On 2018-12-12 09:40, David Holmes wrote:
On 12/12/2018 5:44 pm, Volker Simonis wrote:
On Tue, Dec 11, 2018 at 11:47 PM David Holmes
<david.hol...@oracle.com> wrote:
On 12/12/2018 12:34 am, Magnus Ihse Bursie wrote:
On 2018-12-11 00:23, David Holmes wrote:
Hi Magnus,
On 10/12/2018 11:19 pm, Magnus Ihse Bursie wrote:
I propose that we introduce a new define, available to all JDK
native
files (Hotspot included), called JDK_EXPORT. The behavior of this
symbol will be very similar (as of now, in fact identical) to
JNIEXPORT; however, the semantics will not.
Currently, we "mis-use" the JNIEXPORT define to mark a function for
exporting from the library. The problem with this is that JNIEXPORT
is part of the JNI interface, and is supposed to be used when C
programs interact with Java. And, when doing this, the function
should be fully decorated like this: "JNIEXPORT foo JNICALL".
I've seen a lot of the emails on this issue and I don't fully
understand what has been going wrong. But the intent is obviously the
JNIEXPORT represents what is needed to export this function for
use by
JNI, while JNICALL defines the calling convention. I agree there may
be some mistmatch when functions are actually not intended for
general
export outside the JDK but are only for internal JDK use.
We do have many such JNI exports in our native libraries, but we
also
have a lot of other, non-JNI exports, where one native library just
provides an interface to other libraries. In these cases, we have
still used JNIEXPORT for the functionality of getting the function
exported, but we have not been consistent in our use of JNICALL.
This
has caused us way too much trouble for something that should Just
Work<tm>.
Are you suggesting that the interface between different libraries in
the JDK should not be a JNI interface? Is this because you think the
functions in these libraries are only for JDK internal use or ... ??
I therefore propose that we define "JDK_EXPORT", with the same
behavior as JNIEXPORT (that is, flagging the function for external
visibility in the resulting native library), but which is *not*
supposed to be exported to Java code using JNI, nor supposed to be
decorated with
Just a clarification there. JNI functions are not exported to Java
code, they are exported to native code. Java code can declare native
methods and those native methods must be written as JNI functions,
but
that's not what we are discussing. Libraries expose a JNI
interface (a
set of functions in the library) that can be called by application
native code, using JNI.
We're apparently looking at "JNI" and "exporting" from two opposite
sides here. :-) Just to make everything clear: If I have a Java class
class MyClass {
public static void native myNativeFunc();
}
then I have one half of the JNI function, the Java half. This must be
matched by a corresponding implementation in native, like this:
JNIEXPORT void JNICALL
Java_MyClass_myNativeFunc(void) {
// ... do stuff
}
And this is the native half of the JNI function. Right? Let's leave
aside which side is "exporting" to the other for now. :-)
This way of setting up native functions that can be called from
Java is
what I refer to as JNI. And when you declare a native JNI function,
you
*must* use both JNIEXPORT and JNICALL. Alright?
We do have a lot of those functions in our native libraries. And they
are correct just the way they are.
Yep all well and good. A function declared native in Java must have an
implementation as you describe. But not all native functions exist to
provide the native-half of a Java native function!
However, we also have *other* native functions, that are exported, not
as JNI functions that should be called from Java, but as normal native
library functions that should be called by other native code. Okay so
far? And *those* functions have been problematic in how we decorate
But there are again two cases. Those functions exported from a library
that are expected to be called from external code using the JNI
interface mechanism - such as all the JNI functions and JVM TI
functions
we export from the JVM - and those "exported" for access between
libraries within the JDK (such as all the JVM_* functions in libjvm).
I think it is only the second group that should be addressed by your
JDK_EXPORT proposal - though I'm not completely clear exactly how to
identify them.
them. My proposal is that we *refrain* from using JNIEXPORT for those
functions, and instead use JDK_EXPORT as name for the macro that
decorates them as exported. That way, we can clearly see that a
function
like this:
JDK_EXPORT void
JLI_ReadEnv(char* env);
is correctly declared, and will be exported to other native libraries,
but not to Java.
