ThreadPoolExecutor对象可以通过Executor的工厂方法(newXXX)或者他自己的构造函数来创建。他是Java线程池的实现类。
使用：
//Runnable
fun main(args: Array<String>) {
val pool= Executors.newFixedThreadPool(3)
pool.execute{
Thread.sleep(2000)
println("无返回值")
pool.shutdown()
}
}
//FutureTask
fun main(args: Array<String>) {
val pool= Executors.newFixedThreadPool(3)
val futureTask=FutureTask<String>(Callable<String>{
Thread.sleep(2000)
return@Callable "有返回值"
})
pool.submit(futureTask)
//block current thread
val result=futureTask.get()
println(result)
pool.shutdown()
}
一般用的比较多的都是第一种，创建Runnable实例，通过调用execute()或者submit()，交给线程池内部去处理。
第二种方法用的不多，下文浅析他的工作机制。
submit()
ThreadPoolExecutor的pool.submit(xxx)方法来自于他的父类AbstractExecutorService。
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;
}
在这里可以看到，将传入的无论是Runnable还是Callable类型的变量，统一用一个newTaskFor()方法封装成了RunnableFuture对象，并且统一交给了execute(Runnable command)方法，这样就又回到了最熟悉的使用线程池的方式了。在execute()方法中，让线程池不同的实例去用不同的方法调度工作线程和任务队列。
在这里将RunnableFuture作为Runnable的子类型传递给execute，又作为Future的子类型从submit()方法返回。
他的实例化由newTaskFor()方法负责。
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);
}
全部返回了具体类型FutureTask的子类 。
FutureTask
uml类图如下：
在这里关注FutureTask的三个方法：
- 构造方法
- run()方法。（作为Runnable子类传递给线程池的直接执行方法）
- get()方法。（客户端调用，阻塞当前线程，获取执行结果）
1. 构造方法
public FutureTask(Callable<V> callable) {
if (callable == null)
throw new NullPointerException();
this.callable = callable;
this.state = NEW; // ensure visibility of callable
}
public FutureTask(Runnable runnable, V result) {
//用Executors.callable将runnable转换成callable对象。
this.callable = Executors.callable(runnable, result);
this.state = NEW; // ensure visibility of callable
}
构造方法：注入Callable对象，将当前状态标志为NEW。
2. get()方法
public V get() throws InterruptedException, ExecutionException {
int s = state;
if (s <= COMPLETING)
//阻塞当前线程
s = awaitDone(false, 0L);
//当线程从阻塞状态被唤醒，将结果返回。
return report(s);
}
//Awaits completion or aborts on interrupt or timeout.
private int awaitDone(boolean timed, long nanos)
throws InterruptedException {
final long deadline = timed ? System.nanoTime() + nanos : 0L;
WaitNode q = null;
boolean queued = false;
for (;;) {
if (Thread.interrupted()) {
removeWaiter(q);
throw new InterruptedException();
}
int s = state;
if (s > COMPLETING) {
if (q != null)
q.thread = null;
return s;
}
else if (s == COMPLETING) // cannot time out yet
Thread.yield();
else if (q == null)
q = new WaitNode();
else if (!queued)
queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
q.next = waiters, q);
else if (timed) {
nanos = deadline - System.nanoTime();
if (nanos <= 0L) {
removeWaiter(q);
return state;
}
LockSupport.parkNanos(this, nanos);
}
else
//阻塞当前线程
LockSupport.park(this);
}
}
这里是get()-->awaitDone()-->LockSupport.park(this)。
在LockSupport.park()方法内部，获取当前线程，并阻塞。
那么当线程从阻塞状态唤醒时（run方法执行完毕主动唤醒，或者遇到异常被唤醒），调用report(int s)方法将执行结果返回。
/** The result to return or exception to throw from get() */
private Object outcome; // non-volatile, protected by state reads/writes
private V report(int s) throws ExecutionException {
Object x = outcome;
if (s == NORMAL)
return (V)x;
if (s >= CANCELLED)
throw new CancellationException();
throw new ExecutionException((Throwable)x);
}
读取实例变量outcome的值，并返回。
到这里就应该猜到，outcome的值的写入应该发生在run()方法里面了。
3. run()方法
public void run() {
if (state != NEW ||
!UNSAFE.compareAndSwapObject(this, runnerOffset,
null, Thread.currentThread()))
return;
try {
Callable<V> c = callable;
if (c != null && state == NEW) {
V result;
boolean ran;
try {
//调用callback的方法，并获取执行结果
result = c.call();
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
setException(ex);
}
if (ran)
set(result);//将执行结果设置到实例变量中。
}
} finally {
// runner must be non-null until state is settled to
// prevent concurrent calls to run()
runner = null;
// state must be re-read after nulling runner to prevent
// leaked interrupts
int s = state;
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
}
在这里直接调用callback.call方法执行其内部代码，然后将结果用set()保存起来
protected void set(V v) {
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
//保存执行结果
outcome = v;
UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
//唤醒当前线程。
finishCompletion();
}
}
/**
* Removes and signals all waiting threads, invokes done(), and
* nulls out callable.
*/
private void finishCompletion() {
// assert state > COMPLETING;
for (WaitNode q; (q = waiters) != null;) {
if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
for (;;) {
Thread t = q.thread;
if (t != null) {
q.thread = null;
//唤醒线程
LockSupport.unpark(t);
}
WaitNode next = q.next;
if (next == null)
break;
q.next = null; // unlink to help gc
q = next;
}
break;
}
}
done();
callable = null; // to reduce footprint
}
机制分析结束，在使用上面，FutureTask和Runnable的最大不同就是，可以用get()方法阻塞当前线程并获取执行结果。
