一、线程池的属性
先来看一下线程池的构造方法，总结就是7大属性，这里Doug Lea写的很清楚了，下面简要翻译一下加深印象
/**
* 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;
}
以下属性都是volatile关键字修饰，好处是基于内存屏障避免编译器指令重排造成变量取值不准，同时保证发生更改后的主存立即写入主存，保证其他线程从内存而不是本身私有的栈空间读取准确值
corePoolSize：the number of threads to keep in the pool, even if they are idle, unless {@code allowCoreThreadTimeOut} is set （核心线程数：不管它们创建以后是不是空闲的。线程池需要保持 corePoolSize 数量的线程，除非设置了 allowCoreThreadTimeOut，注意这个数字允许为0。）
maximumPoolSize：the maximum number of threads to allow in the pool。（最大线程数：线程池中最多允许创建 maximumPoolSize 个线程，注意必须大于0，并且需保证maximumPoolSize >= corePoolSize。）
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。（存活时间：根据unit而定，最终转换为单位nanoseconds（纳秒，千分之一微秒），如果经过 keepAliveTime 时间后，超过核心线程数的线程还没有接受到新的任务，那就回收，注意这里没说核心工作线程什么时候回收。）
unit：the time unit for the {@code keepAliveTime} argument （keepAliveTime 的时间单位，不能为负，TimeUnit类定义在J.U.C，里面多处有用到。）
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。（存放等待执行任务的队列：当提交的任务数超过核心线程数大小后，再提交的任务就存放在这里。它仅仅用来存放被 execute 方法提交的 Runnable 任务。）
threadFactory：the factory to use when the executor creates a new thread。（线程工程：用来创建线程工厂。比如这里面可以自定义线程名称，当进行虚拟机栈分析时，看着名字就知道这个线程是哪里来的，不会懵逼。）
handler ：the handler to use when execution is blocked because the thread bounds and queue capacities are reached。（拒绝策略：当队列里面放满了任务、最大线程数的线程都在工作时，这时继续提交的任务线程池就处理不了，应该执行怎么样的拒绝策略。）
二、线程池的状态
/**
* The runState provides the main lifecycle control, taking on values:
*
* RUNNING: Accept new tasks and process queued tasks
* SHUTDOWN: Don't accept new tasks, but process queued tasks
* STOP: Don't accept new tasks, don't process queued tasks,
* and interrupt in-progress tasks
* TIDYING: All tasks have terminated, workerCount is zero,
* the thread transitioning to state TIDYING
* will run the terminated() hook method
* TERMINATED: terminated() has completed
*
* The numerical order among these values matters, to allow
* ordered comparisons. The runState monotonically increases over
* time, but need not hit each state. The transitions are:
*
* RUNNING -> SHUTDOWN
* On invocation of shutdown(), perhaps implicitly in finalize()
* (RUNNING or SHUTDOWN) -> STOP
* On invocation of shutdownNow()
* SHUTDOWN -> TIDYING
* When both queue and pool are empty
* STOP -> TIDYING
* When pool is empty
* TIDYING -> TERMINATED
* When the terminated() hook method has completed
*/
- RUNNING
- 状态说明：线程池处在RUNNING状态时，能够接收新任务，以及对已添加的任务进行处理。
- 状态切换：线程池的初始化状态是RUNNING。换句话说，线程池被一旦被创建，就处于RUNNING状态，并且线程池中的任务数为0！
- SHUTDOWN
- 状态说明：线程池处在SHUTDOWN状态时，不接收新任务，但能处理已添加的任务。
- 状态切换：调用线程池的shutdown()接口时，线程池由RUNNING -> SHUTDOWN。
- STOP
- 状态说明：线程池处在STOP状态时，不接收新任务，不处理已添加的任务，并且会中断正在处理的任务。
- 状态切换：调用线程池的shutdownNow()接口时，线程池由(RUNNING or SHUTDOWN ) -> STOP。
- TIDYING
- 状态说明：当所有的任务已终止，ctl记录的”任务数量”为0，线程池会变为TIDYING状态。当线程池变为TIDYING状态时，会执行钩子函数terminated()。terminated()在ThreadPoolExecutor类中是空的，若用户想在线程池变为TIDYING时，进行相应的处理，可以通过重载terminated()函数来实现。
- 状态切换：当线程池在SHUTDOWN状态下，阻塞队列为空并且线程池中执行的任务也为空时，就会由 SHUTDOWN -> TIDYING。
当线程池在STOP状态下，线程池中执行的任务为空时，就会由STOP -> TIDYING。
- TERMINATED
- 状态说明：线程池彻底终止，就变成TERMINATED状态。
- 状态切换：线程池处在TIDYING状态时，执行完terminated()之后，就会由 TIDYING -> TERMINATED。
三、线程的状态
public enum State {
/**
* Thread state for a thread which has not yet started.
