Java - Make an object modify itself in a thread

Question

Let's assume I have a lot of objects:

public class Main {
    public static DB d1 = new DB(1);
    public static DB d2 = new DB(2);
    public static DB d3 = new DB(3);
    public static DB d4 = new DB(4);

and I want to modify them.

    public static void main(String[] args) {
        d1.modifyObject();
        d2.modifyObject();
        d3.modifyObject();
        d4.modifyObject();
    }
}

and I want them to be modified simultaneously, for it takes some time. Looks like I need multithreading.

This is my DB class:

import java.awt.EventQueue;
import java.util.Date;
import java.util.Random;

public class DB {
    private int id = 0;
    private long field = 0;

    public void DB(int id) {
        this.id = id;
    }

    // The contents of this method are not very important.
    private void modifyField() {
        // [some database interactions which take some seconds to execute]
        // for simplicity, emulated with sleep:
        long newValue = 0;
        try {
            newValue = (this.id + new Date().getTime()) % 42;
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        this.field = newValue;
    }

    public void modifyObject() {
        Runnable r = new Runnable(){ @Override public void run() {
            this.modifyField(); // THIS DOES NOT WORK OF COURSE, I can't access the objects content from a thread
        }};
        EventQueue.invokeLater(r);
    }
}

I want the contents of Main.d1, Main.d2, Main.d3, Main.d4 and so on to be changed without delaying the main thread. I used to do it by accessing Main.d1 within DB itself. This obviously only works if I have just one object. Now, since I have to handle multiple objects, I can't access Main's objects statically any more.

My question is simple but I fear there is no easy answer to it: What options do I have to put this.modifyField(); into its own thread?

How can I make an object modify itself in a thread?

THIS DOES NOT WORK OF COURSE, I can't access the objects content from a thread You need to use DB.this — Falmarri
– Falmarri, Commented Mar 19, 2015 at 23:34

Samuel Edwin Ward · Accepted Answer · 2015-04-03 18:53:53Z

1

I think you simply have the wrong this.

Here:

    Runnable r = new Runnable(){ @Override public void run() {
        this.modifyField(); // THIS DOES NOT WORK OF COURSE, I can't access the objects content from a thread
    }};

this refers to the new Runnable. You can access the enclosing DB object thusly:

    Runnable r = new Runnable(){ @Override public void run() {
        DB.this.modifyField();
    }};

Also, if you are using Java 8, you can write that whole bit more succinctly using a method reference:

Runnable r = this::modifyField;

edited Apr 3, 2015 at 18:53

answered Mar 19, 2015 at 23:37

Samuel Edwin Ward

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Comments

beresfordt · Accepted Answer · 2015-03-19 23:45:45Z

Use an ExecutorServce:

public void foo() throws InterruptedException {
    final ExecutorService executorService = newFixedThreadPool(4); //find what works best for you, setting the number of threads as the number of tasks will not be the best solution in all cases

    final Future<?> runD1Modify = executorService.submit(getModifyObjectRunnable(d1));
    final Future<?> runD2Modify = executorService.submit(getModifyObjectRunnable(d2));
    final Future<?> runD3Modify = executorService.submit(getModifyObjectRunnable(d3));
    final Future<?> runD4Modify = executorService.submit(getModifyObjectRunnable(d4));

    // or in java8

    final Future<?> runD1Modify = executorService.submit(() -> d1.modifyField());
    final Future<?> runD1Modify = executorService.submit(() -> d2.modifyField());
    final Future<?> runD1Modify = executorService.submit(() -> d3.modifyField());
    final Future<?> runD1Modify = executorService.submit(() -> d4.modifyField());
}

private Runnable getModifyObjectRunnable(final DB db) {
    return new Runnable() {
        @Override
        public void run() {
            db.modifyField();
        }
    };
}

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