I do have a WebApp (WAR file) in a Maven project running fine.
I want to call a Java method in a class file which is inside the maven project from a C file. The C program reads the GPIO input of my beagleboard and when it's on, it runs the Java method. I use JNI for doing that.
I can run a classic HelloWorld Java method, but when I want to run a method from my Maven project, I got a fatal error from my JVM when its trying to load the Java static method.
I added the C code in my WebApp, in /WEB-INF/myapp/classes/classfile/
Class file : App
Method : test
my C program:
JNIEnv* create_vm(JavaVM ** jvm) {
JNIEnv *env;
JavaVMInitArgs vm_args;
JavaVMOption options;
//Path to the java source code
options.optionString = "-Djava.class.path=./";
//JDK version. This indicates version 1.6
vm_args.version = JNI_VERSION_1_6;
vm_args.nOptions = 1;
vm_args.options = &options;
vm_args.ignoreUnrecognized = 0;
int ret = JNI_CreateJavaVM(jvm, (void**)&env, &vm_args);
if(ret < 0)
printf("\nUnable to Launch JVM\n");
return env;
}
int main(int argc, char** argv)
{
int fd, ret, data[64];
JNIEnv *env;
JavaVM * jvm;
env = create_vm(&jvm);
if (env == NULL)
return 1;
fd = ret = open(argv[argc - 1], O_RDONLY);
printf("open returned %d\n", ret);
while (1) {
ret = read(fd, &data, sizeof(data));
printf("read returned %d\n", ret);
jclass helloWorldClass = (*env)->FindClass(env, "App");
printf("appel class\n");
jmethodID mainMethod = (*env)->GetStaticMethodID(
env, helloWorldClass, "test", "([Ljava/lang/String;)V");
printf("appel method\n");
(*env)->CallStaticVoidMethod(env, helloWorldClass, mainMethod, NULL);
sleep(2);
}
(*jvm)->DestroyJavaVM(jvm);
exit(0);
}
Java class :
package App;
public class App
{
public static void test (String[] args){
System.out.println("Hello World");
}
}
Have you investigated using Reverse JNI?
I notice that you have the following line:
options.optionString = "-Djava.class.path=./"; //Path to the java source code
The classpath needs to point to either the jar file generated by Maven, or the root directory under which your Java class files are located.
See: Setting the ClassPath,
In a standard Maven project, Maven will create your jar under target/ when you execute mvn package. So you'll likely change -Djava.class.path=./ to -cp /target/App.jar
Related
I am trying to run a java program using c++ code. I tried below code
#include<jni.h>
#include<stdio.h>
int main(int argc, char** argv) {
JavaVM* vm;
JNIEnv* env;
JavaVMInitArgs vm_args;
vm_args.version = JNI_VERSION_1_2;
vm_args.nOptions = 0;
vm_args.ignoreUnrecognized = 1;
// Construct a VM
jint results = JNI_CreateJavaVM(&vm, (void**)& env, &vm_args);
// Construct a String
jstring jstr = env->NewStringUTF("Hello World");
// First get the class that contains the method you need to call
jclass clazz = env->FindClass("java/lang/String");
// Get the method that you want to call
jmethodID to_lower = env->GetMethodID(clazz, "toLowerCase",
"()Ljava/lang/String;");
// Call the method on the object
jobject result = env->CallObjectMethod(jstr, to_lower);
// Get a C-style string
const char* str = env->GetStringUTFChars((jstring)result, NULL);
printf("%s\n", str);
// Clean up
env->ReleaseStringUTFChars(jstr, str);
// Shutdown the VM.
vm->DestroyJavaVM();
}
I used below command to compile the code
g++ LoadJVM.c -I/c/apps64/Java/jdk-11.0.1/include -I/c/apps64/Java/jdk-11.0.1/include/win32 -L/c/apps64/Java/jdk-11.0.1/lib/ -ljvm
It compiles fine, but when i run the executable like below, i am facing error
./a.exe
Error
error while loading shared libraries: jvm.dll: cannot open shared object file: No such file or directory
Any Idea why this jvm.dll is not getting loaded?
PS: I am compiling and running from Git-Bash on windows 10.
It looks like your jvm.dll can not be found.
