Why "jobject" in JNI part is always outside Java heap?

[John Coomes]

Colin(Du Li) (dawn2004 at gmail.com) wrote:
> 
> Hello, I have a question related to JNI of hotspot.
> According to my undestanding, a jobject is representation of java object in
> JNI part, so it should reside in Java heap.

Because the garbage collector may move objects, addresses within the
java heap cannot be visible to native (JNI) code.  There's no way for
the GC to find java object pointers in native code and update them so
they refer to the new locations.  So jobject is just an opaque data
item that the JVM can use to identify the object--a level of
indirection.  See JNIHandles::resolve() in jniHandles.hpp.

-John

> I check the "jobject" on stack, I find it is always outside the java heap.
> (I give an example as follows.)
> Is it correct? 
> 
> e.g.
> for the method: virtual jobject
> java.lang.ClassLoader.findLoadedClass0(jobject)
> The stack is below:
>  - local  [0x73f37e40] ; #0
>  - local  [0x73f38c80] ; #1
>  - stack  [0x73f38c80] ; #1
>  - stack  [0x73f37e40] ; #0
>  - monitor[0xbfd21c78]
>  - bcp    [0xb3f31194] ; @16
>  - locals [0xbfd21cf4]
>  - method [0xb3f311c0] ; virtual jobject
> java.lang.ClassLoader.findLoadedClass(jobject)
> 
> We can see two jobjects' addresses are:0x73f38c80, 0x73f37e40
> 
> But at this time I dump the heap layout, get information as below:
> 
> Heap
>  def new generation   total 7936K, used 285K [0x73f40000, 0x747d0000,
> 0x7b100000)
>   eden space 7104K,   4% used [0x73f40000, 0x73f874c0, 0x74630000)
>   from space 832K,   0% used [0x74630000, 0x74630000, 0x74700000)
>   to   space 832K,   0% used [0x74700000, 0x74700000, 0x747d0000)
>  tenured generation   total 63360K, used 0K [0x7b100000, 0x7eee0000,
> 0xae440000)
>    the space 63360K,   0% used [0x7b100000, 0x7b100000, 0x7b100200,
> 0x7eee0000)
>  tenured generation   total 6976K, used 0K [0xae440000, 0xaeb10000,
> 0xb3f40000)
>    the space 6976K,   0% used [0xae440000, 0xae440000, 0xae440800,
> 0xaeb10000)
>  compacting perm gen  total 16384K, used 1826K [0xb3f40000, 0xb4f40000,
> 0xb7f40000)
>    the space 16384K,  11% used [0xb3f40000, 0xb41089a0, 0xb4108a00,
> 0xb4f40000)
> No shared spaces configured.
> 
> The java heap starts from 0x73f40000, so I can tell two jobjects' addresses
> (0x73f38c80, 0x73f37e40) reside outside Java heap.
> -- 
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>