Java多线程之~~~Fork/Join框架的同步和异步

在Fork/Join框架中，提交任务的时候，有同步和异步两种方式。以前使用的invokeAll()的方法是同步的，也就是任

务提交后，这个方法不会返回直到所有的任务都处理完了。而还有另一种方式，就是使用fork方法，这个是异步的。也

就是你提交任务后，fork方法立即返回，可以继续下面的任务。这个线程也会继续运行。

下面我们以一个查询磁盘的以log结尾的文件的程序例子来说明异步的用法。

package com.bird.concursey.charpet8;

import java.io.File;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.RecursiveTask;

public class FolderProcessor extends RecursiveTask<List<String>> {

	private static final long serialVersionUID = 1L;

	private String path;
	private String extension;

	public FolderProcessor(String path, String extension) {
		super();
		this.path = path;
		this.extension = extension;
	}

	@Override
	protected List<String> compute() {
		List<String> list = new ArrayList<String>();
		List<FolderProcessor> tasks = new ArrayList<FolderProcessor>();
		File file = new File(path);
		File content[] = file.listFiles();
		if(content != null) {
			for(int i = 0; i < content.length; i++) {
				if(content[i].isDirectory()) {
					FolderProcessor task = new FolderProcessor(content[i].getAbsolutePath(), extension);
					//异步方式提交任务
					task.fork();
					tasks.add(task);
				}else{
					if(checkFile(content[i].getName())) {
						list.add(content[i].getAbsolutePath());
					}
				}
			}
		}
		if(tasks.size() > 50) {
			System.out.printf("%s: %d tasks ran.\n",file.getAbsolutePath(),tasks.size());
		}

		addResultsFromTasks(list,tasks);
		return list;
	}

	/**
	 * that will add to the list of files
the results returned by the subtasks launched by this task.
	 * @param list
	 * @param tasks
	 */
	private void addResultsFromTasks(List<String> list,
			List<FolderProcessor> tasks) {
		for(FolderProcessor item: tasks) {
			list.addAll(item.join());
		}
	}

	/**
	 * This method compares if the name of a file
passed as a parameter ends with the extension you are looking for
	 * @param name
	 * @return
	 */
	private boolean checkFile(String name) {
		return name.endsWith(extension);
	}

	public static void main(String[] args) {
		ForkJoinPool pool = new ForkJoinPool();
		FolderProcessor system = new FolderProcessor("C:\\Windows", "log");
		FolderProcessor apps = new FolderProcessor("C:\\Program Files", "log");

		pool.execute(system);
		pool.execute(apps);

		pool.shutdown();

		List<String> results = null;
		results = system.join();
		System.out.printf("System: %d files found.\n",results.size());

		results = apps.join();
		System.out.printf("Apps: %d files found.\n",results.size());

	}
}

The key of this example is in the FolderProcessor class. Each task processes the content

of a folder. As you know, this content has the following two kinds of elements:

ff Files

ff Other folders

If the task finds a folder, it creates another Task object to process that folder and sends it to

the pool using the fork() method. This method sends the task to the pool that will execute it

if it has a free worker-thread or it can create a new one. The method returns immediately, so

the task can continue processing the content of the folder. For every file, a task compares its

extension with the one it‘s looking for and, if they are equal, adds the name of the file to the

list of results.

Once the task has processed all the content of the assigned folder, it waits for the finalization

of all the tasks it sent to the pool using the join() method. This method called in a task

waits for the finalization of its execution and returns the value returned by the compute()

method. The task groups the results of all the tasks it sent with its own results and returns

that list as a return value of the compute() method.

The ForkJoinPool class also allows the execution of tasks in an asynchronous way. You

have used the execute() method to send the three initial tasks to the pool. In the Main

class, you also finished the pool using the shutdown() method and wrote information about

the status and the evolution of the tasks that are running in it. The ForkJoinPool class

includes more methods that can be useful for this purpose. See the Monitoring a Fork/Join

pool recipe to see a complete list of those methods.