用C++调用Java的java.lang.String类为例：
1. Object类出创建JVM。
使用Java类之前必须要创建JVM环境。JDK由java.exe来完成。本文有Object类的静态方法BeginJVM来创建，用EndJVM来关闭。
创建JVM之后会在创建2个变量，分别是JNIEnv* env和JavaVM* jvm，JNIEnv上文已经说明，JavaVM，顾名思义，代表Java虚拟机，用它来关闭JVM。
Object类的头文件
#include "jni.h"
class Object
{
public:
static bool BeginJVM();
static bool EndJVM();
Object();
virtual ~Object();
protected:
static JNIEnv* env;
static JavaVM* jvm;
};
object.cpp代码
#include "stdafx.h"
#include "JavaClasses.h"
#include "Object.h"
Object::Object()
{}
Object::~Object()
{}
JNIEnv* Object::env=NULL;
JavaVM* Object::jvm=NULL;
//创建JVM
bool Object::BeginJVM()
{
JavaVMOption options[3];
JavaVMInitArgs vm_args;
//各种参数
options[0].optionString="-Xmx128m";
options[1].optionString="-Verbose:gc";
options[2].optionString="-Djava.class.path=.";
vm_args.version=JNI_VERSION_1_2;
vm_args.options=options;
vm_args.nOptions=3;
//创建JVM，获得jvm和env
int res = JNI_CreateJavaVM(&jvm,(void **)&env, &vm_args);
return true;
}
bool Object::EndJVM()
{
//关闭JVM
jvm->DestroyJavaVM();
return true;
}
2. C++的String类调用java.lang.String类方法
编写C++版的String类，调用java String类方法。调用的方法如下：
String replaceAll(String regex, String replacement);
boolean endsWith(String str);
int indexOf(String str);
int compareTo(String anotherString);
char charAt(int i);
String的头文件:
class String :public Object
{
public:
//与要调用的Java方法名一致。
const char * replaceAll(char *regex,char *replacement);
bool endsWith(char * str);
int indexOf(char * str);
int compareTo(char *anotherString);
char charAt(int i);
String(char *str);
virtual ~String();
};
实现：
#include "stdafx.h"
#include "String.h"
#include "jni.h"
using namespace std;
jclass clazz; //全局变量，用来传递class
jobject object; //全局变量，用来传递object
String::String(char *str)
{
jstring jstr;
if (Object::env ==NULL)
{
cout << "JVM is not created" << endl;
exit(-1);
}
//获得java.lang.String类
clazz=Object::env->FindClass("java/lang/String");
if (clazz ==0 ){
cout << "Class is not found" << endl;
exit(-1);
}
//获得String(String str)构造体
jmethodID mid= Object::env->GetMethodID(clazz,"<init>", "(Ljava/lang/String;)V");
if (mid==0){
cerr<< "GetMethodID Error for class" << endl;
exit(-1);
}
//将字符串封装为jstring。
jstr = Object::env->NewStringUTF(str);
if (jstr == 0) {
cerr << "Out of memory" <<endl;
exit(-1);
}
cout << "invoking method" << endl;
//创建一个java.lang.String对象。
object=Object::env->NewObject(clazz,mid,jstr);
}
String::~String()
{}
char String::charAt(int i)
{
if (Object::env ==NULL)
{
cout << "JVM is not created" << endl;
exit(-1);
}
if (clazz ==0 ){
cout << "Class is not found" << endl;
exit(-1);
}
if (object ==0 ){
cout << "String object is not created" << endl;
exit(-1);
}
jmethodID mid;
//获得charAt方法，（I）C表示 参数为int型，返回char型。详细参见JNI规范
mid= Object::env->GetMethodID(clazz,"charAt", "(I)C");
if (mid==0){
cerr<< "GetMethodID Error for class" << endl;
exit(-1);
}
jint ji=i;
cout << "invoking method" << endl;
//调用charAt
jchar z=Object::env->CallCharMethod(object,mid,i);
//返回结果。
return z;
}
int String::compareTo(char *anotherString)
{
if (Object::env ==NULL)
{
cout << "JVM is not created" << endl;
exit(-1);
}
if (clazz ==0 ){
cout << "Class is not found" << endl;
exit(-1);
