java并发之Future与Callable使用
这篇文章需要大家知道线程、线程池的知识，尤其是线程池。
有的时候我们要获取线程的执行结果，这个时候就需要用到Callable、Future、FutureTask了
先看下Future、Callable接口、RunnableFuture、FutureTask。
Future
Future是一个接口，能够取消任务、获取任务取消状态、获取结果。
package java.util.concurrent;
public interface Future<V> {
boolean cancel(boolean mayInterruptIfRunning);
boolean isCancelled();
/**
* Returns {@code true} if this task completed.
*
* Completion may be due to normal termination, an exception, or
* cancellation -- in all of these cases, this method will return
* {@code true}.
*
* @return {@code true} if this task completed
*/
boolean isDone();
/**
* Waits if necessary for the computation to complete, and then
* retrieves its result.
*
* @return the computed result
* @throws CancellationException if the computation was cancelled
* @throws ExecutionException if the computation threw an
* exception
* @throws InterruptedException if the current thread was interrupted
* while waiting
*/
V get() throws InterruptedException, ExecutionException;
V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;
}
Callable
Callable只有一个功能就是获取结果的V call()。
@FunctionalInterface
public interface Callable<V> {
/**
* Computes a result, or throws an exception if unable to do so.
*
* @return computed result
* @throws Exception if unable to compute a result
*/
V call() throws Exception;
}
FutureTask
FutureTask是一个实现类，它继承了RunnableFuture，而接口RunnableFuture继承了接口Runable,Future。
因此FutureTask可以通过new Thread(FutureTask task)这样的方式来创建线程。
public class FutureTask<V> implements RunnableFuture<V>
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();
}
通过Future创建线程
获取线程的执行结果，最重要就是FutureTask调用。下面的例子是通过线程池来执行线程，并获取结果 。
1.创建实现Callable接口的类Task，就是返回数字和
2.创建线程池ThreadPoolExecutor，新建Task实例
3.线程池ThreadPoolExecutor执行pool.submit(task)
4.返回了Future
5.调用Future的get()获取结果
难点是在pool.submit(task)。
package com.java.javabase.thread.future;
import lombok.extern.slf4j.Slf4j;
import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Executor;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
/**
* @author
*/
@Slf4j
public class FutureTest {
public static void main(String[] args) {
//ExecutorService pool = Executors.newCachedThreadPool();
ThreadPoolExecutor pool =new ThreadPoolExecutor(1,1,0, TimeUnit.SECONDS,
new ArrayBlockingQueue<Runnable>(1));
Task task =new Task();
Future<Integer> result=pool.submit(task);
/*
public <T> Future<T> submit(Callable<T> task) {
if (task == null) throw new NullPointerException();
RunnableFuture<T> ftask = newTaskFor(task);
execute(ftask);
return ftask;
}
*/
pool.shutdown();
try {
log.info("sum {}",result.get());
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
}
static class Task implements Callable<Integer> {
@Override
public Integer call() throws Exception {
int sum = 0;
for (int i = 0; i < 100; i++) {
sum += i;
}
return sum;
}
}
}
线程池获取结果流程说明
ThreadPoolExecutor.submit方法中执行void execute(Runnable command);
如果了解线程池就应该知道这个方法是能够让command的run方法执行的。实现接口 RunnableFuture的
FutureTask的run方法最后返回线程结果的关键
public <T> Future<T> submit(Callable<T> task) {
if (task == null) throw new NullPointerException();
RunnableFuture<T> ftask = newTaskFor(task);
execute(ftask);
return ftask;
}
ThreadPoolExecutor.execute方法
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
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);
}
FutureTask构造器
public FutureTask(Callable<V> callable) {
if (callable == null)
throw new NullPointerException();
this.callable = callable;
this.state = NEW; // ensure visibility of callable
}
FutureTask的run方法
我们创建FutureTask的时候，传入了实现Callable接口的实现类Task,而run()方法中调用了result = c.call();
也就是说FutureTask调用了Callable接口的实现类Task的call方法获取结果，并保存，然后通过对外的get()方法返回。
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 {
result = c.call();
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
setException(ex);
}
if (ran)
set(result);
}
} 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);
}
}
通过FutureTask创建线程
值得注意的就是获取结果的方式，创建FutureTask，也只能通过FutureTask对象get到结果。
pool.submit(futureTask)返回的Future调用get()返回的是null!
package com.java.javabase.thread.future;
import lombok.extern.slf4j.Slf4j;
import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
import java.util.concurrent.FutureTask;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
/**
* @author
*/
@Slf4j
public class FutureTaskTest {
public static void main(String[] args) {
//ExecutorService pool = Executors.newCachedThreadPool();
ThreadPoolExecutor pool =new ThreadPoolExecutor(1,1,0, TimeUnit.SECONDS,
new ArrayBlockingQueue<Runnable>(1));
Task task =new Task();
FutureTask<Integer> futureTask=new FutureTask<Integer>(task);
//Future<?> result =pool.submit(futureTask);
pool.submit(futureTask);
pool.shutdown();
try {
log.info("sum {}",futureTask.get());
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
Task t2=new Task();
FutureTask<Integer> futureTask2=new FutureTask<Integer>(t2);
new Thread(futureTask2).start();
try {
log.info("sum 2 {}",futureTask2.get());
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
}
static class Task implements Callable<Integer> {
@Override
public Integer call() throws Exception {
int sum = 0;
for (int i = 0; i < 100; i++) {
sum += i;
}
return sum;
}
}
}
只能通过FutureTask对象get到结果的原因
我就简单强调一点下面的：ftask与task的区别就是获取结果不一致的原因
ftask的run方法调用了对象task的run方法，ftask的get()返回的是null，而实际我们看到自己创建的task的run方法才能返回结果
public Future<?> submit(Runnable task) {
if (task == null) throw new NullPointerException();
RunnableFuture<Void> ftask = newTaskFor(task, null);
execute(ftask);
return ftask;
}