First, take a look at the overall structure:

In the main of src, we define a path such as com.lf. There are two packages below. First, let's look at the API package. We define an interface MyPrinterAPI:

package com.lf;

import com.lf.API.MyPrinterAPI;

import java.util.ServiceLoader;

public class testClient {
    public static void main(String[] args) {
        ServiceLoader<MyPrinterAPI> printers = ServiceLoader.load(MyPrinterAPI.class);
//        System.out.println(printers);
        for (MyPrinterAPI printer : printers) {
            printer.sayHello("SPI");
        }
    }
}

Then, under the impl package, we define two implementations of this interface - MyPrinterA and MyPrinterB (C is a comparison, but does not implement the interface). The code is as follows:

// MyPrinterA.java
package com.lf.impl;

import com.lf.API.MyPrinterAPI;

public class MyPrinterA implements MyPrinterAPI {
    @Override
    public String sayHello(String str) {
        String res = "This is printer A!\nHello " + str;
        System.out.println(res);
        return res;
    }
}

// MyPrinterB.java
package com.lf.impl;

import com.lf.API.MyPrinterAPI;

public class MyPrinterB implements MyPrinterAPI {
    @Override
    public String sayHello(String str) {
        String res = "This is printer B!\nHello " + str;
        System.out.println(res);
        return res;
    }
}

After completing these three java files, we create a folder path such as META-INF/services under the resources folder, and then create a file com.lf.API.MyPrinterAPI under the services folder. You can see that this file is the fully qualified name of our defined interface MyPrinterAPI.

After that, we write:

com.lf.impl.MyPrinterA
com.lf.impl.MyPrinterB

(in the above figure, my IDE prompts A and B when I write, but there is no C, because C does not implement the interface.)

After the above steps are completed, we can start our test.

We create a test file testClient under com.lf, in which we write the main function as the test entry:

// testClient.java
package com.lf;

import com.lf.API.MyPrinterAPI;

import java.util.ServiceLoader;

public class testClient {
    public static void main(String[] args) {
        ServiceLoader<MyPrinterAPI> printers = ServiceLoader.load(MyPrinterAPI.class);
        for (MyPrinterAPI printer : printers) {
            printer.sayHello("SPI");
        }
    }
}

Operation result:

This is printer A!
Hello SPI
This is printer B!
Hello SPI

You can see that the two implementation classes we wrote were called successfully.