开篇
ThreadPoolExecutor还是很值得仔细看一看的，所以准备分开两篇文章来说明，这篇文章用来说明ThreadPoolExecutor的构造函数、拒绝策略、以及一些额外的线程池工厂的一些细节。下一篇文章会着重说明ThreadPoolExecutor的执行。
java源码-ThreadPoolExecutor（1）
java源码-ThreadPoolExecutor（2）
java源码-ThreadPoolExecutor（3）
ThreadPoolExecutor类结构
从ThreadPoolExecutor的类继承关系来看：
- 接口Executor和ExecutorService都是接口定义。
- 接口Executor只有一个execute()方法。
- 接口ExecutorService增加了一些状态判断和任务提交接口。
- 类AbstractExecutorService实现了定义中的部分接口包括submit方法且新增doInvokeAny相关方法。
- 类ThreadPoolExecutor继承AbstractExecutorService实现了其他在Executor和ExecutorService中定义的接口
public interface Executor {
void execute(Runnable command);
}
public interface ExecutorService extends Executor {
void shutdown();
List<Runnable> shutdownNow();
boolean isShutdown();
boolean isTerminated();
boolean awaitTermination(long timeout, TimeUnit unit)
throws InterruptedException;
<T> Future<T> submit(Callable<T> task);
<T> Future<T> submit(Runnable task, T result);
Future<?> submit(Runnable task);
<T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
throws InterruptedException;
<T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks,
long timeout, TimeUnit unit)
throws InterruptedException;
<T> T invokeAny(Collection<? extends Callable<T>> tasks)
throws InterruptedException, ExecutionException;
<T> T invokeAny(Collection<? extends Callable<T>> tasks,
long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;
}
public abstract class AbstractExecutorService implements ExecutorService {
protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
return new FutureTask<T>(runnable, value);
}
protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
return new FutureTask<T>(callable);
}
public Future<?> submit(Runnable task) {
if (task == null) throw new NullPointerException();
RunnableFuture<Void> ftask = newTaskFor(task, null);
execute(ftask);
return ftask;
}
public <T> Future<T> submit(Runnable task, T result) {
if (task == null) throw new NullPointerException();
RunnableFuture<T> ftask = newTaskFor(task, result);
execute(ftask);
return ftask;
}
public <T> Future<T> submit(Callable<T> task) {
if (task == null) throw new NullPointerException();
RunnableFuture<T> ftask = newTaskFor(task);
execute(ftask);
return ftask;
}
private <T> T doInvokeAny(Collection<? extends Callable<T>> tasks,
boolean timed, long nanos)
throws InterruptedException, ExecutionException, TimeoutException {
// 省略相关代码
}
public <T> T invokeAny(Collection<? extends Callable<T>> tasks)
throws InterruptedException, ExecutionException {
try {
return doInvokeAny(tasks, false, 0);
} catch (TimeoutException cannotHappen) {
assert false;
return null;
}
}
public <T> T invokeAny(Collection<? extends Callable<T>> tasks,
long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException {
return doInvokeAny(tasks, true, unit.toNanos(timeout));
}
public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
throws InterruptedException {
// 省略相关代码
}
public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks,
long timeout, TimeUnit unit)
throws InterruptedException {
// 省略相关代码
}
}
public class ThreadPoolExecutor extends AbstractExecutorService {
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
private static final int COUNT_BITS = Integer.SIZE - 3;
private static final int CAPACITY = (1 << COUNT_BITS) - 1;
// runState is stored in the high-order bits
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;
private static int runStateOf(int c) { return c & ~CAPACITY; }
private static int workerCountOf(int c) { return c & CAPACITY; }
private static int ctlOf(int rs, int wc) { return rs | wc; }
}
ThreadPoolExecutor的类定义
ThreadPoolExecutor的构造函数
ThreadPoolExecutor的核心构造函数其实只有一个，见下面的代码注释。然后需要解释的是构造函数的几个参数：
corePoolSize
核心线程数，默认情况下核心线程会一直存活，即使处于闲置状态也不会受存keepAliveTime限制。
maximumPoolSize
线程池所能容纳的最大线程数。超过这个数的线程将被阻塞。当任务队列为没有设置大小的LinkedBlockingDeque时，这个值无效。
keepAliveTime
非核心线程的闲置超时时间，超过这个时间就会被回收。
unit
指定keepAliveTime的单位，如TimeUnit.SECONDS。当将allowCoreThreadTimeOut设置为true时对corePoolSize生效。
workQueue
线程池中的任务队列.
