How to Return an ArrayList of Strings from Native Java to JNI

Question

How can I return an ArrayList of Strings from Java to JNI?

JNIEXPORT jobject JNICALL Java_MyClass_getStringList(JNIEnv *env, jobject obj) {
    // Create a new Java ArrayList
    jclass arrayListClass = (*env)->FindClass(env, "java/util/ArrayList");
    jmethodID arrayListInit = (*env)->GetMethodID(env, arrayListClass, "<init>", "()V");
    jobject arrayList = (*env)->NewObject(env, arrayListClass, arrayListInit);

    // Add Strings to the ArrayList
    jmethodID arrayListAdd = (*env)->GetMethodID(env, arrayListClass, "add", "(Ljava/lang/Object;)Z");
    jclass stringClass = (*env)->FindClass(env, "java/lang/String");
    jstring str1 = (*env)->NewStringUTF(env, "Hello");
    (*env)->CallBooleanMethod(env, arrayList, arrayListAdd, str1);
    jstring str2 = (*env)->NewStringUTF(env, "World");
    (*env)->CallBooleanMethod(env, arrayList, arrayListAdd, str2);

    // Return the ArrayList
    return arrayList;
}

Answer

JNI provides a way for Java code to call and be called by native applications and libraries written in other languages such as C and C++. Returning an ArrayList of Strings from native code back to a Java application involves creating an ArrayList instance and adding elements to it through JNI calls.

JNIEXPORT jobject JNICALL Java_MyClass_getStringList(JNIEnv *env, jobject obj) {
    // Create a new Java ArrayList
    jclass arrayListClass = (*env)->FindClass(env, "java/util/ArrayList");
    jmethodID arrayListInit = (*env)->GetMethodID(env, arrayListClass, "<init>", "()V");
    jobject arrayList = (*env)->NewObject(env, arrayListClass, arrayListInit);

    // Add Strings to the ArrayList
    jmethodID arrayListAdd = (*env)->GetMethodID(env, arrayListClass, "add", "(Ljava/lang/Object;)Z");
    jclass stringClass = (*env)->FindClass(env, "java/lang/String");
    jstring str1 = (*env)->NewStringUTF(env, "Hello");
    (*env)->CallBooleanMethod(env, arrayList, arrayListAdd, str1);
    jstring str2 = (*env)->NewStringUTF(env, "World");
    (*env)->CallBooleanMethod(env, arrayList, arrayListAdd, str2);

    // Return the ArrayList
    return arrayList;
}

Causes

Understanding JNI conventions
Properly managing Java objects from native code
Using the right method IDs to interact with Java classes

Solutions

Use the JNI API correctly to access Java class methods and constructors.
Manage the local references to prevent memory leaks in JNI.

Common Mistakes

Mistake: Not releasing local references, leading to memory leaks.

Solution: Use DeleteLocalRef() after you're done with each local reference.

Mistake: Incorrectly calling Java methods leading to exceptions at runtime.

Solution: Check for exceptions using env->ExceptionCheck() and env->ExceptionDescribe() after native calls.

Helpers

JNI
Java Native Interface
ArrayList
C code
return Java objects from JNI
Java to JNI
JNI ArrayList example