {
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firstReader = current;
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firstReaderHoldCount = 1;
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} else if (firstReader == current) {
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firstReaderHoldCount++;
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} else {
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HoldCounter rh = cachedHoldCounter;
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if (rh == null || rh.tid != getThreadId(current))
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cachedHoldCounter = rh = readHolds.get();
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else if (rh.count == 0)
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readHolds.set(rh);
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rh.count++;
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}
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return true;
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}
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}
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}
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final boolean isWriteLocked() {
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|
return exclusiveCount(getState()) != 0;
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}
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}
|
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|
|
/**
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|
* 非公平锁
|
|
|
*/
|
|
|
static final class NonfairSync extends Sync {
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|
final boolean writerShouldBlock() {
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|
return false; // writers can always barge
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|
|
}
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|
|
final boolean readerShouldBlock() {
|
|
|
/* As a heuristic to avoid indefinite writer starvation,
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|
|
* block if the thread that momentarily appears to be head
|
|
|
* of queue, if one exists, is a waiting writer. This is
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|
|
* only a probabilistic effect since a new reader will not
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|
|
* block if there is a waiting writer behind other enabled
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|
|
* readers that have not yet drained from the queue.
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|
|
*/
|
|
|
return apparentlyFirstQueuedIsExclusive();
|
|
|
}
|
|
|
}
|
|
|
|
|
|
/**
|
|
|
* 公平锁
|
|
|
*/
|
|
|
static final class FairSync extends Sync {
|
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|
|
final boolean writerShouldBlock() {
|
|
|
return hasQueuedPredecessors();
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|
|
}
|
|
|
final boolean readerShouldBlock() {
|
|
|
return hasQueuedPredecessors();
|
|
|
}
|
|
|
}
|
|
|
|
|
|
/**
|
|
|
* 读锁
|
|
|
*/
|
|
|
public static class ReadLock implements Lock, java.io.Serializable {
|
|
|
|
|
|
private final Sync sync;
|
|
|
|
|
|
protected ReadLock(ReentrantReadWriteLock lock) {
|
|
|
sync = lock.sync;
|
|
|
}
|
|
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|
|
|
public void lock() {
|
|
|
sync.acquireShared(1);
|
|
|
}
|
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|
|
|
|
public void lockInterruptibly() throws InterruptedException {
|
|
|
sync.acquireSharedInterruptibly(1);
|
|
|
}
|
|
|
|
|
|
public boolean tryLock() {
|
|
|
return sync.tryReadLock();
|
|
|
}
|
|
|
|
|
|
public boolean tryLock(long timeout, TimeUnit unit)
|
|
|
throws InterruptedException {
|
|
|
return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
|
|
|
}
|
|
|
|
|
|
public void unlock() {
|
|
|
sync.releaseShared(1);
|
|
|
}
|
|
|
}
|
|
|
|
|
|
/**
|
|
|
* 写锁
|
|
|
*/
|
|
|
public static class WriteLock implements Lock, java.io.Serializable {
|
|
|
|
|
|
private final Sync sync;
|
|
|
|
|
|
protected WriteLock(ReentrantReadWriteLock lock) {
