Common Pool对象池的中断策略和ThreadPoolExecutor中断策略
题记：
看到摊子里gogole_09同学分享了他阿里的面筋，其中有个问题提到：线程池的中断策略有哪些？各有什么特点？想想自己也看过Common的对象池源码和用过Sun的ThreadPoolExecutor线程池，却没注意到“中断策略”，这是一个所有缓存池需要考虑的异常问题。
Common Pool中断策略
竟然是中断策略，直接看Common Pool中的borrowObject，从池中取对象，当没有可用的对象的时候的策略。
**StackObjectPool（以Stack为存储结构的Pool）
public synchronized Object borrowObject() throws Exception {
....
Object obj = null;// 取对象
...
obj = _factory.makeObject(); // 制造对象
newlyCreated = true;
if (obj == null) {
throw new NoSuchElementException("PoolableObjectFactory.makeObject() returned null.");
}
....
_numActive++;
return obj;
发现_factory.makeObject(); 这个方法是一个abstract，所以猜测Common Pool并未实现自己的中断策略，而由使用者自行扩展。
ThreadPoolExecutor中断策略
JDK提供了4种策略，分别是：
1、CallerRunsPolicy
2、AbortPolicy 中止任务
3、DiscardPolicy 丢弃任务
4、DiscarOldestPolicy 丢弃最老任务
分析中断策略，直接查看ThreadPoolExecutor类中的execute(Runnable command)设计，如下：
/**
* The default rejected execution handler
*/
private static final RejectedExecutionHandler defaultHandler =
new AbortPolicy();
....
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
for (;;) {
if (runState != RUNNING) {
reject(command);
return;
}
if (poolSize < corePoolSize && addIfUnderCorePoolSize(command))
return;
if (workQueue.offer(command))
return;
Runnable r = addIfUnderMaximumPoolSize(command);// 当线程大小超过maximumPoolSize时，返回null
if (r == command)
return;
if (r == null) {
reject(command);// 执行中断策略
return;
}
// else retry
}
}
reject方法为：
void reject(Runnable command) {
handler.rejectedExecution(command, this);// 直接调用接口RejectedExecutionHandler的rejectedExecution方法
}
4种中断策略的实现，首先，默认的AbortPolicy implements RejectedExecutionHandler
public static class AbortPolicy implements RejectedExecutionHandler {
public AbortPolicy() { }
/**
* Always throws RejectedExecutionException.
* @param r the runnable task requested to be executed
* @param e the executor attempting to execute this task
* @throws RejectedExecutionException always.
*/
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
throw new RejectedExecutionException();// 直接抛出异常
}
}这种策略会让程序抛出异常，会一定程度影响系统运行。
然后CallerRunsPolicy策略：
public static class CallerRunsPolicy implements RejectedExecutionHandler {
public CallerRunsPolicy() { }
/**
* Executes task r in the caller's thread, unless the executor
* has been shut down, in which case the task is discarded.
* @param r the runnable task requested to be executed
* @param e the executor attempting to execute this task
*/
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
if (!e.isShutdown()) {// 只要ThreadPoolExecutor没有关闭，就马上执行新线程，注意这里是run，而非start，新线程会立马执行
r.run();
}
}
}这个策略比较保守，不抛出异常，也不忽略线程，而是让线程直接运行抢占CPU而发生阻塞。
DiscardPolicy策略（忽略当前线程）：
public static class DiscardPolicy implements RejectedExecutionHandler {
/**
* Creates a DiscardPolicy.
*/
public DiscardPolicy() { }
/**
* Does nothing, which has the effect of discarding task r.
* @param r the runnable task requested to be executed
* @param e the executor attempting to execute this task
*/
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {// 直接忽略
}
}这种策略被忽略，而不给出任何警告信息。
最后一个DiscardOldestPolicy策略（忽略最老线程）
public static class DiscardOldestPolicy implements RejectedExecutionHandler {
public DiscardOldestPolicy() { }
/**
* Obtains and ignores the next task that the executor
* would otherwise execute, if one is immediately available,
* and then retries execution of task r, unless the executor
* is shut down, in which case task r is instead discarded.
* @param r the runnable task requested to be executed
* @param e the executor attempting to execute this task
*/
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
if (!e.isShutdown()) {
e.getQueue().poll();// 直接移除当前队列中头节点
e.execute(r);// 把新线程加入到队列的尾节点，并调用线程的start方法启动
}
}
}
结论：
从代码中看：
当线程数量小于pool中活动线程数，则创建新线程；
当线程数量等于pool中活动线程数，则尝试把任务加入到任务队列里面；
当任务队列满的时候，则执行中断策略进行调整。
各自的策略都有不同的特点，根据系统的需求选择合适的策略以达到线程利用率的最优。