Javaマルチスレッドの~~~Fork/Joinフレームワークの同期と非同期

Fork/Joinフレームワークでは、タスクをコミットするときに同期と非同期の2つの方法があります.以前使用していたinvokeAll()の方法は、同期化されています.
タスクがコミットされると、このメソッドはすべてのタスクが処理されるまで返されません.もう一つの方法はforkメソッドを使用することです.これは非同期です.同じく
タスクをコミットするとforkメソッドがすぐに戻り、次のタスクを続行できます.このスレッドも引き続き実行されます.
次に、ディスクのlogで終わるファイルをクエリーするプログラムの例で、非同期の使い方を説明します.

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