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在研究ThreadPoolExecutor线程池的时候,发现可以有两种启动线程的方法:submit(Runnable runnable),excute(Runnable runnable)
先说个结论吧:
submit()方法,可以提供Future < T > 类型的返回值。
executor()方法,无返回值。
首先,我们看一下ThreadPoolExecutor的类图:
ThreadPoolExecutor的抽象父类AbstractExecutorService实现了接口ExecutorService和Executor,而submit方法是ExecutorService中的接口,executor方法是Executor中的接口。
查看AbstractExecutorService源码,发现submit方法是在此抽象父类中实现:
/**
* @throws RejectedExecutionException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*/
public Future<?> submit(Runnable task) {
if (task == null) throw new NullPointerException();
RunnableFuture<Void> ftask = newTaskFor(task, null);
execute(ftask);
return ftask;
}
/**
* @throws RejectedExecutionException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*/
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;
}
/**
* @throws RejectedExecutionException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*/
public <T> Future<T> submit(Callable<T> task) {
if (task == null) throw new NullPointerException();
RunnableFuture<T> ftask = newTaskFor(task);
execute(ftask);
return ftask;
}
可以看到,submit方法是先构造出一个RunnableFuture(FutureTask) 然后调用execute方法。不管你submit的时候传入的是Runnable还是Callable最后RunnableFuture(FutureTask)里面都会生成Callable对象。任务调用的时候调用RunnableFuture(FutureTask)的run方法,run方法调用Callable对象的call方法。
submit()方法,可以提供Future < T > 类型的返回值。
ThreadPoolExecutor中的executor方法:
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);
}
executor()方法,无返回值。