Java线程池Executors.newSingleThreadExecutor()
前言：本文先就Java线程池 ThreadPoolExecutor 进行分析，然后逐步分析单线程池的源码工作流程
ThreadPoolExecutor的工作流程
我们执行以下代码：
ExecutorService singleThreadExecutor = Executors.newSingleThreadExecutor();
singleThreadExecutor.execute(new Runnable() {
@Override
public void run() {
...
}
});
当线程池提交Runnable实现时singleThreadExecutor.execute(Runnable command) ，其大致的工作流程如下：
下面跟随源码进入到ThreadPoolExecutor中进行详细分析
线程池初始化
当Executors.newSingleThreadExecutor()执行时，其内部源码如下：
public static ExecutorService newSingleThreadExecutor() {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>()));
}
继续深入new ThreadPoolExecutor方法可以发现，对应的参数的意义分别是：
public ThreadPoolExecutor(int corePoolSize, // 核心线程数
int maximumPoolSize, // 最大线程数
long keepAliveTime, // 保活时间（这里是0，单线程池暂时用不上此参数）
TimeUnit unit, // 保活时间单位
BlockingQueue<Runnable> workQueue // 存放Runnable实现的队列) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
Executors.defaultThreadFactory(), defaultHandler);
}
往后执行，也就是线程池执行初始化的一个过程，给线程池对象的重要参数进行赋值。
线程池提交任务
先看看提交时，最外层的代码：
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();
// 1.
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
c = ctl.get();
}
// 2.
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);
}
// 3.
else if (!addWorker(command, false))
reject(command);
}
翻译翻译Doug Lea大神写的注释:
- 首先判断
Worker的数量是否小于corePoolSize核心线程池数量，小于则newWorker对象，启动线程并以此次的Runnable任务为第一次任务进行run()执行。后一段则说有原子性的检查机制保证Worker线程不会额外创建，否则会返回false； - 上述条件不满足时，会将
Runnale任务放入队列，如果放置成功，且会进行双重校验：先判断线程池是否是SHUTDOWN状态，然后尝试撤回Runnable任务，如果失败，则判断Worker的数量是否为0，如果为0，则新增一个非核心线程池的Worker。根据鄙人目前的理解，这里应该是避免线程池执行shutdown()方法，然后导致Runnable任务没有被正确的处理。 - 如果
Runnable放入队列失败，则添加非核心线程池的Worker。PS：LinkedBlockingQueue无论如何都可以offer()成功，SynchronousQueue的offer()是返回的false。上述两个BlockingQueue分别对应固定线程数的线程池和非固定线程数的线程池，分别对应的典型是Executors.newSingleThreadExecutor()和Executors.newCachedThreadPool()。非固定线程数的线程池放在下一期讲。
addWorker方法与Worker类
单线程的线程池的提交任务的时候，最重要的就2个步骤：addWorker(command, true)添加核心线程Worker与 workQueue.offer(command)将超出核心线程数的任务放入队列中。
那么addWorker方法里面发生了什么呢，我摘取了我认为最重要的一段：上锁、new Worker对象与启动线程
private boolean addWorker(Runnable firstTask, boolean core) {
...
boolean workerStarted = false;
boolean workerAdded = false;
Worker w = null;
try {
w = new Worker(firstTask);
final Thread t = w.thread;
if (t != null) {
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());
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(); // 注意这里
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
那么，这个Worker究竟是怎么执行的呢，调用t.start()发生了什么呢。先看看Worker的类结构：
再看看Worker的成员变量有哪些，参考其构造方法：
Worker(Runnable firstTask) {
setState(-1); // inhibit interrupts until runWorker
this.firstTask = firstTask;
this.thread = getThreadFactory().newThread(this);
}
接下来就是我认为关于Worker最重要核心的地方了，通过上述代码我们可以发现，Worker实现了Runnable接口，然后在构建Worker对象的时候，初始化了它的成员变量thread，并且是以Woker本身赋值进去的！！！看到这里，我们就知道了，t.start()的时候，是执行的Worker.run()方法。
下面是Worker.run()方法内部的部分源码：
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock(); // allow interrupts
boolean completedAbruptly = true;
try {
while (task != null || (task = getTask()) != null)
w.lock();
// If pool is stopping, ensure thread is interr
// if not, ensure thread is not interrupted. T
// requires a recheck in second case to deal wi
// shutdownNow race while clearing interrupt
if ((runStateAtLeast(ctl.get(), STOP) ||
(Thread.interrupted() &&
runStateAtLeast(ctl.get(), STOP))) &&
!wt.isInterrupted())
wt.interrupt();
try {
beforeExecute(wt, task);
Throwable thrown = null;
try {
task.run();
} catch (RuntimeException x) {
thrown = x; throw x;
} catch (Error x) {
thrown = x; throw x;
} catch (Throwable x) {
thrown = x; throw new Error(x);
} finally {
afterExecute(task, thrown);
}
} finally {
task = null;
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally {
processWorkerExit(w, completedAbruptly);
}
}
也就是说，启动Worker的之后，就是循环getTask()与task.run()，这个getTask()得到的task才是提交到线程池中的Runnable实现。
对于单线程池，getTask()最终执行的是LinkedBlockingQueue.take()方法：
- 按照队列的FIFO原则（先进先出），取出头部节点
- 如果没有头部节点，则调用
notEmpty.await();方法等待，等待offer()方法时，调用notEmpty.signal();，这里涉及多线程的其它知识了，可以参考并发包中的AbstractQueuedSynchronizer相关知识点。
至此，单线程池的执行过程大概讲解了一通，现在再来看开篇的工作流程图，又有了更深地理解。