现如今的应用中，有很多算法是用C语言进行开发的，利用C语言进行进行科学计算，矩阵运算等，效率会比较高，外层应用如果使用 java ，就需要有一个桥梁来链接java与C，这种技术就是 jni 。JNI是Java Native Interface的缩写，它提供了若干的API实现了Java和其他语言的通信（主要是C&C++）。从Java1.1开始，JNI标准成为java平台的一部分，它允许Java代码和其他语言写的代码进行交互。
现如今我们已经习惯了使用 maven 来对项目进行打包管理，那么如果加入了C语言这一层，该如何进行集成和打包呢？本文就使用了一个简单的例子来展示如何使用JNI来进行java与C的集成以及如何进行打包管理。
假设现在的算法是求a除以b的结果，由于我们在实际的接口定义中，经常定义复杂的 结构体 来，为了展示常用的参数传递方法，我定义了结构体来表示入参和出参。
相关的接口定义如下：
a)algorithm.h头文件中定义入参ParamIn，出参ParamOut,接口函数ParamOut divide(ParamIn in);
#ifndef ALGORITHM_ALGORITHM_H #define ALGORITHM_ALGORITHM_H #pragma pack(1) typedef Struct { double a; double b; }ParamIn; typedef struct { double result; }ParamOut; /** * 除法运算 * @param in * @return */void divide(ParamIn* in,ParamOut *out); #pragma pack() # endif //ALGORITHM_ALGORITHM_H
b)algorithmCalc.cpp函数实现为
# include "algorithm.h" #include <stdio.h> #include <string.h> void divide(ParamIn* in,ParamOut *out){ double result = in->a/in->b; ParamOut paramOut; memset(¶mOut,0x00, sizeof (ParamOut)); out->result = result; printf("the result is %3.8f",out->result); }
c)使用main函数来进行单元测试
#include <iostream>
#include "src/algorithm.h"
int main() {
std::cout << "Hello, World!" << std::endl;
double a = 10;
double b = 13;
ParamIn paramIn;
memset(¶mIn,0x00, sizeof(ParamIn));
paramIn.a = a;
paramIn.b = b;
ParamOut out;
memset(&out,0x00, sizeof(ParamOut));
divide(¶mIn,&out);
return 0;
}
d)为了使用maven来进行打包管理，在你的c工程当中新增 pom .xml来将应用打包为nar包。注意<packaging>nar</packaging>需要设置为nar，才会打包为nar包。
<?xml version="1.0" encoding="UTF-8"?> <project xmlns="" xmlns:xsi="" xsi:schemaLocation=" org /POM/4.0.0 "> <modelVersion>4.0.0</modelVersion> <groupId>com.alex.algorithm</groupId> <artifactId>alg</artifactId> <version>1.0.0-SNAPSHOT</version> <packaging>nar</packaging> <dependencies> <!-- native loader --> <dependency> <groupId>org.scijava</groupId> <artifactId>native-lib-loader</artifactId> <version>2.1.3</version> </dependency> </dependencies> <properties> <maven. build .timestamp.format>yyyyMMddHHmm</maven.build.timestamp.format> </properties> <build> < plugin Management> <plugins> <plugin> <groupId>com.github.maven-nar</groupId> <artifactId>nar-maven-plugin</artifactId> <version>3.4.0</version> <extensions>true</extensions> < configuration > <cpp> <options> <option>-fPIC</option> <option>-I${project.basedir}</option> <option>-g</option> <option>-ggdb</option> </options> </cpp> </configuration> </plugin> <plugin> <groupId>org.codehaus.mojo</groupId> <artifactId>versions-maven-plugin</artifactId> <version>2.2</version> <configuration> <generateBackupPoms>false</generateBackupPoms> </configuration> </plugin> <plugin> <artifactId>maven-compiler-plugin</artifactId> <version>3.3</version> </plugin> <plugin> <artifactId>maven-source-plugin</artifactId> <version>2.4</version> </plugin> </plugins> </pluginManagement> <resources> <resource> <directory>${project.basedir}</directory> <includes> <include>src/*.h</include> </includes> <targetPath>${project.build.directory}/nar/${project.artifactId}-${project.version}-noarch/include</targetPath> </resource> </resources> <plugins> <plugin> <groupId>org.codehaus.mojo</groupId> <artifactId>versions-maven-plugin</artifactId> </plugin> <plugin> <groupId>com.github.maven-nar</groupId> <artifactId>nar-maven-plugin</artifactId> <extensions>true</extensions> <configuration> <cpp> <sourceDirectory>${project.basedir}</sourceDirectory> <includes> <include>src/*.cpp</include> </includes> <multiThreaded>true</multiThreaded> </cpp> <libraries> <library> < type >static</type> </library> </libraries> </configuration> </plugin> <plugin> <artifactId>maven-source-plugin</artifactId> <executions> <execution> <id>attach-sources</id> <goals> <goal>jar</goal> </goals> </execution> </executions> </plugin> </plugins> </build> </project>