*/
NEW,
/**
* Thread state for a runnable thread. A thread in the runnable
* state is executing in the Java virtual machine but it may
* be waiting for other resources from the operating system
* such as processor.
*/
RUNNABLE,
/**
* Thread state for a thread blocked waiting for a monitor lock.
* A thread in the blocked state is waiting for a monitor lock
* to enter a synchronized block/method or
* reenter a synchronized block/method after calling
* {@link Object#wait() Object.wait}.
*/
BLOCKED,
/**
* Thread state for a waiting thread.
* A thread is in the waiting state due to calling one of the
* following methods:
* <ul>
* <li>{@link Object#wait() Object.wait} with no timeout</li>
* <li>{@link #join() Thread.join} with no timeout</li>
* <li>{@link LockSupport#park() LockSupport.park}</li>
* </ul>
*
* <p>A thread in the waiting state is waiting for another thread to
* perform a particular action.
*
* For example, a thread that has called <tt>Object.wait()</tt>
* on an object is waiting for another thread to call
* <tt>Object.notify()</tt> or <tt>Object.notifyAll()</tt> on
* that object. A thread that has called <tt>Thread.join()</tt>
* is waiting for a specified thread to terminate.
*/
WAITING,
/**
* Thread state for a waiting thread with a specified waiting time.
* A thread is in the timed waiting state due to calling one of
* the following methods with a specified positive waiting time:
* <ul>
* <li>{@link #sleep Thread.sleep}</li>
* <li>{@link Object#wait(long) Object.wait} with timeout</li>
* <li>{@link #join(long) Thread.join} with timeout</li>
* <li>{@link LockSupport#parkNanos LockSupport.parkNanos}</li>
* <li>{@link LockSupport#parkUntil LockSupport.parkUntil}</li>
* </ul>
*/
TIMED_WAITING,
/**
* Thread state for a terminated thread.
* The thread has completed execution.
*/
TERMINATED;
}
- NEW
- 状态说明：创建后未执行
- 状态切换：未调用start()方法
- RUNNABLE
- 状态说明：JVM运行状态，实际是处于运行，或处于就绪（例如等待CPU时间片）
- 状态切换：调用start()方法，或yield()进入就绪，NEW -> RUNABLE <-> BLOCKED
- BLOCKED
- 状态说明：请求monitor lock进入synchronized方法或者程序块，未请求到锁，从而被阻塞
- 状态切换：RUNABLE <-> BLOCKED
- WAITING
- 状态说明：主动挂起等待，看作是无限期
- 状态切换：Object.wait() ，Thread.join() ，LockSupport.park() 进入等待，Object.notify() / Object.notifyAll()，其他线程执行完，LockSupport.unpark(Thread) 解除等待，RUNABLE <-> WAITING
- TIMED_WAITING
- 状态说明：有限期等待
- 状态切换：Thread.sleep() 或调用上面WAITING里面的方法带上超时时间参数，RUNABLE <-> TIMED_WAITING
- TERMINATED
- 状态说明：终止
- 状态切换：run()或主线程执行完毕，RUNABLE -> TERMINATED
