Calling DeleteLocalRef in java native interface

Question

I'm returning a jstring from a JNI method. I delete the local reference to it before returning the value.

JNIEXPORT jstring JNICALL TestJNIMethod( JNIEnv* env, jclass )
{
    jstring  test_string = env->NewStringUTF( "test_string_value" );
    env->DeleteLocalRef( test_string );
    return test_string;
}

Would the calling JAVA method be still able to access the returned jstring or would the Garbage collector cleanup the memory?

Answer 1

No it will not, however your code will work on Android versions earlier than ICS. From ICS on this code will correctly fail.

In general you don't need to delete local references yourself, once the JNI function returns to Java the references will get GC'd.

The exception to that rule is if you create a lot of them, perhaps in a loop. Then it is possible to fill the local reference table. See IBM: Overview of JNI object references.

You should read JNI Local Reference Changes in ICS. Even if you are not writing for Android, it still identifies many common mistakes.