Evaluating the Java Native Interface (JNI): Leveraging Existing Native Code, Libraries and Threads to a Running Java Virtual Machine

Classes and Objects

JNI provides functions that allow the native code to access a field or call a method within a Java object or a Java class. Figure 1 demonstrates the classes and objects JNI methods classification.

Figure 1.

Classification of JNI Array methods

All these functions require a value of jclass type that contains the information for the class in which the field or method is declared. JNI defines two classes namely FindClass() and GetObjectClass(), which can be used from native code to get jclass objects. Next, we outline the characteristics of each function.

• 1.

  The FindClass() method takes as parameter the fully qualified class name with package components and returns the jclass value for the given class or a NULL value. The FindClass() browses all directories and files defined in the Java classpath environment variable for a given class. The statement - jclass strClass = env -> FindClass(“Java/lang/String”) - demonstrates a call based on the FindClass() function to obtain the jclass value for the string class, where Java/lang/String is the fully qualified name for the string class. Another case involves the statement - jclass intClass = env -> FindClass(“[I”) - where the FindClass() obtains the jclass value for an array of Java integer type, with the signature being “I”, while the JNI int[] is represented as “[I”. Finally, the FindClass() function is useful when the native code knows the fully qualified name o the class or the class signature.

• 2.

  The GetObjectClass() function takes an object as parameter and returns the jclass value of the class from which the object was instantiated, or returns a NULL value. This function is useful when the native code requires identifying the class for an object that is passed as an argument. The statement - jclass xClass = env -> GetObjectClass(x) - represents a call using the GetObjectClass() function within a native method to obtain the class of an object passed into a native method as argument.