The issue is not whether it is "exported to Java"** but whether it is
exported using the JNI mechanism such that other native code calls it
using the JNI mechanism.
** There is no way to write a native method declaration in Java such
that it would be linked to the JLI_ReadEnv function. The naming is all
wrong, as is the signature.
But that's exactly what this change is about! Remove the usage of
JNIEXPORT from functions which are NOT exported using the JNI
mechanism. What don't you like about it ?
I'm just saying we need to be clear about what functions we plan on
changing. Taking concrete examples as I don't see JLI_Read anywhere,
we have:
JNIEXPORT void JNICALL
JLI_ReportErrorMessage(const char* fmt, ...) {
JNIEXPORT void JNICALL
JLI_ReportErrorMessageSys(const char* fmt, ...) {
JNIEXPORT void JNICALL
JLI_ReportExceptionDescription(JNIEnv * env) {
JNIEXPORT StdArg JNICALL
*JLI_GetStdArgs()
{
JNIEXPORT int JNICALL
JLI_GetStdArgc() {
return 0;
}
which seems to define the exported interface for (part of) libJLI and
establishes both the export status and the expected calling
convention. Would these be changed?
Yes! Those are exactly the kind of functions that should change.
At this point, I intended to go out and make a full list of all
functions that I wanted to change, but a grep for JNIEXPORT resulted in
over 3800 hits, so I'm not doing it now. (However, if my proposal is
eventually accepted, I will need to go through these 3800 calls and
check them up. Fortunately, with some scripting, the search can be
drastically limited.)
These functions you list *cannot* be called from Java. There is no
corresponding Java native methods. All suchs methods need to follow the
JNI calling convention, which does not *only* put requirements on the
decoration, but also on the name
("Java_fully_qualified_class_name_and_method") and on the argument list
(first argument must be a JNIEnv*). For the purpose of this discussion,
I'm calling functions that fulfil these requirements *JNI functions*.
The functions you listed above is *not* JNI function. OK? Other examples
of non-JNI functions are e.g:
JNIEXPORT void JNICALL SplashSetScaleFactor(float); (from splashscreen)
or
JNIEXPORT jboolean JNICALL doDrawPath(DrawHandler* hnd, ...)
Contrast this with:
JNIEXPORT void JNICALL Java_sun_java2d_loops_DrawPath_DrawPath(JNIEnv
*env, jobject self, ...)
The latter function is called, not from some other native code, but
directly from java, since there is a
sun.java2d.loops.DrawPath.DrawPath() method (it's apparently a native
constructor, a bit odd, but hey... 2d...).
The former function is called from *other native libraries in the JDK*,
from native code, using #import "java2d/loops/ProcessPath.h" and then
calling doDrawPath().
I think there's an obvious difference between these two, and I'm getting
increasingly frustrated that I'm unable to communicate this difference
to you. :-(
Are you with me now, with this classification?
My proposal is that we should *only* use JNIEXPORT and JNICALL in the
latter case, the JNI case. The former case, the non-JNI, "native lib to
native lib" function, should use JDK_EXPORT instead of JNIEXPORT, so we
let JNIEXPORT correctly signify that the function marked is a JNI-call.
*Also*, we should remove JNICALL as well. Since these functions are just
ways for our native libraries to communicate with another, we can
determine to change the calling convention at will, just as if we've
added a new argument to a function signature. Also, for practical
purposes, JNICALL is empty for all platforms except Windows 32.
And, just to be extremely clear, I do *not* propose we change official
APIs. Documented, public function calls will not be affected. This
includes e.g.
JNIEXPORT jint JNICALL JNI_CreateJavaVM(JavaVM **pvm, void **penv, void
*args);
which will continue to look like that, even though it can not be called
from Java. It also applies to all other functions in jni.h, and other
documented header files.
