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Calling Java Methods from JNI results in program crash

Calling get or set methods crashes. Object array is valid. It prints till line 2.Intially was getting error while inserting into the jobjectArray .Also tried with getting the value from Jobject .Both getter and setters fail .
JNICode is as below :
JNIEXPORT jint JNICALL Java_demo_JNIWrapper_pax_1store_1get_1data_1avail_1info
(JNIEnv *env, jclass jclass1, jobjectArray jobj)
{
.....
.....
int len = (*env)->GetArrayLength(env, jobj);
printf ("Incoming object array length = %d\n", len);// - - - > Works.. shows 2 (I am passing 2 objects)
jobject j = (*env)->GetObjectArrayElement(env, jobj,0);
printf("This line 1 \n ");
jmethodID meth1=(*env)->GetMethodID(env,jclass1,"getTimestamp","()Ljava/lang/String;");
printf("This line 2 \n "); // - - - - > Works..
jstring string_from_obj = (*env)->CallObjectMethod(env,j,meth1); // - - - - > Crashes..
printf("This line 3 \n "); // - - -> does not print this..
printf("Contents are =%s\n",(*env)->GetStringUTFChars(env, string_from_obj, 0));
............
}
My Java Object is as Below :
public class DataAvailable {
String timestamp;
public String getTimestamp() {
return timestamp;
}
public void setTimestamp(String timestamp) {
this.timestamp = timestamp;
}
}
Call to JNI is as below :
public native int pax_store_get_data_avail_info(DataAvailable[] stats_array);

The glaring issue is that pax_store_get_data_avail_info() is not a method of DataAvailable. It is a method of some other class? You don't actually say what it is. But let's suppose it is:
class X {
public native int pax_store_get_data_avail_info(DataAvailable[] stats_array);
}
When you call this method, the jclass jclass1 argument you get is for "X", not for DataAvailable. So your call to GetMethodID() probably fails. You don't check the return, so you re probably handing a bad method ID to the next call. JNI fails are hard crashes and very ugly.
You need to query the jclass for DataAvailable and use that.

SIGSEGV when calling Java method from native pthread

In a Java project that uses C code via JNI I have a piece of native C code that obtains references to an object and one of its methods, then starts a native thread, passing these references to it in a struct. When the thread tries to call the method, the code crashes with a SIGSEGV. Calling that same method from the main thread works.
Doing some research I learned that the env reference is only valid within the thread and that any other native thread must be attached first. I did this but the code still crashes on the first call to the method.
Strangely, when I call the same method from the main thread before I create the other thread (just uncomment the line in the main code), things work for some time. The output thread loops for some 10,000 times before it crashes.
The method call is to DataOutputStream.writeShort(). The thread in question is the only one writing to the DataOutputStream. However, the DataOutputStream is connected to a DataInputStream.
Simplified native code:
static void write_output(struct output_state *s) {
int i;
jint sample;
for (i = 0; i < 2 * s->result_len; i += 2) {
sample = (s->result[i] << 8) + s->result[i+1];
(*(s->env))->CallVoidMethod(s->env, s->tunerOut, s->writeShort, sample);
}
}
static void *output_thread_fn(void *arg)
{
struct output_state *s = arg;
(*(s->jvm))->AttachCurrentThread(s->jvm, &(s->env), NULL);
while (!do_exit) {
// use timedwait and pad out under runs
safe_cond_wait(&s->ready, &s->ready_m);
pthread_rwlock_rdlock(&s->rw); //sync access to s with producer thread
write_output(s);
pthread_rwlock_unlock(&s->rw);
}
(*(s->jvm))->DetachCurrentThread(s->jvm);
return 0;
}
JNIEXPORT jboolean JNICALL Java_eu_jacquet80_rds_input_SdrGroupReader_open
(JNIEnv *env, jobject self) {
jclass clsSelf = (*env)->GetObjectClass(env, self);
jfieldID fTunerOut = (*env)->GetFieldID(env, clsSelf, "tunerOut", "Ljava/io/DataOutputStream;");
jobject tunerOut = (*env)->GetObjectField(env, self, fTunerOut);
jclass cls = (*env)->GetObjectClass(env, tunerOut);
jmethodID writeShortID = (*env)->GetMethodID(env, cls, "writeShort", "(I)V");
if (!writeShortID || !cls)
return 0;
(*env)->GetJavaVM(env, &(output.jvm));
output.tunerOut = tunerOut;
output.writeShort = writeShortID;
// (*env)->CallVoidMethod(env, tunerOut, writeShortID, 0); // just for testing
pthread_create(&controller.thread, NULL, controller_thread_fn, (void *)(&controller));
usleep(100000);
pthread_create(&output.thread, NULL, output_thread_fn, (void *)(&output));
pthread_create(&demod.thread, NULL, demod_thread_fn, (void *)(&demod));
pthread_create(&dongle.thread, NULL, dongle_thread_fn, (void *)(&dongle));
return 1;
}
Are JNI references such as jclass, jfieldID, jobject and jmethodID subject to the same limitations as JNIEnv, i.e. valid only within the same thread?
Suspecting this, I moved the JNI reference stuff from open() to output_thread(), right after the call to AttachCurrentThread(). However, I still need to pass a jobject reference (self) across thread borders, and the call to GetObjectClass() crashes.
What is the correct way to create a thread native code and have that thread call a particular method of a given class instance?

