java String,Jni jstring, utf-8/unicode interoperate sample code

继 前一篇 jni基本原理后，这里写了一个sample code,有关java String object 如何传到 jni jstring,native code 如何取得 java String object 的utf-8, unicode character,native 的utf-8, unicode character如何传到 java String等操作。

software enviroment: ubuntu 8.10+jdk 1.5,+gcc 4.3.2

1. after editing java code, compile it to class file: 

javac HelloWorld.java 

public class HelloWorld {public static String java_str="Java String";static {System.loadLibrary("Hello");}public native void DisplayHello();public native String PassToJni(String java_str);public static void main(String[] args){HelloWorld obj=new HelloWorld();obj.DisplayHello();String ret = obj.PassToJni(java_str);System.out.println(ret);}}

2. to generate jni header file:

      javah HelloWorld

then you will get a jni header file named HelloWorld.h

/* DO NOT EDIT THIS FILE - it is machine generated */#include <jni.h>/* Header for class HelloWorld */#ifndef _Included_HelloWorld#define _Included_HelloWorld#ifdef __cplusplusextern "C" {#endif/* * Class:     HelloWorld * Method:    DisplayHello * Signature: ()V */JNIEXPORT void JNICALL Java_HelloWorld_DisplayHello  (JNIEnv *, jobject);/* * Class:     HelloWorld * Method:    PassToJni * Signature: (Ljava/lang/String;)Ljava/lang/String; */JNIEXPORT jstring JNICALL Java_HelloWorld_PassToJni  (JNIEnv *, jobject, jstring);#ifdef __cplusplus}#endif#endif

3.  new a c-source code file to implement the jni header file

     vim helloworldImpl.c

#include <stdio.h>#include "HelloWorld.h"JNIEXPORT void JNICALL Java_HelloWorld_DisplayHello  (JNIEnv *env, jobject obj)  {  printf("in native code,say hello\n");  }JNIEXPORT jstring JNICALL Java_HelloWorld_PassToJni  (JNIEnv *env, jobject obj, jstring str){char buf_utf8[128]="能在别人身上找到自己的一些线索bbbbbbbbbb";const jchar* uni_native="aaaaaa";// jstring--> utf-8 charconst char *c = (*env)->GetStringUTFChars(env,str,0);printf("string=%s\n",c);printf("sizeof(char)=%d,len=%d\n",sizeof(char),(*env)->GetStringUTFLength(env,str));(*env)->ReleaseStringUTFChars(env,str,c);// jstring --> unicode charconst jchar *uni = (*env)->GetStringChars(env,str,0);printf("unicode=%s\n",uni);printf("sizeof(char)=%d,len=%d\n",sizeof(jchar),(*env)->GetStringLength(env,str));(*env)->ReleaseStringChars(env,str,uni);// native utf-8 char --> jstringjstring jstr=(*env)->NewStringUTF(env,buf_utf8);// native unicode  ---> jstring//jstring jstr=(*env)->NewString(env,uni_native,sizeof(uni_native));return jstr;}

4. compile c-source code to generate a share library 

     gcc -shared -I /usr/lib/jvm/java-1.5.0-sun-1.5.0.19/include/ helloworldImpl.c -o libHello.so

5.  run java executive file linked with libHello.so library

    java -Djava.library.path=. HelloWorld