Use of jni native methods

foreword

Today, I was looking at the HashMap source code, and then I clicked on the Object class. I saw several native identification methods. If I don't understand it, I will first popularize the native method. I will get it first, and I will add the analysis of HashMap tomorrow.

native methods are called native methods. It is declared with the keyword "native" in the java source program and does not provide a function body. Its implementation is written in another file using C/C++ language, and the written rules follow the Java native Interface (JNI) specification of the Java native interface. In short, it is a callable method implemented in C/C++ declared in Java.

Implementation

1. Write a java class with methods declared by natives

   Not much to say, the code is as follows

   package local.zcw.demo;
   
   /**
   * 作者 zcw
   * 时间 2017/10/24 18:04
   * 版本 1.0.0
   * 描述 TODO
   */
   public class MyNative {
   
       static {
           System.loadLibrary("MyNative");
       }
   
       public static native void sayHello();
   
       public static void main(String[] args) {
           MyNative.sayHello();
       }
   }

2. Compile java classes using javac command

   cd src/
   javac local/zcw/demo/MyNative.java

3. Use the javah -jni command to generate *.h header files

   javah -jni local.zcw.demo.MyNative

   The content of the generated file is as follows

   /* DO NOT EDIT THIS FILE - it is machine generated */
   
   #include <jni.h>
   
   /* Header for class local_zcw_demo_MyNative */
   
   
   #ifndef _Included_local_zcw_demo_MyNative
   
   
   #define _Included_local_zcw_demo_MyNative
   
   
   #ifdef __cplusplus
   
   extern "C" {
   
   #endif
   
   /*
   * Class:     local_zcw_demo_MyNative
   * Method:    sayHello
   * Signature: ()V
   */
   JNIEXPORT void JNICALL Java_local_zcw_demo_MyNative_sayHello
   (JNIEnv *, jclass);
   
   
   #ifdef __cplusplus
   
   }
   
   #endif
   
   
   #endif
   

4. Implement native methods using c language

   
   #include "jni.h"
   
   
   #include "local_zcw_demo_MyNative.h"
   
   
   #include <stdio.h>
   
   
   JNIEXPORT void JNICALL Java_local_zcw_demo_MyNative_sayHello(JNIEnv * env, jclass obj)
   {
       printf("hello by c\n");
   }

5. Generate a dynamic link library from files written by native methods

   gcc -dynamiclib -I ${JAVA_HOME}/include/  \
       -I ${JAVA_HOME}/include/darwin/ \
       local_zcw_demo_MyNative.c \
       -o libMyNative.jnilib

6. run

   java local.zcw.demo.MyNative

other

The project directory is as shown below, the code download

The complete command is as follows

Reference article

• The native method of Java under Mac and JNI call C language

• Java calls C (implementing Java native methods under Linux)

• Implementing Java native methods in Windows

• The use of JNI under Linux

Supplement (using jni under Linux 2017/12/19)

• Compile dynamic library under linux

  gcc -I ${JAVA_HOME}/include -I ${JAVA_HOME}/include/linux \
      -fPIC -shared -o libMyNative.so \
      local_zcw_demo_MyNative.c

• run

  direct execution

  java local.zcw.demo.MyNative

  throw java.lang.UnsatisfiedLinkError exception

  Shared library path can be specified

  java -Djava.library.path='.' local.zcw.demo.MyNative

  or add environment variable

  export LD_LIBRARY_PATH="libMyNative.so路径":$LD_LIBRARY_PATH