Let's say we have 32bit MinGW installation (this is the version I have).
simple.cc
#include<jni.h>
#include<stdio.h>
int main(int argc, char** argv) {
JavaVM* vm;
JNIEnv* env;
JavaVMInitArgs vm_args;
vm_args.version = JNI_VERSION_1_2;
vm_args.nOptions = 0;
vm_args.ignoreUnrecognized = 1;
// Construct a VM
jint results = JNI_CreateJavaVM(&vm, (void**)& env, &vm_args);
printf("Hello");
// Shutdown the VM.
(*vm).DestroyJavaVM();
}
compilation and execution
> export JAVA_HOME="/c/Program\ Files\ \(x86\)/Java/jdk1.8.0_211/"
> export PATH="/c/Program Files (x86)/Java/jdk1.8.0_211/jre/bin/server/":"$PATH"
> g++ -o simple simple.cc -I"$JAVA_HOME/include/" -I"$JAVA_HOME/include/win32/" -L"$JAVA_HOME/lib" -ljvm
> ./simple
Hello
you have to make sure that jvm.dll is visible on %PATH% - $PATH inside git-bash.
I've just looked at anyone else questions but it seems that nobody has the same problem of mine. I have a Java class HelloWorldHandler.java (in org.eclipse.gemoc.testapplilauncher.handlers package in org.eclipse.gemoc.testapplilauncher project) that launches another java application. Then I have a c file launcherC.c (in the same project but different directory) that is supposed to call the execute method in the java file, through JNI. Now, all my previous JNI applications worked, this one doesn't. The only difference is that HelloWorldHandler.java is not in the default package (so the .class is not directy in /bin) and there are multiple package imports (maybe something changes with the FindClass call (?)).
I launch the c file with
gcc -fPIC -I"/usr/lib/jvm/java-8-oracle/include" -I"/usr/lib/jvm/java-8-oracle/include/linux" -L/usr/lib/jvm/java-8-oracle/lib/amd64/jli/ -L/usr/lib/va-8-oracle/jre/lib/amd64/server/ -o launcherC launcherC.c -ljli -ljvm
and
export LD_LIBRARY_PATH=${LD_LIBRARY_PATH}:/usr/lib/jvm/java-8-oracle/jre/lib/amd64/server/
and there is no problem. But when I execute it, it doesn't do anything, without any Expections and errors. I checked and the FindClass returns NULL.
HelloWorldHandler.java
package org.eclipse.gemoc.testapplilauncher.handlers;
public class HelloWorldHandler {
#Execute
public static void execute() {
ILaunchManager manager = DebugPlugin.getDefault().getLaunchManager();
IWorkspaceRoot root = ResourcesPlugin.getWorkspace().getRoot();
IFile ff = root.getFile(new Path("d/Test.launch"));
ILaunchConfiguration res = manager.getLaunchConfiguration(ff);
DebugUITools.launch(res, ILaunchManager.DEBUG_MODE);
}
}
launcherC.c
#include <jni.h>
#include <string.h>
int main()
{
JavaVMOption options[1];
JNIEnv *env;
JavaVM *jvm;
JavaVMInitArgs vm_args;
long status;
jclass cls;
jmethodID mid;
options[0].optionString = "-Djava.class.path=/home/ezambon/Desktop/modeling/org/eclipse/gemoc/testAppliLauncher/bin/org/eclipse/gemoc/testapplilauncher/handlers";
memset(&vm_args, 0, sizeof(vm_args));
vm_args.version = JNI_VERSION_1_2;
vm_args.nOptions = 1;
vm_args.options = options;
status = JNI_CreateJavaVM(&jvm, (void**)&env, &vm_args);
if (status != JNI_ERR)
{
cls = (*env)->FindClass(env, "HelloWorldHandler");
if(cls != 0) {
//printf("asdfgh\n");
mid = (*env)->GetStaticMethodID(env, cls, "execute", "()V");
if(mid !=0) {
(*env)->CallStaticVoidMethod(env, cls, mid);
}
}
//printf("qwerty");
(*jvm)->DestroyJavaVM(jvm);
return 0;
}
else
return -1;
}
All the suggestions are really welcome, I'm getting crazy. Thank you :)
First problem you setup classpath wrong
Instead of
options[0].optionString = "-Djava.class.path=/home/ezambon/Desktop/modeling/org/eclipse/gemoc/testAppliLauncher/bin/org/eclipse/gemoc/testapplilauncher/handlers";
you should have
options[0].optionString = "-Djava.class.path=/home/ezambon/Desktop/modeling/org/eclipse/gemoc/testAppliLauncher";
and you should use full class name
cls = (*env)->FindClass(env, "org.eclipse.gemoc.testapplilauncher.handlers.HelloWorldHandler");
But I suppose this is not all your problems.
