JNI编程：c++ 调用java 对象

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]\ jre\bin\client

运行结果如下：

invoking method

invoking method

hezzo

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