Turns out my suspicion was correct: jobject and jclass references are indeed local, i.e. valid only within the same thread and only until the current native method returns. See http://developer.android.com/training/articles/perf-jni.html#local_and_global_references.
My approach of moving the reference-related code to the thread function was correct, except that I need to first convert self into a global reference by calling NewGlobalRef().
Updated code:
static void write_output(struct output_state *s) {
int i;
jint sample;
for (i = 0; i < 2 * s->result_len; i += 2) {
sample = (s->result[i] << 8) + s->result[i+1];
(*(s->env))->CallVoidMethod(s->env, s->tunerOut, s->writeShort, sample);
}
}
static void *output_thread_fn(void *arg)
{
struct output_state *s = arg;
(*(s->jvm))->AttachCurrentThread(s->jvm, &(s->env), NULL);
jclass clsSelf = (*(s->env))->GetObjectClass(s->env, s->self);
jfieldID fTunerOut = (*(s->env))->GetFieldID(s->env, clsSelf, "tunerOut", "Ljava/io/DataOutputStream;");
s->tunerOut = (*(s->env))->GetObjectField(s->env, s->self, fTunerOut);
jclass cls = (*(s->env))->GetObjectClass(s->env, s->tunerOut);
s->writeShort = (*(s->env))->GetMethodID(s->env, cls, "writeShort", "(I)V");
while (!do_exit) {
// use timedwait and pad out under runs
safe_cond_wait(&s->ready, &s->ready_m);
pthread_rwlock_rdlock(&s->rw); //sync access to s with producer thread
write_output(s);
pthread_rwlock_unlock(&s->rw);
}
(*(s->jvm))->DetachCurrentThread(s->jvm);
return 0;
}
JNIEXPORT jboolean JNICALL Java_eu_jacquet80_rds_input_SdrGroupReader_open
(JNIEnv *env, jobject self) {
jclass clsSelf = (*env)->GetObjectClass(env, self);
if (!writeShortID || !cls)
return 0;
output.self = (*env)->NewGlobalRef(env, self);
(*env)->GetJavaVM(env, &(output.jvm));
(*env)->CallVoidMethod(env, tunerOut, writeShortID, 0); // just for testing
pthread_create(&controller.thread, NULL, controller_thread_fn, (void *)(&controller));
usleep(100000);
pthread_create(&output.thread, NULL, output_thread_fn, (void *)(&output));
pthread_create(&demod.thread, NULL, demod_thread_fn, (void *)(&demod));
pthread_create(&dongle.thread, NULL, dongle_thread_fn, (void *)(&dongle));
return 1;
}
One thing still missing is a call to DeleteGlobalRef() when the output thread is done. This is to make sure the global reference is released when it is no longer needed, so that the garbage collector can pick it up.

