Java Tip: When to use ForkJoinPool vs ExecutorService

Know your options for multithreaded programming on the Java platform

The Fork/Join library introduced in Java 7 extends the existing Java concurrency package with support for hardware parallelism, a key feature of multicore systems. In this Java Tip Madalin Ilie demonstrates the performance impact of replacing the Java 6 ExecutorService class with Java 7's ForkJoinPool in a web crawler application.

Web crawlers, also known as web spiders, are key to the success of search engines. These programs perpetually scan the web, gathering up millions of pages of data and sending it back to search-engine databases. The data is then indexed and processed algorithmically, resulting in faster, more accurate search results. While they are most famously used for search optimization, web crawlers also can be used for automated tasks such as link validation or finding and returning specific data (such as email addresses) in a collection of web pages.

Architecturally, most web crawlers are high-performance multithreaded programs, albeit with relatively simple functionality and requirements. Building a web crawler is therefore an interesting way to practice, as well as compare, multithreaded, or concurrent, programming techniques.

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In this article I'll walk through two approaches to writing a web crawler: one using the Java 6 ExecutorService, and the other Java 7's ForkJoinPool. In order to follow the examples, you'll need to have (as of this writing) Java 7 update 2 installed in your development environment, as well as the third-party library HtmlParser.

Two approaches to Java concurrency

The ExecutorService class is part of the java.util.concurrent revolution introduced in Java 5 (and part of Java 6, of course), which simplified thread-handling on the Java platform. ExecutorService is an Executor that provides methods to manage the progress-tracking and termination of asynchronous tasks. Prior to the introduction of java.util.concurrent, Java developers relied on third-party libraries or wrote their own classes to manage concurrency in their programs.

Fork/Join, introduced in Java 7, isn't intended to replace or compete with the existing concurrency utility classes; instead it updates and completes them. Fork/Join addresses the need for divide-and-conquer, or recursive task-processing in Java programs (see Resources).

Fork/Join's logic is very simple: (1) separate (fork) each large task into smaller tasks; (2) process each task in a separate thread (separating those into even smaller tasks if necessary); (3) join the results.

The two web crawler implementations that follow are simple programs that demonstrate the features and functionality of the Java 6 ExecutorService and the Java 7 ForkJoinPool.

Building and benchmarking the web crawler

Our web crawler's task will be to find and follow links. Its purpose could be link validation, or it could be gathering data. (You might, for instance, instruct the program to search the web for pictures of Angelina Jolie, or Brad Pitt.)

The application architecture consists of the following:

  1. An interface that exposes basic operations to interact with links; i.e., get the number of visited links, add new links to be visited in queue, mark a link as visited
  2. An implementation for this interface that will also be the starting point of the application
  3. A thread/recursive action that will hold the business logic to check whether a link has already been visited. If not, it will gather all the links in the corresponding page, create a new thread/recursive task, and submit it to the ExecutorService or ForkJoinPool
  4. An ExecutorService or ForkJoinPool to handle waiting tasks

Note that a link is considered "visited" after all links in the corresponding page have been returned.

In addition to comparing ease of development using the concurrency tools available in Java 6 and Java 7, we'll compare application performance based on two benchmarks:

  • Search coverage: Measures the time required to visit 1,500 distinct links
  • Processing power: Measures the time in seconds required to visit 3,000 non-distinct links; this is like measuring how many kilobits per second your Internet connection processes.

While relatively simple, these benchmarks will provide at least a small window into the performance of Java concurrency in Java 6 versus Java 7 for certain application requirements.

A Java 6 web crawler built with ExecutorService

For the Java 6 web crawler implementation we'll use a fixed-thread pool of 64 threads, which we create by calling the Executors.newFixedThreadPool(int) factory method. Listing 1 shows the main class implementation.

Listing 1. Constructing a WebCrawler

package insidecoding.webcrawler;

import java.util.Collection;
import java.util.Collections;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import insidecoding.webcrawler.net.LinkFinder;
import java.util.HashSet;

/**
 *
 * @author Madalin Ilie
 */
public class WebCrawler6 implements LinkHandler {

    private final Collection<String> visitedLinks = Collections.synchronizedSet(new HashSet<String>());
//    private final Collection<String> visitedLinks = Collections.synchronizedList(new ArrayList<String>());    
    private String url;
    private ExecutorService execService;

    public WebCrawler6(String startingURL, int maxThreads) {
        this.url = startingURL;
        execService = Executors.newFixedThreadPool(maxThreads);
    }

    @Override
    public void queueLink(String link) throws Exception {
        startNewThread(link);
    }

    @Override
    public int size() {
        return visitedLinks.size();
    }

    @Override
    public void addVisited(String s) {
        visitedLinks.add(s);
    }

    @Override
    public boolean visited(String s) {
        return visitedLinks.contains(s);
    }

    private void startNewThread(String link) throws Exception {
        execService.execute(new LinkFinder(link, this));
    }

    private void startCrawling() throws Exception {
        startNewThread(this.url);
    }

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) throws Exception {
        new WebCrawler("http://www.javaworld.com", 64).startCrawling();
    }
}

In the above WebCrawler6 constructor, we create a fixed-size thread pool of 64 threads. We then start the program by calling the startCrawling method, which creates the first thread and submits it to the ExecutorService.

Next, we create a LinkHandler interface, which exposes helper methods to interact with URLs. Requirements are as follows: (1) mark a URL as visited using the addVisited() method; (2) get the number of the visited URLs through the size() method; (3) determine whether a URL has been already visited using the visited() method; and (4) add a new URL in the queue through the queueLink() method.