I need to call java method from c program. i have tried below code to call the java method through Java native interface but facing issues while compilation. i m new to C and have experience in java. so, i m not able to think myself what is happening while creating JVM.
Below is the code.
CTest.c
#include <stdio.h>
#include <jni.h>
JNIEnv* create_vm() {
JavaVM* jvm;
JNIEnv* env;
JavaVMInitArgs args;
JavaVMOption options[1];
args.version = JNI_VERSION_1_6;
args.nOptions = 1;
options[0].optionString = "-Djava.class.path=D:\\Ashish_Review\\JNI\\src";
args.options = options;
args.ignoreUnrecognized = JNI_FALSE;
JNI_CreateJavaVM(&jvm, (void **)&env, &args);
return env;
}
void invoke_class(JNIEnv* env) {
jclass helloWorldClass;
jmethodID mainMethod;
jobjectArray applicationArgs;
jstring applicationArg0;
helloWorldClass = (*env)->FindClass(env, "HelloWorld");
mainMethod = (*env)->GetStaticMethodID(env, helloWorldClass, "main", "([Ljava/lang/String;)V");
applicationArgs = (*env)->NewObjectArray(env, 1, (*env)->FindClass(env, "java/lang/String"), NULL);
applicationArg0 = (*env)->NewStringUTF(env, "From-C-program");
(*env)->SetObjectArrayElement(env, applicationArgs, 0, applicationArg0);
(*env)->CallStaticVoidMethod(env, helloWorldClass, mainMethod, applicationArgs);
}
int main(int argc, char **argv) {
JNIEnv* env = create_vm();
invoke_class( env );
}
C:\Users\Desktop\tcc>tcc C:\TurboC++\Disk\TurboC3\BIN\CTest.c -I "C:\Program Files\Java\jdk1.6.0_16\include" -I "C:\Program Files\Java\jdk1.6.0_16\include\win32" -shared -o CTest.dll
tcc: undefined symbol '_JNI_CreateJavaVM#12'
please help me out.
The error message is about the linking stage, not compilation - you have included the header file, but to create the executable you have to specify the .a (library files) also.
You have to link with JVM's library (add some reference to libjvm.a to the tcc's command line).
If you don't have a precompiled jvm.lib file for TurboC++, there is another option - link with the jvm.dll file and export all the methods from JVM manually. This uses the LoadLibrary/GetProcAddress functions.
For a short sample (sorry, couldn't find the entire export source code), look at this:
/* load library */
HMODULE dll = LoadLibraryA("jvm.dll");
/* declare a function pointer and initialize it with the "pointer" to dll's code */
JNI_CreateJavaVM_func JNI_CreateJavaVM_ptr = GetProcAddress(dll, "JNI_CreateJavaVM");
Later use the JNI_CreateJavaVM_ptr instead of JNI_CreateJavaVM. Also you'll have to declare the JNI_CreateJavaVM_func type - you might just copy the signature from "jni.h"
I'm quite new to JNI and right now I'm using this simple C program to create a JVM and call the main() from my Java project:
#include <stdio.h>
#include <jni.h>
JNIEnv* create_vm() {
JavaVM* jvm;
JNIEnv* env;
JavaVMInitArgs args;
JavaVMOption options[1];
args.version = JNI_VERSION_1_6;
args.nOptions = 1;
options[0].optionString = "-Djava.class.path=CLASSPATH"; //This isn't the actual classhpath, but you get the idea
args.options = options;
args.ignoreUnrecognized = JNI_FALSE;
int ret = JNI_CreateJavaVM(&jvm, (void **)&env, &args);
if (ret<0){
printf("\nUnable to Launch JVM");
} else {
printf("\nJVM launched successfully");
}
return env;
}
void invoke_class(JNIEnv* env) {
jclass UncaughtExceptionClass;
jmethodID mainMethod;
jobjectArray applicationArgs;
jstring applicationArg0;
UncaughtExceptionClass = (*env)->FindClass(env, "exceptioncatcher/ExceptionCatcher");
mainMethod = (*env)->GetStaticMethodID(env, UncaughtExceptionClass, "main", "([Ljava/lang/String;)V");
applicationArgs = (*env)->NewObjectArray(env, 1, (*env)->FindClass(env, "java/lang/String"), NULL);
applicationArg0 = (*env)->NewStringUTF(env, "From C");
(*env)->SetObjectArrayElement(env, applicationArgs, 0, applicationArg0);
(*env)->CallStaticVoidMethod(env, UncaughtExceptionClass, mainMethod, applicationArgs);
}
int main(int argc, char **argv) {
JNIEnv* env = create_vm();
invoke_class( env );
}
This works well when running a HelloWorld type java code, but my code actually sets a Default Uncaught Exception Handler. This handler will send the info collected for each uncaught exception to a thread that will process them and send them by email.
