Java7之forkjoin简介_动力节点Java学院整理

java7引入了fork join的概念，来更好的支持并行运算。顾名思义，fork join类似与流程语言的分支，合并的概念。也就是说java7 se原生支持了在一个主线程中开辟多个分支线程，并且根据分支线程的逻辑来等待(或者不等待)汇集，当然你也可以fork的某一个分支线程中再开辟fork join，这也就可以实现fork join的嵌套。

有两个核心类forkjoinpool和forkjointask。

forkjoinpool实现了executorservice接口，起到线程池的作用。所以他的用法和executor框架的使用时一样的，当然fork join本身就是executor框架的扩展。forkjoinpool有3个关键的方法，来启动线程，execute(…),invoke(…),submit(…)。具体描述如下：

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forkjointask是分支合并的执行任何，分支合并的业务逻辑使用者可以再继承了这个抽先类之后，在抽象方法exec()中实现。其中exec()的返回结果和forkjoinpool的执行调用方（execute(…),invoke(…),submit(…)），共同决定着线程是否阻塞，具体请看下面的测试用例。

首先，用户需要创建一个自己的forkjointask。代码如下：

public class myforkjointask extends forkjointask {
 
 /**
  *
  */
 private static final long serialversionuid = 1l;
 private v value;
 private boolean success = false;
 @override
 public v getrawresult() {
  return value;
 }
 @override
 protected void setrawresult(v value) {
  this.value = value;
 }
 @override
 protected boolean exec() {
  system.out.println("exec");
  return this.success;
 }
 public boolean issuccess() {
  return success;
 }
 public void setsuccess(boolean issuccess) {
  this.success = issuccess;
 }
}

测试forkjoinpool.invoke(…)：

 @test
 public void testforkjoininvoke() throws interruptedexception, executionexception {
  forkjoinpool forkjoinpool = new forkjoinpool();
  myforkjointask task = new myforkjointask();
  task.setsuccess(true);
  task.setrawresult("test");
  string invokeresult = forkjoinpool.invoke(task);
  assertequals(invokeresult, "test");
 }
 @test
 public void testforkjoininvoke2() throws interruptedexception, executionexception {
  final forkjoinpool forkjoinpool = new forkjoinpool();
  final myforkjointask task = new myforkjointask();
  new thread(new runnable() {
   public void run() {
    try {
     thread.sleep(1000);
    } catch (interruptedexception e) {
    }
    task.complete("test");
   }
  }).start();
  // exec()返回值是false，此处阻塞，直到另一个线程调用了task.complete(...)
  string result = forkjoinpool.invoke(task);
  system.out.println(result);
 }
 @test
 public void testforkjoinsubmit() throws interruptedexception, executionexception {
  final forkjoinpool forkjoinpool = new forkjoinpool();
  final myforkjointask task = new myforkjointask();
  task.setsuccess(true); // 是否在此任务运行完毕后结束阻塞
  forkjointask result = forkjoinpool.submit(task);
  result.get(); // 如果exec()返回值是false，在此处会阻塞，直到调用complete
 }

测试forkjoinpool.submit(…)：

@test
 public void testforkjoinsubmit() throws interruptedexception, executionexception {
  final forkjoinpool forkjoinpool = new forkjoinpool();
  final myforkjointask task = new myforkjointask();
  task.setsuccess(true); // 是否在此任务运行完毕后结束阻塞
  forkjointask result = forkjoinpool.submit(task);
  result.get(); // 如果exec()返回值是false，在此处会阻塞，直到调用complete
 }
 @test
 public void testforkjoinsubmit2() throws interruptedexception, executionexception {
  final forkjoinpool forkjoinpool = new forkjoinpool();
  final myforkjointask task = new myforkjointask();
  forkjoinpool.submit(task);
  thread.sleep(1000);
 }
 @test
 public void testforkjoinsubmit3() throws interruptedexception, executionexception {
  final forkjoinpool forkjoinpool = new forkjoinpool();
  final myforkjointask task = new myforkjointask();
  new thread(new runnable() {
   public void run() {
    try {
     thread.sleep(1000);
    } catch (interruptedexception e) {
    }
    task.complete("test");
   }
  }).start();
  forkjointask result = forkjoinpool.submit(task);
  // exec()返回值是false，此处阻塞，直到另一个线程调用了task.complete(...)
  result.get();
  thread.sleep(1000);
 }

测试forkjoinpool.execute(…)：

 @test
 public void testforkjoinexecute() throws interruptedexception, executionexception {
  forkjoinpool forkjoinpool = new forkjoinpool();
  myforkjointask task = new myforkjointask();
  forkjoinpool.execute(task); // 异步执行，无视task.exec()返回值。
 }

在实际情况中，很多时候我们都需要面对经典的“分治”问题。要解决这类问题，主要任务通常被分解为多个任务块（分解阶段），其后每一小块任务被独立并行计算。一旦计算任务完成，每一快的结果会被合并或者解决（解决阶段）。forkjointask天然就是为了支持“分治”问题的。

分支/合并的完整过程如下： 

下面列举一个分治算法的实例。

import java.util.random;
import java.util.concurrent.forkjoinpool;
import java.util.concurrent.recursivetask;
public class maximumfinder extends recursivetask<integer> {
 private static final int sequential_threshold = 5;
 private final int[] data;
 private final int start;
 private final int end;
 public maximumfinder(int[] data, int start, int end) {
 this.data = data;
 this.start = start;
 this.end = end;
 }
 public maximumfinder(int[] data) {
 this(data, 0, data.length);
 }
 @override
 protected integer compute() {
 final int length = end - start;
 if (length < sequential_threshold) {
  return computedirectly();
 }
 final int split = length / 2;
 final maximumfinder left = new maximumfinder(data, start, start + split);
 left.fork();
 final maximumfinder right = new maximumfinder(data, start + split, end);
 return math.max(right.compute(), left.join());
 }
 private integer computedirectly() {
 system.out.println(thread.currentthread() + ' computing: ' + start
      + ' to ' + end);
 int max = integer.min_value;
 for (int i = start; i < end; i++) {
  if (data[i] > max) {
  max = data[i];
  }
 }
 return max;
 }
 public static void main(string[] args) {
 // create a random data set
 final int[] data = new int[1000];
 final random random = new random();
 for (int i = 0; i < data.length; i++) {
  data[i] = random.nextint(100);
 }
 // submit the task to the pool
 final forkjoinpool pool = new forkjoinpool(4);
 final maximumfinder finder = new maximumfinder(data);
 system.out.println(pool.invoke(finder));
 }
}

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