jni

STEP1 编写java调用文件

J2C.java

import java.lang.management.ManagementFactory;
import java.lang.management.RuntimeMXBean;

public class J2C
{ 
     static 
     { 
          try{ 
               // 此处即为本地方法所在链接库名
               System.loadLibrary("j2c");
          } catch(UnsatisfiedLinkError e) 
          { 
               System.err.println( "Cannot load J2C library:\n " + 
               e.toString() ); 
          } 
     }

     //声明的本地方法
     public static native int write2proc(int pid);

     public static void main(String[] args){

          //获取本进程(即主线程)的pid
          final RuntimeMXBean runtime = ManagementFactory.getRuntimeMXBean();
          final String info = runtime.getName();
          final int index = info.indexOf("@");


          if (index != -1) {
               final int pid = Integer.parseInt(info.substring(0, index));
               System.out.println(info);
               System.out.println(pid);

               write2proc(pid);
          }


          try{
               Thread.sleep(8000);
          } catch(InterruptedException e){
               e.printStackTrace();
          }
     }
}

Step2 生成J2C.h

bash: javac -h ./ J2C.java

以下内容为自动生成：

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class J2C */

#ifndef _Included_J2C
#define _Included_J2C
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     J2C
 * Method:    write2proc
 * Signature: (I)I
 */
JNIEXPORT jint JNICALL Java_J2C_write2proc
  (JNIEnv *, jclass, jint);

#ifdef __cplusplus
}
#endif
#endif

Step3 编写J2C.c

#include <stdio.h>

#include "J2C.h"

JNIEXPORT int JNICALL Java_J2C_write2proc(JNIEnv * env, jobject arg, jint pid) 
{

     printf("current pid is %d\n", pid);

     return 0;

}

Step4 生成J2C.so

bash: gcc J2C.c -fPIC -shared -o libJ2C.so

利用gcc、g++编译源文件找不到jni.h头文件:

可以在编译时利用-I选项指定jni.h头文件所在目录：

gcc J2C.c -fPIC -shared -o libJ2C.so -I /usr/java/jdk-12.0.1/include/ -I /usr/java/jdk-12.0.1/include/linux/

注意：cmake make 找不到jni.h的情况，可以将文件复制到公共库/usr/include/