J.U.C剖析与解读2（AQS的由来）
2019-12-13 06:36:05来源：博客园 阅读 ()
J.U.C剖析与解读2（AQS的由来）
J.U.C剖析与解读2（AQS的由来）
前言
前面已经通过实现自定义ReentrantLock与自定义ReentrantReadWriteLock，展示了JDK是如何实现独占锁与共享锁的。
那么实际JDK源码中的ReentrantLock与ReentrantReadWritreLock是如何实现的呢？我们现有的自定义代码是否可以更进一步呢？
答案是肯定的。注意看我之前两个Lock的朋友，应该注意到了。自定义ReentrantReadWriteLock的独占锁部分，其实和自定义ReentrantLock是几乎一样的。
也就是说，不同Lock其实现是差不多的。那么是否可以提取公共的部分，是否可以写得更加优雅一些。
那么这篇博客，就是通过提取公共代码，引入模板方法设计模式，并利用Java的一些特性，写出一个自定义的AQS。
当然，最后也会剖析源码中AQS实现与我们自定义AQS的差别所在，并解读源码AQS中一些高级应用，如AQS通过一个state实现读写锁的持有数量（居然通过一个int值的CAS操作，解决了自定义读写锁持有数量的独占操作）。
如果看过源码的朋友，会发现源码中的ReentrantLock会自定义一个Sync，该Sync会继承一个AbstratQueueSynchronizer（简称AQS）。然后源码中的ReentrantLock的tryLock等方法，则是调用Sync的对应子类（FairSync或NonFairSync，也就是是否为公平锁）来实现对应功能。并且，只有tryAcquire与lock两个方法是由ReentrantLock实现的，其它方法是由AQS提供的。lock是由FairSync与NonFairSync分别实现的。而tryAcquire是由FairSync与NonFairSync父类的Sync实现，NonFairSync的tryLock直接调用父类Sync的nonfairTryAcquire方法。
而ReentrantReadWriteLock则是增加了ReadLock与WriteLock，其实现，则是调用Sync的不同方法而已。
有的小伙伴，会觉得这样的关系很复杂，明明一个锁就比较复杂了，还搞得这么抽象。提取一个AQS就够抽象的了，每个锁还整了一个Sync，FairSync，NonFairSync内部类，视情况，还要弄个ReadLock，WriteLock这些内部类。这样做的目的其实是为了封装代码，提高代码复用性。当然，实际源码看多了，反而会觉得这样的代码，看得挺舒服的。比较符合设计理念（想想，你接收的项目中，一个类上千行代码，完全不敢修改）。
关于读源码，简单说一下我的感受。最核心的就是坚持，最重要的是全局观，最需要的是积累。
我陆陆续续阅读源码（不只是Java），也有差不多两年的经验吧。从最早的Windows内核源码，到后面的前端框架源码，到今年的Java源码阅读。最早的Windows内核源码，那真的是无知者无畏啊，简直是一段极其痛苦的经历。那时候一天可能就二十页样子，还得看运气。但是那段时间给我带来了很多，包括什么系统内存管理，内存的用户态与内核态，以及系统上下文等积累，为我后面的提升带来了很多。而后面的前端源码的阅读，也让我开始接触源码的一些思路。最后到今年的Java源码，有了去年golang一些外文博客的翻译（涉及语言设计部分）铺垫，才渐渐有了一些阅读源码的感觉（自我感觉有点上路了）。所以，最核心的是坚持。
至于全局观嘛，就是一方面很多时候源码太多，常常迷路，我们需要把握主线，以及自己的目的。如果可以有一个XMIND，或者比较不错的博客等作为指引就更好了。比如这次AQS拆分，我就是从网易云的大佬James学到的。虽然之前就有了JUC学习的积累，但是大佬的AQS拆分，确实令我对AQS有了更为深入的理解。另一方面就是需要把握自己应该研究的深度（在自己能力层级再深入一些即可），而不是抓着源码的每个字不放。我今年年初的时候，就想研究IOC源码，根据一位大佬的文章，连续啃了两三个星期。但后面陆陆续续就忘了。虽然这段经历对我依旧有着一定的积累价值（对我最近研究SpringApplication的run过程有着不错的价值），但是超出自己能力太多地撸源码，性价比就太低了。
最后就是积累，我非常看重积累。自从高三后，我就对积累这个词有了更深入的理解。很多时候，我们阅读一些书籍，研究一些原理，虽然后来感觉忘记了，但是积累还是在的。就像我学习编程时，就经常感受到大学时期的计算机网络，计算机原理，分布式课题等经历给我带来的积累。而现在很多人都过于看重即时价值（就是我立马学了，立马就要有效果），而我相信技术的攀登，是离不开经年累月的积累的。
如果大家对阅读源码，感兴趣的话，可以告诉我。可以考虑写一篇文章，来简单谈谈源码阅读这件事儿。
一，简易JUC（版本一）：
这里就是将之前实现的简易版ReentrantLock与ReentrantReadWriteLock展现出来，就当是简单回顾一下。
1.JarryReentrantLock：
package tech.jarry.learning.netease.locks2;
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;
/**
* @Description： 仿ReentrantLock，实现其基本功能及特性
* @Author: jarry
*/
public class JarryReentrantLock {
private AtomicInteger count = new AtomicInteger(0);
private AtomicReference<Thread> owner = new AtomicReference<>();
private LinkedBlockingQueue<Thread> waiters = new LinkedBlockingQueue<>();
public void lock() {
int arg = 1;
if (!tryLock(arg)){
waiters.offer(Thread.currentThread());
while (true){
Thread head = waiters.peek();
if (head == Thread.currentThread()){
if (!tryLock(arg)){
LockSupport.park();
} else {
waiters.poll();
return;
}
} else {
LockSupport.park();
}
}
}
}
public void unlock() {
int arg = 1;
if (tryUnlock(arg)){
Thread head = waiters.peek();
if (head != null){
LockSupport.unpark(head);
}
}
}
public boolean tryLock(int acquires) {
int countValue = count.get();
if (countValue != 0){
if (Thread.currentThread() == owner.get()){
count.set(countValue+acquires);
return true;
}else{
return false;
}
}else {
if (count.compareAndSet(countValue,countValue+acquires)){
owner.set(Thread.currentThread());
return true;
} else {
return false;
}
}
}
private boolean tryUnlock(int releases) {
if (Thread.currentThread() != owner.get()){
throw new IllegalMonitorStateException();
} else {
int countValue = count.get();
int countNextValue = countValue - releases;
count.compareAndSet(countValue,countNextValue);
if (countNextValue == 0){
owner.compareAndSet(Thread.currentThread(),null);
return true;
} else {
return false;
}
}
}
public void lockInterruptibly() throws InterruptedException {
}
public boolean tryLock(long time, TimeUnit unit) throws InterruptedException {
return false;
}
public Condition newCondition() {
return null;
}
}
2.JarryReadWriteLock：
package tech.jarry.learning.netease.locks2;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.locks.LockSupport;
/**
* @Description：
* @Author: jarry
*/
public class JarryReadWriteLock {
volatile AtomicInteger readCount = new AtomicInteger(0);
AtomicInteger writeCount = new AtomicInteger(0);
AtomicReference<Thread> owner = new AtomicReference<>();
public volatile LinkedBlockingQueue<WaitNode> waiters = new LinkedBlockingQueue<>();
class WaitNode{
Thread thread = null;
// 表示希望争取的锁的类型。0表示写锁（独占锁），1表示读锁（共享锁）
int type = 0;
int arg = 0;
public WaitNode(Thread thread, int type, int arg) {
this.type = type;
this.thread = thread;
this.arg = arg;
}
}
/**
* 获取独占锁（针对独占锁）
*/
public void lock(){
int arg = 1;
if (!tryLock(arg)){
WaitNode waitNode = new WaitNode(Thread.currentThread(), 0, arg);
waiters.offer(waitNode);
while (true){
WaitNode headNote = waiters.peek();
if (headNote !=null && headNote.thread == Thread.currentThread()){
if (!tryLock(headNote.arg)){
LockSupport.park();
} else {
waiters.poll();
return;
}
}else {
LockSupport.park();
}
}
}
}
/**
* 解锁（针对独占锁）
*/
public void unlock(){
int arg = 1;
if (tryUnlock(arg)){
WaitNode head = waiters.peek();
