JNI Demo In Linux

秒速五厘米 2023-01-19 09:40 9阅读 0赞

# Overview #

JNI (Java Native Interface) is used to call C/C++ libraries by Java code.

The C/C++ library is known as SO library in Linux and is known as DLL in windows.

# Environment Requirment #

*  Linux
 *  GCC
 *  G++
 *  Java 1.8+

If you have not prepared for the environment, you can install them by commands below:

yum -y install gcc
    yum -y install gcc+ gcc-c++
    yum -y install java-1.8.0-openjdk-devel.x86_64

# Let’s Start #

First, I prepared a Java file called JniDemo.java and its content is below:

[root@snail jni_helloworld]# cat JniDemo.java 
    public class JniDemo{ 
    
      public static void main(String[] args){ 
        System.out.println("Hello, I'm Java!");
        new JniDemo().hello();
      }
    
      public native void hello(); // Native key word tell Jvm that the method is from SO libraries instead of Java.
    
      static{ 
        System.loadLibrary("cpp_lib_demo"); // Loading the C library.
      }
    }

Second, use `javah` tool to generate `JniDemo.h` file.

[root@snail jni_helloworld]# javah JniDemo
    [root@snail jni_helloworld]# ls
    JniDemo.h  JniDemo.java

Third, It’s time to code our cpp file.

[root@snail jni_helloworld]# cat cpp_demo.cpp 
    #include "JniDemo.h" // Import generated header file.
    #include "jni.h"
    JNIEXPORT void JNICALL Java_JniDemo_hello
      (JNIEnv * env, jobject jb){ 
          printf("%s","Hello, I'm Cpp.\n");
    }

To compile our cpp file, we need to make our `Makefile` file.

[root@ jni_helloworld]# cat Makefile 
    libcpp_lib_demo.so : cpp_demo.cpp
    	g++ -o $@ $+ -fPIC -shared -I/usr/lib/jvm/java-1.8.0-openjdk-1.8.0.292.b10-1.el7_9.x86_64/include -I/usr/lib/jvm/java-1.8.0-openjdk-1.8.0.292.b10-1.el7_9.x86_64/include/linux
    
    .PHONY : clean
    clean :
    	rm libcpp_lib_demo.so

> Note: You need to modify the java path to your java path.

And now, let’s us to compile our Java file to class and compile our cpp file to `SO` library.

[root@snail jni_helloworld]# javac JniDemo.java 
     [root@snail jni_helloworld]# make
    g++ -o libcpp_lib_demo.so cpp_demo.cpp -fPIC -shared -I/usr/lib/jvm/java-1.8.0-openjdk-1.8.0.292.b10-1.el7_9.x86_64/include -I/usr/lib/jvm/java-1.8.0-openjdk-1.8.0.292.b10-1.el7_9.x86_64/include/linux
     [root@snail jni_helloworld]# ls
    cpp_demo.cpp  JniDemo.class  JniDemo.h  JniDemo.java  libcpp_lib_demo.so  Makefile

Now, we can run our JNI program.

[root@snail jni_helloworld]# java -Djava.library.path='.' JniDemo
    Hello, I'm Java!
    Hello, I'm Cpp.