Parallel programming in Java

Question

How can we do Parallel Programming in Java? Is there any special framework for that? How can we make the stuff work?

I will tell you guys what I need, think that I developed a web crawler and it crawls a lot of data from the internet. One crawling system will not make things work properly, so I need more systems working in parallel. If this is the case can I apply parallel computing? Can you guys give me an example?

Answer 1

Java SE 5 and 6 introduced a set of packages in java.util.concurrent.* which provide powerful concurrency building blocks. check this for more information. http://www.oracle.com/technetwork/articles/java/fork-join-422606.html

Answer 2

There is a library called Habanero-Java (HJ), developed at Rice University that was built using lambda expressions and can run on any Java 8 JVM.

HJ-lib integrates a wide range of parallel programming constructs (e.g., async tasks, futures, data-driven tasks, forall, barriers, phasers, transactions, actors) in a single programming model that enables unique combinations of these constructs (e.g., nested combinations of task and actor parallelism).

The HJ runtime is responsible for orchestrating the creation, execution, and termination of HJ tasks, and features both work-sharing and work-stealing schedulers. You can follow the tutorial to set it up on your computer.

Here is a simple HelloWorld example:

import static edu.rice.hj.Module1.*;

public class HelloWorld {

public static void main(final String[] args) {

    launchHabaneroApp(() -> {

        finish(() -> {
            async(() -> System.out.println("Hello World - 1!"));
            async(() -> System.out.println("Hello World - 2!"));
            async(() -> System.out.println("Hello World - 3!"));
            async(() -> System.out.println("Hello World - 4!"));
        });

    });
}}

Each async method runs in parallel with the other async methods, while the content within these methods run sequentially. The program doesn't continue until all code within the finish method complete.

Answer 3

java.util.concurrency package and the Brian Goetz book "Java concurrency in practice"

There is also a lot of resources here about parallel patterns by Ralph Johnson (one of the GoF design pattern author) : http://parlab.eecs.berkeley.edu/wiki/patterns/patterns

Answer 4

Read the section ón threads in the java tutorial. http://download-llnw.oracle.com/javase/tutorial/essential/concurrency/procthread.html

Answer 5

you can use JCSP (http://www.cs.kent.ac.uk/projects/ofa/jcsp/) the library implements CSP (Communicating Sequential Processes) principles in Java, parallelisation is abstracted from thread level and you deal instead with processes.

Answer 6

As far as I know, on most operating systems the Threading mechanism of Java should be based on real kernel threads. This is good from the parallel programming prospective. Other languages like Python simply do some time multiplexing of the processor (namely, if you run a heavvy multithreaded application on a multiprocessor machine you'll see only one processor running).

You can easily find something just googling it: by example this is the first result for "java threading": http://download-llnw.oracle.com/javase/tutorial/essential/concurrency/

Basically it boils down to extend the Thread class, overload the "run" method with the code belonging to the other thread and call the "start" method on an instance of the class you extended.

Also if you need to make something thread safe, have a look to the synchronized methods.