Just to clarify, this has nothing to do with if this is a officially
supported API or not. In general though, I assume that most (if not
all?) of our exported functions (apart from the JNI_* stuff) is
supposed
to be consumed by other libraries in the JDK, and is not a public API.
I think it varies library by library. You may need native application
code that can call directly into native JDK libraries. JLI is the Java
Launcher Interface - I think it was introduced to make it easier for
other launchers to be created. Native agents may need access to
libmanagement or libjdwp functions. Native graphics code may need
access
to the JDK graphics library. Some of these accesses may be unsupported
and undocumented, but I don't think you can just cut them all off.
Nobody wants to cut off anything. Magnus only proposes to decorate
these required functions with the new JDK_EXPORT macro (instead of
JNIEXPORT) in order to make it clear that they are not exported by
using the JNI mechanism (but they will still be exported, technically
speaking, JDK_EXPORT will even resolve to the exact same function
modifiers!).
Maybe I've misunderstood the proposal. I thought some functions
presently JNIEXPORT and JNICALL would be changed to just JDK_EXPORT as
they are deemed not to be "JNI exported functions" as they are not the
native-half of any Java native method.
If I understand you correctly, this is what I'm proposing.
If the proposal is only to use JDK_EXPORT where JNICALL is missing
(and JDK_EXPORT is identical to JNIEXPORT) that seems fine as it just
serves as "documentation". But if JDKEXPORT differs from JNI_EXPORT
then you may change the ability of external code to link to libraries
it presently does link to.
JDK_EXPORT and JNIEXPORT will be identical. The difference is, we can in
the future change JDK_EXPORT at will, without having to do a CCC or risk
breaking tons of user code. For instance, in AIX, we build native
libraries with all symbols for xlc version 12 or below, but for version
13 doing proper symbol visibility becomes doable. In that case, the AIX
team might find that it would be a good idea to declare JNIEXPORT as a
way to set exported visibility on functions, to help user's developing
native code, but we might still keep JDK_EXPORT as a no-op (and continue
exporting all symbols) for our internal need. (Not saying any of this is
a good idea, but it's an example of the freedom it gives us.)
I think you are trying to use "JNICALL is missing" as a way to measure
either:
1) the function is a JNI-function, an implementation of a Java native
method,
or
2) the function is officially documented
Unfortunately, neither of these are true. And the presence, or absence,
of JNICALL is at this time mostly arbitrary, for non-JNI ("lib to lib")
functions. Which is one thing that has caused us a lot of trouble!
And the change to jdk.h versus jni.h is still unclear to me given
jni.h defines the types that allow interaction with Java and which may
still be needed in a JDK_EXPORT function implementation.
I think there is overloaded use of the term "JNI" function in all this
Yes, that is one part of the confusion I want to address. There is no
need to bring in "JNI" in functions that has nothing to do with JNI.
/Magnus
David
-----
David
/Magnus
JNICALL. All current instances of JNIEXPORT which is not pure JNI
native functions should be changed to use JDK_EXPORT instead.
I further propose that this macro should reside in a new file
"jdk.h", placed in the new directory
src/java.base/share/native/include/internal. This header file path
will automatically be provided to all native libraries, but not
copied to the JDK being built. (The existence of a
"include/internal"
directory with this behavior has been discussed before. There are
more files that ought to be moved there, if/when it is created.) I
believe in many cases the #include "jni.h" can be just modified to
#include "#jdk.h", since most native code will not require "jni.h"
unless actually doing JNI calls -- most have included this file to
get the JNIEXPORT macro, which would explain the pervasive use of
#include "jni.h" in our code base.
jni.h also defines all of the types used by the JNI. Those types are
pervsive to the native code used throughout the JDK.
Thoughts?
I think we need to understand the problems on Windows that prompted
all this. Then I think we need to look at exactly how jni.h and
JNIEXPORT etc are being used and understand whether this is truly an
exported interface or not.
Cheers,
David
/Magnus
