= Java Support in ZOO  Kernel =

This is a sample program which  dynamicaly loads JVM then load a Class to run one of its static methods :

{{{
#!c
//sample.c file

#include <unistd.h>
#include <jni.h>

int main(int argc, char *argv[], char **envp) {
  JavaVMOption options[2];
  JavaVMInitArgs vm_args;
  JavaVM *jvm;
  JNIEnv *env;
  long result;
  jmethodID mid;
  jfieldID fid;
  jobject jobj;
  jclass cls;
  int i, asize;

  options[0].optionString = "-Djava.class.path=.";
  options[1].optionString = "-Djava.compiler=NONE";

  vm_args.version = JNI_VERSION_1_4;
  vm_args.options = options;
  vm_args.nOptions = 2;
  vm_args.ignoreUnrecognized = JNI_FALSE;

  result = JNI_CreateJavaVM(
             &jvm,(void **)&env, &vm_args);
  if(result == JNI_ERR ) {
    printf("Error invoking the JVM");
    return 1;
  }
  else
        printf("JAVA VM Started\n");

  cls = (*env)->FindClass(env,argv[1]);
  if( cls == NULL ) {
    printf("can't find class %s\n",argv[1]);
    return 1;
  }else{
    printf("%s loaded\n",argv[1]);
  }
  (*env)->ExceptionClear(env);
  mid=(*env)->GetStaticMethodID(env,cls, argv[2], "([Ljava/lang/String;)I");
  if(mid==0)
        printf("%s.%s static method not loaded \n", argv[1],argv[2]);
  else{
        printf("%s.%s static method loaded \n",argv[1],argv[2] );
  (*env)->CallStaticVoidMethod(env,cls, mid, "MID");
  }
  (*jvm)->DestroyJavaVM(jvm);
}
}}}

The sample program can be compiled this way (we will use {{{$SDK_ROOT}}} for the java sdk installation directory) :

{{{
#!sh
gcc -I $SDK_ROOT/include/ -I $SDK_ROOT/include/linux/   -o sample sample.c -L $SDK_ROOT/jre/lib/i386/ -ljavaplugin_jni -L $SDK_ROOT/jre/lib/i386/server/ -ljvm -lpthread
}}}

You need now to use a sample Java Class, you can use the simple code bellow :

{{{
#!java
// HelloWorld.java file
import java.util.*;

public class HelloJava {
  public static int HelloWorldJava(String[] args) {
        System.out.println("Hello from JAVA WOrld !");
        return 3;
  }
}
}}}

You have now to compile your small java code using javac which should create a {{{HelloWorld.class}}} file in the current directory :
{{{
#!sh
javac HelloWrold.java
}}}

To run this sample program you can simply use the following command :

{{{
#!sh
LD_LIBRARY_PATH=".:$SDK_ROOT/jre/lib/i386/server/:$SDK_ROOT/jre/lib/i386/"   ./sample HelloJava HelloWorldJava
}}}

Which should result in a similar result :

{{{
#!sh
JAVA VM Started
HelloJava loaded
HelloJava.HelloWorldJava static method loaded 
Hello from JAVA WOrld !
}}}

This method need to load the jvm for every call.

The data structure selected was java.util.HashMap to share data fom/to C to/from JAVA. Sample creation of this data structure in C :

{{{
#!c
 jclass scReplyClass,scInfo_class;
 jmethodID scInfo_constructor;
 jobject scObject,scObject1;
 scReplyClass = (*env)->FindClass(env, "java/util/HashMap");
 scInfo_class = (*env)->NewGlobalRef(env, scReplyClass);
 scInfo_constructor = (*env)->GetMethodID(env, scInfo_class, "<init>", "()V"); // no parameters
 if(scInfo_constructor==NULL)
 scInfo_constructor = (*env)->GetMethodID(env, scReplyClass, "<init>", "()V"); // no parameters
 if(scInfo_constructor!=NULL){
 scObject = (*env)->NewObject(env, scInfo_class, scInfo_constructor);
 scObject1 = (*env)->NewObject(env, scInfo_class, scInfo_constructor);
 jmethodID put_mid = 0;

 put_mid = (*env)->GetMethodID(env,scReplyClass, "put",
                            "(Ljava/lang/Object;Ljava/lang/Object;)"
                            "Ljava/lang/Object;");

 (*env)->CallObjectMethod(env,scObject1, put_mid, (*env)->NewStringUTF(env,"demoKey"), (*env)->NewStringUTF(env,"demoValue"));
 (*env)->CallObjectMethod(env,scObject1, put_mid, (*env)->NewStringUTF(env,"demoKey1"), (*env)->NewStringUTF(env,"demoValue1"));
 (*env)->CallObjectMethod(env,scObject1, put_mid, (*env)->NewStringUTF(env,"demoKey2"), (*env)->NewStringUTF(env,"demoValue2"));
 (*env)->CallObjectMethod(env,scObject1, put_mid, (*env)->NewStringUTF(env,"demoKey3"), (*env)->NewStringUTF(env,"demoValue3"));

 (*env)->CallObjectMethod(env,scObject1, put_mid, (*env)->NewStringUTF(env,"demoInput"), scObject1);
 }
 else
 printf("Unable to find java.util.HashMap constructor !\n");
}}}