if (head == null){
return;
}
LockSupport.unpark(head.thread);
}
}
/**
* 尝试获取独占锁（针对独占锁）
* @param acquires 用于加锁次数。一般传入waitNode.arg（本代码中就是1。为什么不用一个常量1，就不知道了？）
* @return
*/
public boolean tryLock(int acquires){
if (readCount.get() == 0){
int writeCountValue = writeCount.get();
if (writeCountValue == 0){
if (writeCount.compareAndSet(writeCountValue,writeCountValue+acquires)){
owner.set(Thread.currentThread());
return true;
}
} else {
if (Thread.currentThread() == owner.get()){
writeCount.set(writeCountValue+acquires);
return true;
}
}
}
return false;
}
/**
* 尝试解锁（针对独占锁）
* @param releases 用于设定解锁次数。一般传入waitNode.arg
* @return
*/
public boolean tryUnlock(int releases){
if (owner.get() != Thread.currentThread()){
throw new IllegalMonitorStateException();
}
int writeCountValue = writeCount.get();
writeCount.set(writeCountValue-releases);
if (writeCount.get() == 0){
owner.compareAndSet(Thread.currentThread(),null);
return true;
} else {
return false;
}
}
/**
* 获取共享锁（针对共享锁）
*/
public void lockShared(){
int arg = 1;
if (!tryLockShared(arg)){
WaitNode waitNode = new WaitNode(Thread.currentThread(),1,arg);
waiters.offer(waitNode);
while (true){
WaitNode head = waiters.peek();
if (head != null && head.thread == Thread.currentThread()){
if (tryLockShared(head.arg)){
waiters.poll();
WaitNode newHead = waiters.peek();
if (newHead != null && newHead.type == 1){
LockSupport.unpark(newHead.thread);
}
return;
} else {
LockSupport.park();
}
} else {
LockSupport.park();
}
}
}
}
/**
* 解锁（针对共享锁）
*/
public boolean unLockShared(){
int arg = 1;
if (tryUnLockShared(arg)){
WaitNode head = waiters.peek();
if (head != null){
LockSupport.unpark(head.thread);
}
return true;
}
return false;
}
/**
* 尝试获取共享锁（针对共享锁）
* @param acquires
* @return
*/
public boolean tryLockShared(int acquires){
while (true){
if (writeCount.get() == 0 || owner.get() == Thread.currentThread()){
int readCountValue = readCount.get();
if (readCount.compareAndSet(readCountValue, readCountValue+acquires)){
return true;
}
}
return false;
}
}
/**
* 尝试解锁（针对共享锁）
* @param releases
* @return
*/
public boolean tryUnLockShared(int releases){
while (true){
int readCountValue = readCount.get();
int readCountNext = readCountValue - releases;
if (readCount.compareAndSet(readCountValue,readCountNext)){
return readCountNext == 0;
}
}
}
}
二，简易JUC（版本二）：
很明显，上面的代码中，JarryReentrantLock的tryLock等方法与JarryReadWriteLock中共享锁的tryLock等方法是类似的（本来就是从JarryReentrantLock复制过来的嘛）。那么，这里就需要引入模板方法（详见笔记《设计模式》-模板方法）。通过一个commonMask类，来提取公共方法。
1.CommonMask：
package tech.jarry.learning.netease.locks3;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.locks.LockSupport;
/**
* @Description：
* @Author: jarry
*/
public class CommonMask {
volatile AtomicInteger readCount = new AtomicInteger(0);
AtomicInteger writeCount = new AtomicInteger(0);
AtomicReference<Thread> owner = new AtomicReference<>();
public volatile LinkedBlockingQueue<WaitNode> waiters = new LinkedBlockingQueue<>();
class WaitNode{
Thread thread = null;
// 表示希望争取的锁的类型。0表示写锁（独占锁），1表示读锁（共享锁）
int type = 0;
int arg = 0;
public WaitNode(Thread thread, int type, int arg) {
this.type = type;
this.thread = thread;
this.arg = arg;
}
}
/**
* 获取独占锁（针对独占锁）
*/
public void lock(){
int arg = 1;
if (!tryLock(arg)){
WaitNode waitNode = new WaitNode(Thread.currentThread(), 0, arg);
waiters.offer(waitNode);
while (true){
WaitNode headNote = waiters.peek();
if (headNote !=null && headNote.thread == Thread.currentThread()){
if (!tryLock(headNote.arg)){
LockSupport.park();
} else {
waiters.poll();
return;
}
}else {
LockSupport.park();
}
}
}
}
/**
* 解锁（针对独占锁）
*/
public void unlock(){
int arg = 1;
if (tryUnlock(arg)){
WaitNode head = waiters.peek();
if (head == null){
return;
}
LockSupport.unpark(head.thread);
}
}
/**
* 尝试获取独占锁（针对独占锁）
* @param acquires 用于加锁次数。一般传入waitNode.arg（本代码中就是1。为什么不用一个常量1，就不知道了？）
* @return
*/
public boolean tryLock(int acquires){
if (readCount.get() == 0){
int writeCountValue = writeCount.get();
if (writeCountValue == 0){
if (writeCount.compareAndSet(writeCountValue,writeCountValue+acquires)){
owner.set(Thread.currentThread());
return true;
}
} else {
if (Thread.currentThread() == owner.get()){
writeCount.set(writeCountValue+acquires);
return true;
}
}
}
return false;
}
/**
* 尝试解锁（针对独占锁）
* @param releases 用于设定解锁次数。一般传入waitNode.arg
* @return
*/
public boolean tryUnlock(int releases){
if (owner.get() != Thread.currentThread()){
throw new IllegalMonitorStateException();
}
int writeCountValue = writeCount.get();
writeCount.set(writeCountValue-releases);
if (writeCount.get() == 0){
owner.compareAndSet(Thread.currentThread(),null);
return true;
} else {
return false;
}
}
/**
* 获取共享锁（针对共享锁）
*/
public void lockShared(){
int arg = 1;
if (!tryLockShared(arg)){
WaitNode waitNode = new WaitNode(Thread.currentThread(),1,arg);
waiters.offer(waitNode);
while (true){
WaitNode head = waiters.peek();
if (head != null && head.thread == Thread.currentThread()){
if (tryLockShared(head.arg)){
waiters.poll();
WaitNode newHead = waiters.peek();
if (newHead != null && newHead.type == 1){
LockSupport.unpark(newHead.thread);
}
return;
} else {
LockSupport.park();
}
} else {
