Jni C++ findclass function return null - c++

Question

I found a problem with jni about C calling Java code.
Environment WIN10 JDK1.8
Currently I need C++ code to call Java code. At first I wrote a demothat was successful. Code show as below:
public class Sample2 {
public String name;
public static String sayHello(String name) {
return "Hello, " + name + "!";
}
public String sayHello() {
return "Hello, " + name + "!";
}
}
Some of the C++ code is as follows:
int main(){
printf("hello world");
JavaVMOption options[3];
JNIEnv* env;
JavaVM* jvm;
JavaVMInitArgs vm_args;
long status;
jclass cls;
jmethodID mid;
jfieldID fid;
jobject obj;
char opt1[] = "-Djava.compiler=NONE";
char opt2[] = "-Djava.class.path=.";
char opt3[] = "-verbose:NONE";
options[0].optionString = opt1; options[0].extraInfo = NULL;
options[1].optionString = opt2; options[1].extraInfo = NULL;
options[2].optionString = opt3; options[2].extraInfo = NULL;
memset(&vm_args, 0, sizeof(vm_args));
vm_args.version = JNI_VERSION_1_8;
vm_args.nOptions = 1;
vm_args.options = options;
vm_args.ignoreUnrecognized = 0;
// 启动虚拟机
status = JNI_CreateJavaVM(&jvm, (void**)& env, &vm_args);
if (status != JNI_ERR){
// 先获得class对象
cls = env->FindClass("Sample2");
}
}
I used Eclipse to compile the Java code into a .class file, copy the .class file into my C++ project, the above DEMO C++ call Java function is successful, and the findclass function returns to normal.
Because I have to introduce a third-party JAR package org.eclipse.paho.client.mqttv3-1.2.0.jar in my own Java, based on the above example, I modified the Java code in DEMO, but when I want to reference the JAR package function, And then run successfully in Eclipse, when I copy the .class file to the C++ project. JNI_CreateJavaVM in the C++ code is returned successfully, but FINDCLASS always returns null, I don't know why. I have not changed the other parts code.
Some Java code:
import javax.net.ssl.TrustManager;
import javax.net.ssl.X509TrustManager;
import org.eclipse.paho.client.mqttv3.*;
import org.eclipse.paho.client.mqttv3.persist.MemoryPersistence;
public class Sample2 {
public String name;
static MqttAsyncClient mqttClient = null;
static String username = "xxx";
static String password = "xxx";
static String broker = "xxx";
public static void main(String[] args) throws InterruptedException {
System.out.print("hello");
}
public static void start() {
String clientId = "mqttserver" + String.valueOf(System.currentTimeMillis());
try {
mqttClient = new MqttAsyncClient(broker, clientId, new MemoryPersistence());
} catch (Exception me) {
me.printStackTrace();
}
}
When in start function is added
mqttClient = new MqttAsyncClient(broker, clientId, new MemoryPersistence()); After the code, there will be problems

Answer 1

Take a look here
char opt1[] = "-Djava.compiler=NONE";
char opt2[] = "-Djava.class.path=.";
char opt3[] = "-verbose:NONE";
options[0].optionString = opt1; options[0].extraInfo = NULL;
options[1].optionString = opt2; options[1].extraInfo = NULL;
options[2].optionString = opt3; options[2].extraInfo = NULL;
memset(&vm_args, 0, sizeof(vm_args));
vm_args.version = JNI_VERSION_1_8;
vm_args.nOptions = 1;
You are passing three options (array of options with three options defined) but then, you say something like this
vm_args.nOptions = 1;
which means you are passing just one option. It means your options
char opt2[] = "-Djava.class.path=.";
char opt3[] = "-verbose:NONE";
are not even read. You have to change your code to
vm_args.nOptions = 3;
Also, make sure to put on java.class.path all the JARs, folders, where classes required by your code are.