I am writing a small C program using Cygwin that launches a Java Virtual Machine (libraries I am using require POSIX environment). So far, I have been able to get it to work as long as I place all of my classes in the same folder as the executable. However, I want to specify an actual JAR file that contains the application I want to run. This does not seem to work though, FindClass simply returns a null. I've narrowed it down to a problem with the classpath setting, like I said, because I can extract my jar file in the same directory as the executable and it will work. Here is a subset of my code:
I've loosely been following this guide: http://www.inonit.com/cygwin/jni/invocationApi/
int main( int argc, char *argv[] )
{
void* jvmDllHandle;
JNIEnv* jenv;
JavaVM* jvm;
JavaVMInitArgs args;
JavaVMOption options[1];
jclass cls;
jmethodID mainMethod;
jobjectArray appArgs;
jstring arg0;
assert( cygwin_internal( CW_SYNC_WINENV ) != 1UL );
jvmDllHandle = LoadLibrary( "c:\\Path\\To\\Application\\jre\\bin\\server\\jvm.dll" );
createJavaVM = dlsym( jvmDllHandle, "JNI_CreateJavaVM" );
args.version = JNI_VERSION_1_6;
args.nOptions = 1;
options[0].optionString = "-Djava.class.path=c:\\Path\\To\\Application\\TheJarFile.jar";
args.options = options;
args.ignoreUnrecognized = JNI_FALSE;
createJavaVM( &jvm, (void **) &jenv, &args );
cls = (*jenv)->FindClass( jenv, "some/package/MainClass" );
assert( cls != NULL ); // This fails.
/// Omitted...
return 0;
}
Tried using -classpath and -cp
int main( int argc, char *argv[] )
{
void* jvmDllHandle;
JNIEnv* jenv;
JavaVM* jvm;
JavaVMInitArgs args;
JavaVMOption options[1];
jclass cls;
jmethodID mainMethod;
jobjectArray appArgs;
jstring arg0;
assert( cygwin_internal( CW_SYNC_WINENV ) != 1UL );
jvmDllHandle = LoadLibrary( "c:\\Path\\To\\Application\\jre\\bin\\server\\jvm.dll" );
createJavaVM = dlsym( jvmDllHandle, "JNI_CreateJavaVM" );
args.version = JNI_VERSION_1_6;
args.nOptions = 1;
options[0].optionString = "-classpath c:\\Path\\To\\Application\\TheJarFile.jar";
args.options = options;
args.ignoreUnrecognized = JNI_FALSE;
createJavaVM( &jvm, (void **) &jenv, &args );
cls = (*jenv)->FindClass( jenv, "some/package/MainClass" );
assert( cls != NULL ); // This fails.
/// Omitted...
return 0;
}
How am I specifying the classpath incorrectly?
On x86-64, the Oracle Windows JDK headers define jint as long. This is 32 bits with Microsoft compilers (which the Oracle JDK is written for) but 64 bits with Cygwin gcc. Since JavaVMInitArgs contains some fields of this type, its binary layout is changed by this discrepancy.
I worked around this by providing a local jni.h header:
#include "stdint.h"
#define __int64 int64_t
#define long int32_t
#include "jni_md.h"
#undef long
#include_next "jni.h"
I'm including only jni_md.h within the scope of the long redefinition because it doesn't include any other headers, whereas jni.h includes a couple of standard headers which we would not want to be affected as well.
To ensure this is always included ahead of the Oracle header, use the -I compiler option to add its directory to the #include path.
Related
New to JNI. I am trying to call Element::getChild and Element::getChildText APIs (java org.jdom.Element) to get the version number of a system that is stored in "settings.xml" This xml file is archived in JAR file. Assuming the root element is available, here is what I am doing:
jstring fileNameStr = env->NewStringUTF("settings.xml");
jobject rootElement = env->CallStaticObjectMethod(a_class, xmlRootElement_mid, fileNameStr);
jclass cls_element = env->FindClass("org/jdom/Element");
/*get method ID for getChild() & getChildText() */
jmethodID getChild_mid = env->GetMethodID(cls_element, "getChild", "(Ljava/lang/String;)Lorg/jdom/Element;");
jmethodID getChildText_mid = env->GetMethodID(cls_element, "getChildText", "(Ljava/lang/String;)Ljava/lang/String;");
jstring aboutStr = env->NewStringUTF("about");
jobject about = env->CallStaticObjectMethod(cls_element, getChild_mid, aboutStr); ---> Seg Faults!!!