LockSupport.park();
}
}
}
}
/**
* 解锁（针对共享锁）
*/
public boolean unLockShared(){
int arg = 1;
if (tryUnLockShared(arg)){
WaitNode head = waiters.peek();
if (head != null){
LockSupport.unpark(head.thread);
}
return true;
}
return false;
}
/**
* 尝试获取共享锁（针对共享锁）
* @param acquires
* @return
*/
public boolean tryLockShared(int acquires){
while (true){
if (writeCount.get() == 0 || owner.get() == Thread.currentThread()){
int readCountValue = readCount.get();
if (readCount.compareAndSet(readCountValue, readCountValue+acquires)){
return true;
}
}
return false;
}
}
/**
* 尝试解锁（针对共享锁）
* @param releases
* @return
*/
public boolean tryUnLockShared(int releases){
while (true){
int readCountValue = readCount.get();
int readCountNext = readCountValue - releases;
if (readCount.compareAndSet(readCountValue,readCountNext)){
return readCountNext == 0;
}
}
}
}
2.JarryReentrantLock：
package tech.jarry.learning.netease.locks3;
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;
/**
* @Description： 仿ReentrantLock，实现其基本功能及特性
* @Author: jarry
*/
public class JarryReentrantLock {
private CommonMask commonMask = new CommonMask();
public void lock() {
commonMask.lock();
}
public void unlock() {
commonMask.unlock();
}
public boolean tryLock(int acquire) {
return commonMask.tryLock(acquire);
}
private boolean tryUnlock(int release) {
return commonMask.tryUnlock(release);
}
}
3.JarryReadWriteLock：
package tech.jarry.learning.netease.locks3;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.locks.LockSupport;
/**
* @Description：
* @Author: jarry
*/
public class JarryReadWriteLock {
private CommonMask commonMask = new CommonMask();
/**
* 获取独占锁（针对独占锁）
*/
public void lock(){
commonMask.lock();
}
/**
* 解锁（针对独占锁）
*/
public void unlock(){
commonMask.unlock();
}
/**
* 尝试获取独占锁（针对独占锁）
* @param acquires 用于加锁次数。一般传入waitNode.arg（本代码中就是1。为什么不用一个常量1，就不知道了？）
* @return
*/
public boolean tryLock(int acquires){
return commonMask.tryLock(acquires);
}
/**
* 尝试解锁（针对独占锁）
* @param releases 用于设定解锁次数。一般传入waitNode.arg
* @return
*/
public boolean tryUnlock(int releases){
return commonMask.tryUnlock(releases);
}
/**
* 获取共享锁（针对共享锁）
*/
public void lockShared(){
commonMask.lockShared();
}
/**
* 解锁（针对共享锁）
*/
public boolean unLockShared(){
return commonMask.unLockShared();
}
/**
* 尝试获取共享锁（针对共享锁）
* @param acquires
* @return
*/
public boolean tryLockShared(int acquires){
return tryLockShared(acquires);
}
/**
* 尝试解锁（针对共享锁）
* @param releases
* @return
*/
public boolean tryUnLockShared(int releases){
return commonMask.tryUnLockShared(releases);
}
}
到了这里，大家就可以明显看出，总体代码量的下降（这还只是两个Lock）。但是问题也出来了，那就是这样将所有方法都放在父类CommonMask，子类进行调用，是不是显得过于死板（说得直接点，就是这种操作，完全就是将代码往父类一抛而已）。这说明，之前代码公共提取做得并不好。
重新整理思路，JarryReentrantLock与JarryReadWriteLock的共同之处到底在哪里。细想一下，发现这两个方法的lock，unlock等操作是一致的，只是实际的运行逻辑方法tryLock，tryUnlock，tryLockShared，tryUnLockShared四个方法（在框架源码中，常常用doxxx方法，表示实际运行逻辑的方法）。所以CommonMask应该实现的是这四个方法之外的方法，而这四个方法交由子类，来根据具体需要来实现（CommonMask中，这四个方法直接抛出对应异常）。
最后，ReentrantLock是有公平锁，非公平锁之分的。而通过上面的调整，现在的JarryReentrantLock可以实现自己对应方法，来展现特性（公平锁/非公平锁的选择）了。
三，简易JUC（版本三）：
1.CommonMask：
package tech.jarry.learning.netease.locks4;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.locks.LockSupport;
/**
* @Description：
* @Author: jarry
*/
public class CommonMask {
volatile AtomicInteger readCount = new AtomicInteger(0);
AtomicInteger writeCount = new AtomicInteger(0);
AtomicReference<Thread> owner = new AtomicReference<>();
public volatile LinkedBlockingQueue<WaitNode> waiters = new LinkedBlockingQueue<>();
class WaitNode{
Thread thread = null;
// 表示希望争取的锁的类型。0表示写锁（独占锁），1表示读锁（共享锁）
int type = 0;
int arg = 0;
public WaitNode(Thread thread, int type, int arg) {
this.type = type;
this.thread = thread;
this.arg = arg;
}
}
/**
* 获取独占锁（针对独占锁）
*/
public void lock(){
int arg = 1;
if (!tryLock(arg)){
WaitNode waitNode = new WaitNode(Thread.currentThread(), 0, arg);
waiters.offer(waitNode);
while (true){
WaitNode headNote = waiters.peek();
if (headNote !=null && headNote.thread == Thread.currentThread()){
if (!tryLock(headNote.arg)){
LockSupport.park();
} else {
waiters.poll();
return;
}
}else {
LockSupport.park();
}
}
}
}
/**
* 解锁（针对独占锁）
*/
public void unlock(){
int arg = 1;
if (tryUnlock(arg)){
WaitNode head = waiters.peek();
if (head == null){
return;
}
LockSupport.unpark(head.thread);
}
}
/**
* 获取共享锁（针对共享锁）
*/
public void lockShared(){
int arg = 1;
if (!tryLockShared(arg)){
WaitNode waitNode = new WaitNode(Thread.currentThread(),1,arg);
waiters.offer(waitNode);
while (true){
WaitNode head = waiters.peek();
if (head != null && head.thread == Thread.currentThread()){
if (tryLockShared(head.arg)){
waiters.poll();
WaitNode newHead = waiters.peek();
if (newHead != null && newHead.type == 1){
LockSupport.unpark(newHead.thread);
}
return;
} else {
LockSupport.park();
}
} else {
LockSupport.park();
}
}
}
}
/**
* 解锁（针对共享锁）
*/
public boolean unLockShared(){
int arg = 1;
if (tryUnLockShared(arg)){
WaitNode head = waiters.peek();
if (head != null){
LockSupport.unpark(head.thread);
}
return true;
}
return false;
}
/**
* 尝试获取独占锁（针对独占锁）
* @param acquires
* @return
*/
public boolean tryLock(int acquires){