|
|
|
sync = lock.sync;
|
|
|
}
|
|
|
|
|
|
public void lock() {
|
|
|
sync.acquire(1);
|
|
|
}
|
|
|
|
|
|
public void lockInterruptibly() throws InterruptedException {
|
|
|
sync.acquireInterruptibly(1);
|
|
|
}
|
|
|
|
|
|
public boolean tryLock( ) {
|
|
|
return sync.tryWriteLock();
|
|
|
}
|
|
|
|
|
|
public boolean tryLock(long timeout, TimeUnit unit)
|
|
|
throws InterruptedException {
|
|
|
return sync.tryAcquireNanos(1, unit.toNanos(timeout));
|
|
|
}
|
|
|
|
|
|
public void unlock() {
|
|
|
sync.release(1);
|
|
|
}
|
|
|
}
|
|
|
|
|
|
public final boolean isFair() {
|
|
|
return sync instanceof FairSync;
|
|
|
}
|
|
|
|
|
|
public boolean isWriteLocked() {
|
|
|
return sync.isWriteLocked();
|
|
|
}
|
|
|
}
|
|
|
```
|
|
|
|
|
|
## AbstractQueuedSynchronizer
|
|
|
|
|
|
最后看一下抽象类 AbstractQueuedSynchronizer，在同步组件的实现中，AQS 是核心部分，同步组件的实现者通过使用 AQS 提供的模板方法实现同步组件语义，AQS 则实现了对同步状态的管理，以及对阻塞线程进行排队，等待通知等等一些底层的实现处理。AQS 的核心包括：同步队列，独占式锁的获取和释放，共享锁的获取和释放以及可中断锁，超时等待锁获取这些特性的实现，而这些实际上则是 AQS 提供出来的模板方法。源码如下。
|
|
|
|
|
|
```java
|
|
|
public abstract class AbstractQueuedSynchronizer extends AbstractOwnableSynchronizer
|
|
|
implements java.io.Serializable {
|
|
|
|
|
|
/**
|
|
|
* 当共享资源被某个线程占有，其他请求该资源的线程将会阻塞，从而进入同步队列。
|
|
|
* 就数据结构而言，队列的实现方式无外乎两者一是通过数组的形式，另外一种则是链表的形式。
|
|
|
* AQS中的同步队列则是通过链式方式进行实现，下面的内部类Node便是其实现的载体
|
|
|
*/
|
|
|
static final class Node {
|
|
|
|
|
|
/** Marker to indicate a node is waiting in shared mode */
|
|
|
static final Node SHARED = new Node();
|
|
|
/** Marker to indicate a node is waiting in exclusive mode */
|
|
|
static final Node EXCLUSIVE = null;
|
|
|
|
|
|
// 节点从同步队列中取消
|
|
|
static final int CANCELLED = 1;
|
|
|
// 后继节点的线程处于等待状态，如果当前节点释放同步状态会通知后继节点，
|
|
|
// 使得后继节点的线程能够运行；
|
|
|
static final int SIGNAL = -1;
|
|
|
// 当前节点进入等待队列中
|
|
|
static final int CONDITION = -2;
|
|
|
// 表示下一次共享式同步状态获取将会无条件传播下去
|
|
|
static final int PROPAGATE = -3;
|
|
|
|
|
|
// 节点状态
|
|
|
volatile int waitStatus;
|
|
|
|
|
|
// 当前节点/线程的前驱节点
|
|
|
volatile Node prev;
|
|
|
|
|
|
// 当前节点/线程的后驱节点
|
|
|
volatile Node next;
|
|
|
|
|
|
// 加入同步队列的线程引用
|
|
|
volatile Thread thread;
|
|
|
|
|
|
// 等待队列中的下一个节点
|
|
|
Node nextWaiter;
|
|
|
|
|
|
final boolean isShared() {
|
|
|
return nextWaiter == SHARED;
|
|
|
}
|
|
|
|
|
|
final Node predecessor() throws NullPointerException {
|
|
|
Node p = prev;
|
|
|
if (p == null)
|
|
|
throw new NullPointerException();
|
|
|
else
|
|
|
return p;
|
|
|
}
|
|
|
|
|
|
Node() { // Used to establish initial head or SHARED marker
|
|
|
}
|
|
|
|
|
|
Node(Thread thread, Node mode) { // Used by addWaiter
|
|
|
this.nextWaiter = mode;
|
|
|
this.thread = thread;
|
|
|
}
|
|
|
|
|
|
Node(Thread thread, int waitStatus) { // Used by Condition
|
|
|
this.waitStatus = waitStatus;
|
|
|
this.thread = thread;
|
|
|
}
|
|
|
}
|
|
|
|
|
|
/**
|
|
|
* AQS实际上通过头尾指针来管理同步队列，同时实现包括获取锁失败的线程进行入队，
|
|
|
* 释放锁时对同步队列中的线程进行通知等核心方法。
|
|
|
*/
|
|
|
private transient volatile Node head;
|
|
|
private transient volatile Node tail;
|
|
|
|
|
|
/**
|
|
|
* 获取独占式锁
|
|
|
*/
|
|
|
public final void acquire(int arg) {
|
|
|
if (!tryAcquire(arg) &&
|
|
|
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
|
|
|
selfInterrupt();
|
|
|
}
|
|
|
|
|
|
/**
|
|
|
* 释放独占式锁
|
|
|
*/
|
|
|
public final boolean release(int arg) {
|
|
|
if (tryRelease(arg)) {
|
|
|
Node h = head;
|
|
|
if (h != null && h.waitStatus != 0)
|
|
|
unparkSuccessor(h);
|
|
|
return true;
|
|
|
}
|
|
|
return false;
|
|
|
}
|
|
|
|
|
|
/**
|
|
|
* 获取可中断式锁
|
|
|
*/
|
|
|
public final void acquireInterruptibly(int arg)
|
|
|
throws InterruptedException {
|
|
|
if (Thread.interrupted())
|
|
|
throw new InterruptedException();
|
|
|
if (!tryAcquire(arg))
|
|
|
doAcquireInterruptibly(arg);
|
|
|
}
|
|
|
|
|
|
/**
|
|
|
* 获取共享锁
|
|
|
*/
|
|
|
public final void acquireShared(int arg) {
|
|
|
if (tryAcquireShared(arg) < 0)
|
|
|
doAcquireShared(arg);
|
|
|
}
|
|
|
|
|
|
/**
|
|
|
* 释放共享锁
|
|
|
*/
|
|
|
public final boolean releaseShared(int arg) {
|
|
|
if (tryReleaseShared(arg)) {
|
|
|
doReleaseShared();
|
|
|
return true;
|
|
|
}
|
|
|
return false;
|
|
|
}
|
|
|
}
|
|
|
```
|