Basically, I want to do Java equivalent of this:
In Java:
import org.jdom.Element;
...
Element element = SomeMethodToReadXmlFile("settings.xml");
version = element.getChild("about").getChildText("version"); <---- works
How should I do this?
Using JNI, here is what worked for me that uses org.jdom.Element. My settings.xml file looks like this:
<?xml version="1.0">
<settings>
<about>
<version>1.0</version>
</about>
</settings>
JNI C++:
jstring fileNameStr = env->NewStringUTF("settings.xml");
// assuming xmlRootElement_mid is known
jobject element_obj = env->CallStaticObjectMethod(a_class, xmlRootElement_mid, fileNameStr);
jclass cls_element = env->FindClass("org/jdom/Element");
/*get method ID for getChild() & getChildText() */
jmethodID getChild_mid = env->GetMethodID(cls_element, "getChild", "(Ljava/lang/String;)Lorg/jdom/Element;");
jmethodID getChildText_mid = env->GetMethodID(cls_element, "getChildText", "(Ljava/lang/String;)Ljava/lang/String;");
jstring jstr = env->NewStringUTF("about");
element_obj = env->CallObjectMethod(element_obj, getChild_mid, jstr)
jstr = env->NewStringUTF("version");
jstring version_str = (jstring)env->CallObjectMethod(element_obj, getChildText_mid, jstr);
//Convert to std::string
std::string version_std_str;
if (version_str)
{
const char *c = env->GetStringUTFChars(version_str, 0);
version_std_str = std::string(c);
env->ReleaseStringUTFChars(version_str, c);
}
std::cout << version_std_str << std::endl
Outputs:
1.0
I'll start with my code:
#include <jni.h>
int main(int argc, char const *argv[]) {
JavaVMInitArgs args;
JNIEnv *env;
JavaVM *vm;
args.version = JNI_VERSION_1_8;
args.ignoreUnrecognized = JNI_FALSE;
JNI_CreateJavaVM(&vm, (void **)&env, &args);
jclass System;
jclass PrintStream;
jobject out;
jfieldID outID;
jstring someText;
jmethodID println;
someText = (*env)->NewStringUTF(env, "Hello World");
System = (*env)->FindClass(env, "java/lang/System");
PrintStream = (*env)->FindClass(env, "java/io/PrintStream");
outID = (*env)->GetStaticFieldID(env, System, "out", "Ljava/io/PrintStream;");
out = (*env)->GetStaticObjectField(env, System, outID);
println = (*env)->GetMethodID(env, PrintStream, "println", "(Ljava/lang/String;)V");
(*env)->CallVoidMethod(env, out, println, someText);
return 0;
}
I would expect it to print "Hello World" but it does not, instead I get Segmentation fault (core dumped) annoying error. I could not figure out what is wrong with this code, I tried to comment out everything after someText = (*env)->NewStringUTF(env, "Hello World"); and the program didn't crash, I also tried to comment only the someText = (*env)->NewStringUTF(env, "Hello World"); and I worked too. I even changed the println signature to "boolean" and passed 0 to it, program printed "false" as I would expect so I guess this is something wrong with NewStringUTF method.