throw new UnsupportedOperationException();
}
/**
* 尝试解锁（针对独占锁）
* @param releases 用于设定解锁次数。一般传入waitNode.arg
* @return
*/
public boolean tryUnlock(int releases){
throw new UnsupportedOperationException();
}
/**
* 尝试获取共享锁（针对共享锁）
* @param acquires
* @return
*/
public boolean tryLockShared(int acquires){
throw new UnsupportedOperationException();
}
/**
* 尝试解锁（针对共享锁）
* @param releases
* @return
*/
public boolean tryUnLockShared(int releases){
throw new UnsupportedOperationException();
}
}
2.JarryReentrantLock：
package tech.jarry.learning.netease.locks4;
/**
* @Description： 仿ReentrantLock，实现其基本功能及特性
* @Author: jarry
*/
public class JarryReentrantLock {
private boolean isFair;
// 默认采用非公平锁，保证效率（就是参照源码）
public JarryReentrantLock() {
this.isFair = false;
}
public JarryReentrantLock(boolean isFair) {
this.isFair = isFair;
}
private CommonMask commonMask = new CommonMask(){
@Override
public boolean tryLock(int acquires){
if (isFair){
return tryFairLock(acquires);
} else {
return tryNonFairLock(acquires);
}
}
private boolean tryFairLock(int acquires){
// 这里简单注释一下，如何实现公平锁，其关键在于新的线程到来时，不再直接尝试获取锁，而是直接塞入队列（队列为空，也是殊途同归的）
// 1.判断读锁（共享锁）是否被占用
if (readCount.get() == 0){
// 2.判断写锁（独占锁）是否被占用
int writeCountValue = writeCount.get();
if (writeCountValue == 0){
// 2.1 （核心区别）如果写锁未被占用，需要先对等待队列waiters进行判断
WaitNode head = waiters.peek();
if (head !=null && head.thread == Thread.currentThread()){
if (writeCount.compareAndSet(writeCountValue,writeCountValue+acquires)){
owner.set(Thread.currentThread());
return true;
} // 竞争失败就直接返回false了
}
} else {
// 2.2 如果写锁已经被占用了，就判断是否为当前线程持有，是否进行重入操作
if (owner.get() == Thread.currentThread()){
// 如果持有独占锁的线程就是当前线程，那么不需要改变owner，也不需要CAS，只需要修改writeCount的值即可
writeCount.set(writeCountValue + acquires);
return true;
}
}
}
// 以上操作失败，就返回false，表示竞争锁失败
return false;
}
private boolean tryNonFairLock(int acquires){
if (readCount.get() == 0){
int writeCountValue = writeCount.get();
if (writeCountValue == 0){
if (writeCount.compareAndSet(writeCountValue,writeCountValue+acquires)){
owner.set(Thread.currentThread());
return true;
}
} else {
if (Thread.currentThread() == owner.get()){
writeCount.set(writeCountValue+acquires);
return true;
}
}
}
return false;
}
@Override
public boolean tryUnlock(int releases) {
if (owner.get() != Thread.currentThread()){
throw new IllegalMonitorStateException();
}
int writeCountValue = writeCount.get();
writeCount.set(writeCountValue-releases);
if (writeCount.get() == 0){
owner.compareAndSet(Thread.currentThread(),null);
return true;
} else {
return false;
}
}
// 其它诸如共享锁的相关操作，就不进行了。如果强行调用，只会发生UnsupportedOperationException
};
public void lock() {
commonMask.lock();
}
public void unlock() {
commonMask.unlock();
}
public boolean tryLock(int acquire) {
return commonMask.tryLock(acquire);
}
private boolean tryUnlock(int release) {
return commonMask.tryUnlock(release);
}
}
3.JarryReadWriteLock：
package tech.jarry.learning.netease.locks4;
/**
* @Description：
* @Author: jarry
*/
public class JarryReadWriteLock {
private CommonMask commonMask = new CommonMask(){
@Override
public boolean tryLock(int acquires){
if (readCount.get() == 0){
int writeCountValue = writeCount.get();
if (writeCountValue == 0){
if (writeCount.compareAndSet(writeCountValue,writeCountValue+acquires)){
owner.set(Thread.currentThread());
return true;
}
} else {
if (Thread.currentThread() == owner.get()){
writeCount.set(writeCountValue+acquires);
return true;
}
}
}
return false;
}
@Override
public boolean tryUnlock(int releases) {
if (owner.get() != Thread.currentThread()){
throw new IllegalMonitorStateException();
}
int writeCountValue = writeCount.get();
writeCount.set(writeCountValue-releases);
if (writeCount.get() == 0){
owner.compareAndSet(Thread.currentThread(),null);
return true;
} else {
return false;
}
}
@Override
public boolean tryLockShared(int acquires) {
while (true){
if (writeCount.get() == 0 || owner.get() == Thread.currentThread()){
int readCountValue = readCount.get();
if (readCount.compareAndSet(readCountValue, readCountValue+acquires)){
return true;
}
}
return false;
}
}
@Override
public boolean tryUnLockShared(int releases) {
while (true){
int readCountValue = readCount.get();
int readCountNext = readCountValue - releases;
if (readCount.compareAndSet(readCountValue,readCountNext)){
return readCountNext == 0;
}
}
}
};
/**
* 获取独占锁（针对独占锁）
*/
public void lock(){
commonMask.lock();
}
/**
* 解锁（针对独占锁）
*/
public void unlock(){
commonMask.unlock();
}
/**
* 尝试获取独占锁（针对独占锁）
* @param acquires 用于加锁次数。一般传入waitNode.arg（本代码中就是1。为什么不用一个常量1，就不知道了？）
* @return
*/
public boolean tryLock(int acquires){
return commonMask.tryLock(acquires);
}
/**
* 尝试解锁（针对独占锁）
* @param releases 用于设定解锁次数。一般传入waitNode.arg
* @return
*/
public boolean tryUnlock(int releases){
return commonMask.tryUnlock(releases);
}
/**
* 获取共享锁（针对共享锁）
*/
public void lockShared(){
commonMask.lockShared();
}
/**
* 解锁（针对共享锁）
*/
public boolean unLockShared(){
return commonMask.unLockShared();
}
/**
* 尝试获取共享锁（针对共享锁）
* @param acquires
* @return
*/
public boolean tryLockShared(int acquires){
return tryLockShared(acquires);
}
/**
* 尝试解锁（针对共享锁）
* @param releases
* @return
*/
public boolean tryUnLockShared(int releases){