常用的有三种队列，SynchronousQueue,LinkedBlockingDeque,ArrayBlockingQueue。
threadFactory
线程工厂，提供创建新线程的功能。
RejectedExecutionHandler
当线程池中的资源已经全部使用，添加新线程被拒绝时，会调用RejectedExecutionHandler的rejectedExecution方法。
public class ThreadPoolExecutor extends AbstractExecutorService {
private static final RejectedExecutionHandler defaultHandler = new AbortPolicy();
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue) {
// 使用了Executors.defaultThreadFactory()创建了线程池工厂
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
Executors.defaultThreadFactory(), defaultHandler);
}
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
threadFactory, defaultHandler);
}
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
RejectedExecutionHandler handler) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
Executors.defaultThreadFactory(), handler);
}
// 核心构造函数，核心构造函数，核心构造函数，重要事情说三遍！！！
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.acc = System.getSecurityManager() == null ?
null :
AccessController.getContext();
this.corePoolSize = corePoolSize;
this.maximumPoolSize = maximumPoolSize;
this.workQueue = workQueue;
this.keepAliveTime = unit.toNanos(keepAliveTime);
this.threadFactory = threadFactory;
this.handler = handler;
}
}
在ThreadPoolExecutor源码中额外的收获就是线程池工厂的实现方法了，其实我们有时候在使用Executors的多线程的时候一定要传入线程池工厂的实现方式，这样当我们通过jstack等方法定位问题的时候方便鉴别出线程归属。
Executors的DefaultThreadFactory方法内部通过静态变量tatic final AtomicInteger poolNumber来保证DefaultThreadFactory的实例能够有唯一的id标识，至于线程池工厂创建的线程会自带线程池的前缀以及自增id。
这也算额外的收获吧。
public class Executors {
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}
public static ThreadFactory defaultThreadFactory() {
return new DefaultThreadFactory();
}
static class DefaultThreadFactory implements ThreadFactory {
private static final AtomicInteger poolNumber = new AtomicInteger(1);
private final ThreadGroup group;
private final AtomicInteger threadNumber = new AtomicInteger(1);
private final String namePrefix;
DefaultThreadFactory() {
SecurityManager s = System.getSecurityManager();
group = (s != null) ? s.getThreadGroup() :
Thread.currentThread().getThreadGroup();
namePrefix = "pool-" +
poolNumber.getAndIncrement() +
"-thread-";
}
public Thread newThread(Runnable r) {
Thread t = new Thread(group, r,
namePrefix + threadNumber.getAndIncrement(),
0);
if (t.isDaemon())
t.setDaemon(false);
if (t.getPriority() != Thread.NORM_PRIORITY)
t.setPriority(Thread.NORM_PRIORITY);
return t;
}
}
}
RejectedExecutionHandler是一个接口，里面只有一个方法。当要创建的线程数量大于线程池的最大线程数的时候，就会调用这个接口里的这个方法实现任务的拒绝。可以自定义这个接口，实现对这些超出数量的任务的处理。
ThreadPoolExecutor自己已经提供了四个拒绝策略，分别是
- CallerRunsPolicy：CallerRunsPolicy在任务被拒绝添加后，会调用当前线程池的所在的线程去执行被拒绝的任务
- AbortPolicy：ThreadPoolExecutor中默认的拒绝策略就是AbortPolicy，直接抛出异常。
- DiscardPolicy：让被线程池拒绝的任务直接抛弃，不会抛异常也不会执行。
- DiscardOldestPolicy：抛弃任务队列中最旧的任务也就是最先加入队列的，再把这个新任务添加进去。
public interface RejectedExecutionHandler {
void rejectedExecution(Runnable r, ThreadPoolExecutor executor);
}
public static class AbortPolicy implements RejectedExecutionHandler {
public AbortPolicy() { }
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
throw new RejectedExecutionException("Task " + r.toString() +
" rejected from " +
e.toString());
}
}
public static class DiscardPolicy implements RejectedExecutionHandler {
public DiscardPolicy() { }
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
}
}
public static class DiscardOldestPolicy implements RejectedExecutionHandler {
public DiscardOldestPolicy() { }
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
if (!e.isShutdown()) {
e.getQueue().poll();
e.execute(r);
}
}
}
public static class CallerRunsPolicy implements RejectedExecutionHandler {
public CallerRunsPolicy() { }
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
if (!e.isShutdown()) {
r.run();
}
}
}
参考文章
Java多线程-线程池ThreadPoolExecutor构造方法和规则
java多线程-ThreadPoolExecutor的拒绝策略RejectedExecutionHandler
