从线程池使用进行实现分析
一.自定义线程池
1.自定义线程池
2.构造完成之后状态
3.关键参数介绍
二.执行任务
1.execute一个任务
2.执行分析
三.线程池停止
1.shutDown分析
2.shutDownNow分析
四.线程池常见问题
一.自定义线程池
1.自定义线程池
//阻塞队列
LinkedBlockingQueue blockingQueue= new LinkedBlockingQueue(10);
//线程工厂
ThreadFactory threadFactory=new ThreadFactory() {
@Override
public Thread newThread(Runnable r) {
Thread t = new Thread(r);
t.setName("test");
return t;
}
};
//拒绝策略
RejectedExecutionHandler rejectedExecutionHandler=new RejectedExecutionHandler() {
@Override
public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
System.out.println("自定义拒绝策略");
}
};
//核心线程数
Integer corePoolSize =3;
//最大线程数
Integer maxPoolSize=10;
//空闲线程等待时间
Integer keepAliveTime=7;
//构造方法
ExecutorService threadPoolExecutor=new ThreadPoolExecutor(corePoolSize, maxPoolSize, keepAliveTime,
TimeUnit.SECONDS, blockingQueue, threadFactory, rejectedExecutionHandler);
2.构造完成之后状态
构造方法完成之后状态
3.几个关键参数
ctl:保存线程池存活状态,线程池内线程数
workqueque:自定义的任务队列
mainLock:线程池的锁
termination:mainLock.newCondition()阻塞/通知线程;
keepAliveTime:非核心线程的存活时间
corePoolSize:3
maximumPoolSize:10
workers:线程集合 线程池内存活的线程数=0
二.执行任务
1.execute一个任务
execute之后状态
workers: 0 到 1
2.执行分析
1.首先检查存活线程数量
2.根据存活数量进行不同处理
3.开辟新的线程/添加到任务队列/开新线程至最大线程数/拒绝任务
4.如果成功的开了线程,调用线程的start()开始处理
5.worker用while循环不断获取任务.
6.直到当前任务和任务队列都为空,判断是否阻塞直至有新任务到来.
下面开始就源码进行分析
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.
*/
1.获取线程池状态
int c = ctl.get();
2.取workers数量 和如果小于核心线程数,生成一个新的worker
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
c = ctl.get();
}
3.workers.count到达了核心线程数,线程池是运行状态,把任务添加到任务队列
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
4.再次检查状态,如果不是运行状态,将任务移除出队列
if (! isRunning(recheck) && remove(command))
reject(command);
5.如果此时线程池内没有线程了,再生成一个线程
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
6.如果worker加不上去了,执行拒绝策略
else if (!addWorker(command, false))
reject(command);
}
从上面可以看到每来一个任务都线程数量会判断使用策略
1.如果没有到核心线程,添加新线程执行任务
2.如果到核心线程,添加到任务队列
3.如果队列都满了,那就添加新线程,知道最大线程数.
分析addworker之前需要对worker这个内部类进行了解
worker结构
可以看到,本质上就是对线程的包装.主要是继承了AQS,通过锁对线程进行保护.
下面开始看addWorks方法
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
for (;;) {
1.进来先获取线程池状态
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
2.校验线程池状态
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
for (;;) {
int wc = workerCountOf(c);
3.先看是否达到最大容量.再判断是否到达自定义规定容量
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
if
4.这里采取的cas算法让worker计数器+1.如果cas失败,重复1234.
(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
}
}
5.下面开始正式创建worker
boolean workerStarted = false;
boolean workerAdded = false;
Worker w = null;
try {
6.获得线程池的锁
final ReentrantLock mainLock = this.mainLock;
7.初始化worker,构造方法里创建了线程
w = new Worker(firstTask);
final Thread t = w.thread;
if (t != null) {
8.因为works是hashSet不是线程安全的,所以这里需要加锁处理.里面添加worker,更新相关计数器
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();
}
9.如果worker添加成功.直接启动线程
if (workerAdded) {
11.start方法实际调用worker runWorker()方法
t.start();
workerStarted = true;
}
}
} finally {
10.如果线程启动失败,worker数量减少,尝试中断失败线程.
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
对runWorker进行分析
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
Runnable task = w.firstTask;
w.firstTask = null;
猜测这里unlock主要避免执行shutdownNow时并发问题
w.unlock(); // allow interrupts
boolean completedAbruptly = true;
try {
1.这里有两种方式获取任务 getTask会根据情况是否阻塞线程.
getTask下面分析
while (task != null || (task = getTask()) != null) {
2.这里采用的独占锁,保证了任务开始执行后,只有在锁之前已经产生的终止状态才能使自我中断。
w.lock();
// If pool is stopping, ensure thread is interrupted;
// if not, ensure thread is not interrupted. This
// requires a recheck in second case to deal with
// 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 {
3.这里终于开始执行任务了！！！
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 {
4.最后根据状态判断线程是否可以正常退出.
processWorkerExit(w, completedAbruptly);
}
}
线程不断获取任务执行,并且用独占锁保护线程执行过程,这样就保证了shutdown时后,正在执行任务的县城不会被中断,但是shutdownNow不受此影响.可以直接粗暴的中断线程.
getTask()分析
private Runnable getTask() {
boolean timedOut = false; // Did the last poll() time out?