return commonMask.tryUnLockShared(releases);
}
}
这样看来，顺眼不少。但是，还是存在两点问题。一方面，两个Lock并没有如实际源码那样，实现Lock接口与ReadWriteLock接口。另一方面，JarryReadWriteLock并没有如实际源码那样，通过获取对应Lock（如ReadLock与WriteLock），再进行对应锁操作（其实，就是实现ReadWriteLock接口）。
那么就来进行改造吧。这里直接采用James大佬的最终版CommonMask-JameAQS了。 这里采用自己的AQS，因为自己的AQS有一些关键注解。
四，简易JUC（版本四）：
1.JarryAQS：
package tech.jarry.learning.netease.locks6;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.locks.LockSupport;
/**
* @Description：
* @Author: jarry
*/
public class JarryAQS {
volatile AtomicInteger readCount = new AtomicInteger(0);
AtomicInteger writeCount = new AtomicInteger(0);
AtomicReference<Thread> owner = new AtomicReference<>();
public volatile LinkedBlockingQueue<WaitNode> waiters = new LinkedBlockingQueue<>();
class WaitNode{
Thread thread = null;
// 表示希望争取的锁的类型。0表示写锁（独占锁），1表示读锁（共享锁）
int type = 0;
int arg = 0;
public WaitNode(Thread thread, int type, int arg) {
this.type = type;
this.thread = thread;
this.arg = arg;
}
}
/**
* 获取独占锁（针对独占锁）
*/
public void lock(){
int arg = 1;
if (!tryLock(arg)){
WaitNode waitNode = new WaitNode(Thread.currentThread(), 0, arg);
waiters.offer(waitNode);
while (true){
WaitNode headNote = waiters.peek();
if (headNote !=null && headNote.thread == Thread.currentThread()){
if (!tryLock(headNote.arg)){
LockSupport.park();
} else {
waiters.poll();
return;
}
}else {
LockSupport.park();
}
}
}
}
/**
* 解锁（针对独占锁）
*/
public void unlock(){
int arg = 1;
if (tryUnlock(arg)){
WaitNode head = waiters.peek();
if (head == null){
return;
}
LockSupport.unpark(head.thread);
}
}
/**
* 获取共享锁（针对共享锁）
*/
public void lockShared(){
int arg = 1;
if (!tryLockShared(arg)){
WaitNode waitNode = new WaitNode(Thread.currentThread(),1,arg);
waiters.offer(waitNode);
while (true){
WaitNode head = waiters.peek();
if (head != null && head.thread == Thread.currentThread()){
if (tryLockShared(head.arg)){
waiters.poll();
WaitNode newHead = waiters.peek();
if (newHead != null && newHead.type == 1){
LockSupport.unpark(newHead.thread);
}
return;
} else {
LockSupport.park();
}
} else {
LockSupport.park();
}
}
}
}
/**
* 解锁（针对共享锁）
*/
public boolean unLockShared(){
int arg = 1;
if (tryUnLockShared(arg)){
WaitNode head = waiters.peek();
if (head != null){
LockSupport.unpark(head.thread);
}
return true;
}
return false;
}
/**
* 尝试获取独占锁（针对独占锁）
* @param acquires
* @return
*/
public boolean tryLock(int acquires){
throw new UnsupportedOperationException();
}
/**
* 尝试解锁（针对独占锁）
* @param releases 用于设定解锁次数。一般传入waitNode.arg
* @return
*/
public boolean tryUnlock(int releases){
throw new UnsupportedOperationException();
}
/**
* 尝试获取共享锁（针对共享锁）
* @param acquires
* @return
*/
public boolean tryLockShared(int acquires){
throw new UnsupportedOperationException();
}
/**
* 尝试解锁（针对共享锁）
* @param releases
* @return
*/
public boolean tryUnLockShared(int releases){
throw new UnsupportedOperationException();
}
}
2.JarryReentrantLock：
package tech.jarry.learning.netease.locks6;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
/**
* @Description： 仿ReentrantLock，实现其基本功能及特性
* @Author: jarry
*/
public class JarryReentrantLock implements Lock {
private boolean isFair;
// 默认采用非公平锁，保证效率（就是参照源码）
public JarryReentrantLock() {
this.isFair = false;
}
public JarryReentrantLock(boolean isFair) {
this.isFair = isFair;
}
private JarryAQS jarryAQS = new JarryAQS(){
@Override
// 源码中，则是将FairSync与NonFairSync作为两个单独内布类（extend Sync），来实现的。那样会更加优雅，耦合度更低，扩展性更好（而且实际源码，需要重写的部分也会更多，而不像这个自定义demo，只有一个tryLock方法需要重写）
public boolean tryLock(int acquires){
if (isFair){
return tryFairLock(acquires);
} else {
return tryNonFairLock(acquires);
}
}
private boolean tryFairLock(int acquires){
// 这里简单注释一下，如何实现公平锁，其关键在于新的线程到来时，不再直接尝试获取锁，而是直接塞入队列（队列为空，也是殊途同归的）
// 1.判断读锁（共享锁）是否被占用
if (readCount.get() == 0){
// 2.判断写锁（独占锁）是否被占用
int writeCountValue = writeCount.get();
if (writeCountValue == 0){
// 2.1 （核心区别）如果写锁未被占用，需要先对等待队列waiters进行判断
WaitNode head = waiters.peek();
if (head !=null && head.thread == Thread.currentThread()){
if (writeCount.compareAndSet(writeCountValue,writeCountValue+acquires)){
owner.set(Thread.currentThread());
return true;
} // 竞争失败就直接返回false了
}
} else {
// 2.2 如果写锁已经被占用了，就判断是否为当前线程持有，是否进行重入操作
if (owner.get() == Thread.currentThread()){
// 如果持有独占锁的线程就是当前线程，那么不需要改变owner，也不需要CAS，只需要修改writeCount的值即可
writeCount.set(writeCountValue + acquires);
return true;
}
}
}
// 以上操作失败，就返回false，表示竞争锁失败
return false;
}
private boolean tryNonFairLock(int acquires){
if (readCount.get() == 0){
int writeCountValue = writeCount.get();
if (writeCountValue == 0){
if (writeCount.compareAndSet(writeCountValue,writeCountValue+acquires)){
owner.set(Thread.currentThread());
return true;
}
} else {
if (Thread.currentThread() == owner.get()){
writeCount.set(writeCountValue+acquires);
return true;
}
}
}
return false;
}
@Override
/**
*
先通过临时变量c，判断是否接下来的操作会完全解锁。
如果完全解锁，先释放owner，再通过setState将count（源码中为state)修改为0。
这样调换了一下顺序，但是避免了owner的原子性问题（毕竟别的线程是通过state来判断是否可以竞争锁，修改owner的）。
*/
public boolean tryUnlock(int releases) {
if (owner.get() != Thread.currentThread()){
throw new IllegalMonitorStateException();
}
int writeCountNextValue = writeCount.get() - releases;
