About the location of data structure and its inner objects

Lijie Xu csxulijie at gmail.com
Tue Aug 27 19:46:18 PDT 2013


Thank Tao, I think I have a graph of memory layout now. I have another five
questions:

1) Can old space's max size be adjusted dynamically by GC while the JVM is
running?

2) I want to know if FullGC will definitely trigger MinorGC, or just
reclaim the unreferenced objects in Old/Eden/S0/S1 without object promotion.

3) Whether MinorGC can copy an object directly into Old if the Survior
hasn't enough space to hold it currently?

4) Can JVM heap use virtual memory or just physical memory?

5) Can DirectBuffer use virtual memory or just physical memory?





On Tue, Aug 27, 2013 at 2:23 AM, Tao Mao <tao.mao at oracle.com> wrote:

>  Java always conceptually holds a reference rather than the whole object
> body, but you can directly see and use the body in C++.
>
> Below let me illustrate on the HashMap example?
>
> Tao
>
>
> On 8/26/13 1:20 AM, Lijie Xu wrote:
>
>  Hi, folks. I’m confused with the concrete locations of the data
> structure and its inner objects in the heap. The questions are below.
>
> A general question:
>
> If an object X is decided to be copied into old from new gen by GC, all
> the objects which can be reached from X are copied into old too. Or X’s
> retained set. Or this statement is wrong.
>
>
>  Two concrete questions.
>
> Q1: Can an array such as byte[], String[] and Object[] span two
> generations?
>
> I think primitive arrays such as byte[] and int[] cannot span (e.g., a
> part of the array exists in eden and the other part exists in old space).
> For reference arrays such as Object[], the array itself cannot span but the
> items in the arrays can span (i.e., some items exist in new gen while
> others exist in old gen). I’m not sure if I’m right and if String[] is as
> same as byte[].
>
>
>
> Q2: Can ArrayList, LinkedList, HashMap span two generations?
>
> For example, I initialize some data structures as follows.
>
> Say, hashMap = {str1: obj1; str2: obj2; ...; strn: objn;}
>
> The object body of hashMap is in the same generation, including the data
> structure containing references of str(i) and obj(i); however, the object
> bodies of str(i) and obj(i) may span different generations.
>
> BTW, hashMap itself (not its referenced object body) is a reference and,
> hence, on stack (not on java heap) since it's a local variable.
>
> Hope this helps build a concrete picture of memory layout.
>
>
>
> --------------------------------------------------------------------------------------------------
>
> *import* java.util.ArrayList;
>
> *import* java.util.HashMap;
>
> *import* java.util.LinkedList;
>
> *import* java.util.List;
>
> *import* java.util.Map;
>
>
>
> *public* *class* ObjectTest {
>
>     *public* *static* *void* main(String[] args) {
>
>        List<Obj> arrayList = *new* ArrayList<Obj>();
>
>        List<Obj> linkedList = *new* LinkedList<Obj>();
>
>        Map<String, Obj> hashMap = *new* HashMap<String, Obj>();
>
>
>
>        *for*(*int* i = 0; i < 10000; i++) {
>
>            Obj arrayObj = *new* Obj();
>
>            arrayList.add(arrayObj);
>
>        }
>
>
>
>        *for*(*int* i = 0; i < 10000; i++) {
>
>            Obj linkedObj = *new* Obj();
>
>            linkedList.add(linkedObj);
>
>        }
>
>
>
>        *for*(*int* i = 0; i < 10000; i++) {
>
>            String str = i + "";
>
>            Obj hashObj = *new* Obj();
>
>            hashMap.put(str, hashObj);
>
>        }
>
>     }
>
> }
>
>
>
> *class* Obj {
>
>     *byte*[] bytes;
>
>     *public* Obj() {
>
>        bytes = *new* *byte*[16];
>
>     }
>
>  }
>
>
> --------------------------------------------------------------------------------------------------
>
> If new gen cannot hold all the objects, GC will occur. I want to know if
> all the items in the data structure are copied into old gen.
>
> For example, arrayList itself exists in old while some of its arrayObjs
> exist in new gen. A arrayObj exists in old gen while its bytes exists in
> new gen.
>
>
>
>
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