线程池不允许使用Executors去创建，而是通过ThreadPoolExecutor的方式，这样的处理方式让写的同学更加明确线程池的运行规则，规避资源耗尽的风险。
Executors返回的线程池对象的弊端如下：
1）FixedThreadPool和SingleThreadPool:
允许的请求队列长度为Integer.MAX_VALUE，可能会堆积大量的请求，从而导致OOM。
2）CachedThreadPool和ScheduledThreadPool:
允许的创建线程数量为Integer.MAX_VALUE，可能会创建大量的线程，从而导致OOM。
以前代码里面 Executors使用的多一些，ThreadPoolExecutor也有使用，但是对其原理和代码了解的却不多。最近通过看源码，了解了一下。
ThreadPoolExecutor的创建
可以通过调用构造函数来创建一个线程池。它有4个构造函数，其中参数最全的是下面这个
/**
* Creates a new {@code ThreadPoolExecutor} with the given initial
* parameters.
*
* @param corePoolSize the number of threads to keep in the pool, even
* if they are idle, unless {@code allowCoreThreadTimeOut} is set
* @param maximumPoolSize the maximum number of threads to allow in the
* pool
* @param keepAliveTime when the number of threads is greater than
* the core, this is the maximum time that excess idle threads
* will wait for new tasks before terminating.
* @param unit the time unit for the {@code keepAliveTime} argument
* @param workQueue the queue to use for holding tasks before they are
* executed. This queue will hold only the {@code Runnable}
* tasks submitted by the {@code execute} method.
* @param threadFactory the factory to use when the executor
* creates a new thread
* @param handler the handler to use when execution is blocked
* because the thread bounds and queue capacities are reached
* @throws IllegalArgumentException if one of the following holds:<br>
* {@code corePoolSize < 0}<br>
* {@code keepAliveTime < 0}<br>
* {@code maximumPoolSize <= 0}<br>
* {@code maximumPoolSize < corePoolSize}
* @throws NullPointerException if {@code workQueue}
* or {@code threadFactory} or {@code handler} is null
*/
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler) {
if (corePoolSize < 0 ||
maximumPoolSize <= 0 ||
maximumPoolSize < corePoolSize ||
keepAliveTime < 0)
throw new IllegalArgumentException();
if (workQueue == null || threadFactory == null || handler == null)
throw new NullPointerException();
this.corePoolSize = corePoolSize;
this.maximumPoolSize = maximumPoolSize;
this.workQueue = workQueue;
this.keepAliveTime = unit.toNanos(keepAliveTime);
this.threadFactory = threadFactory;
this.handler = handler;
}
具体解释一下上述参数：
-
corePoolSize线程池核心线程数大小，初始化是核心线程数也是0，除非先调用prestartCoreThread或者prestartAllCoreThreads先创建核心线程；在没有设置allowCoreThreadTimeOut为true情况下，核心线程不会销毁 -
maximumPoolSize线程池线程数最大值，达到最大值后线程池不会再增加线程执行任务 -
keepAliveTime线程池空闲时，线程存活的时间 -
TimeUnit时间单位 -
ThreadFactory线程工厂 -
BlockingQueue任务队列 -
RejectedExecutionHandler任务拒绝策略；负责处理当线程饱后、线程池正在关闭时的新提交任务；
ThreadPoolExecutor内部有实现4个拒绝策略：
- (1)、CallerRunsPolicy，由调用execute方法提交任务的线程来执行这个任务；
- (2)、AbortPolicy，抛出异常RejectedExecutionException拒绝提交任务；
- (3)、DiscardPolicy，直接抛弃任务，不做任何处理；
- (4)、DiscardOldestPolicy，去除任务队列中的第一个任务，重新提交；
一个变量控制线程状态和线程池容量
ThreadPoolExecutor有如下 5种状态：
private static final int RUNNING = -1 << COUNT_BITS;
private static final int SHUTDOWN = 0 << COUNT_BITS;
private static final int STOP = 1 << COUNT_BITS;
private static final int TIDYING = 2 << COUNT_BITS;
private static final int TERMINATED = 3 << COUNT_BITS;
其中
