Java JNI开发时常用数据类型与C++中数据类型转换

JavaJNI开发时常用数据类型与C++中数据类型转换

常见的数据类型对应关系如下：

此外我们经常用String类型，它可以通过如下的API实现jstring到char*之间的相互转换constchar* szString = env-> GetStringUTFChars(jstring, 0);这种转换出来的char*类型数据，使用完之后一定要调用

env-> ReleaseStringUTFChars(jstring, szString);释放掉，不然会导致内存泄漏，如果忘记JVM会崩溃的。从C++中创建一个新的字符串然后返回的代码如下：

// create jstring	jstring computerName = env->NewStringUTF(chRtn);
	return computerName;

其中chRtn是char数组类型的指针。

下面是一个Java通过JNI接口调用C++的Windows接口实现电脑名称和用户名称查询的例子，首先定义JNI接口类如下

package com.gloomyfish.jnidemo;public class HelloJNI {		public native int helloJNI(String name);		public native String getComputerName();		public native String getUserName();		public native double calculateDistance(double x, double y);		public static void main(String[] args) {		System.loadLibrary("jnitest");		HelloJNI happ = new HelloJNI();		happ.helloJNI("gloomyfish");		double sum = happ.calculateDistance(3, 4);		String computerName = happ.getComputerName();		String user = happ.getUserName();				System.out.println("sum : " + sum);		System.out.println("computer name : " + computerName);		System.out.println("current user : " + user);		System.out.println();	}}

C++中实现如下：

// jnitest.cpp : 定义 DLL 应用程序的导出函数。//#include "stdafx.h"#include <windows.h>#include <stdlib.h>#include "math.h"#include "com_gloomyfish_jnidemo_HelloJNI.h"JNIEXPORT jint JNICALL Java_com_gloomyfish_jnidemo_HelloJNI_helloJNI(JNIEnv *env, jobject obj, jstring param) {	// convert java string type to c++ char* type	const char* name = env->GetStringUTFChars(param, 0);	printf("%s %s\n", "Hello JNI, I am ", name);	// release memory	env->ReleaseStringUTFChars(param, name);	// return 0;	return 0;}JNIEXPORT jstring JNICALL Java_com_gloomyfish_jnidemo_HelloJNI_getComputerName(JNIEnv *env, jobject obj) {	// define the buffer size	const int MAX_BUFFER_LEN = 500;	TCHAR  infoBuf[MAX_BUFFER_LEN];	DWORD  bufCharCount = MAX_BUFFER_LEN;	GetComputerName(infoBuf, &bufCharCount);	// conver to jstring	printf("computer name : %ls\n", infoBuf);	int nLen = WideCharToMultiByte(CP_ACP, 0, infoBuf, -1, NULL, 0, NULL, NULL);	char* chRtn = new char[nLen];	WideCharToMultiByte(CP_ACP, 0, infoBuf, -1, chRtn, nLen, NULL, NULL);	// create jstring	jstring computerName = env->NewStringUTF(chRtn);	return computerName;}JNIEXPORT jstring JNICALL Java_com_gloomyfish_jnidemo_HelloJNI_getUserName(JNIEnv *env, jobject obj) {	// define the buffer size	const int MAX_BUFFER_LEN = 500;	TCHAR  infoBuf[MAX_BUFFER_LEN];	DWORD  bufCharCount = MAX_BUFFER_LEN;	GetUserName(infoBuf, &bufCharCount);	// conver to jstring	printf("current user : %ls\n", infoBuf);	int nLen = WideCharToMultiByte(CP_ACP, 0, infoBuf, -1, NULL, 0, NULL, NULL);   	char* chRtn = new char[nLen];	WideCharToMultiByte(CP_ACP, 0, infoBuf, -1, chRtn, nLen, NULL, NULL);	// create jstring	jstring username = env->NewStringUTF(chRtn);	return username;}JNIEXPORT jdouble JNICALL Java_com_gloomyfish_jnidemo_HelloJNI_calculateDistance(JNIEnv *env, jobject obj, jdouble x, jdouble y) {	double cx = x;	double cy = y;	double sum = pow(cx, 2) + pow(cy, 2);	return sqrt(sum);}

运行结果如下：

其中通过命令行实现JNI头文件生成，命令如下：

javah com.gloomyfish.jnidemo.HelloJNI