Java Memory Model - reg.

Discussion in 'Java' started by jakarta.ant@gmail.com, Feb 26, 2008.

  1. Guest

    Hi Friends,

    I am new to Java Memory Model. I read that when I instantiate an
    object the memory for the object will be allocated in Heap and the
    identifier will be allocated in Stack.

    For example:

    Object myObj = new Object();

    The memory for the myObj will be allocated in Heap.

    and the reference memory address will be stored in Stack.

    Is my understanding correct? can anybody suggest some tools that
    visulises all these memory modeling concepts.

    Thanks and Regards,
    Dinesh.V
     
    , Feb 26, 2008
    #1
    1. Advertising

  2. Guest

    I guess you looking out for this:
    http://java.sun.com/j2se/1.5.0/docs/tooldocs/index.html#manage

    On Feb 26, 2:31 am, wrote:
    > Hi Friends,
    >
    > I am new to Java Memory Model. I read that when I instantiate an
    > object the memory for the object will be allocated in Heap and the
    > identifier will be allocated in Stack.
    >
    > For example:
    >
    > Object myObj = new Object();
    >
    > The memory for the myObj will be allocated in Heap.
    >
    > and the reference memory address will be stored in Stack.
    >
    > Is my understanding correct? can anybody suggest some tools that
    > visulises all these memory modeling concepts.
    >
    > Thanks and Regards,
    > Dinesh.V
     
    , Feb 26, 2008
    #2
    1. Advertising

  3. Lew Guest

    wrote:
    >> I am new to Java Memory Model. I read that when I instantiate an
    >> object the memory for the object will be allocated in Heap and the
    >> identifier will be allocated in Stack.
    >>
    >> For example:
    >>
    >> Object myObj = new Object();
    >>
    >> The memory for the myObj will be allocated in Heap.
    >>
    >> and the reference memory address will be stored in Stack.


    That's sort-of true but not really.

    First, one use of the term "Java memory model" has nothing to do with heap vs.
    stack - it refers to how multiple threads make memory activity visible to each
    other. Bear that in mind while Googling. It is a very important topic, just
    not what you are asking.

    Now, Java does not make any promises about heap vs. stack - the JVM is free to
    optimize certain allocations to the stack, or even out of existence altogether.

    Conceptually all object instances live on the heap, and references from
    objects, i.e., class and instance variables, will also live on the heap
    because they are part of the class or object. However, there is no semantic
    impact to that, and the physical reality can shift and change.

    Automatic variables, those of method or block scope, are more likely to live
    on the stack, but might in practice live their entire lives in registers. Or
    both, depending on run-time circumstances at the point of invocation.
    Conceptually one might as well think of automatic variables, return values,
    arguments and such as living on the stack.

    So the conceptual, and generally useful picture is similar to your guess, but
    not quite the same. Not all references live on the "stack" - allocated memory
    lives on the "heap", and may include references as class or instance
    variables. Method arguments, return values and block-scoped variables live on
    the "stack".

    Much more important than phantasms of "stack" and "heap" are the Java
    realities of "references" and "instances". References keep instances alive,
    and immune to garbage collection (except for weak references and weaker - a
    separate topic). The ideas of reference and instance will solve a lot more
    bugs than the ideas of stack and heap.

    --
    Lew
     
    Lew, Feb 26, 2008
    #3
  4. Roedy Green Guest

    On Tue, 26 Feb 2008 02:31:20 -0800 (PST), wrote,
    quoted or indirectly quoted someone who said :

    >I am new to Java Memory Model. I read that when I instantiate an
    >object the memory for the object will be allocated in Heap and the
    >identifier will be allocated in Stack.


