How to implement Java underlying technology operating system calls and JNI

In Java programming, we usually use high-level language features and APIs for development. But sometimes, we need to access the underlying functions of the operating system to meet specific needs. In order to achieve this purpose, Java provides a mechanism-operating system calls and JNI (Java Native Interface).

Operating system calls refer to Java programs accessing the underlying functions of the operating system by calling functions provided by the operating system. JNI is a mechanism for Java programs to interact with underlying C/C code. Through JNI, we can write low-level code and call these low-level functions in Java programs.

Below, I will introduce in detail how to use operating system calls and JNI to implement Java underlying technology.

1. Operating system calls
Operating system calls are Java programs that access the underlying functions of the operating system by calling the underlying API. Java provides a series of classes to encapsulate operating system functions, such as the java.lang.Runtime and java.lang.Process classes.

1. Using the java.lang.Runtime class
   The java.lang.Runtime class is the entry point for Java programs to interact with the operating system. It provides many methods to execute operating system commands, obtain memory information, load local libraries, etc.

The following is a simple example that demonstrates how to use the java.lang.Runtime class to execute operating system commands:

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class OSCommandExample {
    public static void main(String[] args) {
        String command = "ls -l";
        try {
            Process process = Runtime.getRuntime().exec(command);
            BufferedReader reader = new BufferedReader(new InputStreamReader(process.getInputStream()));
            String line;
            while ((line = reader.readLine()) != null) {
                System.out.println(line);
            }
            reader.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

In the above example, we pass Runtime.getRuntime The ().exec(command) method executes an operating system command and reads the output of the command through BufferedReader.

2. Using the java.lang.Process class
   The java.lang.Process class is an abstract class that represents the executing process. It provides a series of methods to obtain the input/output stream of the process, wait for the process to complete, etc.

The following is an example that demonstrates how to use the java.lang.Process class to execute operating system commands:

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class ProcessExample {
    public static void main(String[] args) {
        try {
            ProcessBuilder processBuilder = new ProcessBuilder("ls", "-l");
            Process process = processBuilder.start();

            BufferedReader reader = new BufferedReader(new InputStreamReader(process.getInputStream()));
            String line;
            while ((line = reader.readLine()) != null) {
                System.out.println(line);
            }
            reader.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

In the above example, we use ProcessBuilder to create Create a process and execute operating system commands through the start() method. Then, we read the output of the command through BufferedReader.

2. JNI (Java Native Interface)
If we need to access lower-level functions, such as hardware interfaces or specific operating system functions, we can use JNI. JNI is a mechanism for Java programs to interact with underlying C/C code.

The following is a simple example that demonstrates how to use JNI to implement Java underlying technology:

1. Create Java classes and Native methods
   First, we need to declare in the Java class native method to implement related functions at the bottom layer. Then, we implement the underlying functionality by implementing these native methods in the native library.

The following is an example of a Java class:

public class NativeExample {
    public native void printMessage(String message);

    static {
        System.loadLibrary("mylib");
    }

    public static void main(String[] args) {
        NativeExample example = new NativeExample();
        example.printMessage("Hello from Java!");
    }
}

In the above example, we declared a native method printMessage and passed System.loadLibrary () method loads the local library. Then, call this native method in the main method.

2. Create native library
   In the local library, we can use C/C language to implement the native method. Then, we must use Java's JNI standard header file jni.h to compile the native library.

The following is an example of the native library:

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

JNIEXPORT void JNICALL Java_NativeExample_printMessage(JNIEnv *env, jobject obj, jstring message) {
    const char *c_message = (*env)->GetStringUTFChars(env, message, NULL);
    printf("%s
", c_message);
    (*env)->ReleaseStringUTFChars(env, message, c_message);
}

In the above example, we implemented the native method Java_NativeExample_printMessage and used printf Function prints a string.

Then, we can use Java's JNI tool chain to compile the C/C code into a native library. For example, under Linux, we can use the following command:

gcc -shared -fpic -o libmylib.so NativeExample.c

In the above command, we use the gcc compiler to compile the C code into libmylib.so local library.

3. Run Java program
   When compiling and running a Java program, make sure that the local library exists in the runtime library path. The library path can be set by specifying the java.library.path system property.

For example, under Linux, you can use the following command to run a Java program:

java -Djava.library.path=. NativeExample

In the above command, we add the current directory (.) to Library path and specify the Java class to run via NativeExample.

Summary
Through operating system calls and JNI, we can implement Java underlying technology and access the underlying functions of the operating system. Operating system calls enable us to access the operating system using high-level APIs, while JNI enables us to write low-level code and call it in Java programs.

It should be noted that you should be cautious when using the underlying technology to ensure the security and reliability of the code.

I hope this article will help you understand how to use operating system calls and JNI to implement Java underlying technology. If you have any questions, please feel free to ask.

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