JNIJAVA和C/C++相互呼叫

1、JAVA可以通過JNI呼叫C/C++函式

2、C/C++也可以通過JNI相關函式，來呼叫JAVA。並且可以訪問到JAVA類中所有成員變數和成員函式。

要實現C/C++呼叫JAVA相關功能，則需要掌握以下知識點：

1、C/C++如何找到JAVA類

2、C/C++如何獲取到JAVA變數

3、C/C++如何設定Java變數

4、C/C++如何獲取到JAVA函式

3、帶著這些疑問，我還是來寫個簡單程式。

該程式主要實現以下功能：

1、java傳遞兩個數，讓C/C++做運算。

2、C/C++將運算結果，讓JAVA顯示。

3、java傳遞兩個字串，讓C/C++做連線操作

4、C/C++將運算結果，讓JAVA

顯示。

5、C/C++獲取JAVA的變數，並且修改JAVA變數值。

6、Java計算並且顯示。

7、C/C++獲取JAVAString物件，並且修改物件值。

8、Java將修改後的值連線並且顯示。

4、jnicalleachother.java程式碼如下：

publicclass jnicalleachother

{

float f1;

float f2;

Stringstr1;

Stringstr2;

StringstrResult;

static

{

System.loadLibrary("jnicalleachother");

}

publicnative int AddFloat(float fFirst, float fSecond);

publicnative int AddJavaFloat();

publicnative int CatString(String fFirst, String fSecond);

publicnative int CatJavaString();

publicint PrintFloat(float fFirst, float fSecond, float fResult)

{

System.out.println("fFirst:"+ fFirst);

System.out.println("fSecond:"+ fSecond);

System.out.println("Result:"+ fResult);

return0;

}

publicint PrintString(String strFirst, String strSecond, String strResult)

{

System.out.println(strFirst);

System.out.println(strSecond);

System.out.println(strResult);

return0;

}

publicint PrintModifyFloat()

{

floatfResult = (float)0.0;

System.out.println("fFirst:"+ f1);

System.out.println("fSecond:"+ f2);

fResult= f1 + f2;

System.out.println("Result:"+ fResult);

return0;

}

publicint PrintModifyString()

{

System.out.println(str1);

System.out.println(str2);

strResult= str1 + str2;

System.out.println(strResult);

return0;

}

publicstatic void main(String[] args)

{

jnicalleachotherdata = new jnicalleachother();

data.f1= (float)100.00;

data.f2= (float)1000.00;

data.AddFloat(data.f1,data.f2);

data.str1= "Java call";

data.str2= "c/c++";

data.CatString(data.str1,data.str2);

data.AddJavaFloat();

data.CatJavaString();

}

}

5、通過javah命令生成的標頭檔案（jnicalleachother.h）如下：

/*DO NOT EDIT THIS FILE - it is machine generated */

#include<jni.h>

/*Header for class jnicalleachother */

#ifndef_Included_jnicalleachother

#define_Included_jnicalleachother

#ifdef__cplusplus

extern"C" {

#endif

/*

*Class: jnicalleachother

*Method: AddFloat

*Signature: (FF)I

*/

JNIEXPORTjint JNICALL Java_jnicalleachother_AddFloat

(JNIEnv*, jobject, jfloat, jfloat);

/*

*Class: jnicalleachother

*Method: AddJavaFloat

*Signature: ()I

*/

JNIEXPORTjint JNICALL Java_jnicalleachother_AddJavaFloat

(JNIEnv*, jobject);

/*

*Class: jnicalleachother

*Method: CatString

*Signature: (Ljava/lang/String;Ljava/lang/String;)I

*/

JNIEXPORTjint JNICALL Java_jnicalleachother_CatString

(JNIEnv*, jobject, jstring, jstring);

/*

*Class: jnicalleachother

*Method: CatJavaString

*Signature: ()I

*/

JNIEXPORTjint JNICALL Java_jnicalleachother_CatJavaString

(JNIEnv*, jobject);

#ifdef__cplusplus

}

#endif

#endif

6、本地實現原始檔（jnicalleachother.c）

#include"jnicalleachother.h"

/*

*Class: jnicalleachother

*Method: AddFloat

*Signature: (FF)I

*/

JNIEXPORTjint JNICALL Java_jnicalleachother_AddFloat(JNIEnv * pEnv, jobjectpObject, jfloat fFirst, jfloat fSecond)

