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Java programming with JNI

Presented by developerWorks, your source for great tutorials

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Table of Contents

If you're viewing this document online, you can click any of the topics below to link directly to that section.

1. About this tutorial....................................................... 2

2. Calling C/C++ code from Java programs .......................... 4

3. Calling Java code from C/C++ programs .......................... 11

4. Advanced topics........................................................ 19

5. Wrap-up and resources ............................................... 25

6. Appendices.............................................................. 28

Java programming with JNI Page 1 of 30

Section 1. About this tutorial

What is this tutorial about?

The Java Native Interface (JNI) is a native programming interface that is part of the Java

Software Development Kit (SDK). JNI lets Java code use code and code libraries written in

other languages, such as C and C++. The Invocation API, which is part of JNI, can be used

to embed a Java virtual machine (JVM) into native applications, thereby allowing

programmers to call Java code from within native code.

This tutorial deals with the two most common applications of JNI: calling C/C++ code from

Java programs, and calling Java code from C/C++ programs. We'll cover both the essentials

of the Java Native Interface and some of the more advanced programming challenges that

can arise.

Should I take this tutorial?

This tutorial will walk you through the steps of using the Java Native Interface. You'll learn

how to call native C/C++ code from within a Java application and how to call Java code from

within a native C/C++ application.

All the examples use Java, C, and C++ code, and are written to be portable to both Windows

and UNIX-based platforms. To follow the examples, you must have some experience

programming in the Java language. In addition, you will also need some experience

programming in C or C++. Strictly speaking, a JNI solution could be broken down between

Java programming tasks and C/C++ programming tasks, with separate programmers doing

each task. However, to fully understand how JNI works in both programming environments,

you'll need to be able to understand both the Java and C/C++ code.

We'll also cover a number of advanced topics, including exception handling and

multithreading with native methods. To get the most out of this part of the tutorial, you should

be familiar with the Java platform's security model and have some experience in

multithreaded application development.

The section on Advanced topics on page 19 is separate from the more basic step-by-step

introduction to JNI. Beginning Java programmers may benefit from taking the first two parts

of the tutorial now and returning to the advanced topics at a later time.

See Resources on page 25 for a listing of tutorials, articles, and other references that expand

upon the material presented here.

Tools and components

To run the examples in this tutorial, you will need the following tools and components:

• A Java compiler: javac.exe ships with the SDK.

• A Java virtual machine (JVM): java.exe ships with the SDK.

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Presented by developerWorks, your source for great tutorials

Page 2 of 30 Java programming with JNI

Section 2. Calling C/C++ code from Java programs

Overview

JNI allows you to use native code when an application cannot be written entirely in the Java

language. The following are typical situations where you might decide to use native code:

• You want to implement time-critical code in a lower-level, faster programming language.

• You have legacy code or code libraries that you want to access from Java programs.

• You need platform-dependent features not supported in the standard Java class library.

Six steps to call C/C++ from Java code

The process of calling C or C ++ from Java programs consists of six steps. We'll go over

each step in depth in the panels that follow, but let's start with a quick look at each one.

1. Write the Java code. We'll start by writing Java classes to perform three tasks: declare

the native method we'll be calling; load the shared library containing the native code; and

call the native method.

2. Compile the Java code. We must successfully compile the Java class or classes to

bytecode before we can use them.

3. Create the C/C++ header file. The C/C++ header file will declare the native function

signature that we want to call. This header will then be used with the C/C++ function

implementation (see Step 4) to create the shared library (see Step 5).

4. Write the C/C++ code. This step consists of implementing the function in a C or C++

source code file. The C/C++ source file must include the header file we created in Step 3.

5. Create the shared library file. We'll create a shared library file from the C source code

file we created in Step 4.

6. Run the Java program. We'll run the code and see if it works. We'll also go over some

tips for dealing with the more commonly occurring errors.

Step 1: Write the Java code

We'll start by writing the Java source code file, which will declare the native method (or

methods), load the shared library containing the native code, and actually call the native

method.

Here's our example Java source code file, called Sample1.java:

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Page 4 of 30 Java programming with JNI

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2014-10-09

学学如何从上层跳转到下层。

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