建站
咨詢
售后
建站咨詢
新聞中心
原理解析：fork分解，join結(jié)合。這個(gè)框架的本質(zhì)是將一個(gè)任務(wù)分解成多個(gè)子任務(wù)，每個(gè)子任務(wù)用單獨(dú)的線程去處理。這里用到了遞歸的思想。框架的結(jié)構(gòu)圖可以參考
為巴彥淖爾等地區(qū)用戶提供了全套網(wǎng)頁(yè)設(shè)計(jì)制作服務(wù)，及巴彥淖爾網(wǎng)站建設(shè)行業(yè)解決方案。主營(yíng)業(yè)務(wù)為網(wǎng)站建設(shè)、成都做網(wǎng)站、巴彥淖爾網(wǎng)站設(shè)計(jì)，以傳統(tǒng)方式定制建設(shè)網(wǎng)站，并提供域名空間備案等一條龍服務(wù)，秉承以專業(yè)、用心的態(tài)度為用戶提供真誠(chéng)的服務(wù)。我們深信只要達(dá)到每一位用戶的要求，就會(huì)得到認(rèn)可，從而選擇與我們長(zhǎng)期合作。這樣，我們也可以走得更遠(yuǎn)！
圖片來(lái)源(http://www.ibm.com/developerworks/cn/java/j-lo-forkjoin/index.html)
使用fork/join 框架很簡(jiǎn)單，
1.實(shí)現(xiàn)子問題的一般求解算法
2.如何分解問題
3.繼承 RecursiveAction ，實(shí)現(xiàn)compute()方法
偽代碼代碼
- Result solve(Problem problem) {
- if (problem is small)
- directly solve problem
- else {
- split problem into independent parts
- fork new subtasks to solve each part
- join all subtasks
- compose result from subresults
- }
這里我通過一個(gè)改進(jìn)的二分查找來(lái)講解fork/join的使用。(后面才發(fā)現(xiàn)，選用這個(gè)案例是非常失敗的，因?yàn)槎植檎业臅r(shí)間是logn,而創(chuàng)建線程的開銷更大，這樣并不能體現(xiàn)多線程二分查找的優(yōu)勢(shì)，所以這個(gè)代碼不具有實(shí)用性，只是為了說明如何使用框架：)
代碼如下：
BinarySearchProblem.java
Java代碼
- package testjdk7;
- import java.util.Arrays;
- /**
- * @author kencs@foxmail.com
- */
- public class BinarySearchProblem {
- private final int[] numbers;
- private final int start;
- private final int end;
- public final int size;
- public BinarySearchProblem(int[] numbers,int start,int end){
- this.numbers = numbers;
- this.start = start;
- this.end = end;
- this.size = end -start;
- }
- public int searchSequentially(int numberToSearch){
- //偷懶，不自己寫二分查找了
- return Arrays.binarySearch(numbers, start, end, numberToSearch);
- }
- public BinarySearchProblem subProblem(int subStart,int subEnd){
- return new BinarySearchProblem(numbers,start+subStart,start+subEnd);
- }
- }
BiSearchWithForkJoin.java
Java代碼
- package testjdk7;
- import java.util.concurrent.ForkJoinPool;
- import java.util.concurrent.RecursiveAction;
- /**
- * @author kencs@foxmail.com
- */
- public class BiSearchWithForkJoin extends RecursiveAction {
- private final int threshold;
- private final BinarySearchProblem problem;
- public int result;
- private final int numberToSearch;
- public BiSearchWithForkJoin(BinarySearchProblem problem,int threshold,int numberToSearch){
- this.problem = problem;
- this.threshold = threshold;
- this.numberToSearch = numberToSearch;
- }
- @Override
- protected void compute() {
- if(problem.size < threshold){ //小于閥值，就直接用普通的二分查找
- result = problem.searchSequentially(numberToSearch);
- }else{
- //分解子任務(wù)
- int midPoint = problem.size/2;
- BiSearchWithForkJoin left = new BiSearchWithForkJoin(problem.subProblem(0, midPoint),threshold,numberToSearch);
- BiSearchWithForkJoin right = new BiSearchWithForkJoin(problem.subProblem(midPoint+1, problem.size),threshold,numberToSearch);
- invokeAll(left,right);
- result = Math.max(left.result, right.result);
- }
- }
- //構(gòu)造數(shù)據(jù)
- private static final int[] data = new int[1000_0000];
- static{
- for(int i = 0;i<1000_0000;i++){
- data[i] = i;
- }
- }
- public static void main(String[] args){
- BinarySearchProblem problem = new BinarySearchProblem(data,0,data.length);
- int threshold = 100;
- int nThreads = 10;
- //查找100_0000所在的下標(biāo)
- BiSearchWithForkJoin bswfj = new BiSearchWithForkJoin(problem,threshold,100_0000);
- ForkJoinPool fjPool = new ForkJoinPool(nThreads);
- fjPool.invoke(bswfj);
- System.out.printf("Result is:%d%n",bswfj.result);
- }
- }
RecursiveTask 還可以帶返回值，這里給出一段代碼作為參考(斐波那契函數(shù))
(來(lái)自http://www.ibm.com/developerworks/cn/java/j-lo-forkjoin/index.html)
Java代碼
- class Fibonacci extends RecursiveTask {
- final int n;
- Fibonacci(int n) {
- this.n = n;
- }
- private int compute(int small) {
- final int[] results = { 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89 };
- return results[small];
- }
- public Integer compute() {
- if (n <= 10) {
- return compute(n);
- }
- Fibonacci f1 = new Fibonacci(n - 1);
- Fibonacci f2 = new Fibonacci(n - 2);
- System.out.println("fork new thread for " + (n - 1));
- f1.fork();
- System.out.println("fork new thread for " + (n - 2));
- f2.fork();
- return f1.join() + f2.join();
- }
- }
用途
只要問題能夠分解成類似子問題的，都可以使用這個(gè)框架。對(duì)于大批量的數(shù)據(jù)尤其合適
參考資料
Jdk7官網(wǎng) http://openjdk.java.net/projects/jdk7/
(注：這篇文章發(fā)表時(shí)，JDK7未正式公布，可能有誤差，具體以官方正式版為準(zhǔn))
名稱欄目：探秘JDK7新特性之fork/join框架
網(wǎng)站路徑：http://m.5511xx.com/article/dhiseci.html