private static final int COUNT_BITS = Integer.SIZE - 3; //Integer.SIZE=32
所以COUNT_BITS=29。
实际上线程池的5种状态是使用Integer的高三位。其10机制数分别是
RUNNING=111
SHUTDOWN=000
STOP=001
TIDYING=010
TERMINATED=110
这样线程池的状态和线程数量就尽由一个变量存储：
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0)); //使用AtomicInteger 当然是为了保证多线程同步问题
ctl 可以理解为control（控制），初始值为线程数0，状态RUNNING；
线程池的执行
线程池执行一个线程有两种方法: execute 和submit。下面看一下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) {
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);
}
通过代码可以看出流程图如下：
- 1、调用ThreadPoolExecutor的execute提交线程，首先检查CorePool，如果CorePool内的线程小于CorePoolSize，新创建线程执行任务。
- 2、如果当前CorePool内的线程大于等于CorePoolSize，那么将线程加入到BlockingQueue。
- 3、如果不能加入BlockingQueue，在小于MaxPoolSize的情况下创建线程执行任务。
- 4、如果线程数大于等于MaxPoolSize，那么执行拒绝策略。
核心方法addWorker
/**
* Checks if a new worker can be added with respect to current
* pool state and the given bound (either core or maximum). If so,
* the worker count is adjusted accordingly, and, if possible, a
* new worker is created and started, running firstTask as its
* first task. This method returns false if the pool is stopped or
* eligible to shut down. It also returns false if the thread
* factory fails to create a thread when asked. If the thread
* creation fails, either due to the thread factory returning
* null, or due to an exception (typically OutOfMemoryError in
* Thread.start()), we roll back cleanly.
*
* @param firstTask the task the new thread should run first (or
* null if none). Workers are created with an initial first task
* (in method execute()) to bypass queuing when there are fewer
* than corePoolSize threads (in which case we always start one),
* or when the queue is full (in which case we must bypass queue).
* Initially idle threads are usually created via
* prestartCoreThread or to replace other dying workers.
*
* @param core if true use corePoolSize as bound, else
* maximumPoolSize. (A boolean indicator is used here rather than a
* value to ensure reads of fresh values after checking other pool
* state).
* @return true if successful
*/
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 {
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();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
代码中首先进行一次线程池状态的检测
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
这段代码初看有点难懂。如果把代码改成如下可能会好理解一些
if (rs >= SHUTDOWN &&
(rs == !SHUTDOWN ||
firstTask == !null ||
workQueue.isEmpty()))
{
return false;
}
大致逻辑是：
- 如果rs>=SHUTDOWN,同时不等于SHUTDOWN，即为SHUTDOWN以上的状态，那么不接受新线程。
- 如果rs>=SHUTDOWN，同时等于SHUTDOWN（其实就是SHUTDOWN），同时firstTask！=null，那么拒绝新线程；
- 如果rs>=SHUTDOWN，同时等于SHUTDOWN（其实就是SHUTDOWN），同时firstTask ==null，那么可能是新增加线程消耗Queue中的线程。但是同时还要检测workQueue是否isEmpty()，如果为Empty，那么队列已空，不需要增加消耗线程，如果队列没有空那么运行增加first=null的Worker。
从这里是可以总结出线程池增加线程的策略：
首先，在rs>SHUTDOWN时，拒绝一切线程的增加，因为STOP是会终止所有的线程，同时移除Queue中所有的待执行的线程的，所以也不需要增加first=null的Worker了;
其次，在SHUTDOWN状态时，是不能增加first！=null的Worker的，同时即使first=null，但是此时Queue为Empty也是不允许增加Worker的，SHUTDOWN下增加的Worker主要用于消耗Queue中的任务。
SHUTDOWN状态时，是不允许向workQueue中增加线程的，isRunning(c) && workQueue.offer(command) 每次在offer之前都要做状态检测，也就是线程池状态变为>=SHUTDOWN时不允许新线程进入线程池了。
通过上面的分析，对线程池的运行基本了解了。后续会补充一下线程状态的切换。
