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From "LvJimmy,Jing" <firep...@gmail.com>
Subject Re: problem about System.loadLibrary and object's address
Date Sun, 29 Apr 2007 10:32:55 GMT
2007/4/29, Tingpeng Wu <wutingpeng@gmail.com>:
>
> ----- Original Message -----
> From: "LvJimmy,Jing" <firepure@gmail.com>
> To: <dev@harmony.apache.org>
> Sent: Sunday, April 29, 2007 5:24 PM
> Subject: Re: problem about System.loadLibrary and object's address
>
>
> > 2007/4/29, Tingpeng Wu <wutingpeng@gmail.com>:
> >> Hi, xiaofeng,
> >>
> >> 1. I also feel very unnatural to call it through MyRuntime class, but I don't
know how to call it directly. Since drlvm needs HotSpot to help it to compile the java source
code, if I don't call it though an instance, the HotSpot couldn't distinguish it. If I let
the Jit to add the free() call in a future time, I will not need it any more.
> >>
> >
> > :) Java static method can be called as SomeClass.StaticMethod, so your
> > program can be written as (it is nothing related with compiler)
> > public static void main(String[] args)
> > {
> >    set(10);
> >    System.out.println(get());
> > }
> >
>
> >> 2. I added an method named "unsigned int get_object_addr(jobject jobj)" in object_handle.cpp
to return the address of jobj . I can call it directly to get the object address in free()
method. Since I could not assure whether it can get the right address, so I add the get_obj_addr()
for test purpose. I will delete it if it can work corredtly.
> >>
> >
> > Ah, I don't catch well here. Why do you need a free() function here?
> > If you alloc a java object (no matter in java or native), you can just
> > leave it alone and let GC do everything for you. Free() is too c-style
> > for Java developer :)
> > In the other way, if you want to alloc some memory for other use, for
> > an example, char array in c/c++, you can just keep its address(you can
> > keep a jlong as void* ) and free it at last(you'd better free in this
> > case or you'll get memory leak). It is such easier to keep/pass a
> > jlong value than jobject handle.
> >
> My thesis is to redesign the heap layout, and provide the free() method to support reclaiming
the object explicitly. Then in jit module, I will add some analysis to insert some free call
to reclaim the dead object in advance. The purpose is to try to improve the efficiency of
program's execution.
>

Got it, I remember we've already discussed this problem (free() in
java) and IIRC, some expert mark this problem as "mission impossible"
... :)
However study something about GC may benefit your thesis. :)