e)进入项目下，执行打包命令mvn clean package就可以在项目下的target目录当中生成nar包了，由于我的电脑是MAC的，所以生成的nar包是alg-1.0.0-SNAPSHOT-x86_64-MacOSX-gpp-static.nar，如果是linux，生成的nar包就是alg-1.0.0-SNAPSHOT-amd64-Linux-gpp-jni.nar
下面是需要在java层与C进行集成，定义一个jni层的java应用名字就是jni
a)定义与C结构体对应的ParamIn,ParamOut,以及对应的Native函数
package com.alex;
import javolution.io.Struct;
public class ParamIn extends Struct {
public final Float64 a = new Float64();
public final Float64 b = new Float64();
}
package com.alex;
import javolution.io.Struct;
public class ParamOut extends Struct {
public final Float64 result = new Float64();
}
package com.alex;
public class Native {
static
{
try {
NarSystem.loadLibrary();
}catch (Throwable e){
}
}
}
package com.alex;
public class Divide extends Native{
public static native void divide(long in,long out);
}
b)执行mvn clean package生成对应的class，在target/class下面执行javah -classpath . com.alex.Divide，会生成对应的.h文件com_alex_Divide.h
c)在src/main下面新增c++的文件夹，新增divide.cpp函数，函数内部如下：
#include <jni.h>
#include <stdio.h>
#include "com_alex_Divide.h"
#include "src/algorithm.h"
#ifndef _Included_com_alex_Divide
#define _Included_com_alex_Divide
#ifdef __cplusplus
extern "C" {
#endif
/*
* Class: com_alex_Divide
* Method: divide
* Signature: (JJ)V
*/JNIEXPORT void JNICALL Java_com_alex_Divide_divide
(JNIEnv *env, jclass clazz, jlong in, jlong out){
ParamIn* a = (ParamIn*)in;
ParamOut* b = (ParamIn*)out;
//此处为src/algorithm.h中的函数
divide(a,b);
}
#ifdef __cplusplus
}
#endif
#endif
最后新增新的工程，引入该jni层的nar包即可实现对C程序的调用
public class TestDivide {
public static void main(String[] args) {
int a = 11;
int b = 12;
ParamIn paramIn = new ParamIn();
paramIn.a.set(a);
paramIn.b.set(b);
ParamOut out = new ParamOut();
Divide.divide(paramIn.address(),out.address());
System.out.println(out.result);
}
}
对应的pom如下
<?xml version="1.0" encoding="UTF-8"?> <project xmlns="" xmlns:xsi="" xsi:schemaLocation=" "> <modelVersion>4.0.0</modelVersion> <groupId>com.alex</groupId> <artifactId>testjni</artifactId> <version>1.0.0-SNAPSHOT</version> <dependencies> <dependency> <groupId>org.scijava</groupId> <artifactId>native-lib-loader</artifactId> <version>2.1.3</version> </dependency> <dependency> <groupId>org.javolution</groupId> <artifactId>javolution-core-java</artifactId> <version>6.0.0</version> </dependency> <dependency> <groupId>com.alex</groupId> <artifactId>jni</artifactId> <version>1.0.0-SNAPSHOT</version> <classifier>epoch</classifier> <type>jar</type> </dependency> <dependency> <groupId>com.alex</groupId> <artifactId>jni</artifactId> <version>1.0.0-SNAPSHOT</version> <type>nar</type> </dependency> <dependency> <groupId>com.alex</groupId> <artifactId>jni</artifactId> <version>1.0.0-SNAPSHOT</version> <classifier>x86_64-MacOSX-gpp-jni</classifier> <type>nar</type> </dependency> </dependencies> </project>
这样一个jni工程就结束了
jni应用的 git hub地址为git@github.com:kangyz/jni.git
调用jni的应用为git@github.com:kangyz/testjni.git
jni调用的c应用为git@github.com:kangyz/algorithm.git