openjdk version "1.8.0_131"
OpenJDK Runtime Environment (build 1.8.0_131-b11)
OpenJDK 64-Bit Server VM (build 25.131-b11, mixed mode)
(gdb) bt
#0 0x00007ffff713b2ff in ?? () from /usr/lib/jvm/java-8-openjdk/jre/lib/amd64/server/libjvm.so
#1 0x00007ffff78955c4 in ?? () from /usr/lib/jvm/java-8-openjdk/jre/lib/amd64/server/libjvm.so
#2 0x00007ffff7507e71 in JNI_CreateJavaVM () from /usr/lib/jvm/java-8-openjdk/jre/lib/amd64/server/libjvm.so
#3 0x0000000000400558 in main (argc=1, argv=0x7fffffffe678) at main.c:11
(gdb) info f
Stack level 0, frame at 0x7fffffffe310:
rip = 0x7ffff713b2ff; saved rip = 0x7ffff78955c4
called by frame at 0x7fffffffe490
Arglist at 0x7fffffffe2c8, args:
Locals at 0x7fffffffe2c8, Previous frame's sp is 0x7fffffffe310
Saved registers:
rbx at 0x7fffffffe2d8, rbp at 0x7fffffffe300, r12 at 0x7fffffffe2e0, r13 at 0x7fffffffe2e8, r14 at 0x7fffffffe2f0,
r15 at 0x7fffffffe2f8, rip at 0x7fffffffe308
after commenting out the NewStringUTF
(gdb) bt
#0 0x00007fffe61082b4 in ?? ()
#1 0x0000000000000246 in ?? ()
#2 0x00007fffe6108160 in ?? ()
#3 0x00007fffffffe0f0 in ?? ()
#4 0x00007fffffffe090 in ?? ()
#5 0x00007ffff78f6748 in ?? () from /usr/lib/jvm/java-8-openjdk/jre/lib/amd64/server/libjvm.so
Backtrace stopped: previous frame inner to this frame (corrupt stack?)
(gdb) info f
Stack level 0, frame at 0x7fffffffde80:
rip = 0x7fffe61082b4; saved rip = 0x246
called by frame at 0x7fffffffde88
Arglist at 0x7fffffffde70, args:
Locals at 0x7fffffffde70, Previous frame's sp is 0x7fffffffde80
Saved registers:
rip at 0x7fffffffde78
After some observations, looks like the function JNI_CreateJavaVM is crashing after NewStringUTF was added, but it's "working" after it's removed. How weird is that?
This is JDK and JRE I am using: https://www.archlinux.org/packages/extra/x86_64/jdk8-openjdk/
I am compiling with this command:
gcc \
-I /usr/lib/jvm/java-8-openjdk/include/ \
-I /usr/lib/jvm/java-8-openjdk/include/linux/ \
-L /usr/lib/jvm/java-8-openjdk/jre/lib/amd64/server/ \
-l jvm \
main.c
And running the file with
export LD_LIBRARY_PATH=/usr/lib/jvm/java-8-openjdk/jre/lib/amd64/server/
./a.out
Next Day
Different code, same problem:
#include <jni.h>
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char const *argv[]) {
JavaVMInitArgs args;
JavaVM *jvm;
JNIEnv *env;
args.version = JNI_VERSION_1_8;
args.ignoreUnrecognized = JNI_FALSE;
printf("%s\n", "Creating VM");
JNI_CreateJavaVM(&jvm, (void **)&env, &args);
printf("%s\n", "VM Was Created");
jclass String = (*env)->FindClass(env, "java/lang/String");
if (String == NULL) {
printf("%s\n", "String was NULL");
exit(1);
}
jmethodID method = (*env)->GetMethodID(env, String, "codePointAt", "(I)I");
if (method == NULL) {
printf("%s\n", "method was NULL");
exit(1);
}
printf("%s\n", "I am finishing here");
(*jvm)->DestroyJavaVM(jvm);
return 0;
}
Compile and run:
$ gcc \
> -I /usr/lib/jvm/java-8-openjdk/include/ \
> -I /usr/lib/jvm/java-8-openjdk/include/linux/ \
> -L /usr/lib/jvm/java-8-openjdk/jre/lib/amd64/server/ \
> -l jvm \
> main.c && ./a.out
Creating VM
Segmentation fault (core dumped)
But if I comment the part of the code:
#include <jni.h>
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char const *argv[]) {
JavaVMInitArgs args;
JavaVM *jvm;
JNIEnv *env;
args.version = JNI_VERSION_1_8;
args.ignoreUnrecognized = JNI_FALSE;
printf("%s\n", "Creating VM");
JNI_CreateJavaVM(&jvm, (void **)&env, &args);
printf("%s\n", "VM Was Created");
jclass String = (*env)->FindClass(env, "java/lang/String");
if (String == NULL) {
printf("%s\n", "String was NULL");
exit(1);
}
/*jmethodID method = (*env)->GetMethodID(env, String, "codePointAt", "(I)I");
if (method == NULL) {
printf("%s\n", "method was NULL");
exit(1);
}*/
printf("%s\n", "I am finishing here");
(*jvm)->DestroyJavaVM(jvm);
return 0;
}
Compile and run:
$ gcc \
> -I /usr/lib/jvm/java-8-openjdk/include/ \
> -I /usr/lib/jvm/java-8-openjdk/include/linux/ \
> -L /usr/lib/jvm/java-8-openjdk/jre/lib/amd64/server/ \
> -l jvm \
> main.c && ./a.out
Creating VM
VM Was Created
I am finishing here
So, for me, it works as expected:
> gdb ./main
...
