简单 带返回值的线程 callable

测试带返回值的线程，要实现callable接口，重写call方法，在返回值的时候用get方法获取到返回值。

1.内部类实现接口

import java.util.ArrayList;import java.util.List;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;public class TestCallableAndFuture {TestCallableAndFuture() {}public static void main(String[] args) {ExecutorService pool = Executors.newScheduledThreadPool(3);List<Future<String>> futureList = new ArrayList<>();TestCallableAndFuture t = new TestCallableAndFuture();for(int i = 0; i < 5; i++) {int leng = i * 1000;futureList.add(pool.submit(t.new myCallable(leng)));}pool.shutdown();for(int i = 0; i < 5; i++) {try {System.out.println(futureList.get(i).get());} catch (InterruptedException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (ExecutionException e) {// TODO Auto-generated catch blocke.printStackTrace();};}}class myCallable implements Callable {private int length;myCallable(int length) {this.length = length;}@Overridepublic String call() throws Exception {// 线程调用call（）方法for(int i = 0; i < length; i++) {System.out.println("线程" + length + ", i = " + i);}return "线程" + length;}}}

代码中实现多线程输出。

这是截图输出，可以看见，2,、3、4分别运行。
2.直接实现callable接口

注意会报Cannot refer to a non-final variable j inside an inner class defined in a different method这个错误，因为callable要求内部的数据为final类型。如果，两个final指针指向同一个对象地址，会在线程执行过程中改变值，发生错误。要保证对象不变形。可用clone来复制对象。

final int[] leng = {10000,20000,30000,40000};for(int j = 0; j < 4; j++) {futureList.add(pool.submit(new Callable() {@Overridepublic String call() throws Exception {for(int i = 0; i < leng.length; i++) {System.out.println("线程" + leng[j] + ", i = " + i);}return "线程" + leng[j];}}));}

这样会报错，因为j不是final

3.用futuretask接收执行的callable方法。

与future不同的是futuretask实现了Runnable接口，可以对线程的状态进行查询。在我的使用中不常见。

下面的例子执行和1的结果类似

import java.util.ArrayList;import java.util.List;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.FutureTask;public class TestCallableAndFuture {TestCallableAndFuture() {}public static void main(String[] args) {ExecutorService pool = Executors.newScheduledThreadPool(3);List<FutureTask<String>> futuretaskList = new ArrayList<>();TestCallableAndFuture t = new TestCallableAndFuture();for(int i = 0; i < 6; i++) {int leng = i * 1000 * 100;FutureTask<String> thisFutureTask = new FutureTask<String>(t.new myCallable(leng));futuretaskList.add(thisFutureTask);pool.execute(thisFutureTask);//futureList.add(pool.submit(t.new myCallable(leng)));}pool.shutdown();for(int i = 0; i < 6; i++) {try {System.out.println(futuretaskList.get(i).get());} catch (InterruptedException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (ExecutionException e) {// TODO Auto-generated catch blocke.printStackTrace();};}}class myCallable implements Callable {private int length;myCallable(int length) {this.length = length;}@Overridepublic String call() throws Exception {// 线程调用call（）方法for(int i = 0; i < length; i++) {System.out.println("线程" + length + ", i = " + i);}return "线程" + length;}}}