JEP 178 - working example?

[Bob Vandette]

On Jun 4, 2013, at 3:53 AM, Steve Poole wrote:

> Thanks Bob -  appreciate your time to give me more information.  I have one more question!
> 
> I  could interpret  this statement in your spec change 
> 
> "A library L whose image has been combined with the VM is defined as statically linked if and only if the library exports a function called JNI_OnLoad_L."
> 
> as indicating that I can take an existing dynamic library L and by adding an JNI_OnLoad_L entry point I can expect that it will be called at 'dynamic' load time. 
> 
> Since the wider definition of 'statically linked' is implementation dependent  it would be reasonable for an implementor to decide they wanted this sort of behaviour. 
> However I'm not sure if that is also your intention / expectation for the OpenJDK reference implementation?   
> 

I'm not exactly sure what you are asking?

The fact that we left the details of how to link a library statically up to the implementation, 
should not alter the fact that a static library is NOT a dynamically linked library.  

In the OpenJDK implementation, we limit the scope of where we look for the JNI_OnLoad_L symbol 
to the currently running executable image and will not attempt to find this symbol in any "not yet" loaded 
shared libraries.   The dynamic library load process will first load the image into memory and then
search for a JNI_OnLoad entrypoint.  The static library implementation will look for a JNI_OnLoad_L
before attempting any loading (dlopen on Linux).  If found, it declares this library to be static and
treats it as such.  Otherwise it processes the library as a dynamic library.

Bob.

> 
> 
> On 30 May 2013, at 15:30, Bob Vandette <bob.vandette at oracle.com> wrote:
> 
>> My primary goal for implementing JEP 178 was to eliminate any specification barriers in JDK 8 that would prohibit the
>> implementation of a Java application that might want to link the entire Java runtime into a single application binary.
>> 
>> This allows us to implement the changes that you're suggesting in a JDK 8 implementation without having to 
>> alter the specification.  This type of spec change is typically only done in major Java releases.
>> 
>> We don't yet have any plans to provide the build system changes that would allow an openjdk build to produce
>> a complete static library implementation.  It's getting pretty late to push these types of changes into JDK8 as we're pretty
>> close to feature freeze.
>> 
>> As for use cases for this change, we're trying to enable the creation of single Java application binaries that can take 
>> advantage of newer compiler technologies such as link time optimization.   This feature coupled with the JDK 8 profiles will allow 
>> smaller applications that bundle Java binaries to be deployed on app stores.
>> 
>> 
>> Bob.
>> 
>> 
>> On May 30, 2013, at 6:15 AM, Steve Poole wrote:
>> 
>>> Thanks for the quick response Bob.
>>> 
>>> This all seems quite manual - are there any plans to add support to OpenJDK to help with this process?   
>>> 
>>> 
>>> 
>>> On 29 May 2013, at 00:06, Bob Vandette <bob.vandette at oracle.com> wrote:
>>> 
>>>> 
>>>> On May 28, 2013, at 7:06 AM, Steve Poole wrote:
>>>> 
>>>>> 
>>>>> hi everyone,  
>>>>> 
>>>>> I've been reading JEP 178  (Statically-Linked JNI Libraries )  http://openjdk.java.net/jeps/178
>>>>> 
>>>>> There is a sentence  in the  section on JNI Specification changes that says
>>>>> 
>>>>> "A native library may be statically linked with the VM. The manner in which the library and VM image are combined is implementation-dependent."
>>>>> 
>>>>> I can't find anything that explains the process  in more detail so posting here.
>>>>> 
>>>>> Basically - if I want to statically link my JNI library to Hotspot what are the instructions for the OpenJDK implementation?  
>>>> 
>>>> Since the procedure for statically linking binaries into a executable is very platform specific, the specification 
>>>> intentionally leaves out these details.
>>>> 
>>>> The key functional changes that were implemented to support this spec change, were:
>>>> 
>>>> 1. Require JNI_OnLoad_{libraryname} for static libraries.
>>>> 2. Modify the Java API's that load native JNI libraries to support static libraries by detecting the
>>>> presence of the library specific OnLoad function.
>>>> 3. Support JNI_OnUnLoad_{libraryname}
>>>> 
>>>> So you can either build the entire JDK as static libraries, change every JNI_OnLoad function to be 
>>>> unique and link them all together with a java launcher.
>>>> 
>>>> OR
>>>> 
>>>> You can create a single static library, making sure to give it a unique JNI_OnLoad_{libraryname}
>>>> entrypoint, link it with a program executable that loads the VM via the Java launcher or Invocation APIs.
>>>> 
>>>> For example:  If your library name is speciallibrary.a, you would then call System.loadLibrary("speciallibrary")
>>>> 
>>>> Once the loadLibrary call succeeds, you can then call any native Java methods defined by the static library.
>>>> 
>>>> Bob.
>>>> 
>>>> 
>>>>> 
>>>>> 
>>>>> Cheers
>>>>> 
>>>>> Steve
>>>> 
>>> 
>> 
>