Java applet & native method (JNI) -- Snippets

1. Java applet

>javac -d . HelloApplet.java

>appletviewer HelloApplet.html 

 或者直接用IE打开

//  HelloApplet.java

public   class  HelloApplet  extends  java.applet.Applet

{

     public   void  paint(java.awt.Graphics g)

    {

        g.drawString( " Hello Applet! " ,  50 ,  25 );

    }

     public   static   void  main(String[] args)

    {

        System.out.println( " Hello "   +   "   "   +   " World "   +   " ! " );

    }

}

<!--  HelloApplet.html  -->

< html >

< head >

< title > Hello Applet </ title >

</ head >

< body >

Here is the "Hello Applet":

< applet  code ="HelloApplet.class"  width ="150"  height ="25" ></ applet >     

</ body >

</ html >

2. native method

 1) Java文件, HelloJni.java, 并编译

>javac -d . HelloJni.java

package  tony.test;

public   class  HelloJni

{

     private   static   final  String jniLib  =   " HelloJni " ;

     public   native   int  add( int  i1,  int  i2);

     static

    {

        System.loadLibrary(jniLib);

    }

     public   static   void  main(String[] args)

    {

         int  i1  =   100 ;

         int  i2  =   200 ;

        System.out.println( new  HelloJni().add(i1, i2));

    }

}

2) 生成C++ (Native) 文件tony_test_HelloJni.h

>javah -jni tony.test.HelloJni

3) 编写HelloJni.cpp

#include  " tony_test_HelloJni.h "

JNIEXPORT jint JNICALL Java_tony_test_HelloJni_add(JNIEnv *  env, jobject obj, jint i1, jint i2)

{

     int  i  =  i1  +  i2;

     return  i;

}

4) 编译生成dll

Windows:

>cl -I%JAVA_HOME%/include -I%JAVA_HOME%/include/win32 -LD HelloJni.cpp -FeHelloJni.dll

Solaris:

#cc -G -I/usr/local/java/include -I/usr/local/java/include/solaris HelloJni.cpp / -o libHelloJni.so

-G选项指生成动态链接库.

这里需要说明的是, 在windows下和solaris动态链接库的名称分别为HelloJni.dll和libHelloJni.so.

在java.lang.System下有三个与加载动态链接库有关的函数: load, mapLibraryName, loadLibrary.

load参数为动态链接库绝对路径, 如System.load("F://codes//jnicode//HelloJni.dll");

mapLibraryName会根据操作系统和参数动态链接库名来返回对应的动态链接库文件名, 比如, System.mapLibraryName("HelloJni"); 在Windows下返回HelloJni.dll, 在Solaris下返回libHelloJni.so;

loadLibrary参数为动态链接库名, 如System.load("HelloJni"); 对于不同的系统它会加载对应的库(windows下加载HelloJni.dll, Solaris下加载libHelloJni.so), 而不需改Java代码.

5) 运行Java程序

>java tony.test.HelloJni

两个不错的参考:

Jni中C++和Java的参数传递

[JNI] Eclipse直接完成JAVA调用C/C++ (Eclipse上使用CDT结合MinGW)

推荐一个开源项目: Jace. 极大地方便JNI编程.

http://sourceforge.net/projects/jace/

jace.1.1rc1_05

"Jace is a toolkit designed to make it easy to write JNI-based programs. Jace consists of a C++ runtime library, and a set of tools written in Java. Because Jace is not a framework, it leaves all possible options open to the developer.

Jace's C++ runtime library provides an easy to use API that allows user to manipulate Java objects as if they were C++ objects. Behind the scenes, the runtime library automatically manages the lifetimes of Java references, the mapping of Java exceptions to C++ exceptions, and the binding of C++ threads to the JVM.

Jace's Java tools can automatically generate C++ Proxy classes and C++ Peer classes from Java classes. This means that you can easily access any Java object or implement any Java native function from your C++ code. "