Adding a native method to OpenJDK source code

Question

I am trying add a custom native method(free) to OpenJDK source, so I can call that method as System.free() from inside the user Application.

I referred online resources to do so and did following changes :

{JDK13ROOT}/src/java.base/share/classes/java/lang/Runtime.java
public static native void free();

{JDK13ROOT}/src/java.base/share/classes/java/lang/System.java
public static void free() {
        Runtime.getRuntime().free();
}

{JDK13ROOT}/src/java.base/share/native/libjava/Runtime.c
JNIEXPORT void JNICALL
Java_java_lang_Runtime_free(){
     printf("Caught !\n");
}

After above changes, I was able to successfully compile the Driver program which directly calls System.free(), and prints Caught ! after execution.

I want to call JVM_Free() inside Java_java_lang_Runtime_free() as below:

JNIEXPORT void JNICALL
Java_java_lang_Runtime_free(){
     printf("Caught !\n");
     JVM_Free();
}

I have included the declaration in {JDK13ROOT}/src/hotspot/share/include/jvm.h

JNIEXPORT void JNICALL
JVM_Free(void);

And the definition of same in {JDK13ROOT}/src/hotspot/share/prims/jvm.cpp

JVM_ENTRY_NO_ENV(void, JVM_Free(void))
  JVMWrapper("JVM_Free");
  printf("SUCCESS\n");
JVM_END

I took reference of implementation of GC() and did the same changes., but when I build the program I get the following error.

Updating support/modules_libs/java.base/libjava.so due to 1 file(s)
/usr/bin/ld: {JDK13ROOT}/build/linux-x86_64-server-slowdebug/support/native/java.base/libjava/Runtime.o: in function `Java_java_lang_Runtime_free':
{JDK13ROOT}src/java.base/share/native/libjava/Runtime.c:71: undefined reference to `JVM_Free'

I'm not able to figure out why am I not able to call any functions from Runtime.c.

PS: I also tried adding the native method from System.c instead of Runtime.c and I see the same build failure. What am I missing or doing wrong here ?.

Answer 1

Your symbol (JVM_Free) is not exported. Take a look here (t vs. T)

> nm libjvm.dylib | grep JVM_GC
000000000041508c T _JVM_GC

vs.

> nm libjvm.dylib | grep JVM_Free
000000000041517a t _JVM_Free

in order to export your new symbol, you have to add it here

{JDK13ROOT}/make/hotspot/symbols/symbols-unix

once it's there, you will be able to "see" it from the outside of libjvm.dylib (or .so).