Java: access to static final field in JNI

Question

Is there a way to access the static final field in JNI?

public class TryMe {
    public int a = 1;
    public final int b = 2;
    public static int c = 3;
    public static final int d = 4;
}

C ++ JNI Code:

jclass cls = env->FindClass("my/package/TryMe");

jfieldID a = env->GetFieldID(cls, "a", "I"); //OK
jfieldID b = env->GetFieldID(cls, "b", "I"); //OK
jfieldID c = env->GetStaticFieldID(cls, "c", "I"); //OK
jfieldID d = env->GetStaticFieldID(cls, "d", "I"); //Error!

The last call returns NULL and fails. Any idea why?

Answer 1

SOLVED: I was bitten by a bogeyman.

Initially, I thought it might be a runtime bug, but it is not. I believe that proguard does not know and does not care about JNI, so I decided to optimize the "persistent" one. By adding this to proguard.cfg:

keep class my.package.TryMe {
    public static final int d;
}

the problem is gone.

Answer 2

After use javap -p -s TryMe, does the variable appear d:

...
public static final int d;
    :
...

?

, , . ? Arch x86_64, :

Test.java:

public class Test {
    public int a = 1;
    public final int b = 2;
    public static int c = 3;
    public static final int d = 4;
}

ctest.cpp:

#include <iostream>
#include <jni.h>

using namespace std;

int main(){
     JavaVM *jvm;
     JNIEnv *env;

    JavaVMInitArgs vm_args;
    JavaVMOption options[1];
    options[0].optionString = const_cast<char*>("-Djava.class.path=/home/fishi/Workspace/tmp/");
    vm_args.version = JNI_VERSION_1_8;
    vm_args.options = options;
    vm_args.nOptions = 1;
    vm_args.ignoreUnrecognized = JNI_FALSE;

    /* Create the Java VM */
    int res = JNI_CreateJavaVM(&jvm, (void **)&env, &vm_args); 

     if (res == JNI_ERR){
         cout << "Fail: Unable to load JVM \t Exit" << endl;
        return 1;
     }
         else if (res == JNI_OK){
         cout << "CreateVM:\t\tJVM loaded successfully!" << endl;
     }

    /* Find class and static field */
    jclass cls = env->FindClass("Test");
    if(cls == NULL)
    {
        cout << "FindClass:\t\tCould not load class!" << endl;
        return 1;
    }
    else
    {
        cout << "FindClass:\t\tLoaded class successfully!" << endl;
    }
    jfieldID field = env->GetStaticFieldID(cls, "d", "I");
    if(field == NULL)
    {
        cout << "GetField:\t\tError!" << endl;
        return 1;
    }

    cout << "Success" << endl;
}

Makefile:

build-jni:
    g++ -g -I/usr/lib/jvm/java-8-openjdk/include/ -I/usr/lib/jvm/java-8-openjdk/include/linux/ -L/usr/bin/java -L/usr/lib/jvm/java-8-openjdk/jre/lib/amd64/server/ ctest.cpp -ljvm -o ctest

run-jni:
    LD_LIBRARY_PATH=/usr/bin/java:/usr/lib/jvm/java-8-openjdk/jre/lib/amd64/server/ ./ctest

:

$make run-jni
LD_LIBRARY_PATH =/usr/bin/java:/usr/lib/jvm/java-8-openjdk/jre/lib/amd64/server/./ctest
CreateVM: JVM !
FindClass: !

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