How to Fix \'java.lang.NoSuchMethodError: messageMe\' When Calling Java Methods from C in Android?

Calling Java Methods from C in Android: A Detailed Walkthrough

Calling Java methods from C is a common task in Android development. This article aims to provide a comprehensive solution to this problem by delving into the Java and C code and addressing a common error that may arise during this process.

Java Code Setup

The provided Java code defines a class named MainActivity with a method getJniString() that returns a string from C . The messageMe() method, which we intend to call from C , simply prints the received text.

<code class="java">public class MainActivity extends Activity {
    ...
    public void messageMe(String text) {
        System.out.println(text);
    }
    ...
}</code>

C Code

The C code in native.cpp attempts to find the MainActivity class, retrieve the messageMe() method, and invoke it on the jstr string object. However, the error message "java.lang.NoSuchMethodError: messageMe" indicates that the messageMe() method is not found.

<code class="cpp">jstring Java_the_package_MainActivity_getJniString( JNIEnv* env, jobject obj, jint depth ){
    jstring jstr = env->NewStringUTF("This string comes from JNI");
    jclass clazz = env->FindClass("the/package/MainActivity");
    jmethodID messageMe = env->GetMethodID(clazz, "messageMe", "(Ljava/lang/String;)V");
    env->CallObjectMethod(jstr, messageMe);
    ...
}</code>

The Fix

The error is caused by two issues:

• Incorrect Object Invocation: The messageMe() method requires an object reference, which should be passed as the first argument to CallObjectMethod(). The incorrect code invokes the method on jstr directly, which is not valid.
• Invalid JNI Signature: The C code uses (Ljava/lang/String;)V as the signature for messageMe() method, indicating that it returns void. However, the actual Java method returns a String value. To fix this, change the C code to:

<code class="cpp">jstring Java_the_package_MainActivity_getJniString( JNIEnv* env, jobject obj, jint depth ){
    jstring jstr = env->NewStringUTF("This string comes from JNI");
    ...
    jstring result = env->CallObjectMethod(obj, messageMe, jstr);
    ...
}</code>

Updated C Method

<code class="cpp">jstring Java_the_package_MainActivity_getJniString( JNIEnv* env, jobject obj, jint depth ){
    jstring jstr = env->NewStringUTF("This string comes from JNI");
    ...
    jstring result = env->CallObjectMethod(obj, messageMe, jstr);
    const char* str = env->GetStringUTFChars(result, NULL);
    printf("%s\n", str);
    ...
}</code>

Conclusion

By addressing the object invocation and JNI signature mismatch issues, we can successfully call the messageMe() method from C code.

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