Right now these are pretty much all the classes created and it, obviously, won't catch a lot of uncaught exceptions right now. But I use the main() to try it out:
public static void main(String[] args){
Thread.setDefaultUncaughtExceptionHandler(new DefaultExceptionHandler());
Integer i = null;
i++;
}
It works fine when I simply run it from netbeans, but the only thing that appears when using the C app is "JVM launched successfully".
Can anyone help on this one?
Managed to make it work in the end (never closed the question before), simply had to build the project and change the classpath to the .jar generated.
I am trying to get my feet wet with JNI because I have an application in C that needs to access a single Java library function (no C-equivalent library). I've written a very simple test program to load a Java VM from C and call a static function and get the return value.
Unfortunately, I am unable to get the class to properly load. Although it will probably boil down to it, I think my ClassPath is correct: when I use the java command with the same ClassPath in the same directory, the class loads and executes perfectly.
Environment:
Ubuntu 8.04 server
Java JRE&SDK 1.6
gcc
My present working directory is always /home/me/project.
Here is what I get when I run the java command (java -Djava.class.path=/home/me/project/ -verbose my.ClassABC):
[Loaded ...] (many loads)
[Loaded my.ClassABC from file:/home/me/project/]
Hello test
[Loaded java.lang.Shutdown from shared objects file]
[Loaded java.lang.Shutdown$Lock from shared objects file]
Here is what I get when I run my C program (./myClassABC):
[Loaded ...]
[Loaded my.ClassABC from file:/home/me/project/]
Exception in thread "main" java.lang.NoClassDefFoundError: my.ClassABC
Failed to get class
Here is my gcc command line:
gcc -o myClassABC myClassABC.c -I/usr/lib/jvm/java-6-sun-1.6.0.16/include/ -I/usr/lib/jvm/java-6-sun-1.6.0.16/include/linux -L/usr/lib/jvm/java-6-sun-1.6.0.16/jre/lib/i386/server/ -ljvm
My C code (myClassABC.c):
int main(int argc,char **argv)
{
JNIEnv *env;
JavaVM *jvm;
jint res;
jclass cls;
jmethodID mid;
jstring jstr;
jclass stringClass;
jobjectArray args;
JavaVMInitArgs vm_args;
JavaVMOption options[2];
options[0].optionString =
"-Djava.class.path=."; // or "-Djava.class.path=/home/me/project/";
options[1].optionString =
"-verbose";
vm_args.version = JNI_VERSION_1_6;
vm_args.options = options;
vm_args.nOptions = 2;
vm_args.ignoreUnrecognized = JNI_FALSE;
/* Create the Java VM */
res = JNI_CreateJavaVM(&jvm, (void**)&env, &vm_args);
if (res < 0) {
fprintf(stderr, "Can't create Java VM\n");
exit(1);
}
if ((*env)->ExceptionOccurred(env)) {
(*env)->ExceptionDescribe(env);
}
cls = (*env)->FindClass(env,"my.ClassABC");
if (cls == NULL) {
if ((*env)->ExceptionOccurred(env)) {
(*env)->ExceptionDescribe(env);
}
printf("Failed to get class\n");
exit(1);
}
[call methods, etc.]
}
And my java code, just for #$%#s and giggles (gets compiled to /home/me/project/my/ClassABC.class):
package my;
class ClassABC {
public static void main(String[] args) {
System.out.println(ClassABC.getPassword("test"));
return;
}
static String getPassword(String filename)
{
return "Hello "+filename;
}
}
Thanks,
Brian
Just replace this
cls = (*env)->FindClass(env,"my.ClassABC");
with
cls = (*env)->FindClass(env,"my/ClassABC");
and you should be fine.
Else try adding
...
JavaVMOption options[3];
...
options[2].optionString = "-verbose:jni";
...