Core Java Interview - Jni

Front 1

What is JNI? Where is it useful?

Back 1

JNI is a mechanism to invoke native methods typically written in C++ from inside a JVM.

Front 2

What are the strengths and weaknesses of using JNI?

Back 2

JNI can be used to boost performance. It gets used when we want to do something close to hardware like manipulating a framebuffer in a graphics application. The down-side of JNI is that the code is not portable. It is tied to the hardware platform to which the native code is built for.

Front 3

Outline the steps involved in writing a JNI module.

Back 3

Declare a method in a class using the native keyword. Load the library that implements the native method using a static initializer block. Generate the native header file using the javah utility. Implement the native method using the same signature generated by the header file. Compile the native module into a library and run the application.

Front 4

Explain JNI methods and pointers.

Back 4

Java generates the native method name by concatenating the following items. The prefix Java_, encoded fully qualified class name, underscore separator, encoded method name. Each native method gets a JNIEnv interface pointer. This gives the ability to get more information about the JVM from which the method is being invoked. Instances methods get a jobject parameter that represents the Java object on which the native method is invoked. For a static method a jclass parameter is present that represents the class.