boolean result = false;
if (writeCountNextValue == 0){
result = true;
owner.set(null);
}
writeCount.set(writeCountNextValue);
return result;
}
// 其它诸如共享锁的相关操作，就不进行了。如果强行调用，只会发生UnsupportedOperationException
};
@Override
public void lock() {
jarryAQS.lock();
}
@Override
public void lockInterruptibly() throws InterruptedException {
}
@Override
public boolean tryLock() {
return jarryAQS.tryLock(1);
}
@Override
public boolean tryLock(long time, TimeUnit unit) throws InterruptedException {
return false;
}
@Override
public void unlock() {
jarryAQS.unlock();
}
@Override
public Condition newCondition() {
return null;
}
}
3.JarryReadWriteLock：
package tech.jarry.learning.netease.locks6;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReadWriteLock;
/**
* @Description：
* @Author: jarry
*/
public class JarryReadWriteLock implements ReadWriteLock {
private JarryAQS jarryAQS = new JarryAQS(){
@Override
// 实际源码，是通过Sync类，继承AQS，再进行Override的。
public boolean tryLock(int acquires){
if (readCount.get() == 0){
int writeCountValue = writeCount.get();
if (writeCountValue == 0){
if (writeCount.compareAndSet(writeCountValue,writeCountValue+acquires)){
owner.set(Thread.currentThread());
return true;
}
} else {
if (Thread.currentThread() == owner.get()){
writeCount.set(writeCountValue+acquires);
return true;
}
}
}
return false;
}
@Override
public boolean tryUnlock(int releases) {
if (owner.get() != Thread.currentThread()){
throw new IllegalMonitorStateException();
}
int writeCountNextValue = writeCount.get() - releases;
boolean result = false;
if (writeCountNextValue == 0){
result = true;
owner.set(null);
}
writeCount.set(writeCountNextValue);
return result;
}
@Override
public boolean tryLockShared(int acquires) {
while (true){
if (writeCount.get() == 0 || owner.get() == Thread.currentThread()){
int readCountValue = readCount.get();
if (readCount.compareAndSet(readCountValue, readCountValue+acquires)){
return true;
}
}
return false;
}
}
@Override
public boolean tryUnLockShared(int releases) {
while (true){
int readCountValue = readCount.get();
int readCountNext = readCountValue - releases;
if (readCount.compareAndSet(readCountValue,readCountNext)){
return readCountNext == 0;
}
}
}
};
/**
* 获取独占锁（针对独占锁）
*/
public void lock(){
jarryAQS.lock();
}
/**
* 解锁（针对独占锁）
*/
public void unlock(){
jarryAQS.unlock();
}
/**
* 尝试获取独占锁（针对独占锁）
* @param acquires 用于加锁次数。一般传入waitNode.arg（本代码中就是1。为什么不用一个常量1，就不知道了？）
* @return
*/
public boolean tryLock(int acquires){
return jarryAQS.tryLock(acquires);
}
/**
* 尝试解锁（针对独占锁）
* @param releases 用于设定解锁次数。一般传入waitNode.arg
* @return
*/
public boolean tryUnlock(int releases){
return jarryAQS.tryUnlock(releases);
}
/**
* 获取共享锁（针对共享锁）
*/
public void lockShared(){
jarryAQS.lockShared();
}
/**
* 解锁（针对共享锁）
*/
public boolean unLockShared(){
return jarryAQS.unLockShared();
}
/**
* 尝试获取共享锁（针对共享锁）
* @param acquires
* @return
*/
public boolean tryLockShared(int acquires){
return tryLockShared(acquires);
}
/**
* 尝试解锁（针对共享锁）
* @param releases
* @return
*/
public boolean tryUnLockShared(int releases){
return jarryAQS.tryUnLockShared(releases);
}
@Override
public Lock readLock() {
return new Lock() {
@Override
public void lock() {
jarryAQS.lockShared();
}
@Override
public void lockInterruptibly() throws InterruptedException {
}
@Override
public boolean tryLock() {
return jarryAQS.tryLockShared(1);
}
@Override
public boolean tryLock(long time, TimeUnit unit) throws InterruptedException {
return false;
}
@Override
public void unlock() {
jarryAQS.unLockShared();
}
@Override
public Condition newCondition() {
return null;
}
};
}
@Override
public Lock writeLock() {
return new Lock() {
@Override
public void lock() {
jarryAQS.lock();
}
@Override
public void lockInterruptibly() throws InterruptedException {
}
@Override
public boolean tryLock() {
return jarryAQS.tryLock(1);
}
@Override
public boolean tryLock(long time, TimeUnit unit) throws InterruptedException {
return false;
}
@Override
public void unlock() {
jarryAQS.unlock();
}
@Override
public Condition newCondition() {
return null;
}
};
}
}
到了这里，其实JUC的核心-AQS，已经揭露出来了。通过这个，就可以把握住AQS核心运行机制。而实际的AQS，无非就是修改了存储线程的WaitNodes，采用了Node形成链表。并通过head与tail的应用，来提高效率。当然还有lockInterruptibly等没有提及，也有Condition这样的大头没有说。这部分就留待以后有机会，再深入吧。
另外，再给出这方面的提升道路。如果希望更加深入理解AQS源码，可以一边阅读源码（思考源码实现与自己实现的区别），一边扩展自定义简易AQS。
如，我了解到AQS是通过一个state来同时实现独占锁与共享锁的持有数量。那么我就在JarryAQS中，去尝试实现，从而进一步理解它。
五，简易JUC（版本X-扩展state）：
1.JarryAQS：
package tech.jarry.learning.netease.locks7;
import sun.misc.Unsafe;
import java.lang.reflect.Field;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.locks.LockSupport;
/**
* @Description：
* @Author: jarry
*/
public class JarryAQS {
static final int SHARED_SHIFT = 16;
static final int SHARED_UNIT = (1 << SHARED_SHIFT);
static final int MAX_COUNT = (1 << SHARED_SHIFT) - 1;
static final int EXCLUSIVE_MASK = (1 << SHARED_SHIFT) - 1;
/** Returns the number of shared holds represented in count */
static int sharedCount(int c) { return c >>> SHARED_SHIFT; }
/** Returns the number of exclusive holds represented in count */
static int exclusiveCount(int c) { return c & EXCLUSIVE_MASK; }
/**
* The synchronization state.
