文章目录
1、Runnable缺陷
Runnable主要有两个缺陷:
不能返回一个返回值
不能抛出异常
2、Callable接口
这两点Callable都能做到;
-
Future和Callable配合使用:可以用Future.get来获取Callable接口返回的执行结果,还可以通过Future.isDone()来判断任务是否已完成以及取消这个任务,限时获取任务的结果
-
在call()未执行完毕之前,调用get()的线程(假定此时是主线程)会被阻塞,直到call()方法返回了结果后,此时future.get()才会得到该结果,然后主线程才会切换到runnable状态
-
所以Future是一个存储器,它存储了call()这个任务的结果,而这个任务的执行时间是无法提前确定的,因为这完全取决于call()方法执行的情况
3、Future类
get()方法:获取结果,future一旦调用get方法,就会阻塞,直到返回结果
get方法的行为取决于Callable任务的状态,只有以下这5种情况:
4、两种用法
- future的方法:
get、带时间参数get、isDone、cancel、isCancelled
4.1 用法1:线程池submit方法返回future对象
示例1:get的基本用法 Onefuture.java
package future;
import java.util.Random;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
/**
* 描述: 演示一个Future的使用方法
*/
public class OneFuture {
public static void main(String[] args) {
ExecutorService service = Executors.newFixedThreadPool(10);
Future<Integer> future = service.submit(new CallableTask());
try {
System.out.println(future.get());
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
service.shutdown();
}
static class CallableTask implements Callable<Integer> {
@Override
public Integer call() throws Exception {
Thread.sleep(3000);
return new Random().nextInt();
}
}
}
运行结果
示例2: Future用法的lamda形式
package future;
import java.util.Random;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
/**
* 描述: 演示一个Future的使用方法,lambda形式
*/
package future;
import java.util.Random;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
/**
* 描述: 演示一个Future的使用方法,lambda形式
*/
public class OneFutureLambda {
public static void main(String[] args) {
ExecutorService service = Executors.newFixedThreadPool(10);
Callable<Integer> callable = ()->{
Thread.sleep(3000);
return new Random().nextInt();
};
Future<Integer> future = service.submit(callable);
try {
System.out.println(future.get());
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
service.shutdown();
}
}
运行结果
示例3: 多个任务,用future的数组获取结果
package future;
import java.util.ArrayList;
import java.util.Random;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
/**
* 描述: 演示批量提交任务时,用List来批量接收结果
*/
public class MultiFutures {
public static void main(String[] args) throws InterruptedException {
ExecutorService service = Executors.newFixedThreadPool(20);
ArrayList<Future> futures = new ArrayList<>();
for (int i = 0; i < 20; i++) {
Future<Integer> future = service.submit(new CallableTask());
futures.add(future);
}
Thread.sleep(5000);
for (int i = 0; i < 20; i++) {
Future<Integer> future = futures.get(i);
try {
Integer integer = future.get();
System.out.println(integer);
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
}
}
static class CallableTask implements Callable<Integer> {
@Override
public Integer call() throws Exception {
Thread.sleep(3000);
return new Random().nextInt();
}
}
}
运行结果
示例4:抛出异常
package future;
import java.util.Random;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
/**
* 描述: 演示get方法过程中抛出异常,for循环为了演示抛出Exception的时机:并不是说一产生异常就抛出,直到我们get执行时,才会抛出。
*/
public class GetException {
public static void main(String[] args) {
ExecutorService service = Executors.newFixedThreadPool(20);
Future<Integer> future = service.submit(new CallableTask());
try {
for (int i = 0; i < 5; i++) {
System.out.println(i);
Thread.sleep(500);
}
System.out.println(future.isDone());
future.get();
} catch (InterruptedException e) {
e.printStackTrace();
System.out.println("InterruptedException异常");
} catch (ExecutionException e) {
e.printStackTrace();
System.out.println("ExecutionException异常");
}
}
static class CallableTask implements Callable<Integer> {
@Override
public Integer call() throws Exception {
throw new IllegalArgumentException("Callable抛出异常");
}
}
}
运行结果
示例5 超时获取
package future;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
/**
* 描述: 演示get的超时方法,需要注意超时后处理,调用future.cancel()。演示cancel传入true和false的区别,代表是否中断正在执行的任务。
*/
public class Timeout {
private static final Ad DEFAULT_AD = new Ad("无网络时候的默认广告");
private static final ExecutorService exec = Executors.newFixedThreadPool(10);
static class Ad {
String name;
public Ad(String name) {
this.name = name;
}
@Override
public String toString() {
return "Ad{" +
"name='" + name + '\'' +
'}';
}
}
static class FetchAdTask implements Callable<Ad> {
@Override
public Ad call() throws Exception {
try {
Thread.sleep(3000);
} catch (InterruptedException e) {
System.out.println("sleep期间被中断了");
return new Ad("被中断时候的默认广告");
}
return new Ad("旅游订票哪家强?找某程");
}
}
public void printAd() {
Future<Ad> f = exec.submit(new FetchAdTask());
Ad ad;
try {
ad = f.get(2000, TimeUnit.MILLISECONDS);
} catch (InterruptedException e) {
ad = new Ad("被中断时候的默认广告");
} catch (ExecutionException e) {
ad = new Ad("异常时候的默认广告");
} catch (TimeoutException e) {
ad = new Ad("超时时候的默认广告");
System.out.println("超时,未获取到广告");
boolean cancel = f.cancel(true);
System.out.println("cancel的结果:" + cancel);
}
exec.shutdown();
System.out.println(ad);
}
public static void main(String[] args) {
Timeout timeout = new Timeout();
timeout.printAd();
}
}
运行结果1:当cancel参数为false
运行结果2: 当cancel参数为true
4.2 用法2:用FutureTask创建future
可以用FutureTask获得future和任务的结果
package future;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.FutureTask;
/**
* 描述: 演示FutureTask的用法
*/
public class FutureTaskDemo {
public static void main(String[] args) {
Task task = new Task();
FutureTask<Integer> integerFutureTask = new FutureTask<>(task);
// new Thread(integerFutureTask).start();
ExecutorService service = Executors.newCachedThreadPool();
service.submit(integerFutureTask);
try {
System.out.println("task运行结果:"+integerFutureTask.get());
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
}
}
class Task implements Callable<Integer> {
@Override
public Integer call() throws Exception {
System.out.println("子线程正在计算");
Thread.sleep(3000);
int sum = 0;
for (int i = 0; i < 100; i++) {
sum += i;
}
return sum;
}
}
5、Future的注意点
- future生命周期不能后退
- for循环批量获取future结果,容易发生一部分线程慢的情况,get方法调用使用timeout限制