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
1.如果线程池调用了shutDown,线程池状态就是shutdown.如果调用shutdown线程池状态就是stop,这2个策略主要是后续任务处理方式不一样.
如果是shutdown状态,队列必须不为空,那么可以继续从任务队列获取任务进行处理
如果是stop状态,队列不为空也不进行处理.(shutDownNow时任务队列已经把任务返回了)
if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
decrementWorkerCount();
return null;
}
boolean timed; // Are workers subject to culling?
for (;;) {
int wc = workerCountOf(c);
timed = allowCoreThreadTimeOut || wc > corePoolSize;
if (wc <= maximumPoolSize && ! (timedOut && timed))
break;
if (compareAndDecrementWorkerCount(c))
return null;
c = ctl.get(); // Re-read ctl
if (runStateOf(c) != rs)
continue retry;
// else CAS failed due to workerCount change; retry inner loop
}
try {
2.判断是阻塞一定时间还是一直阻塞直到取到任务.
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
if (r != null)
return r;
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}
这里通过多重判断,来决定线程是阻塞在这里等任务,还是存活keepAliveTime.
1.如果线程数是最大线程数,并且任务队列为空,那么阻塞空闲线程还是存活keepAliveTime时间,期间没有任务到来,让其向后执行,自然中断.
2.如果线程数是核心线程数,根据allowCoreThreadTimeOut处理。true情况下阻塞keepAliveTime时间,期间没有任务到来,让其向后执行,自然中断.false情况下,一直阻塞,直到有任务加入队列.
三.线程池停止
1.shutDown分析
public void shutdown() {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
1.这里应该是做一些权限检查
checkShutdownAccess();
2.利用CAS将变线程池转变成shutDown状态
advanceRunState(SHUTDOWN);
3.中断空闲线程
interruptIdleWorkers();
onShutdown(); // hook for ScheduledThreadPoolExecutor
} finally {
mainLock.unlock();
}
4.最后尝试将线程池状态设置为TIDYING状态
tryTerminate();
}
interruptIdleWorkers
private void interruptIdleWorkers(boolean onlyOne) {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
for (Worker w : workers) {
Thread t = w.thread;
1.首先线程不是中断状态,尝试获取线程的锁.这里tryLock的实现就决定了
if (!t.isInterrupted() && w.tryLock()) {
try {
2.如果能拿到锁终端线程
t.interrupt();
} catch (SecurityException ignore) {
} finally {
w.unlock();
}
}
if (onlyOne)
break;
}
} finally {
mainLock.unlock();
}
}
这里是先获取线程锁,获取不到不终止,所以不能中断任务中的线程,并且会把队列中的任务处理完
tryLock
tryLock里面调用了tryAcquire
protected boolean tryAcquire(int unused) {
1.这里采用了独占锁的实现方式 0到1才能获取锁.所以保证了正在执行任务的线程不会被shutdown中断
if (compareAndSetState(0, 1)) {
setExclusiveOwnerThread(Thread.currentThread());
return true;
}
return false;
}
这是自定义了aqs获取锁的方式
2.shutDownNow分析
public List shutdownNow() {
List tasks;
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
checkShutdownAccess();
advanceRunState(STOP);
1.第1个区别尝试中断所有线程
interruptWorkers();
1.第2个区别,把队列中的任务返回给调用者处理
tasks = drainQueue();
} finally {
mainLock.unlock();
}
tryTerminate();
return tasks;
}
interruptWorkers();
private void interruptWorkers() {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
for (Worker w : workers)
1.这里就粗暴多了,直接终止,可能导致有些线程不能正常终止,处于异常状态.
w.interruptIfStarted();
} finally {
mainLock.unlock();
}
}
====================================================
void interruptIfStarted() {
Thread t;
if (getState() >= 0 && (t = thread) != null && !t.isInterrupted()) {
try {
t.interrupt();
} catch (SecurityException ignore) {
}
}
}
直接粗暴的终止线程,不管线程上没上锁.
线程池常见问题
1.线程池对线程包装的作用
2.关键参数(allowCoreThreadTimeOut,keepAliveTime,corePoolSize,maximumPoolSize,ctl)
3.关键技术(CAS,AbstractQueuedSynchronizer,ReentrantLock,LinkedB lockingQueue)
4.线程池线程数动态变化过程
5.shutDown和shutDownNow执行上有什么区别
6.excute和submit区别
7.核心线程是什么情况下减少到0