Java JNI调用c/c++程序

1. 创建带native的Java类 eg：Sample1.java

class Sample1 {

    // --- Native methods
    public native int intMethod(int n);
    public native boolean booleanMethod(boolean bool);
    public native String stringMethod(String text);
    public native int intArrayMethod(int[] intArray);


    // --- Main method to test our native library
    public static void main(String[] args) {
        System.loadLibrary("Sample1");
        Sample1 sample = new Sample1();
        int square = sample.intMethod(5);
        boolean bool = sample.booleanMethod(true);
        String text = sample.stringMethod("java");
        int sum = sample.intArrayMethod(new int[] {1, 1, 2, 3, 5, 8, 13});

        System.out.println("intMethod: " + square);
        System.out.println("booleanMethod: " + bool);
        System.out.println("stringMethod: " + text);
        System.out.println("intArrayMethod: " + sum);
    }
}

2.　保存编译, 会生成Sample1.class

javac Sample1.java

3.　使用javah 命令生成头文件，此时会出现 Sample1.h文件

javah Sample1

//Sample1.h文件内容为：

* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class Sample1 */

#ifndef _Included_Sample1
#define _Included_Sample1
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     Sample1
 * Method:    intMethod
 * Signature: (I)I
 */
JNIEXPORT jint JNICALL Java_Sample1_intMethod
  (JNIEnv *, jobject, jint);

/*
 * Class:     Sample1
 * Method:    booleanMethod
 * Signature: (Z)Z
 */
JNIEXPORT jboolean JNICALL Java_Sample1_booleanMethod
  (JNIEnv *, jobject, jboolean);

/*
 * Class:     Sample1
 * Method:    stringMethod
 * Signature: (Ljava/lang/String;)Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_Sample1_stringMethod
  (JNIEnv *, jobject, jstring);

/*
 * Class:     Sample1
 * Method:    intArrayMethod
 * Signature: ([I)I
 */
JNIEXPORT jint JNICALL Java_Sample1_intArrayMethod
  (JNIEnv *, jobject, jintArray);

#ifdef __cplusplus
}
#endif
#endif</jni.h>

4.　创建本地实现，新建C源码文件，Sample1.cpp

#include "Sample1.h"
#include <ctype.h>
#include <string.h>

// Mutate array to uppercase
void uppercase(char* str) {
    size_t n = strlen(str);
    for (size_t i = 0; i < n; i++) {
        str[i] = toupper(str[i]); 
    }
}

JNIEXPORT jint JNICALL Java_Sample1_intMethod
(JNIEnv* env, jobject obj, jint num) {
    return num * num;
}


JNIEXPORT jboolean JNICALL Java_Sample1_booleanMethod
(JNIEnv* env, jobject obj, jboolean boolean) {
    return !boolean;
}

JNIEXPORT jstring JNICALL Java_Sample1_stringMethod
(JNIEnv* env, jobject obj, jstring string) {
    const char* str = env->GetStringUTFChars(string, 0);
    char cap[128];
    strcpy(cap, str);
    env->ReleaseStringUTFChars(string, str);
    uppercase(cap);
    return env->NewStringUTF(cap);
}

JNIEXPORT jint JNICALL Java_Sample1_intArrayMethod
(JNIEnv* env, jobject obj, jintArray array) {
    int sum = 0;
    jsize len = env->GetArrayLength(array);
    jint* body = env->GetIntArrayElements(array, 0);
    for (int i = 0; i < len; i++) {
        sum += body[i];
    }
    env->ReleaseIntArrayElements(array, body, 0);
    return sum;
}

5.　接下来对c++文件进行编译，生成函数库。库有静态库和动态库之分。静态库是以.a结尾的文件，例如：libXXX.a；动态库是以.so结尾的文件，例如： libXXX.so

比如，要编译生成动态链接库时：

gcc -c -fpic Sample1.cpp         //会生成Sample1.o的中间文件
gcc -shared Sample1.o -o libSample1.so
//或者直接；
gcc -fpic -shared Sample1.c -o libSample1.so

完成之后就会生成动态链接库：libSample1.so
需要注意的是：
Java JNI的头文件 jni.h需要在两个地方包含:
其一：$JAVA_HOME/include 。
其二：$JAVA_HOME/include/linux
6.运行java程序：

//pwd 为包含libSample1.so文件的路径。
java -Djava.library.path=$(pwd) -cp . Sample1