Usage of native keyword in Java

Usage of native keyword in Java

In Java, the native keyword is used to indicate that a method is implemented using an underlying language (such as C or C) , rather than implemented in Java language. Using the native keyword, you can call functions implemented in other non-Java languages ​​in Java, such as specific functions of the operating system, control of hardware devices, etc.

The declaration of the native method does not contain the implementation code, only the declaration of the method. The specific implementation code needs to be completed in other languages. Java calls these methods native methods.

The following is an example of usage of the native keyword:

Declare the native method in the Java class:

public class NativeDemo {
    public native void printHello();    // native方法声明

    static {
        System.loadLibrary("NativeLibrary");    // 加载本地库
    }

    public static void main(String[] args) {
        NativeDemo demo = new NativeDemo();
        demo.printHello();    // 调用native方法
    }
}

In the above example, printHello() Methods are declared as local methods using the native keyword. In the static code block of the class, load the native library NativeLibrary through the System.loadLibrary() method. Then create the NativeDemo object in the main() method, and the code in the local library will be executed when the printHello() method is called.

Native library that implements native methods:

#include <stdio.h>
#include "NativeDemo.h"

JNIEXPORT void JNICALL Java_NativeDemo_printHello(JNIEnv *env, jobject obj) {
    printf("Hello from native code!
");
}

In the above example, a local method printHello() is implemented, and a message is printed in this method. The naming rule of this method is Java class name method name, with specific parameters (JNIEnv* and jobject).

To compile and run this example, you can follow these steps:

1. Write the Java file: NativeDemo.java
2. Use the javac command Compile the Java file: javac NativeDemo.java
3. Use the javah command to generate the C header file: javah -jni NativeDemo
4. Write a C file: NativeDemo.c
5. Use a C compiler to compile the C file and generate a local library: gcc -I"$JAVA_HOME/include" -I"$JAVA_HOME/include/win32" -shared - o NativeLibrary NativeDemo.c
6. Run the Java program: java NativeDemo

The running result will output Hello from native code!, Indicates that the local method was successfully called.

It should be noted that the implementation code of the native method may be different on different operating systems and needs to be modified accordingly according to the actual situation.

Summary:
The native keyword allows Java to call functions implemented in non-Java languages. It is a way to realize the interaction between Java and the underlying language (such as C or C). By using the native keyword, you can call functions implemented by the underlying language in Java, such as specific functions of the operating system, control of hardware devices, etc. In methods declared using the native keyword, specific functions need to be implemented through local libraries. The writing and use of local libraries need to be configured and adjusted according to different operating systems and requirements. Mastering the usage of the native keyword can provide Java programs with a wider range of application scenarios and higher performance optimization potential.

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