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Beginning JNI with NetBeans C/C++ Pack 5.5(2)
摘要: The tutorial will take you through the creation of a sample application which uses JNI to execute some native code written in the C programming language. For the Java? part of the application you will use NetBeans? IDE 5.5; for the C part - NetBeans? C/C++ Pack 5.5.
You will start off by creating a simple Java project, adding a native
method to it and then implementing this method in C using
NetBeans C/C++ Pack 5.5.
This is the second part of the tutorial that covers the creation of a sample
JNI application in the Sun™ Linux
Expected duration: 30 minutes
This tutorial assumes you have some basic knowledge of, or
programming experience with, the following technologies:
Software Required for Tutorial
Before you begin, you need to install the following software on
Notations Used in Tutorial
- <JAVA_HOME> - the JDK installation directory
- <PROJECTS_ROOT> - directory that contains the Java project you create
Exercise 0: Installing and Configuring Tutorial Environment
This exercise will guide you through the process of performing
required configuration changes that should be applied to your
system, prior to starting the tutorial.
Append the paths to the directories that contain the gcc and make utilities to the PATH environment variable.
Start the NetBeans IDE.
Setting Up General Java Application Project
The goal of this exercise is to create and configure the Java part
of the JNI application you will be developing. You will create a
new Java application project, initialize its main class and add a
native method to this class.
Creating General Java Application Project
- Choose File > New Project. Under Categories, select
General. Under Projects, select Java Application, and
- Under Project Name, enter HelloWorld.
- Change the Project Location to any directory on your
computer (hereinafter, this directory is referred to as
- Leave the Create Main Class checkbox selected and accept
the default value for the corresponding text field.
- Leave the Set as Main Project checkbox selected. Then click
The IDE creates the
Editing Main Class Source
- To open the Main class source in the editor, right-click the Main class node, and choose Open from the context menu.
- Replace the body
of the main method with
- Press Alt - Enter and choose the Create method nativePrint() in helloworld.Main line from the drop-down list.
A compilation error was caused by referencing a non-existing method in Step 2 above and the IDE automatically proposes a fix for this problem.
- Modify the body of the nativePrint() method by deleting its contents and inserting the native keyword into the method signature as follows:
private native void nativePrint();
The native keyword indicates that the method
has an implementation located in an external native
library, thus the code is going to compile correctly. However
at runtime the library location is not clear. Refer to the Configuring the Java Project section below for details.
- Press Ctrl - F11 to clean and build the project.
The project should build successfully.
Creating Native Library Header File
- Switch to the console and navigate to the
- Type the following:
<JAVA_HOME>/bin/javah -o HelloWorldNative.h -jni
-classpath <PROJECTS_ROOT>/HelloWorld/build/classes helloworld.Main
A HelloWorldNative.h C header file is
generated. It is required to provide correct function
declaration for the native implementation of the
- Switch back to the NetBeans IDE window.
In this exercise you created a new General Java Application
Project, specified its location and defined the package and
name of the main class of the project. You also added a new
method to the main class and marked it as a method having a
native implementation. As a final step we created a C header
file which is required later for the native library compilation.
Setting Up New C/C++ Dynamic Library Project
This exercise will lead you though the process of creating the
native part of the sample application. You will create the C++
Dynamic Library project and configure it to be able to build JNI
After the project has been set up, you will create the
implementation for the native method, you have declared earlier in
the Java part of the application.
Creating New C/C++ Dynamic Library Project
- Choose File > New Project. Under Categories, select
C/C++ Development. Under Projects, select C/C++ Dynamic
Library, and click Next.
- Under Project Name, type HelloWorldNative.
- Under Project Location, type the same location as you
entered for the General Java Application project,
<PROJECTS_ROOT>. The required value should
be already there as the default value.
- Accept the defaults for all other fields.
- Leave the Set as Main Project checkbox selected.
Then click Finish.
The IDE creates the
Setting Project Properties
- Right-click the project node and choose Properties
from the context menu.
- In the opened dialog box navigate to Configuration
Properties > C/C++ > GNU C Compiler > General.
Edit the value of the Additional Include Libraries
property. Set it to be <JAVA_HOME>/include,
These settings are required to enable references to the
Java jni.h library from your C code.
- Navigate to Configuration Properties > C/C++ > GNU C
Compiler > Command Line. Edit the value of the
Additional Options property. Set it to -shared -m32.
The -shared option tells the compiler to
generate a dynamic library.
-m32 tells the compiler to create a 32-bit
binary. By default on 64-bit systems the compiled
binaries are 64-bit, which causes a lot of problems
with 32-bit JDKs.
- Navigate to Configuration Properties > Linker >
General. Edit the Output property value. Set it to dist/HelloWorldNative.so
The goal of this step is to simplify the
path of the resulting so file, in order to make referencing it from Java easier for
- Click OK.
The defined settings are saved.
Adding Header File
- Copy the generated <PROJECTS_ROOT>/HelloWorldNative.h header file
C/C++ Library project directory,
- In the Projects view, navigate to HelloWorldNative >
Source Files. Right-click the Source Files node and
choose Add Existing Item from the context menu. Point the
IDE to the HelloWorldNative.h file.
The HelloWorldNative.h file appears
under Source Files.
- Right-click the Source Files node and choose New > Add
Empty C File from the context menu. Under File Name type
HelloWorldNative, and click Finish.
The editor opens the HelloWorldNative.c file.
- Edit the HelloWorldNative.c file by typing the
JNIEXPORT void JNICALL Java_helloworld_Main_nativePrint
(JNIEnv *env, jobject obj)
printf("\nHello World from C\n");
- Right-click the HelloWorldNative project node
and choose Build Project
The Output dialog box displays Build successful.
Exit value 0.
In this exercise you created a new C/C++ Dynamic Library,
specified its location and configured it to be able to build
JNI implementation of your Java method. You added the generated
header file for the native method you have declared in the Java
application and implemented it.
Building and Running Application
In this exercise you will perform some final alterations to the Java
part of the application. This is required to ensure the Java part properly loads the native
library you had compiled in the previous exercise. After that you
will compile and run the resulting application.
Configuring Java Project
- Open the Main.java file in the editor.
- Add the following initialization code:
- To set the HelloWorld Java project as the main
project, right-click the project node and
choose Set As Main Project from the context menu.
- Press F6 to run the application.
The program should execute correctly and the
Output dialog box should say:
Hello World from C
BUILD SUCCESSFUL (total time: 0 seconds)
In this exercise you made some final configuration steps and
ran the application to verify that the implementation of the
native method comes from the native C library.
You can download the sources for this tutorial from
You can use the following documents to get more information: