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Java Native Interface
Application Notes
Da Ke
4/5/2009
ECE 480 Spring 2009, Design Team 3
Department of Electrical and Computer Engineering
Michigan State University
Abstract
Java is one of most widely used programming language today due to its openness, ability to
scale up and portability. The language is design to have a simpler object model and fewer low-level
facilities like pointer methods in C/C++. Sometime this could impose a limitation on what a program
can do. This application note will explore various ways of implementation to allow Java interact with
some programs that are written in lower level language.
Keywords: Java, JVM, JNI, Linux, Windows, Operating System, Native interface, Native code
Introduction
Most modern high level programming languages today are capable of handling what once
trivial tasks for developers like memory management. However, this doesn't come without cost. A
Java application does not interact with underlying system infrastructure directly instead it first
compile source code to bytecode and run on Java virtual machine which itself is an application that
run on top of the operating system. The Java virtual machine acts like a mediator between the
application and the operating system. Most modern implementations of Java Virtual Machine does
include several interfaces and libraries that we can utilize to allow Java applications to perform
some crucial system level tasks and even speed up the execution of the program by handing over
some more resource-intense operations to programs that are running natively on the machine.
Objective
This application note will explain different approaches to implement system level functions
using Java Programming Language. The goal is for the reader with basic understanding of Java to
understand different ways of interacting with operating system in Java and what are the trade-offs.
Background
The ability to access system resource is essential for programs that are design to be flexible
and expandable.
Approaches
Java Native Interface (JNI)
http://java.sun.com/j2se/1.5.0/docs/guide/jni/spec/design.html#wp16696
JNI for C/C++ Program
Most modern java compiler provides an interface library jni.h that allows C/C++ program to
access and manipulate Java objects. This is all done by C/C++ memory pointers as shown in figure
1. JNI allows the programmer to pass different Java objects as parameters to C/C++ program and
gets the return value from the native program converts it back to the java object.
Figure 1: JNI Pointers and Functions
Figure 2: Java Native Interface illustration in C/C++
Get it to work
There are 5 steps that we need to follow:
1. Write the Java Code
2. Compile the Java Code
3. Create the header (.h) File
4. Implement the Native Method
5. Create a Shared Library
Alternative Approach:
Java Runtime Object
Java also provides another simple and easy interface to interact with operating system – the
Runtime object. The Runtime object is a special object that associated with current Java
application. It’s able to send commands as a string to system command shell. The command shell
is different across different operating system.
Figure 3: Java Runtime Command Execution Illustration
Conclusion
A Java application with system-specific implementation usually scarifies the portability of the
application in exchange for the efficiency of the program. However, there are different ways to go
around that. For example, you can provide different implementations of native code for various
platforms and let Java Virtual Machine decide which implementation to use. This way we can have
a relative more efficient application while still able enjoy many great features provides by Java.to
implement many general methods on top of Java.
References:
JNI Design (Sun Microsystem):
http://java.sun.com/j2se/1.5.0/docs/guide/jni/spec/design.html
Java Runtime Object Reference (Sun Microsystem):
http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Runtime.html
JNI in Wikipedia:
http://en.wikipedia.org/wiki/Java_Native_Interface