Java常見面試題—Future與FutureTask
Future
Future就是對於具體的Runnable或者Callable任務的執行結果進行取消、查詢是否完成、獲取結果等操作。必要時可以通過get方法獲取執行結果,該方法會阻塞直到任務返回結果。
Future類位於java.util.concurrent包下,它是一個介面:
/**
* @see FutureTask
* @see Executor
* @since 1.5
* @author Doug Lea
* @param <V> The result type returned by this Future's <tt>get</tt> method
*/
public interface Future<V> {
/**
* Attempts to cancel execution of this task. This attempt will
* fail if the task has already completed, has already been cancelled,
* or could not be cancelled for some other reason. If successful,
* and this task has not started when <tt>cancel</tt> is called,
* this task should never run . If the task has already started,
* then the <tt>mayInterruptIfRunning</tt> parameter determines
* whether the thread executing this task should be interrupted in
* an attempt to stop the task. *
*/
boolean cancel(boolean mayInterruptIfRunning);
/**
* Returns <tt>true </tt> if this task was cancelled before it completed
* normally.
*/
boolean isCancelled();
/**
* Returns <tt>true</tt> if this task completed.
*
*/
boolean isDone();
/**
* Waits if necessary for the computation to complete, and then
* retrieves its result.
*
* @return the computed result
*/
V get() throws InterruptedException, ExecutionException;
/**
* Waits if necessary for at most the given time for the computation
* to complete, and then retrieves its result, if available.
*
* @param timeout the maximum time to wait
* @param unit the time unit of the timeout argument
* @return the computed result
*/
V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;
}
在Future介面中聲明瞭5個方法,下面依次解釋每個方法的作用:
cancel()
方法用來取消任務,如果取消任務成功則返回true,如果取消任務失敗則返回false。引數mayInterruptIfRunning表示是否允許取消正在執行卻沒有執行完畢的任務,如果設定true,則表示可以取消正在執行過程中的任務。如果任務已經完成,則無論mayInterruptIfRunning為true還是false,此方法肯定返回false,即如果取消已經完成的任務會返回false;如果任務正在執行,若mayInterruptIfRunning設定為true,則返回true,若mayInterruptIfRunning設定為false,則返回false;如果任務還沒有執行,則無論mayInterruptIfRunning為true還是false,肯定返回true。
isCancelled()
方法表示任務是否被取消成功,如果在任務正常完成前被取消成功,則返回 true。
isDone()
方法表示任務是否已經完成,若任務完成,則返回true;
get()
方法用來獲取執行結果,這個方法會產生阻塞,會一直等到任務執行完畢才返回;
get(long timeout, TimeUnit unit)
用來獲取執行結果,如果在指定時間內,還沒獲取到結果,就直接返回null。
也就是說Future提供了三種功能:
1)判斷任務是否完成;
2)能夠中斷任務;
3)能夠獲取任務執行結果。
因為Future只是一個介面,所以是無法直接用來建立物件使用的,因此就有了下面的FutureTask。
FutureTask
FutureTask的實現:
public class FutureTask<V> implements RunnableFuture<V>
FutureTask類實現了RunnableFuture介面,RunnableFuture介面:
public interface RunnableFuture<V> extends Runnable, Future<V> {
/**
* Sets this Future to the result of its computation
* unless it has been cancelled.
*/
void run();
}
可以看出RunnableFuture繼承了Runnable介面和Future介面,而FutureTask實現了RunnableFuture介面。所以它既可以作為Runnable被執行緒執行,又可以作為Future得到Callable的返回值。
FutureTask提供了2個構造器:
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
}
可以看到,Runnable注入會被Executors.callable()函式轉換為Callable型別,即FutureTask最終都是執行Callable型別的任務。該適配函式的實現如下:
public static <T> Callable<T> callable(Runnable task, T result) {
if (task == null)
throw new NullPointerException();
return new RunnableAdapter<T>(task, result);
}
RunnableAdapter介面卡
/**
* A callable that runs given task and returns given result
*/
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;
}
}
FutureTask是Future介面的一個唯一實現類。
FutureTask實現了Runnable,因此它既可以通過Thread包裝來直接執行,也可以提交給ExecuteService來執行。
FutureTask實現了Futrue可以直接通過get()函式獲取執行結果,該函式會阻塞,直到結果返回。
例項:
Callable+Future獲取執行結果
public class Test {
public static void main(String[] args) {
ExecutorService executor = Executors.newCachedThreadPool();
Task task = new Task();
Future<Integer> result = executor.submit(task);
executor.shutdown();
try {
Thread.sleep(1000);
} catch (InterruptedException e1) {
e1.printStackTrace();
}
System.out.println("主執行緒在執行任務");
try {
System.out.println("task執行結果"+result.get());
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
System.out.println("所有任務執行完畢");
}
}
class Task implements Callable<Integer>{
@Override
public Integer call() throws Exception {
System.out.println("子執行緒在進行計算");
Thread.sleep(3000);
int sum = 0;
for(int i=0;i<100;i++)
sum += i;
return sum;
}
}
執行結果:
子執行緒在進行計算
主執行緒在執行任務
task執行結果4950
所有任務執行完畢
Callable+FutureTask獲取執行結果
public class Test {
public static void main(String[] args) {
//第一種方式
ExecutorService executor = Executors.newCachedThreadPool();
Task task = new Task();
FutureTask<Integer> futureTask = new FutureTask<Integer>(task);
executor.submit(futureTask);
executor.shutdown();
//第二種方式,注意這種方式和第一種方式效果是類似的,只不過一個使用的是ExecutorService,一個使用的是Thread
/*Task task = new Task();
FutureTask<Integer> futureTask = new FutureTask<Integer>(task);
Thread thread = new Thread(futureTask);
thread.start();*/
try {
Thread.sleep(1000);
} catch (InterruptedException e1) {
e1.printStackTrace();
}
System.out.println("主執行緒在執行任務");
try {
System.out.println("task執行結果"+futureTask.get());
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
System.out.println("所有任務執行完畢");
}
}
class Task implements Callable<Integer>{
@Override
public Integer call() throws Exception {
System.out.println("子執行緒在進行計算");
Thread.sleep(3000);
int sum = 0;
for(int i=0;i<100;i++)
sum += i;
return sum;
}
}