Jni FindClass returns NULL

I got c++ structs in header file,
struct StatusLine
{
static jclass Class; // Lorg/apache/http/StatusLine;
static jmethodID GetStatusCode; // ()I
};
struct ByteArrayOutputStream
{
static jclass Class; // Ljava/io/ByteArrayOutputStream;
static jmethodID Constructor; // ()V
static jmethodID Close; // ()V
static jmethodID ToByteArray; // ()[B
};
struct HttpEntity
{
static jclass Class; // Lorg/apache/http/HttpEntity;
static jmethodID WriteTo; // (Ljava/io/OutputStream;)V
static jmethodID GetContent; // ()Ljava/io/InputStream;
};
and cpp file is
#define JAVA_STATUS_LINE_CLASS "org/apache/http/StatusLine"
#define JAVA_HTTP_ENTITY_CLASS "org/apache/http/HttpEntity"
#define JAVA_BYTE_ARRAY_OUTPUT_STREAM_CLASS "java/io/ByteArrayOutputStream"
jclass StatusLine::Class = 0;
jmethodID StatusLine::GetStatusCode = 0;
jclass ByteArrayOutputStream::Class = 0;
jmethodID ByteArrayOutputStream::Constructor = 0;
jmethodID ByteArrayOutputStream::Close = 0;
jmethodID ByteArrayOutputStream::ToByteArray = 0;
jclass HttpEntity::Class = 0;
jmethodID HttpEntity::WriteTo = 0;
jmethodID HttpEntity::GetContent = 0;
void initializeJniPointers()
{
StatusLine::Class = GetJniEnv()->FindClass(JAVA_STATUS_LINE_CLASS);
StatusLine::GetStatusCode = GetJniEnv()->GetMethodID(StatusLine::Class, "getStatusCode", "()I");
ByteArrayOutputStream::Class = GetJniEnv()->FindClass(JAVA_BYTE_ARRAY_OUTPUT_STREAM_CLASS);
ByteArrayOutputStream::Constructor = GetJniEnv()->GetMethodID(ByteArrayOutputStream::Class, "<init>", "()V");
ByteArrayOutputStream::Close = GetJniEnv()->GetMethodID(ByteArrayOutputStream::Class, "close", "()V");
ByteArrayOutputStream::ToByteArray = GetJniEnv()->GetMethodID(ByteArrayOutputStream::Class, "toByteArray", "()[B");
HttpEntity::Class = GetJniEnv()->FindClass(JAVA_HTTP_ENTITY_CLASS);
HttpEntity::WriteTo = GetJniEnv()->GetMethodID(HttpEntity::Class, "writeTo", "(Ljava/io/OutputStream;)V");
HttpEntity::GetContent = GetJniEnv()->GetMethodID(HttpEntity::Class, "getContent", "()Ljava/io/InputStream;");
}
function initializeJniPointers() crushes on line StatusLine::GetStatusCode = GetJniEnv()->GetMethodID(); because StatusLine::Class is NULL.
But! I notice that:
If I write this in some java file of the project
StatusLine l = new StatuLine()
{
...
}
Function crushes on ByteArrayOutputStream::Constructor because ByteArrayOutputStream::Class is NULL, if I create an object of ByteArrayOutputStream in java, function will go further to the next object, etc... I notice that: If I just declare a variable of ByteArrayOutputStream, findClass will return NULL.
Could someone explain me what to do? BTW I use Android 2.3.5 device Samsung GT-S5363, I tried other vertions of android (elder) and devices and it works fine.

Basically this can occur if the thread where you ask FindClass is not the main thread and in your thread system does not build a map of java class IDs.
Check this out, probably you have to ask FindClass first in the main thread (when JNI loads or somewhere else), and then you will have ability to do that in any thread.
http://discuss.cocos2d-x.org/t/jni-findclass-cannot-find-a-class-if-its-called-by-a-new-pthread/1873/4
Also try this out, this worked for me:
https://svn.apache.org/repos/asf/mesos/branches/0.10.x/src/java/jni/convert.cpp
The solution (taken from link above) is finding a Java class loader from your app in JNI_OnLoad and ask him to find class then from any thread. Otherwise, after calling env->FindClass JNI can fall back to the system class loader which loads only system classes like String.

Another technique that has worked for me is to instantiate an object of the needed class, Java-side, just before the native method call (e.g. in AsyncTask doInBackground()).

Passing an int from native code (jni) to java, modify it in java and pass back via arguments

I have a thread in my native Code (c++) and this thread calls a callback function in java with a passed integer in agument.
The callback function in java should modify this integer and returns true or false.
The native code sould work with the new (modified) integer.
Example Code:
C-Code:
doSomething()
{
int id = 5;
callback_fct(&id)
}
callback_fct(int* id)
{
jclass integerClass = (env)->FindClass("java/lang/Integer");
jmethodID midConstructor = (env)->GetMethodID(integerClass, "<init>", "(I)V");
jmethodID midIntValue = (env)->GetMethodID(integerClass, "intValue", "()I");
jobject integerObject = (env)->NewObject(integerClass, midConstructor, *id);
//id should be 5
int res = env->CallIntMethod(appl_object, mid, integerObject);
*id= env->CallIntMethod(integerObject, midIntValue);
//id should be 99 now
}
Java-Code:
private int callback( Integer ID)
{
Log.i("JavaWrapper", "callback");
Log.i("JavaWrapper", "Old ID: " + ID); //should be 5
ID = 99;
Log.i("JavaWrapper", "New ID: " + ID); //should be 99
return 0;
}
Can someone give me a short example for this?
Thanks

Why does my JNI code not successfully find a jthrowable's getMessage method?

