如何解决ForkJoin正在1个线程上运行
我有以下代码,它是对某种站点抓取工具的模拟,它可以抓取页面/子页面并将结果连接到包含页面内容的字符串中。
我使用了Runtime.getRuntime().availableProcessors()
,所以我假设它将在多个线程上运行。但这似乎并非如此。
package Concurrency;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.RecursiveTask;
public class ForkJoinPoolDemo {
public static class MyTask extends RecursiveTask<String>
{
private String url;
public MyTask(String url)
{
this.url = url;
}
@Override
protected String compute() {
System.out.println(Thread.currentThread().getName() + "/" + url);
if(url.equals("http://google.com/b1")) {
return "Content from /b1";
} else if(url.equals("http://google.com/b2")) {
return "Content from /b2";
} else if(url.equals("http://google.com/b")) {
List<MyTask> tasks = new ArrayList<>();
tasks.add(new MyTask("http://google.com/b1"));
tasks.add(new MyTask("http://google.com/b2"));
String result = "Content from /b\n";
for(MyTask task : tasks) {
task.fork();
result += task.join() + "\n";
}
return result;
} else if(url.equals("http://google.com")) {
List<MyTask> tasks = new ArrayList<>();
tasks.add(new MyTask("http://google.com/a"));
tasks.add(new MyTask("http://google.com/b"));
tasks.add(new MyTask("http://google.com/c"));
tasks.add(new MyTask("http://google.com/d"));
tasks.add(new MyTask("http://google.com/e"));
tasks.add(new MyTask("http://google.com/f"));
tasks.add(new MyTask("http://google.com/g"));
tasks.add(new MyTask("http://google.com/h"));
tasks.add(new MyTask("http://google.com/i"));
tasks.add(new MyTask("http://google.com/j"));
String result = "Content from /\n";
for (MyTask task : tasks) {
task.fork();
result += task.join() + "\n";
}
return result;
} else {
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printstacktrace();
}
return "Content from " + url;
}
}
}
public static void main(String[] args) {
ForkJoinPool pool = new ForkJoinPool(Runtime.getRuntime().availableProcessors());
String result = pool.invoke(new MyTask("http://google.com"));
System.out.println(result);
}
}
为什么每个fork都在同一线程上运行?
ForkJoinPool-1-worker-19/http://google.com
ForkJoinPool-1-worker-19/http://google.com/a
ForkJoinPool-1-worker-19/http://google.com/b
ForkJoinPool-1-worker-19/http://google.com/b1
ForkJoinPool-1-worker-19/http://google.com/b2
ForkJoinPool-1-worker-19/http://google.com/c
ForkJoinPool-1-worker-19/http://google.com/d
ForkJoinPool-1-worker-19/http://google.com/e
ForkJoinPool-1-worker-19/http://google.com/f
ForkJoinPool-1-worker-19/http://google.com/g
ForkJoinPool-1-worker-19/http://google.com/h
ForkJoinPool-1-worker-19/http://google.com/i
ForkJoinPool-1-worker-19/http://google.com/j
解决方法
每次生成新任务等待其完成然后再提交另一任务时,都会阻止加入。相反,请先生成所有任务,然后收集其结果:
for(MyTask task : tasks) {
task.fork();
}
for(MyTask task : tasks) {
result += task.join() + "\n";
}
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