How to create a shared library from Java code

As you may know, GraalVM’s native-image command is capable of creating a shared library from Java code

An SVM image can be built as a standalone executable, which is the default, or as a shared library by passing --shared to native-image

https://github.com/oracle/graal/blob/master/substratevm/README.md#images-and-entry-points

It sounds very useful to call a Java library from C or something. Let’s try!

First, write a static method in Java:

Static methods that will be converted to a shared library need to have a specific annotation and accept Isolate/IsolateThread as the first argument.

Next, compile the class using javac

$ javac org/komamitsu/foobar/SharedFooBar.java
$

Then, create a shared library using native-image command

$ native-image --shared -H:Name=libfoobar
Build on Server(pid: 66099, port: 64714)
   classlist:     250.62 ms
       (cap):   1,967.46 ms
       setup:   2,497.16 ms
  (typeflow):   3,190.13 ms
   (objects):   1,842.08 ms
  (features):      43.43 ms
    analysis:   5,209.07 ms
    universe:     216.93 ms
     (parse):   1,053.24 ms
    (inline):     561.35 ms
   (compile):   3,761.77 ms
     compile:   5,768.34 ms
       image:     890.81 ms
       write:   1,345.32 ms
     [total]:  16,224.53 ms
$ ls -ltr
total 10328
drwxr-xr-x   3 komamitsu  staff      102 Jul 16 21:43 org
-rw-r--r--   1 komamitsu  staff      334 Jul 17 22:05 libfoobar_dynamic.h
-rw-r--r--   1 komamitsu  staff      310 Jul 17 22:05 libfoobar.h
-rwxr-xr-x   1 komamitsu  staff  5227120 Jul 17 22:05 libfoobar.dylib
-rw-r--r--   1 komamitsu  staff     3035 Jul 17 22:05 graal_isolate_dynamic.h
-rw-r--r--   1 komamitsu  staff     2939 Jul 17 22:05 graal_isolate.h
$

Of course, we need to write main function in C

This program does:

• Create isolate
• Call a function in the shared library that is generated from the Java static method. You can find the method name in libfoobar.h created by native-image --shared
• Release the isolate

Let’s compile the C code with the static library and execute it.

$ cc -Wall '-Wl,-rpath,$ORIGIN' -I. -L. -lfoobar main.c
$ ./a.out
42 + 13 = 55

It works!

(Update: appended an example to show how to receive String arguments)