*/
public volatile int state;
private static Unsafe unsafe;
private static long stateOffset;
static{
try {
Field field = Unsafe.class.getDeclaredField("theUnsafe");
field.setAccessible(true);
unsafe = (Unsafe) field.get(null);
Field fieldi = JarryAQS.class.getDeclaredField("state");
stateOffset = unsafe.objectFieldOffset(fieldi);
} catch (NoSuchFieldException | IllegalAccessException e) {
e.printStackTrace();
}
}
protected final boolean compareAndSetState(int expect, int update) {
// See below for intrinsics setup to support this
return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
}
volatile AtomicInteger readCount = new AtomicInteger(0);
AtomicInteger writeCount = new AtomicInteger(0);
AtomicReference<Thread> owner = new AtomicReference<>();
public volatile LinkedBlockingQueue<WaitNode> waiters = new LinkedBlockingQueue<>();
class WaitNode{
Thread thread = null;
// 表示希望争取的锁的类型。0表示写锁（独占锁），1表示读锁（共享锁）
int type = 0;
int arg = 0;
public WaitNode(Thread thread, int type, int arg) {
this.type = type;
this.thread = thread;
this.arg = arg;
}
}
/**
* 获取独占锁（针对独占锁）
*/
public void lock(){
int arg = 1;
if (!tryLock(arg)){
WaitNode waitNode = new WaitNode(Thread.currentThread(), 0, arg);
waiters.offer(waitNode);
while (true){
WaitNode headNote = waiters.peek();
if (headNote !=null && headNote.thread == Thread.currentThread()){
if (!tryLock(headNote.arg)){
LockSupport.park();
} else {
waiters.poll();
return;
}
}else {
LockSupport.park();
}
}
}
}
/**
* 解锁（针对独占锁）
*/
public void unlock(){
int arg = 1;
if (tryUnlock(arg)){
WaitNode head = waiters.peek();
if (head == null){
return;
}
LockSupport.unpark(head.thread);
}
}
/**
* 获取共享锁（针对共享锁）
*/
public void lockShared(){
int arg = 1;
if (!tryLockShared(arg)){
WaitNode waitNode = new WaitNode(Thread.currentThread(),1,arg);
waiters.offer(waitNode);
while (true){
WaitNode head = waiters.peek();
if (head != null && head.thread == Thread.currentThread()){
if (tryLockShared(head.arg)){
waiters.poll();
WaitNode newHead = waiters.peek();
if (newHead != null && newHead.type == 1){
LockSupport.unpark(newHead.thread);
}
return;
} else {
LockSupport.park();
}
} else {
LockSupport.park();
}
}
}
}
/**
* 解锁（针对共享锁）
*/
public boolean unLockShared(){
int arg = 1;
if (tryUnLockShared(arg)){
WaitNode head = waiters.peek();
if (head != null){
LockSupport.unpark(head.thread);
}
return true;
}
return false;
}
/**
* 尝试获取独占锁（针对独占锁）
* @param acquires
* @return
*/
public boolean tryLock(int acquires){
throw new UnsupportedOperationException();
}
/**
* 尝试解锁（针对独占锁）
* @param releases 用于设定解锁次数。一般传入waitNode.arg
* @return
*/
public boolean tryUnlock(int releases){
throw new UnsupportedOperationException();
}
/**
* 尝试获取共享锁（针对共享锁）
* @param acquires
* @return
*/
public boolean tryLockShared(int acquires){
throw new UnsupportedOperationException();
}
/**
* 尝试解锁（针对共享锁）
* @param releases
* @return
*/
public boolean tryUnLockShared(int releases){
throw new UnsupportedOperationException();
}
}
2.JarryReentrantLock：
package tech.jarry.learning.netease.locks7;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
/**
* @Description： 仿ReentrantLock，实现其基本功能及特性
* @Author: jarry
*/
public class JarryReentrantLock implements Lock {
private boolean isFair;
// 默认采用非公平锁，保证效率（就是参照源码）
public JarryReentrantLock() {
this.isFair = false;
}
public JarryReentrantLock(boolean isFair) {
this.isFair = isFair;
}
// 实际源码，是通过Sync类，继承AQS，再进行Override的。
private JarryAQS jarryAQS = new JarryAQS(){
@Override
// 源码中，则是将FairSync与NonFairSync作为两个单独内布类（extend Sync），来实现的。那样会更加优雅，耦合度更低，扩展性更好（而且实际源码，需要重写的部分也会更多，而不像这个自定义demo，只有一个tryLock方法需要重写）
public boolean tryLock(int acquires){
if (isFair){
return tryFairLock(acquires);
} else {
return tryNonFairLock(acquires);
}
}
private boolean tryFairLock(int acquires){
// 这里简单注释一下，如何实现公平锁，其关键在于新的线程到来时，不再直接尝试获取锁，而是直接塞入队列（队列为空，也是殊途同归的）
// 1.判断读锁（共享锁）是否被占用
if (readCount.get() == 0){
// 2.判断写锁（独占锁）是否被占用
int writeCountValue = writeCount.get();
if (writeCountValue == 0){
// 2.1 （核心区别）如果写锁未被占用，需要先对等待队列waiters进行判断
WaitNode head = waiters.peek();
if (head !=null && head.thread == Thread.currentThread()){
if (writeCount.compareAndSet(writeCountValue,writeCountValue+acquires)){
owner.set(Thread.currentThread());
return true;
} // 竞争失败就直接返回false了
}
} else {
// 2.2 如果写锁已经被占用了，就判断是否为当前线程持有，是否进行重入操作
if (owner.get() == Thread.currentThread()){
// 如果持有独占锁的线程就是当前线程，那么不需要改变owner，也不需要CAS，只需要修改writeCount的值即可
writeCount.set(writeCountValue + acquires);
return true;
}
}
}
// 以上操作失败，就返回false，表示竞争锁失败
return false;
}
private boolean tryNonFairLock(int acquires){
if (readCount.get() == 0){
int writeCountValue = writeCount.get();
if (writeCountValue == 0){
if (writeCount.compareAndSet(writeCountValue,writeCountValue+acquires)){
owner.set(Thread.currentThread());
return true;
}
} else {
if (Thread.currentThread() == owner.get()){
