Java guarantees initialization safety for immutable objects?

Question

To prove initialization safety for immutable objects, I wrote a small program. Even though fields were final, second thread was able to view the half-constructed object being created by the first thread. Have I got the concept wrong or "object escaping" can cause it? Second thread first printed "10 null" and then after half of the iterations it started printing "10 apple".

package test;

import java.util.ArrayList;
import java.util.List;

public final class ObjectEscape implements Runnable {

    private final int a;
    private final String b;

    //this list is defined somewhere else
    public static volatile List<ObjectEscape> globalList = new ArrayList<ObjectEscape>();

    public ObjectEscape () throws InterruptedException {

        a = 10;
        globalList.add(this);
        Thread.sleep(10);
        b = "apple";
        System.out.println("done");

    }

    public ObjectEscape(int a) {
        this.a = 1;
        b = "";
    }

    public static void main(String are[]) throws InterruptedException{

        Thread t = new Thread(new ObjectEscape(1));
        t.start();
        ObjectEscape oe1 = new ObjectEscape();


    }


    @Override
    public void run() {
        int i=0;
        while(i<10) {
            if(globalList.get(0) != null)
            System.out.println(globalList.get(0).a+"        "+globalList.get(0).b);
        i++;
        }
    }
}

The collection is not thread safe, and there is no guarantee the thread won't a) start long after you add the object, or b) finish long before you add the objects. You have to remember that a thread can start run and die in less than 100 micro-seconds. — Peter Lawrey
– Peter Lawrey, Commented Sep 22, 2013 at 20:12
@PeterLawrey I fully agree, I just wanted to prove my point. Can you please comment on the comment I made on Paul's answer? — Abidi
– Abidi, Commented Sep 22, 2013 at 20:14

Community · Accepted Answer · 2017-05-23 12:22:46Z

9

final fields are guaranteed to have been initialized when the contructor call finishes. Naturally, a "leaking this" will subvert the value of this guarantee:

globalList.add(this); // ObjectEscape.<init> has not finished yet

See also: Java leaking this in constructor

edited May 23, 2017 at 12:22

CommunityBot

11 silver badge

answered Sep 22, 2013 at 20:02

Paul Bellora

55.4k18 gold badges135 silver badges185 bronze badges

Sign up to request clarification or add additional context in comments.

6 Comments

Abidi Over a year ago

Thanks Paul! A different but related question, since list is declared volatile so as soon as "this" is put in the list, it will be visible to second thread, correct? If yes, is it true for the state of "this" as well or both "a" and "b" have to be defined volatile to be made immediately visible to second thread?

Peter Lawrey Over a year ago

the volatile is on the List reference. This means the thread will see a reference to the list just fine, but not the contents of the object (s). Making a and b final is enough to ensure all threads see the correct state when the object is created (when the constructor returns)

Abidi Over a year ago

@PeterLawrey I thought, since list is static, I don't need to declare it "volatile" because it is constructed and visible to all threads. Are you saying, for the list contents (in this example "this" object) to be visible to second thread, it has to be declared volatile?

Peter Lawrey Over a year ago

@Abidi the only way to make the contents of the collection visible is to use locking. Making a reference to a list you don't change volatile won't make any difference.

jtahlborn Over a year ago

@Abidi - one way to make the list usage safe would be to make the reference final and make the list synchronized (or use a thread-safe implementation).

|

Collectives™ on Stack Overflow

Java guarantees initialization safety for immutable objects?

1 Answer 1

6 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

6 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related