JAVA線程池 submit方法返回值
AbstractExecutorService
public abstract class AbstractExecutorService implements ExecutorService {
// RunnableFuture 是用於獲取執行結果的,我們常用它的子類 FutureTask
// 下面兩個 newTaskFor 方法用於將我們的任務包裝成 FutureTask 提交到線程池中執行
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);
}
// 提交任務
public Future<?> submit(Runnable task) {
if (task == null) throw new NullPointerException();
// 1. 將任務包裝成 FutureTask
RunnableFuture<Void> ftask = newTaskFor(task, null);
// 2. 交給執行器執行,execute 方法由具體的子類來實現
// 前面也說了,FutureTask 間接實現了Runnable 接口。
execute(ftask);
return ftask;
}
public <T> Future<T> submit(Runnable task, T result) {
if (task == null) throw new NullPointerException();
// 1. 將任務包裝成 FutureTask
RunnableFuture<T> ftask = newTaskFor(task, result);
// 2. 交給執行器執行
execute(ftask);
return ftask;
}
public <T> Future<T> submit(Callable<T> task) {
if (task == null) throw new NullPointerException();
// 1. 將任務包裝成 FutureTask
RunnableFuture<T> ftask = newTaskFor(task);
// 2. 交給執行器執行
execute(ftask);
return ftask;
}
}
RunnableFuture
public interface RunnableFuture<V> extends Runnable, Future<V> {
/**
* Sets this Future to the result of its computation
* unless it has been cancelled.
*/
void run();
}
FutureTask
public class FutureTask<V> implements RunnableFuture<V> {
private volatile int state;
private static final int NEW = 0; //初始狀態
private static final int COMPLETING = 1; //結果計算完成或響應中斷到賦值給返回值之間的狀態。
private static final int NORMAL = 2; //任務正常完成,結果被set
private static final int EXCEPTIONAL = 3; //任務拋出異常
private static final int CANCELLED = 4; //任務已被取消
private static final int INTERRUPTING = 5; //線程中斷狀態被設置ture,但線程未響應中斷
private static final int INTERRUPTED = 6; //線程已被中斷
//將要執行的任務
private Callable<V> callable; //用於get()返回的結果,也可能是用於get()方法拋出的異常
private Object outcome; // non-volatile, protected by state reads/writes //執行callable的線程,調用FutureTask.run()方法通過CAS設置
private volatile Thread runner; //棧結構的等待隊列,該節點是棧中的最頂層節點。
private volatile WaitNode waiters;
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) {
this.callable = Executors.callable(runnable, result);
this.state = NEW; // ensure visibility of callable
}
protected void set(V v) {
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = v;
UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
finishCompletion();
}
}
public V get() throws InterruptedException, ExecutionException {
int s = state;
if (s <= COMPLETING)
s = awaitDone(false, 0L);
return report(s);
}
/**
* @throws CancellationException {@inheritDoc}
*/
public V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException {
if (unit == null)
throw new NullPointerException();
int s = state;
if (s <= COMPLETING &&
(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
throw new TimeoutException();
return report(s);
}
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);
}
public void run() {
//保證callable任務只被運行一次
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 {
//執行任務,上面的例子我們可以看出,call()裏面可能是一個耗時的操作,不過這裏是同步的
result = c.call();
//上面的call()是同步的,只有上面的result有了結果纔會繼續執行
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
setException(ex);
}
if (ran)
//執行完了,設置result
set(result);
}
}
finally {
runner = null;
int s = state;
//判斷該任務是否正在響應中斷,如果中斷沒有完成,則等待中斷操作完成
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
}
....
}
Excutor內部方法
public static <T> Callable<T> callable(Runnable task, T result) {
if (task == null)
throw new NullPointerException();
return new RunnableAdapter<T>(task, result);
}
Excutor內部類RunnableAdapter
static final class RunnableAdapter<T> implements Callable<T> {
final Runnable task;
final T result;
RunnableAdapter(Runnable task, T result) {
this.task = task;
this.result = result;
}
public T call() {
task.run();
return result;
}
}
爲什麼AbstractExecutorService中submit方法內部使用的是excute,但是卻有返回值
傳入的任務如果是實現Runnable接口的,會通過RunnableAdapter方式轉換成Callable
最終執行的是FutureTask中的run方法,結果賦值給Object outcome,通過get獲取,返回outcome.