java using JNI to call C under Linux++
1.JNI introduction
JNI is Java Native Interface(Java local interface), which is a protocol. Its main function is to realize java calling C / C + + code (Class Library), or C / C + + calling java code
2. prepare JNI
First, use java h to generate. H header file according to. java file, and then use C + + to implement this interface
2.1 first create the JNI.java file in the directory
The contents are as follows
//JNI.java file
public class JNI{
//Create a native interface method, which is implemented in C + + code
public native int doSomething();
//Static code block, load the. dll Dynamic link file generated by C + + code (. dll is equivalent to the jar package in Java...)
static{
System.loadLibrary("JNIDLL");
}
}
2.2 use javac to compile JNI.java to generate JNI.class, and then use Java h to compile JNI.class to get JNI.h file. The steps are as follows
- javac compilation
javac JNI.java - javah compiles JNI.class (without suffix)
javah JNI - Open JNI.h to view
/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class JNI */
#ifndef _Included_JNI
#define _Included_JNI
#ifdef __cplusplus
extern "C" {
#endif
/*
* Class: JNI
* Method: doSomething
* Signature: ()I
*/
JNIEXPORT jint JNICALL Java_JNI_doSomething
(JNIEnv *, jobject);
#ifdef __cplusplus
}
#endif
#endif
2.3 write C + + code (JNIDLL.cpp) to implement the doSomething() method in JNI.java (native decoration)
Note that the method name is Java? JNI? Dosomething in JNI.h
//JNIdll.cpp file
#include<stdio.h>
#include<jni.h>
#include "JNI.h"
JNIEXPORT jint JNICALL Java_JNI_doSomething
(JNIEnv *, jobject){
//Implementation code
int i = 777;
printf("in doSomething \n");
return i;
}
2.4 using g + + compiler to generate. so dynamic library file
The command is as follows:
My jdk path is different from yours. You need to modify it when you use it
g++ -I /java_android/jdk1.8.0_131/include -I /java_android/jdk1.8.0_131/include/linux/ -fPIC -shared -o libJNIDLL.so JNIDLL.cpp
You can also copy jni.h and JNI ENU md.h to the current directory, and then compile:
The two files are under jdk1.8.0 and jdk1.8.0.131/include/linux/
g++ -fPIC -shared -o libJNIDLL.so JNIDLL.cpp
3. use JNI
In the java main program, call the JNI file that is prepared above.
3.1 write the main program HelloWorld.java file
//HelloWorld.java file
import java.lang.Thread;
import java.util.Scanner;
public class HelloWorld {
public static void main(String []args) {
System.out.println("Hello World");
//Create JNI's object call
JNI jni = new JNI();
int count =1;
while (true)
{
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
//call() method;
int i = jni.doSomething();
System.out.println("The first " + count + " second sleep 2s ");
count++;
}
}
}
3.2 compilation and operation
- Compile
javac HelloWorld.java - Function
java HelloWorld
4. Complete process
virtual-machine:~/codeTest/javaTest$ ll -rw-rw-r-- 1 tsh tsh 534 Mar 29 17:03 HelloWorld.java -rw-rw-r-- 1 tsh tsh 229 Mar 29 17:07 JNIDLL.cpp -rw-rw-r-- 1 tsh tsh 309 Apr 1 11:14 JNI.java virtual-machine:~/codeTest/javaTest$ javac JNI.java virtual-machine:~/codeTest/javaTest$ javah JNI virtual-machine:~/codeTest/javaTest$ ll -rw-rw-r-- 1 tsh tsh 534 Mar 29 17:03 HelloWorld.java -rw-rw-r-- 1 tsh tsh 349 Apr 1 14:43 JNI.class -rw-rw-r-- 1 tsh tsh 229 Mar 29 17:07 JNIDLL.cpp -rw-rw-r-- 1 tsh tsh 354 Apr 1 14:44 JNI.h -rw-rw-r-- 1 tsh tsh 309 Apr 1 11:14 JNI.java virtual-machine:~/codeTest/javaTest$ g++ -I /java_android/jdk1.8.0_131/include -I /java_android/jdk1.8.0_131/include/linux/ -fPIC -shared -o libJNIDLL.so JNIDLL.cpp virtual-machine:~/codeTest/javaTest$ ll -rw-rw-r-- 1 tsh tsh 534 Mar 29 17:03 HelloWorld.java -rw-rw-r-- 1 tsh tsh 349 Apr 1 14:43 JNI.class -rw-rw-r-- 1 tsh tsh 229 Mar 29 17:07 JNIDLL.cpp -rw-rw-r-- 1 tsh tsh 354 Apr 1 14:44 JNI.h -rw-rw-r-- 1 tsh tsh 309 Apr 1 11:14 JNI.java -rwxrwxr-x 1 tsh tsh 8136 Apr 1 14:46 libJNIDLL.so* virtual-machine:~/codeTest/javaTest$ javac HelloWorld.java virtual-machine:~/codeTest/javaTest$ ll -rw-rw-r-- 1 tsh tsh 955 Apr 1 14:46 HelloWorld.class -rw-rw-r-- 1 tsh tsh 534 Mar 29 17:03 HelloWorld.java -rw-rw-r-- 1 tsh tsh 349 Apr 1 14:43 JNI.class -rw-rw-r-- 1 tsh tsh 229 Mar 29 17:07 JNIDLL.cpp -rw-rw-r-- 1 tsh tsh 354 Apr 1 14:44 JNI.h -rw-rw-r-- 1 tsh tsh 309 Apr 1 11:14 JNI.java -rwxrwxr-x 1 tsh tsh 8136 Apr 1 14:46 libJNIDLL.so* virtual-machine:~/codeTest/javaTest$ java HelloWorld Hello World in doSomething //The first sleep 2s in doSomething //The second sleep 2s in doSomething //The third sleep 2s