ExecutorService service = Executors.newCachedThreadPool();
service.execute(new Runnable() {
@Override
public void run() {
System.out.println("原理解析");
}
});
这是一个很简单的线程池案例，第一步就是初始化一个线程池，然后进行各种参数的设置，主要有四种线程池类型，可以去看下源码，主要是队列的不同。具体细节可以去网上搜。我们主要看下execute（）方法。其实是调用了ThreadPoolTaskExecutor类的execute方法，参数也就是我们需要执行的任务
public void execute(Runnable command) {
** 判断提交的任务是否为空
if (command == null)
throw new NullPointerException();
int c = ctl.get();
**判断当前有效的线程数是否小于核心线程数
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
c = ctl.get();
}
**判断下当前线程池是否处于Running状态，线程池状态不同，执行的策略也是不同的，参考上面这幅图，并且能成功加入队列
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
**这里又判断了线程池的状态，因为在多线程环境下，有可能线程一执行到此处的时候线程二修改了线程池状态，所以要再判断一下
**如果线程不处于Running状态，就拒绝接受新任务，就将该任务移除掉
if (! isRunning(recheck) && remove(command))
**执行拒绝策略
reject(command);
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
**如何任务添加到队列失败，执行拒绝策略
else if (!addWorker(command, false))
reject(command);
}
在这里主要还是addWorker()方法，我们看下这个方法的实现
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
**也是对线程池状态进行判断
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
**判断当前有效线程数是否大于线程池容纳的最大线程池
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对象是实现了Runnable接口的，所以肯定有run方法，这里是重点，先记着
Worker w = null;
try {
w = new Worker(firstTask);
**创建一个新的线程
final Thread t = w.thread;
if (t != null) {
final ReentrantLock mainLock = this.mainLock;
**加锁
mainLock.lock();
try {
int rs = runStateOf(ctl.get());
**判断线程池状态
if (rs < SHUTDOWN ||
(rs == SHUTDOWN && firstTask == null)) {
if (t.isAlive())
throw new IllegalThreadStateException();
workers.add(w);
int s = workers.size();
**largestPoolSize参数是用来记录线程池出现过的最大线程数
if (s > largestPoolSize)
largestPoolSize = s;
workerAdded = true;
}
} finally {
mainLock.unlock();
}
if (workerAdded) {
**这里的start方法就是调用Worker类的run方法,所以我们进入run方法看看
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
继续看下runWorker()方法的实现
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
**获取需要执行的任务，也就是我们需要执行的业务代码
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock();
boolean completedAbruptly = true;
try {
**判断当前任务是否为空，或者队列中是否为空，这个getTash（）方法很重要，线程存活时间就是在这里判断的
while (task != null || (task = getTask()) != null) {
w.lock();
if ((runStateAtLeast(ctl.get(), STOP) ||
(Thread.interrupted() &&
runStateAtLeast(ctl.get(), STOP))) &&
!wt.isInterrupted())
wt.interrupt();
try {
beforeExecute(wt, task);
Throwable thrown = null;
try {
**这里的run方法就是在我们文章开头那个案例中的run方法，不信的话测试可以在那里打个断点
**到这里线程池执行方法到这里就结束了，但是还没结束，因为线程还需要回收啊，往下看
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 {
**这个方法才是线程池中的核心
processWorkerExit(w, completedAbruptly);
}
}
我们看下这个processWorkerExit方法做了什么
private void processWorkerExit(Worker w, boolean completedAbruptly) {
if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted
decrementWorkerCount();
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
completedTaskCount += w.completedTasks;
workers.remove(w);
} finally {
mainLock.unlock();
}
tryTerminate();
int c = ctl.get();
if (runStateLessThan(c, STOP)) {
if (!completedAbruptly) {
int min = allowCoreThreadTimeOut ? 0 : corePoolSize;
if (min == 0 && ! workQueue.isEmpty())
min = 1;
if (workerCountOf(c) >= min)
return; // replacement not needed
}
**前面的就不做解释，重点这这行代码，它又执行了addWorker方法，之前说到这个方法是创建个线程执行任务的，但是这里第一个参数为null,我们知道addWorker会执行Worker类中的runWorker方法，那里有个很重要的判断
** while (task != null || (task = getTask()) != null) 在这里task = null，所以我们看下getTask方法
addWorker(null, false);
}
}
接下来看下getTask方法做了什么
**这个方法是从队列中获取需要执行的任务
private Runnable getTask() {
boolean timedOut = false; // Did the last poll() time out?
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
decrementWorkerCount();
return null;
}
int wc = workerCountOf(c);
boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
if ((wc > maximumPoolSize || (timed && timedOut))
&& (wc > 1 || workQueue.isEmpty())) {
if (compareAndDecrementWorkerCount(c))
return null;
continue;
}
try {
**从队列中获取任务，keepAliveTime这个参数就是在我们初始化线程池的时候给的线程存活时间，如果在这个时间内没有获取到任务，直接返回null，如果有任务继续执行
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
if (r != null)
return r;
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}
以上纯属个人理解，有错误欢迎提出，其中还有另外三种线程池，四种的拒绝策略
