Java线程池是java cocurrent包下提供的类，使用非常方便。本文希望整理下Java 线程池相关的知识以及其实现原理。
类结构
Executors, ExecutorService, ThreadPoolExecutor等概念，各个类容易混淆，其实类关系整理如图。ExecutorService接口扩展了Executors接口，在基本的execute()方法上，提供了一些线程池管理方法、同步／异步获取线程执行结果的方法。
屏幕快照 2017-06-28 下午11.10.32.png
通常，我们使用线程池的模版代码如下
ExecutorService executorService = Executors.newFixedThreadPool(5,
new ThreadFactory() {
private AtomicInteger count = new AtomicInteger(0);
@Override
public Thread newThread(Runnable r) {
return new Thread("task thread-" + count.incrementAndGet());
}
});
for (int i = 0; i < 10; i++) {
executorService.submit(new Task());
}
从类图结构就可以很好的理解了， Executors即为工厂方法类，提供多种类型线程池的构造。ExecutorService即为所有线程池的接口类。
我们使用的线程池可以分为两类ScheduledExecutorService、ThreadPoolExecutor。实际所有线程池均为其实现类或者wrapper类。所以分析线程池可以从这两类入手。
ThreadPoolExecutor
常用的代表有Executors.newFixedThreadPool(),newCachedThreadPool() 等，构造出来的都是设定了不同参数的ThreadPoolExecutor实例。
ThreadPoolExecutor的核心参数有：
- corePoolSize
- maxPoolSize
- keepAliveTime
- workingQueue: 同步队列，无界队列，有界队列
具体每个参数的定义可以看Java doc,文档对于何时创建线程、何时将任务排队、以及排队策略的说明都写的非常详细。本人经常忘记的一点就是线程创建的策略：
- alive线程数< corePoolSize，来一个新task则新建一个线程
- alive线程数>corePoolSize，则新task先排队，排队排不下了再尝试新建线程。
- 排队时队满了，尝试新建线程。线程数目到达maxPoolSize,根据定义的策略方式来处理后面继续到来的任务，比如直接拒绝任务。
从基本使用代码 executorService.submit(new Task());入手分析，threadPool到底做了什么？是如何让一个任务跑起来的？
submit
代码中可以看到，逻辑为将传入的runnable封装成一个runnableFuture,再调用execute方法执行该任务。runnableFuture可提供接口，将任务取消执行等。其他相关runnableFuture变种可以提供一些异步获取task执行返回结果的接口，具体需要看callable,future的说明。
/**
* @throws RejectedExecutionException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*/
public Future<?> submit(Runnable task) {
if (task == null) throw new NullPointerException();
RunnableFuture<Void> ftask = newTaskFor(task, null);
execute(ftask);
return ftask;
}
execute
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
/*
* Proceed in 3 steps:
*
* 1. If fewer than corePoolSize threads are running, try to
* start a new thread with the given command as its first
* task. The call to addWorker atomically checks runState and
* workerCount, and so prevents false alarms that would add
* threads when it shouldn't, by returning false.
*
* 2. If a task can be successfully queued, then we still need
* to double-check whether we should have added a thread
* (because existing ones died since last checking) or that
* the pool shut down since entry into this method. So we
* recheck state and if necessary roll back the enqueuing if
* stopped, or start a new thread if there are none.
*
* 3. If we cannot queue task, then we try to add a new
* thread. If it fails, we know we are shut down or saturated
* and so reject the task.
*/
int c = ctl.get();
if (workerCountOf(c) < corePoolSize) {
/* worker线程数 < corePoolSize，直接新建一个线程，且该task作为新建线程的第一个任务直接执行*/
if (addWorker(command, true))
return;
c = ctl.get();
}
if (isRunning(c) && workQueue.offer(command)) {
/*线程池running状态且队列中还能排下该task,则task进入队列。此时有二次检查，若二次检查不过，则task移除队列，成功移出后，调用reject策略。否则，task成功在排队队列中等待执行，若此时worker线程数＝0，则新建个线程来处理该任务*/
int recheck = ctl.get();
if (! isRunning(recheck) && remove(command))
reject(command);
else if (workerCountOf(recheck) == 0)
/* 注意此时addWorker里第一个参数为null
addWorker(null, false);
}
/*尝试直接线程，该task作为该新建线程的第一个任务*/
else if (!addWorker(command, false))
reject(command);
}
看完有3个疑问：
- 线程数目>corePoolSize时且队列中有剩余槽位时，task在队列中等待执行，则何时会被worker线程执行？
2.线程数目>corePoolSize且队列中无剩余槽位时，task直接由新建一个线程执行（�< maxPoolSize），那是否是造成了后提交的task是可能先执行的？
addWorker
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
for (;;) {
int wc = workerCountOf(c);
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
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 {
final ReentrantLock mainLock = this.mainLock;
w = new Worker(firstTask);
final Thread t = w.thread;
if (t != null) {
mainLock.lock();
try {
// Recheck while holding lock.
// Back out on ThreadFactory failure or if
// shut down before lock acquired.
int c = ctl.get();
int rs = runStateOf(c);
if (rs < SHUTDOWN ||
(rs == SHUTDOWN && firstTask == null)) {
if (t.isAlive()) // precheck that t is startable
throw new IllegalThreadStateException();
workers.add(w);
int s = workers.size();
if (s > largestPoolSize)
largestPoolSize = s;
workerAdded = true;
}
} finally {
mainLock.unlock();
}
if (workerAdded) {
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