Listing 2. The LinkHandler interface

package insidecoding.webcrawler;

/**
 *
 * @author Madalin Ilie
 */
public interface LinkHandler {

    /**
     * Places the link in the queue
     * @param link
     * @throws Exception
     */
    void queueLink(String link) throws Exception;

    /**
     * Returns the number of visited links
     * @return
     */
    int size();

    /**
     * Checks if the link was already visited
     * @param link
     * @return
     */
    boolean visited(String link);

    /**
     * Marks this link as visited
     * @param link
     */
    void addVisited(String link);
}

Now, as we crawl pages, we need to start up the rest of the threads, which we do via the LinkFinder interface, as shown in Listing 3. Note the linkHandler.queueLink(l) line.

Listing 3. LinkFinder

package insidecoding.webcrawler.net;

import java.net.URL;
import org.htmlparser.Parser;
import org.htmlparser.filters.NodeClassFilter;
import org.htmlparser.tags.LinkTag;
import org.htmlparser.util.NodeList;
import insidecoding.webcrawler.LinkHandler;

/**
 *
 * @author Madalin Ilie
 */
public class LinkFinder implements Runnable {

    private String url;
    private LinkHandler linkHandler;
    /**
     * Used fot statistics
     */
    private static final long t0 = System.nanoTime();

    public LinkFinder(String url, LinkHandler handler) {
        this.url = url;
        this.linkHandler = handler;
    }

    @Override
    public void run() {
        getSimpleLinks(url);
    }

    private void getSimpleLinks(String url) {
        //if not already visited
        if (!linkHandler.visited(url)) {
            try {
                URL uriLink = new URL(url);
                Parser parser = new Parser(uriLink.openConnection());
                NodeList list = parser.extractAllNodesThatMatch(new NodeClassFilter(LinkTag.class));
                List<String> urls = new ArrayList<String>();

                 for (int i = 0; i < list.size(); i++) {
                    LinkTag extracted = (LinkTag) list.elementAt(i);

                    if (!extracted.getLink().isEmpty()
                            && !linkHandler.visited(extracted.getLink())) {

                        urls.add(extracted.getLink());
                    }

                }
                //we visited this url
                linkHandler.addVisited(url);

                if (linkHandler.size() == 1500) {
                    System.out.println("Time to visit 1500 distinct links = " + (System.nanoTime() - t0));                   
                }

                for (String l : urls) {
                    linkHandler.queueLink(l);
                }

             } catch (Exception e) {
                //ignore all errors for now
            }
        }
    }
}

The logic of the LinkFinder is simple: (1) we start parsing a URL; (2) after we gather all the links within the corresponding page, we mark the page as visited; and (3) we send each found link to a queue by calling the queueLink() method. This method will actually create a new thread and send it to the ExecutorService. If "free" threads are available in the pool, the thread will be executed; otherwise it will be placed in a waiting queue. After we reach 1,500 distinct links visited, we print the statistics and the program continues to run.

A Java 7 web crawler with ForkJoinPool

The Fork/Join framework introduced in Java 7 is actually an implementation of the Divide and Conquer algorithm (see Resources), in which a central ForkJoinPool executes branching ForkJoinTasks. For this example we'll use a ForkJoinPool "backed" by 64 threads. I say backed because ForkJoinTasks are lighter than threads. In Fork/Join, a large number of tasks can be hosted by a smaller number of threads.

Similar to the Java 6 implementation, we start by instantiating in the WebCrawler7 constructor a ForkJoinPool object backed by 64 threads.

Listing 4. Java 7 LinkHandler implementation

package insidecoding.webcrawler7;

import java.util.Collection;
import java.util.Collections;
import java.util.concurrent.ForkJoinPool;
import insidecoding.webcrawler7.net.LinkFinderAction;
import java.util.HashSet;

/**
 *
 * @author Madalin Ilie
 */
public class WebCrawler7 implements LinkHandler {

    private final Collection<String> visitedLinks = Collections.synchronizedSet(new HashSet<String>());
//    private final Collection<String> visitedLinks = Collections.synchronizedList(new ArrayList<>());
    private String url;
    private ForkJoinPool mainPool;

    public WebCrawler7(String startingURL, int maxThreads) {
        this.url = startingURL;
        mainPool = new ForkJoinPool(maxThreads);
    }

    private void startCrawling() {
        mainPool.invoke(new LinkFinderAction(this.url, this));
    }

    @Override
    public int size() {
        return visitedLinks.size();
    }

    @Override
    public void addVisited(String s) {
        visitedLinks.add(s);
    }

    @Override
    public boolean visited(String s) {
        return visitedLinks.contains(s);
    }

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) throws Exception {
        new WebCrawler7("http://www.javaworld.com", 64).startCrawling();
    }
}

Note that the LinkHandler interface in Listing 4 is almost the same as the Java 6 implementation from Listing 2. It's only missing the queueLink() method. The most important methods to look at are the constructor and the startCrawling() method. In the constructor, we create a new ForkJoinPool backed by 64 threads. (I've chosen 64 threads instead of 50 or some other round number because in the ForkJoinPool Javadoc it states that the number of threads must be a power of two.) The pool invokes a new LinkFinderAction, which will recursively invoke further ForkJoinTasks. Listing 5 shows the LinkFinderAction class:

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