I'm trying to access the message in a jthrowable while handing an exception generated when I fail to find a class. However, I am unable to access the message ID of getMessage() on the jthrowable object, and I don't know why. I've tried changing the signature of getMessage to "()Ljava/lang/String" (without the semicolon at the end, but that's necessary, right?) with no joy. I'm confused as hell about this. I even tried replacing getMessage with toString, and that didn't work. Obviously I'm doing something trivially wrong here.
Here's the code I'm using:
jthrowable java_exception;
jclass java_class;
jmethodID method;
java_exception = (*jEnv)->ExceptionOccurred(jEnv);
assert (java_exception != NULL);
java_class = (*jEnv)->GetObjectClass (jEnv, java_exception);
assert (java_class != NULL);
method = (*jEnv)->GetMethodID (jEnv, java_class, "getMessage", "()Ljava/lang/String;");
if (method == NULL) {
printf ("Seriously, how do I get here?!\n");
(*jEnv)->ExceptionDescribe (jEnv);
return;
}
The output of this code (amongst other things) looks like this:
Seriously, how do I get here?!
Exception in thread "main" java.lang.NoClassDefFoundError: com/planet/core360/docgen/Processor
javap -p -s java.lang.Throwable gives me this:
Compiled from "Throwable.java"
public class java.lang.Throwable extends java.lang.Object implements java.io.Serializable{
...
public java.lang.String getMessage();
Signature: ()Ljava/lang/String;
...

Okay, so it looks like my problem was that GetObjectClass doesn't act the way you'd expect it to on a jthrowable, or at least the results of it are not useful for the purposes of getting methods. Replacing that portion of the code with this works:
java_class = (*jEnv)->FindClass (jEnv, "java/lang/Throwable");
method = (*jEnv)->GetMethodID (jEnv, java_class, "getMessage", "()Ljava/lang/String;");
Damnably odd, that. I hope this helps someone else in the future, though.

I tried your approach, and it worked for me. A few things though: I'm using the C++ interface (though that shouldn't make a difference), and I'm using Java 6 update 10, x64 edition, on Ubuntu 8.04. Perhaps the Java version and/or platform used will make a difference.
#include <cstdio>
#include <jni.h>
int
main(int argc, char** argv)
{
if (argc != 3) {
std::fprintf(stderr, "usage: %s class message\n", argv[0]);
return 1;
}
JavaVM* jvm;
void* penv;
JavaVMInitArgs args = {JNI_VERSION_1_6};
if (jint res = JNI_CreateJavaVM(&jvm, &penv, &args)) {
std::fprintf(stderr, "Can's create JVM: %d\n", res);
return -res;
}
JNIEnv* env(static_cast<JNIEnv*>(penv));
jint vers(env->GetVersion());
std::printf("JNI version %d.%d\n", vers >> 16, vers & 0xffff);
env->ThrowNew(env->FindClass(argv[1]), argv[2]);
jthrowable exc(env->ExceptionOccurred());
std::printf("Exception: %p\n", exc);
if (exc) {
jclass exccls(env->GetObjectClass(exc));
jclass clscls(env->FindClass("java/lang/Class"));
jmethodID getName(env->GetMethodID(clscls, "getName", "()Ljava/lang/String;"));
jstring name(static_cast<jstring>(env->CallObjectMethod(exccls, getName)));
char const* utfName(env->GetStringUTFChars(name, 0));
jmethodID getMessage(env->GetMethodID(exccls, "getMessage", "()Ljava/lang/String;"));
jstring message(static_cast<jstring>(env->CallObjectMethod(exc, getMessage)));
char const* utfMessage(env->GetStringUTFChars(message, 0));
std::printf("Exception: %s: %s\n", utfName, utfMessage);
env->ReleaseStringUTFChars(message, utfMessage);
env->ReleaseStringUTFChars(name, utfName);
}
return -jvm->DestroyJavaVM();
}
I've used jnitest java/lang/InternalError 'Hello, world!' for my testing; feel free to try with different exception types!