Using threads in Java

Question

I have a portion of code dealing with threads and I want to understand its function in detail. The run method is empty in my example, but lets assume it has some operations to do on a global variable:

import java.io.File;

public class DigestThread extends Thread {
   private File input;

   public DigestThread(File input) {
      this.input = input;
   }

   public void run() {
   }

   public static void main(String[] args) {
      for (int i = 0; i < args.length; i++) {
         File f = new File(args[i]);
         Thread t = new DigestThread(f);
         t.start();
      }
   }
}

After creating a thread and starting it, will it wait to finish the tasks in the run method before creating/running another thread ?

second question

if a variable has declared in run method that means it will be declared many times because every thread created will do the task in run method , is every thread handles its own varible although variable in each thread are same ?

Please edit your question to format your code more readably, and also work on your English - I don't understand your actual question at all. (I realize English may not be your native language, but I'm sure you can improve this with a bit more time and effort.) — Jon Skeet
– Jon Skeet, Commented Jan 18, 2013 at 20:40

Community · Accepted Answer · 2017-05-23 11:51:26Z

will it waitng for finish the tasks in run method to creat another method ?

No. That's the whole point of Multithreading. Every thread has it's own stack of execution. So, when you start a thread, it enters the run() method, and executes it in a different stack, and at the same time the main thread continues execution, in it's own stack.

After that, main thread, can spawn another thread. Thus all the threads run simultaneously, but of course, one at a time, sharing the CPU amongst them, based on the specific CPU allocation algorithm being used.

It's not possible to write down all the details about the execution process of multiple threads here. I would rather suggest to read some tutorials or search for some online resources, regarding the concept of Multithreading. Once clear in concept, move ahead with the implementation in Java.

Here's some tutorials links you can start with: -

Actually, nvm, a link to a good tutorial on threads would be a much better use of time :D (It's easy to say read tutorials, but very few tutorials/articles are actually good .. standard Trails, of course .. and perhaps links in the Java Info page?)
@user1841718. You're welcome :) If you want, there is on nice book - Java Concurrency in Practice. You can search for it. It will give you nice idea about this topic.

Pankaj · Accepted Answer · 2013-01-18 20:47:55Z

2

Once you start a thread, it will run in a separate thread and the main() thread will continue (it may end before the child thread ends).

If you want the thread to finish before main then you can use the Thread.join method on thread to wait for it.

edited Jan 18, 2013 at 20:47

user166390

answered Jan 18, 2013 at 20:42

Pankaj

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2 Comments

Rohit Jain Over a year ago

Well, even without using join, it's not pretty sure that your new thread will end after main thread. There is absolutely no relation between the two threads. They execute independently, and any one can finish first, based on CPU allocation algorithm. So, I would say, your answer is somewhat misleading.

Pankaj Over a year ago

@RohitJain yes you are right, Thread scheduling is OS dependent. We can use join to make sure main() finishes last.

Grambot · Accepted Answer · 2013-01-18 20:43:22Z

Assuming your question is "After creating a thread and starting it, will the program wait for the thread to finish its run method before creating another thread?"

If that's the case, No the program will not wait. t.start() kicks off the thread and it gets its own chunk of memory and thread priority to run. It will do its operations and then exist accordingly. Your main thread will start the number of threads specified in args before terminating.

If you need the application to wait on the thread then use t.join(). That way the parent thread (the one that runs main) will be joined to the child thread and block until its operation is complete. In this case it sort of defeats the purpose of threading but you can store the thread ID for whatever logic you need and join() later.

jarlh · Accepted Answer · 2015-08-26 09:27:33Z

First, Thread is a single sequential flow of control within a program. We can execute more than one thread in a program, but there is a life cycle, where you can find out how threads work... enter image description here

Whenever we call the start() method of Thread it automatically calls the overriden method public void run() and executes the code of the run() method...

Here is a simple example of Thread in Java with ABC and XYZ thread classes

/* Write a program to print having two Thread Class 1) ABC with 1 second  2) XYZ 2 second. 10 times with Extend Constructor */
class ABCThreadConstructor extends Thread {
    ABCThreadConstructor(String name) {
        super(name);
    }
    public void run() {
        try {
            for(int i = 0; i < 10; i++) {
                System.out.println("ABC");
                Thread.sleep(1000);
            }
        } catch(InterruptedException ie) {
            System.out.println("Interrupted Exception : "+ie);
        }
    }
}

class XYZThreadConstructor extends Thread {
    XYZThreadConstructor(String name) {
        super(name);
    }
    public void run() {
        try{
            for(int i = 0; i < 10; i++) {
                System.out.println("XYZ");
                Thread.sleep(2000);
            }
        } catch(InterruptedException ie) {
            System.out.println("Interrupted Exception : "+ie);
        }
    }
}

class AbcXyzThreadConstructorDemo {
    public static void main(String args[]) {
        ABCThreadConstructor atc = new ABCThreadConstructor("ABCThreadConstructor");
        System.out.println("Thread Name : " + atc.getName());
        atc.start();
        XYZThreadConstructor xtc = new XYZThreadConstructor("XYZThreadConstructor");
        System.out.println("Thread Name : " + xtc.getName());
        xtc.start();
    }
}

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