线程池 ThreadPoolExecutor 类的源码解析：
1：数据结构的分析：
private final BlockingQueue<Runnable> workQueue; //用于存储未被线程池处理的任务
private final ReentrantLock mainLock = new ReentrantLock(); //维护一个lock来保证线程安全
private final HashSet<Worker> workers = new HashSet<Worker>();
private final Condition termination = mainLock.newCondition(); //通过Condition来进行线程之间的通信
private volatile ThreadFactory threadFactory; //维护一个线程工厂，用于生成线程
private volatile RejectedExecutionHandler handler; //拒绝任务的句柄对象
private volatile int corePoolSize; //核心池大小
private volatile int maximumPoolSize; //最大池的大小
2：构造方法：将用户自定义的参数赋值给成员变量，并且使用默认的线程工厂，默认的任务拒绝
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
Executors.defaultThreadFactory(), defaultHandler);
}
3：下面看看 execute 执行方法的原理：
public void execute(Runnable command) {
if (command == null) //判空操作
throw new NullPointerException();
int c = ctl.get();
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
c = ctl.get();
}
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
if (! isRunning(recheck) && remove(command))
reject(command);
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
else if (!addWorker(command, false))
reject(command);
}
下面主要分析下addWorker这个方法：
//以2种场景为例进行分析：
//1第一次添加任务到线程池中 2 第6次添加 corePoolsize=5
private boolean addWorker(Runnable firstTask, boolean core) { // firstTask就是Runnable任务 core=true
retry:
for (;;) { //相当于while循环
int c = ctl.get(); //初始换后状态位Running 线程池中任务数为0
int rs = runStateOf(c); //获取线程池的状态 这里是RUNNING
// Check if queue empty only if necessary.
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
for (;;) { //相当于while循环
int wc = workerCountOf(c); //获取worker数量 wc=0
//判断线程池中线程是否超出了限制，若超出了则返回false
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
//CAS 将c的值+1 操作失败退出循环
if (compareAndIncrementWorkerCount(c))
break retry;
c = ctl.get(); // Re-read ctl
if (runStateOf(c) != rs)
continue retry;
// else CAS failed due to workerCount change; retry inner loop
}
}
boolean workerStarted = false;
boolean workerAdded = false;
Worker w = null;
try {
//将传进来的Runnable 任务构建成Worker对象
w = new Worker(firstTask);
//获取worker对应的线程
final Thread t = w.thread;
if (t != null) {
//获取锁
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
// Recheck while holding lock.
// Back out on ThreadFactory failure or if
// shut down before lock acquired.
int rs = runStateOf(ctl.get());//获取线程池状态 这里是RUNNING
//判断线程池状态
if (rs < SHUTDOWN ||
(rs == SHUTDOWN && firstTask == null)) {
if (t.isAlive()) // precheck that t is startable
throw new IllegalThreadStateException();
//将工作任务添加到workers集合中
workers.add(w);
int s = workers.size();
if (s > largestPoolSize) //初始化时largestPoolSize=0
largestPoolSize = s; //赋值 largestPoolSize=1
workerAdded = true; //这里true表明添加成功的标识，后面执行该线程
}
} finally {
mainLock.unlock();
}
if (workerAdded) {
t.start(); //线程开始执行
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
下面看看构建 Worker对象的逻辑：
Worker(Runnable firstTask) {
setState(-1); // 设置AQS state=-1
this.firstTask = firstTask; //任务赋值给firstTask全局变量
this.thread = getThreadFactory().newThread(this); //从线程工程创建新的线程
}
当第6次添加的时候
// command Runnable 任务
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
int c = ctl.get(); //获取线程池的状态 这里是Running
if (workerCountOf(c) < corePoolSize) { //这里c的任务数是5 corePoolSize=5
if (addWorker(command, true))
return;
c = ctl.get();
}
// 判断线程池的状态是不是Ruuning 将任务添加到队列中
if (isRunning(c) && workQueue.offer(command)) {//进入这个逻辑
int recheck = ctl.get(); //再次获取ctl对象
if (! isRunning(recheck) && remove(command)) //线程池状态不是Running 或者任务被移除则局拒绝任务
reject(command);
else if (workerCountOf(recheck) == 0)
addWorker(null, false); //添加空任务到worker中
}
else if (!addWorker(command, false))
reject(command);
}
到这里线程池的分析已经结束了；
