[concurrency-interest] do constructors ever involve threading under the covers?

Andy Nuss andrew_nuss at yahoo.com
Wed Oct 3 10:59:59 EDT 2012


My assumption: I am never 
getting seeded values from somewhere else.  All seed values are passed 
into the constructor with argument containers.

My first question is:  why is it ok to assign the final reference to the newly created ArrayList before filling it with values?
Second question is: how is data race for the newly created values in the ArrayList solved just by making ArrayList member variable final (rather than volatile)?

So I am getting from you guys that I just have to deep copy the argument containers into my own newly created containers, and make the reference to the new containers final, AND make sure that no values are ever added to the containers after the constructor finishes.  Is this correct?

Would it be possible to eliminate the deep copy step if my apis required that the caller never change the contents of the containers after passing them to my constructor?  In that case, maybe I could just assign the references to the argument containers to "final volatile" members, and all my getters would access thru the final volatile members.



________________________________
 From: Aleksey Shipilev <aleksey.shipilev at oracle.com>
To: Andy Nuss <andrew_nuss at yahoo.com> 
Cc: "concurrency-interest at cs.oswego.edu" <concurrency-interest at cs.oswego.edu> 
Sent: Wednesday, October 3, 2012 7:44 AM
Subject: Re: [concurrency-interest] do constructors ever involve threading under the covers?
 
Andy,

On 10/03/2012 06:22 PM, Andy Nuss wrote:
> So if I follow you correctly, for a constructor that has an array or
> ArrayList member, creates that array/ArrayList, and fills it with a
> bunch of seeded values, which are effectively final: another thread will
> not see the values in the array/ArrayList unless the constructed object
> is "published" to the other thread, such as with a volatile variable or
> concurrent map.

If you are reading those "seeded" values from somewhere else, then there
is no guarantees you will read the actual values in the constructor
unless you take the precaution of publishing those values safely. If you
are populating the list in the constructor with generated values and
store it in the field, then you are fine:

public class A {
    private final List<Integer> list;
    public A() {
        list = new ArrayList<>();
        list.add(1); list.add(2); list.add(3);
    }
}

// everything is visible
List<Integer> l = new A().list;

---------

private int myRogueField;

public class B {
    private final List<Integer> list;
    public B() {
        list = new ArrayList<>();
        list.add(myRogueField);
    }
}

// no guarantees actual $myRogueField value is there
List<Integer> l = new B().list;

---------

private volatile int myAwesomeField;

public class C {
    private final List<Integer> list;
    public C() {
        list = new ArrayList<>();
        list.add(myAwesomeField);
    }
}

// extra care was given when reading $myAwesomeField
// guaranteed to be visible in the list.
List<Integer> l = new C().list;

---------

Note that you have to make sure you *read* the correct values when
passing those to the constructor. Target constructor itself could not
enforce this.

> In that case, can someone explain to me how java String, which holds an
> array of characters, is threadsafe without the need for publishing the
> string to the other thread?  I.e. even if the array reference is final,
> the "final" attribute does not apply to the elements of the array.  Same
> with a final ArrayList reference.

Sorry, can't see the problem.

String is immutable, you can not write into the backing char[] array
there. Once string is created, it copies the contents into its own array
(modulo contrived cases of subString()) in constructor, gets everything
freezed as required by JMM, and uses the array then. If you are
providing the String with safely-acquired source char[] array, it goes
the "class A" as described above.

-Aleksey.
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