一、常见的线程池
FixedThreadPool、CachedThreadPool、ScheduledThreadPool、SingleThreadExecutor
这些常见的线程池，基本都是通过Executors中对应的new方法进行创建的。
1.FixedThreadPool
核心线程数固定，没有非核心线程，LinkedBlockingQueue 无界的Queue
public static ExecutorService newFixedThreadPool(int nThreads, ThreadFactory threadFactory) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue(),
threadFactory);
}
2.CachedThreadPool
只有非核心线程，60s的空闲线程保活，SynchronousQueue，直接提交给线程运行。OkHttp中的线程池就是用的CachedThreadPool，核心线程数是0，最大线程数是Integer.MAX_VALUE，使用SynchronousQueue队列，SynchronousQueue队列是0容量的阻塞队列，那么就会直接交给线程执行
public static ExecutorService newCachedThreadPool(ThreadFactory threadFactory) {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue(),
threadFactory);
}
3.ScheduledThreadPool
有延迟执行和周期重复执行的线程池，new DelayedQueue ()
DelayedQueue ： 优先队列 PriorityQueue 存储元素
public static ScheduledExecutorService newScheduledThreadPool(
int corePoolSize, ThreadFactory threadFactory) {
return new ScheduledThreadPoolExecutor(corePoolSize, threadFactory);
}
// ScheduledThreadPoolExecutor.java
public ScheduledThreadPoolExecutor(int corePoolSize,
ThreadFactory threadFactory) {
super(corePoolSize, Integer.MAX_VALUE,
DEFAULT_KEEPALIVE_MILLIS, MILLISECONDS,
new DelayedWorkQueue(), threadFactory);
}
ScheduledThreadPoolExecutor继承自ThreadPoolExecutor
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue 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;
}
ScheduledThreadPool实现周期性原理
public ScheduledFuture scheduleWithFixedDelay(Runnable command,
long initialDelay,
long delay,
TimeUnit unit) {
if (command == null || unit == null)
throw new NullPointerException();
if (delay <= 0L)
throw new IllegalArgumentException();
ScheduledFutureTask sft =
new ScheduledFutureTask(command,
null,
triggerTime(initialDelay, unit),
-unit.toNanos(delay),
sequencer.getAndIncrement());
RunnableScheduledFuture t = decorateTask(command, sft);
sft.outerTask = t;
// 将任务添加到DelayWorkQueue队列中
delayedExecute(t);
return t;
}
获取DelayWorkQueue队列中的任务
public RunnableScheduledFuture take() throws InterruptedException {
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
for (;;) {
RunnableScheduledFuture first = queue[0];
if (first == null)
available.await();
else {
long delay = first.getDelay(NANOSECONDS);
if (delay <= 0L)
return finishPoll(first);
first = null; // don't retain ref while waiting
if (leader != null)
available.await();
else {
Thread thisThread = Thread.currentThread();
leader = thisThread;
try {
available.awaitNanos(delay);
} finally {
if (leader == thisThread)
leader = null;
}
}
}
}
} finally {
if (leader == null && queue[0] != null)
available.signal();
lock.unlock();
}
}
执行任务
public void run() {
boolean periodic = isPeriodic();
if (!canRunInCurrentRunState(periodic))
cancel(false);
else if (!periodic)
super.run();
else if (super.runAndReset()) {
// 判断是否需要周期性执行，如果是
// 则重置下一次要执行的时间
setNextRunTime();
reExecutePeriodic(outerTask);
}
}
private void setNextRunTime() {
long p = period;
if (p > 0)
time += p;
else
time = triggerTime(-p);
}
放回到队列中
public boolean offer(Runnable x) {
if (x == null)
throw new NullPointerException();
RunnableScheduledFuture e = (RunnableScheduledFuture)x;
final ReentrantLock lock = this.lock;
lock.lock();
try {
int i = size;
if (i >= queue.length)
grow();
size = i + 1;
if (i == 0) {
queue[0] = e;
setIndex(e, 0);
} else {
siftUp(i, e);
}
if (queue[0] == e) {
leader = null;
available.signal();
}
} finally {
lock.unlock();
}
return true;
}
主要就是通过修改ScheduledFutureTask的time值，而time是一个volatile修饰，time的值的修改则是通过优先级period的值来判断对time加上还是减去period值
4.SingleThreadExecutor
LinkedBlockingQueue
一个核心线程，不需要处理同步
public static ExecutorService newSingleThreadExecutor(ThreadFactory threadFactory) {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue(),
threadFactory));
}
二、饱和策略
RejectedExecutionHandler 饱和策略
（1）DiscardOldestPolicy：直接丢弃最老的那个任务，执行当前任务。如果执行程序尚未关闭，则位于工作队列头部的任务将被删除，然后重试执行程序（如果再次失败，则重复此过程） 意思就是在pool没有关闭的前提下首先丢掉缓存在队列中的最早的任务，然后重新尝试运行该任务。这个策略需要适当小心。
（2）AbortPolicy：直接抛出异常，默认的使用，即拒绝执行任务RejectedExecutionException。
（3）CallerRunsPolicy：让调用者线程去执行任务，即谁往线程池中提交任务，就由谁来执行这个任务。此策略提供简单的反馈控制机制，能够减缓新任务的提交速度
（4）DiscardPolicy：把最新提交的任务丢弃
如果这这四种拒绝策略都不满足，则自己实现RejectedExecutionHandler接口，自己定义一个拒绝策略。