(gdb) run
Starting program: ..../issue/main
[New Thread 0x1403 of process 2600]
[New Thread 0x1503 of process 2600]
warning: unhandled dyld version (15)
Hello World
[Inferior 1 (process 2600) exited normally]
(gdb)
However, I have slightly different env. macOS and I use Oracle's JDK.
Maybe you can try installing debug info and check what exactly happens inside JDK?
So I'm having some java-Code that I want to call from C++ via JNI. It's done by the following code:
int main(int argc, char **argv) {
JavaVM * jvm;
JNIEnv* env = create_vm(&jvm);
invoke_class( env );
}
JNIEnv* create_vm(JavaVM ** jvm) {
JNIEnv* env;
JavaVMInitArgs vm_args;
JavaVMOption options;
vm_args.version = JNI_VERSION_1_6;
vm_args.nOptions = 1;
options.optionString = "-Djava.class.path=<path_to_java_class>:<path_to_jars>";
vm_args.options = &options;
vm_args.ignoreUnrecognized = JNI_FALSE;
int ret = JNI_CreateJavaVM(jvm, (void **)&env, &vm_args);
if(ret < 0)
printf("\nUnable to Launch JVM\n");
return env;
}
void invoke_class(JNIEnv* env) {
jclass helloWorldClass;
helloWorldClass = env->FindClass("A");
if (env->ExceptionCheck()){
std::cout << "EXCEPTION OCCURED: " << std::endl;
env->ExceptionDescribe();
}
}
call_main(env, helloWorldClass);
void call_main(JNIEnv* env, jclass hClass){
jmethodID mainMethod;
jobjectArray applicationArgs;
jstring applicationArg0;
jstring applicationArg1;
mainMethod = env->GetStaticMethodID(hClass, "main", "([Ljava/lang/String;)V");
applicationArgs = env->NewObjectArray(2, env->FindClass("java/lang/String"), NULL);
applicationArg0 = env->NewStringUTF("planes.txt");
applicationArg1 = env->NewStringUTF("out.txt");
env->SetObjectArrayElement(applicationArgs, 0, applicationArg0);
env->SetObjectArrayElement(applicationArgs, 1, applicationArg1);
env->CallStaticVoidMethod(hClass, mainMethod, applicationArgs);
}
The corresponding Java-Code that is called is as follows:
public class A{
import org.mindswap.pellet.jena.PelletReasonerFactory;
import com.hp.hpl.jena.rdf.model.ModelFactory;
private OntModel model = ModelFactory.createOntologyModel(PelletReasonerFactory.THE_SPEC);
public A() {
System.out.println("hi2");
}
public static void main(String[] args) throws Exception {
System.out.println("hi1");
A app = new A();
long start = System.currentTimeMillis();
long stop = System.currentTimeMillis();
System.out.println("Processing took " + (stop - start) + " ms");
System.out.println("Normal program end.");
}
}
Okay, so if I'm now executing the C++-Code I get the following output:
hi1
without any error-messages, the program terminates correctly. However, I would expect the output to be something like:
hi1
hi2
Processing took 0ms
Normal program end.
It seems like calling the constructor just doesn't happen and the code after that isn't executed at all.
However, if I remove the line
private OntModel model = ModelFactory.createOntologyModel(PelletReasonerFactory.THE_SPEC);
the output is correct.
Since leaving out this isn't an option, what is happening here? How is it even possible that the program is terminating without an error-message although it shouldn't be finished at all?
I've tried to put in something like Thread.sleep(10000) to see if it has something to do with the time that given call takes, but that doesn't seem to be a problem at all.
EDIT: sry, I forgot to change the class-name.
EDIT2: I should mention that the java-code ist running perfectly fine when it is not called out of a C++-environment
Problem solved. It had something to do with the jar-files and some file-locations. Sorry for bothering for nothing!