{

jfloatfResult = 0.0;

jintiResult = 0;

fResult= fFirst + fSecond;

jclasscls = (* pEnv)->GetObjectClass(pEnv,pObject);

jmethodIDprintFloat = (* pEnv)->GetMethodID(pEnv,cls,"PrintFloat","(FFF)I");

iResult= (* pEnv)->CallIntMethod(pEnv, pObject,printFloat, fFirst,fSecond, fResult);

}

/*

*Class: jnicalleachother

*Method: AddJavaFloat

*Signature: ()I

*/

JNIEXPORTjint JNICALL Java_jnicalleachother_AddJavaFloat(JNIEnv * pEnv,jobject pObject)

{

jfloatfResult = 0.0;

intiResult = 0;

jclasscls= (* pEnv)->GetObjectClass(pEnv, pObject);

jfieldIDffield1 =(*pEnv)->GetFieldID(pEnv, cls,"f1","F");

(*pEnv)->SetFloatField(pEnv,pObject,ffield1,15848.00);

jfieldIDffield2 =(*pEnv)->GetFieldID(pEnv, cls,"f2","F");

(*pEnv)->SetFloatField(pEnv,pObject,ffield2,125454.00);

jmethodIDprintFloat = (* pEnv)->GetMethodID(pEnv,cls,"PrintModifyFloat","()I");

iResult= (* pEnv)->CallIntMethod(pEnv, pObject,printFloat);

}

/*

*Class: jnicalleachother

*Method: CatString

*Signature: (Ljava/lang/String;Ljava/lang/String;)I

*/

JNIEXPORTjint JNICALL Java_jnicalleachother_CatString(JNIEnv * pEnv, jobjectpObject, jstring strFirst, jstring strSecond)

{

charszResult[128] = {0};

char* pszFirst = NULL;

char* pszSecond = NULL;

intiResult = 0;

jstringstrResult = NULL;

pszFirst= (* pEnv)->GetStringUTFChars(pEnv, strFirst, NULL);

pszSecond= (* pEnv)->GetStringUTFChars(pEnv, strSecond, NULL);

strcpy(szResult,pszFirst);

strcat(szResult," ");

strcat(szResult,pszSecond);

jclasscls = (* pEnv)->GetObjectClass(pEnv,pObject);

jmethodIDprintString = (* pEnv)->GetMethodID(pEnv,cls,"PrintString","(Ljava/lang/String;Ljava/lang/String;Ljava/lang/String;)I");

strResult= (* pEnv)->NewStringUTF(pEnv, szResult);

iResult= (* pEnv)->CallIntMethod(pEnv, pObject,printString, strFirst,strSecond, strResult);

(*pEnv)->ReleaseStringUTFChars(pEnv, strFirst, pszFirst);

(*pEnv)->ReleaseStringUTFChars(pEnv, strSecond, pszSecond);

}

/*

*Class: jnicalleachother

*Method: CatJavaString

*Signature: ()I

*/

JNIEXPORTjint JNICALL Java_jnicalleachother_CatJavaString(JNIEnv * pEnv,jobject pObject)

{

intiResult = 0;

jstringstrFirst= (*pEnv)->NewStringUTF(pEnv, "C/C++ call ");

jstringstrSecond= (*pEnv)->NewStringUTF(pEnv, "Java");

jclasscls= (* pEnv)->GetObjectClass(pEnv, pObject);

jfieldIDffield1 =(*pEnv)->GetFieldID(pEnv,cls,"str1","Ljava/lang/String;");

(*pEnv)->SetObjectField(pEnv,pObject,ffield1,strFirst);

jfieldIDffield2 =(*pEnv)->GetFieldID(pEnv,cls,"str2","Ljava/lang/String;");

(*pEnv)->SetObjectField(pEnv,pObject,ffield2,strSecond);

jmethodIDprintString = (* pEnv)->GetMethodID(pEnv,cls,"PrintModifyString","()I");

iResult= (* pEnv)->CallIntMethod(pEnv, pObject,printString);

}

7、執行結果：

[email protected]:~/work/jni/jnicalleachother$java jnicalleachother

fFirst:100.0

fSecond:1000.0

Result:1100.0

Javacall

c/c++

Javacall c/c++

fFirst:15848.0

fSecond:125454.0

Result:141302.0

C/C++call

Java

C/C++call Java