> >
> >> 3. Another quesetion: where is runtime helper you speak of? I am afraid I need
do some investigation on it.
> >>
> >> Thanks,
> >> tingpeng
> >> ----- Original Message -----
> >> From: "Xiao-Feng Li" <xiaofeng.li@gmail.com>
> >> To: <dev@harmony.apache.org>
> >> Sent: Sunday, April 29, 2007 4:05 PM
> >> Subject: Re: problem about System.loadLibrary and object's address
> >>
> >>
> >> > Tingpeng,
> >> >
> >> > 1. Your native is static native, why do you call it through an instance?
> >> > 2. You probably need to use runtime helper for the object direct free
> >> > in Java app, because the default JNI call path will save the object
> >> > handle (and passes the reference address as a root entry during GC
> >> > enumeration). For your purpose, there is no GC happening in free(),
> >> > so it probably works with a stale object reference (freed) in the
> >> > object handle.
> >> > 3. You can use free() directly to retrieve its object address from the
> >> > handle. Why do you need the get_obj_addr() method?
> >> >
> >> > Thanks,
> >> > xiaofeng
> >> >
> >> > On 4/29/07, Tingpeng Wu <wutingpeng@gmail.com> wrote:
> >> >> Thanks, xiaofeng,
> >> >> My test program is simple as follows:
> >> >>
> >> >> public class JNItest
> >> >>
> >> >> {
> >> >>
> >> >> static
> >> >>
> >> >> {
> >> >>
> >> >> System.loadLibrary("goodluck");
> >> >>
> >> >> }
> >> >>
> >> >>
> >> >>
> >> >> public native static int get();
> >> >>
> >> >> public native static void set(int i);
> >> >>
> >> >>
> >> >>
> >> >> public static void main(String[] args)
> >> >>
> >> >> {
> >> >>
> >> >> JNItest test = new JNItest();
> >> >>
> >> >> test.set(10);
> >> >>
> >> >> System.out.println(test.get());
> >> >>
> >> >> }
> >> >>
> >> >> }
> >> >>
> >> >> Then I use javah to produce JNItest.h and implement two method in JNItest.cpp.
Then use them to build goodluck.dll. I use the latest verson of drlvm to test it, there is
still an error which reports "Vm launcher meets error and needs shut up (this is translated
from chinese)".
> >> >>
> >> >> The reason I asked the second problem is I want to get the address
of obj in java to support the reclaimation. I plan to
> >> >> MyRuntime class, which has a native method,
> >> >> My thought now is as follows:
> >> >> 1. provide a new class named MyRuntime which has two native methods.
> >> >>     class MyRuntime
> >> >>     {
> >> >>          static
> >> >>          {
> >> >>               System.loadLibrary("runtime");
> >> >>          }
> >> >>
> >> >>         //public native static void alloc(……);
> >> >>
> >> >>          public native static void get_obj_addr(Object obj);
> >> >>
> >> >>           //public native static void free(Object obj);
> >> >>     }
> >> >>
> >> >> 2.    export the get_object_addr(jobject jobj) in vmcore to support
MyRuntime.get_obj_addr(Object obj)
> >> >> 3.    export the free(unsigned size, void* address) in gc to support
MyRuntime.free(Object obj)
> >> >> 4.    if above is right, I can call MyRuntime.free(obj) in java method.
> >> >>
> >> >>
> >> >>
> >> >> Is it feasible to get address that way to support the reclaimation
method?
> >> >>
> >> >> Thanks,
> >> >> tingpeng
> >> >>
> >> >> ----- Original Message -----
> >> >> From: "Xiao-Feng Li" <xiaofeng.li@gmail.com>
> >> >> To: <dev@harmony.apache.org>
> >> >> Sent: Sunday, April 29, 2007 11:06 AM
> >> >> Subject: Re: problem about System.loadLibrary and object's address
> >> >>
> >> >>
> >> >> > Tingpeng, can you post your program if there is no legal issue?
(e.g.,
> >> >> > open a JIRA issue and attach your code there).
> >> >> >
> >> >> > For your second question, itis pretty the core part of JVM native
> >> >> > interface design. Yes, the handle is used to access Java object
> >> >> > indirectly. The idea is to support object movement during GC,
then the
> >> >> > real new address of the same object can be stored to the handle.
It's
> >> >> > not supposed to be used everywhere in the JVM, because that may
break
> >> >> > the protocol of JNI, causing GC to fail to update the object new
> >> >> > address, e.g., if it is put into a register by your C compiler.
You
> >> >> > can access it in two ways: either always use JNI interface, or
> >> >> > guarantee there is GC happening when you access it.
> >> >> >
> >> >> > Thanks,
> >> >> > xiaofeng
> >> >> >
> >> >> > On 4/29/07, 吴廷鹏 <wutingpeng@gmail.com> wrote:
> >> >> >> Hi, all,
> >> >> >> when I use drlvm to execute my program, I found that when
program has System.loadLibrary call, there is always an error which reports "java.lang.outofmemoryerror
<no stack trace available>". The same program can run on Hotspot. Why this happens and
how to solve it?
> >> >> >>
> >> >> >> Another question, I read the implementation code of Object.clone
method in Object_generic.cpp.
> >> >> >>
> >> >> >> jobject object_clone(JNIEnv *jenv, jobject jobj)
> >> >> >> {
> >> >> >>     ObjectHandle h = (ObjectHandle) jobj;
> >> >> >>
> >> >> >>     //aquire the target address and assign it to variable
named result
> >> >> >>
> >> >> >>     memcpy(result, h->object, size);
> >> >> >>
> >> >> >> }
> >> >> >> According to my comprehension, h->object is the address
of java object. Is it ture? Does this means I can use the same way to get the address of object
in vmcore's other place provided the necessary head file is included?
> >> >> >>
> >> >> >> Thanks,
> >> >> >> tingpeng
> >> >> >>
> >> >> >
> >> >> >
> >> >> > --
> >> >> > http://xiao-feng.blogspot.com
> >> >> >
> >> >
> >> >
> >> > --
> >> > http://xiao-feng.blogspot.com
> >> >
> >
> >
> > --
> >
> > Best Regards!
> >
> > Jimmy, Jing Lv
> > China Software Development Lab, IBM
> >


-- 

Best Regards!

Jimmy, Jing Lv
China Software Development Lab, IBM
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