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
