Introduction to the simple use of JNI in java (details)

This article brings you a simple introduction to the use of JNI (detailed). It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you.

JNI allows us to call the functions of local libraries in java code.

The following is a simple way to use JNI

1. Create a java-side interface

public class JNIIterface {
    // 导入最终生成的dll文件
    static {
        System.loadLibrary("JNIDemo");
    }

    // 声明dll文件中实现的接口
    public native int MyMethod();
    
    public native int add(int x, int y);
}

2. Generate the C header file corresponding to the interface

Above Created JNIIterface class, the path of the JNIIterface .class file: \JNIDemo\bin\base\JNIIterface .class

Execute the command in the bin directory in the project directory: javah -classpath . -jni base.JNIIterface

Then the base_JNIIterface.h file will be generated in the \JNIDemo\bin\ directory

3. Use visual studio to write the implementation of the interface

Create a C project. The specific steps are as follows:

Step one:

Step two:

##Step three:

Import the required header files

Put the header file generated in the second step above, as well as jni.h and

## in the include directory under the jdk root directory #include the jni_md.h file in the win32 directory and copy it to the header file directory in the project directory

Create the function implementation file

Right-click on the source file in the project directory and click Add - 》Class

and write the following code:

#include "base_JNIIterface.h"
#include "jni.h"
#include <stdio.h>

JNIEXPORT jint JNICALL Java_base_JNIIterface_MyMethod
  (JNIEnv *, jobject){
      printf("成功实现JNI测试DEMO");
      return 0;
}

JNIEXPORT jint JNICALL Java_base_JNIIterface_add
  (JNIEnv *, jobject, jint a, jint b){
      return a + b;
}

4. Generate the dll file

on the "Solution" in the project directory Right-click, select Properties, and then construct the project as shown below

Then after compiling the project, it will be converted into a dll file in the Release directory under the project directory

5. Configure the dll file into the java project

First place the dll file in the lib directory under the root directory of the java project

Then follow the steps below Add the dll file into the project as a Native Library

Edit the Native Library Location shown in the picture above, and then select the folder where the Native Library is located in the pop-up interface:

6. Write a test class to verify whether JNI is successfully applied public class HelloJNI {

    public static void main(String[] args) {
        JNIIterface hw = new JNIIterface();
        int t = hw.add(3, 7);
        int s = hw.MyMethod();
        System.out.println(t);
        System.out.println(s);
    }
}

The test results after running are as follows:

0
0
成功??JNI??DEMO

The above records the most basic steps for using JNI.

How to use the functions of already written third-party dll files, such as system dll files, requires in-depth exploration.

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