说一下 runnable 和 callable 有什么区别？
答：
不同点：
-
两者最大的不同点是：实现Callable接口的任务线程能返回执行结果；而实现Runnable接口的任务线程不能返回结果；
-
Callable接口的call()方法允许抛出异常；而Runnable接口的run()方法的异常只能在内部消化，不能继续上抛；
注意点：
- Callable接口支持返回执行结果，此时需要调用FutureTask.get()方法实现，此方法会阻塞主线程直到获取‘将来’结果；当不调用此方法时，主线程不会阻塞！
拓展：
Callable接口
public interface Callable<V> {
V call() throws Exception;
}
Runnable接口
public interface Runnable {
public abstract void run();
}
相同点
-
两者都是接口；（废话）
-
两者都可用来编写多线程程序；
-
两者都需要调用Thread.start()启动线程；
Callable工作的Demo：
package com.callable.runnable;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.FutureTask;
/**
* Created on 2016/5/18.
*/
public class CallableImpl implements Callable<String> {
public CallableImpl(String acceptStr) {
this.acceptStr = acceptStr;
}
private String acceptStr;
@Override
public String call() throws Exception {
// 任务阻塞 1 秒
Thread.sleep(1000);
return this.acceptStr + " append some chars and return it!";
}
public static void main(String[] args) throws ExecutionException, InterruptedException {
Callable<String> callable = new CallableImpl("my callable test!");
FutureTask<String> task = new FutureTask<>(callable);
long beginTime = System.currentTimeMillis();
// 创建线程
new Thread(task).start();
// 调用get()阻塞主线程，反之，线程不会阻塞
String result = task.get();
long endTime = System.currentTimeMillis();
System.out.println("hello : " + result);
System.out.println("cast : " + (endTime - beginTime) / 1000 + " second!");
}
}
测试结果：
hello : my callable test! append some chars and return it!
cast : 1 second!
Process finished with exit code 0
Runnable工作的Demo：
package com.callable.runnable;
/**
* Created on 2016/5/18.
*/
public class RunnableImpl implements Runnable {
public RunnableImpl(String acceptStr) {
this.acceptStr = acceptStr;
}
private String acceptStr;
@Override
public void run() {
try {
// 线程阻塞 1 秒，此时有异常产生，只能在方法内部消化，无法上抛
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
// 最终处理结果无法返回
System.out.println("hello : " + this.acceptStr);
}
public static void main(String[] args) {
Runnable runnable = new RunnableImpl("my runable test!");
long beginTime = System.currentTimeMillis();
new Thread(runnable).start();
long endTime = System.currentTimeMillis();
System.out.println("cast : " + (endTime - beginTime) / 1000 + " second!");
}
}
测试结果：
cast : 0 second!
hello : my runable test!
Process finished with exit code 0
原文作者：在一次面试中问到Callable与Runnable的区别，他说他当时用的多的是Runnable，而Callable使用很少！
比较了两者后（网上查了不少），发现Callable在很多特殊的场景下还是很有用的！最后留点抄的代码，加深对Callable的认识！
package com.inte.fork;
/**
* Created on 2016/4/20.
*/
import java.util.*;
import java.util.concurrent.*;
import static java.util.Arrays.asList;
public class Sums {
static class Sum implements Callable<Long> {
private final long from;
private final long to;
Sum(long from, long to) {
this.from = from;
this.to = to;
}
@Override
public Long call() {
long acc = 0;
for (long i = from; i <= to; i++) {
acc = acc + i;
}
System.out.println(Thread.currentThread().getName() + " : " + acc);
return acc;
}
}
public static void main(String[] args) throws Exception {
ExecutorService executor = Executors.newFixedThreadPool(3);
List<Future<Long>> results = executor.invokeAll(asList(
new Sum(0, 10), new Sum(0, 1_000), new Sum(0, 1_000_000)
));
executor.shutdown();
for (Future<Long> result : results) {
System.out.println(result.get());
}
}
}