I'm trying to do the following :
Run Java app on Android
Call native C++ code from that Java app
Create and run a native thread from that natice code
return immediatly while the thread keeps running
Call a java method from native code once the thread is finished
Here is my code :
#include <jni.h>
#include <pthread.h>
#include <unistd.h> /* sleep() */
#include <stdio.h>
#ifdef __cplusplus
extern "C"
{
#endif
JNIEXPORT jstring JNICALL Java_xxx_asynctasktest_MainActivity_runAsyncTask( JNIEnv * env, jobject that, jstring oDummyStr );
#ifdef __cplusplus
}
#endif
JavaVM * g_jvm;
jobject g_obj;
jmethodID g_mid;
void * threadProc( void * pArg )
{
JNIEnv * env = 0;
int res = g_jvm->AttachCurrentThread( & env, NULL );
sleep( 3 );
jobject oStr = (jobject)pArg;
env->CallObjectMethod( g_obj, g_mid, oStr );
env->DeleteGlobalRef( oStr );
env->DeleteGlobalRef( g_obj );
g_jvm->DetachCurrentThread();
return 0;
}
JNIEXPORT jstring JNICALL Java_xxx_asynctasktest_MainActivity_runAsyncTask( JNIEnv * env, jobject that, jstring oDummyStr )
{
env->GetJavaVM( & g_jvm );
g_obj = env->NewGlobalRef( that );
g_mid = env->GetMethodID( env->GetObjectClass( that ), "onTaskFinished", "(Ljava/lang/String;)Ljava/lang/String;" );
if( g_mid == NULL )
{
fprintf( stderr, "No such method" );
}
else
{
pthread_t thread = 0;
pthread_create( & thread, NULL, threadProc, (void *)env->NewGlobalRef( oDummyStr ) );
}
return env->NewStringUTF( "lolol" );
}
My problem is that when i call env->CallObjectMethod( g_obj, g_mid, oStr ); eclipse tries to open Handler.class in the class file editor and says not found and the thread doesn't call the Java method.
What am I doing wrong ? Thank you.
EDIT : Forgot to mention that my Java callback creates a dialog as follows :
AlertDialog.Builder dlgAlert = new AlertDialog.Builder( this );
dlgAlert.setMessage( msgBoxText );
dlgAlert.setTitle( "" );
dlgAlert.setPositiveButton( "OK", null );
dlgAlert.setCancelable( true );
dlgAlert.create().show();
And i'm having a runtime exception somewhere in "Handler" at line 121, source file not available.
EDIT : I'm actually reaching the Java callback, but it crashes when i start creating my dialog.
I'm writing a simple program to call a Java function from my C program.
Following is my code:
#include <jni.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>
#include <pthread.h>
#include <time.h>
#include <stdlib.h>
JNIEnv* create_vm() {
JavaVM* jvm;
JNIEnv *env;
JavaVMInitArgs vm_args;
JavaVMOption options[1];
options[0].optionString - "-Djava.class.path=/home/chanders/workspace/Samples/src/ThreadPriorityTest";
vm_args.version = JNI_VERSION_1_6;
vm_args.nOptions = 1;
vm_args.options = &options;
vm_args.ignoreUnrecognized = JNI_FALSE;
JNI_CreateJavaVM(&jvm, (void**)&env, &vm_args);
return env;
}
void invoke_class(JNIEnv* env) {
jclass helloWorldClass;
jmethodID mainMethod;
jobjectArray applicationArgs;
jstring applicationArg0;
helloWorldClass = (*env)->FindClass(env, "InvocationHelloWorld");
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() {
JNIEnv* env = create_vm();
invoke_class(env);
}
I'm compiling the above program using:
gcc -o invoke -I$JAVA_HOME/include/ -I$JAVA_HOME/include/linux -L$JAVA_HOME/jre/lib/amd64/server/ ThreadPriorityTest.c
and i'm getting the following error:
/tmp/ccllsK5O.o: In function `create_vm': ThreadPriorityTest.c:(.text+0x35): undefined reference to `JNI_CreateJavaVM' collect2: ld returned 1 exit status
I'm not really sure what is causing this problem
UPDATE 1
Included the -ljvm in the command line and then got a undefined reference to FUNCTION_NAME
I'm running it on Rhel 6.2
You've got the path to the Java library (the -L option), but not the library itself. You need to include -ljvm on the link line as well.