}
if (object ==0 ){
cout << "String object is not created" << endl;
exit(-1);
}
jmethodID mid;
//(Ljava/lang/String;)I表示参数为java.lang.String，返回int
mid= Object::env->GetMethodID(clazz,"compareTo", "(Ljava/lang/String;)I");
if (mid==0){
cerr<< "GetMethodID Error for class" << endl;
exit(-1);
}
jstring jstr = Object::env->NewStringUTF(anotherString);
cout << "invoking method" << endl;
//调用方法
jint z=Object::env->CallIntMethod(object,mid,jstr);
//返回结果
return z;
}
int String::indexOf(char *str)
{
if (Object::env ==NULL)
{
cout << "JVM is not created" << endl;
exit(-1);
}
if (clazz ==0 ){
cout << "Class is not found" << endl;
exit(-1);
}
if (object ==0 ){
cout << "String object is not created" << endl;
exit(-1);
}
jmethodID mid;
mid= Object::env->GetMethodID(clazz,"indexOf", "(Ljava/lang/String;)I");
if (mid==0){
cerr<< "GetMethodID Error for class" << endl;
exit(-1);
}
jstring jstr = Object::env->NewStringUTF(str);
cout << "invoking method" << endl;
jint z=Object::env->CallIntMethod(object,mid,jstr);
return z;
}
bool String::endsWith(char *str)
{
if (Object::env ==NULL)
{
cout << "JVM is not created" << endl;
exit(-1);
}
if (clazz ==0 ){
cout << "Class is not found" << endl;
exit(-1);
}
if (object ==0 ){
cout << "String object is not created" << endl;
exit(-1);
}
jmethodID mid;
mid= Object::env->GetMethodID(clazz,"endsWith", "(Ljava/lang/String;)Z");
if (mid==0){
cerr<< "GetMethodID Error for class" << endl;
exit(-1);
}
jstring jstr = Object::env->NewStringUTF(str);
cout << "invoking method" << endl;
bool z=Object::env->CallBooleanMethod(object,mid,jstr);
return z;
}
const char * String::replaceAll(char *regex, char *replacement)
{
if (Object::env ==NULL)
{
cout << "JVM is not created" << endl;
exit(-1);
}
if (clazz ==0 ){
cout << "Class is not found" << endl;
exit(-1);
}
if (object ==0 ){
cout << "String object is not created" << endl;
exit(-1);
}
jmethodID mid;
mid= Object::env->GetMethodID(clazz,"replaceAll", "(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String;");
if (mid==0){
cerr<< "GetMethodID Error for class" << endl;
exit(-1);
}
jvalue array[2];
jstring jreg = Object::env->NewStringUTF(regex);
jstring jstr = Object::env->NewStringUTF(replacement);
array[0].l=jreg;
array[1].l=jstr;
cout << "invoking method" << endl;
//传入参数，调用replaceAll方法
jobject z=Object::env->CallObjectMethodA(object,mid,array);
const char *result=Object::env->GetStringUTFChars((jstring)z, 0);
return (const char *)result;
}
3．测试
编写测试代码
using namespace std;
int _tmain(int argc, TCHAR* argv[], TCHAR* envp[])
{
int nRetCode = 0;
if (!AfxWinInit(::GetModuleHandle(NULL), NULL, ::GetCommandLine(), 0))
{
cerr << _T("Fatal Error: MFC initialization failed") << endl;
nRetCode = 1;
}
else
{
//创建JVM
Object::BeginJVM();
String test("hello");
//调用replaceAll
const char *result=test.replaceAll("l","z");
//返回结果
cout<< result <<endl;
//关闭JVM
Object::EndJVM();
}
return nRetCode;
}
4．运行
编译需要 jni.h和jvm.lib文件。
jni.h在[JAVA_HOME]include
jvm.lib在[JAVA_HOME]lib
运行需要jvm.dll
jvm.dll在[JAVA_HOME] jreinclient
运行结果如下：
invoking method
invoking method
hezzo
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