    Conceptually yes, but Jet for example sometimes allocated objects on
    the Stack. This is an optimisation. The code behaves identically.

    primitive local variables go on the stack. Local references and
    parameters go on the stack. Objects go in the heap.

    see http://mindprod.com/jgloss/reference.html
    http://mindprod.com/jgloss/stack.html
    http://mindprod.com/jgloss/heap.html
    --

    Roedy Green Canadian Mind Products
    The Java Glossary
    http://mindprod.com
     
    Roedy Green, Feb 27, 2008
    #4
  5. > Much more important than phantasms of "stack" and "heap" are the Java
    > realities of "references" and "instances". References keep instances alive,
    > and immune to garbage collection (except for weak references and weaker - a
    > separate topic). The ideas of reference and instance will solve a lot more
    > bugs than the ideas of stack and heap.
    >
    > --
    > Lew


    Great, looks good,
    but I am confused about the term weak and weaker references.
    Could you please brief these terms in reference to memory model.

    --Deepak
     
    Deepak Srivastava, Feb 27, 2008
    #5
  6. On 27.02.2008 16:28, Deepak Srivastava wrote:
    >> Much more important than phantasms of "stack" and "heap" are the Java
    >> realities of "references" and "instances". References keep instances alive,
    >> and immune to garbage collection (except for weak references and weaker - a
    >> separate topic). The ideas of reference and instance will solve a lot more
    >> bugs than the ideas of stack and heap.


    > Great, looks good,
    > but I am confused about the term weak and weaker references.
    > Could you please brief these terms in reference to memory model.


    http://www.pawlan.com/Monica/refobjs/

    http://java.sun.com/developer/Books/performance/performance2/appendixa.pdf

    http://www.ibm.com/developerworks/library/j-jtp01274.html

    http://www.ibm.com/developerworks/java/library/j-jtp09275.html

    Cheers

    robert
     
    Robert Klemme, Mar 2, 2008
    #6
  7. In article
    <>,
    Deepak Srivastava <> wrote:

    > > Much more important than phantasms of "stack" and "heap" are the Java
    > > realities of "references" and "instances". References keep instances alive,
    > > and immune to garbage collection (except for weak references and weaker - a
    > > separate topic). The ideas of reference and instance will solve a lot more
    > > bugs than the ideas of stack and heap.
    > >
    > > --
    > > Lew

    >
    > Great, looks good,
    > but I am confused about the term weak and weaker references.
    > Could you please brief these terms in reference to memory model.
    >
    > --Deepak


    There are no "weaker" references.

    Normal - Referenced object can not be GCed

    SoftReference - Referenced object can not be GCed at the JVM's
    discretion. For Sun's, it's the bonehead XX:SoftRefLRUPolicyMSPerMB
    setting or several consecutive GCs. The object referred by the
    SoftReference becomes null upon GC. You can be notified when the
    referenced object has been GCed. These are often used for caches.

    WeakReference - Referenced object will be GCed nearly immediately after
    stronger references are gone. The object referred by the WeakReference
    becomes null upon GC. You can be notified when the referenced object
    has been GCed. These are often used to associate metadata with another
    object.

    PhantomReference - There is no reference available but you are notified
    when the reference has been GCed. This might be used to free system
    resources that were associated with a Java object.

    SoftReference, WeakReference, and PhantomReference should be extended so
    that they still contain useful data after the reference is purged. For
    example, a SoftReference of cached data may hold a cache key, a
    WeakReference some metadata, or a PhantomReference may hold a JNI
    pointer allocated by the object it refers to.

    --
    I don't read Google's spam. Reply with another service.
     
    Kevin McMurtrie, Mar 2, 2008
    #7
    1. Advertising

Want to reply to this thread or ask your own question?

It takes just 2 minutes to sign up (and it's free!). Just click the sign up button to choose a username and then you can ask your own questions on the forum.
Similar Threads
  1. PKJ
    Replies:
    1
    Views:
    389
    Victor Bazarov
    Oct 22, 2004
  2. Replies:
    2
    Views:
    445
    =?ISO-8859-1?Q?Arne_Vajh=F8j?=
    Oct 3, 2006
  3. Replies:
    1
    Views:
    490
    Andreas Wollschlaeger
    Oct 6, 2006
  4. sunil
    Replies:
    10
    Views:
    610
    Phlip
    May 12, 2006
  5. raghu

    Reg:Memory allocation

    raghu, Dec 29, 2007, in forum: C Programming
    Replies:
    20
    Views:
    775
    J. J. Farrell
    Jan 14, 2008
Loading...

Share This Page