writeCount.set(writeCountValue+acquires);
return true;
}
}
}
return false;
}
@Override
/**
*
先通过临时变量c，判断是否接下来的操作会完全解锁。
如果完全解锁，先释放owner，再通过setState将count（源码中为state)修改为0。
这样调换了一下顺序，但是避免了owner的原子性问题（毕竟别的线程是通过state来判断是否可以竞争锁，修改owner的）。
*/
public boolean tryUnlock(int releases) {
if (owner.get() != Thread.currentThread()){
throw new IllegalMonitorStateException();
}
int writeCountNextValue = writeCount.get() - releases;
boolean result = false;
if (writeCountNextValue == 0){
result = true;
owner.set(null);
}
writeCount.set(writeCountNextValue);
return result;
}
// 其它诸如共享锁的相关操作，就不进行了。如果强行调用，只会发生UnsupportedOperationException
};
@Override
public void lock() {
jarryAQS.lock();
}
@Override
public void lockInterruptibly() throws InterruptedException {
}
@Override
public boolean tryLock() {
return jarryAQS.tryLock(1);
}
@Override
public boolean tryLock(long time, TimeUnit unit) throws InterruptedException {
return false;
}
@Override
public void unlock() {
jarryAQS.unlock();
}
@Override
public Condition newCondition() {
return null;
}
}
3.JarryReadWriteLock：
package tech.jarry.learning.netease.locks7;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReadWriteLock;
/**
* @Description：
* @Author: jarry
*/
public class JarryReadWriteLock implements ReadWriteLock {
// 实际源码，是通过Sync类，继承AQS，再进行Override的。
private JarryAQS jarryAQS = new JarryAQS(){
@Override
public boolean tryLock(int acquires){
int stateTemp = state;
if (sharedCount(stateTemp) == 0){
int writeCountValue = exclusiveCount(stateTemp);
if (writeCountValue == 0){
if (compareAndSetState(stateTemp,stateTemp+acquires)){
owner.set(Thread.currentThread());
return true;
}
} else {
if (Thread.currentThread() == owner.get()){
compareAndSetState(stateTemp,stateTemp+acquires);
return true;
}
}
}
return false;
}
@Override
public boolean tryUnlock(int releases) {
int stateTemp = state;
if (owner.get() != Thread.currentThread()){
throw new IllegalMonitorStateException();
}
int writeCountNextValue = exclusiveCount(stateTemp) - releases;
boolean result = false;
if (writeCountNextValue == 0){
result = true;
owner.set(null);
}
compareAndSetState(stateTemp,stateTemp - releases);
return result;
}
@Override
public boolean tryLockShared(int acquires) {
while (true){
int stateTemp = state;
if (exclusiveCount(stateTemp) == 0 || owner.get() == Thread.currentThread()){
if (compareAndSetState(stateTemp, stateTemp+SHARED_UNIT*acquires)){
return true;
}
}
return false;
}
}
@Override
public boolean tryUnLockShared(int releases) {
while (true){
int stateTemp = state;
int readCountValue = sharedCount(stateTemp);
int readCountNext = readCountValue - releases;
if (compareAndSetState(stateTemp, stateTemp-SHARED_UNIT*readCountNext)){
return readCountNext == 0;
}
}
}
};
/**
* 获取独占锁（针对独占锁）
*/
public void lock(){
jarryAQS.lock();
}
/**
* 解锁（针对独占锁）
*/
public void unlock(){
jarryAQS.unlock();
}
/**
* 尝试获取独占锁（针对独占锁）
* @param acquires 用于加锁次数。一般传入waitNode.arg（本代码中就是1。为什么不用一个常量1，就不知道了？）
* @return
*/
public boolean tryLock(int acquires){
return jarryAQS.tryLock(acquires);
}
/**
* 尝试解锁（针对独占锁）
* @param releases 用于设定解锁次数。一般传入waitNode.arg
* @return
*/
public boolean tryUnlock(int releases){
return jarryAQS.tryUnlock(releases);
}
/**
* 获取共享锁（针对共享锁）
*/
public void lockShared(){
jarryAQS.lockShared();
}
/**
* 解锁（针对共享锁）
*/
public boolean unLockShared(){
return jarryAQS.unLockShared();
}
/**
* 尝试获取共享锁（针对共享锁）
* @param acquires
* @return
*/
public boolean tryLockShared(int acquires){
return tryLockShared(acquires);
}
/**
* 尝试解锁（针对共享锁）
* @param releases
* @return
*/
public boolean tryUnLockShared(int releases){
return jarryAQS.tryUnLockShared(releases);
}
@Override
public Lock readLock() {
return new Lock() {
@Override
public void lock() {
jarryAQS.lockShared();
}
@Override
public void lockInterruptibly() throws InterruptedException {
}
@Override
public boolean tryLock() {
return jarryAQS.tryLockShared(1);
}
@Override
public boolean tryLock(long time, TimeUnit unit) throws InterruptedException {
return false;
}
@Override
public void unlock() {
jarryAQS.unLockShared();
}
@Override
public Condition newCondition() {
return null;
}
};
}
@Override
public Lock writeLock() {
return new Lock() {
@Override
public void lock() {
jarryAQS.lock();
}
@Override
public void lockInterruptibly() throws InterruptedException {
}
@Override
public boolean tryLock() {
return jarryAQS.tryLock(1);
}
@Override
public boolean tryLock(long time, TimeUnit unit) throws InterruptedException {
return false;
}
@Override
public void unlock() {
jarryAQS.unlock();
}
@Override
public Condition newCondition() {
return null;
}
};
}
}
六，总结：
如果是从ReentrantLock实现，一步步走到这里，手动撸到这里，那么你对AQS的认知，就有了非常坚实的基础。如果能够在学习过程中，对照源码学习（一边自己实现，一边了解源码是怎么解决相关问题的），那么你对AQS的理解就很不错了。即使有所欠缺，也只是AQS阅读积累方面了。
原文链接:https://www.cnblogs.com/Tiancheng-Duan/p/12010076.html
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