Constructor Injection for Dependency Injection

illustrate:

　　Compared with the previous setter injection, the constructor injection mainly modifies the computer class, adds a constructor to the computer class, and directly passes the instance to the constructor in the test class.

 

1. Project screenshot

2. Printer interface class

package com.example.demo.printer;

public interface Printer {
    void init();

    void print(String txt);
}

3. Color printers

package com.example.demo.printer;

public class ColorPrinter implements Printer {
    @Override
    public void init() {
        System.out.println( "Start the color printer!" );
    }

    @Override
    public void print(String txt) {
        System.out.println( "Print color text: " .concat(txt));
    }
}

4. Black and white printer

package com.example.demo.printer;

public class GrayPrinter implements Printer{

    @Override
    public void init() {
        System.out.println( "Start the printer" );
    }

    @Override
    public void print(String txt) {
        System.out.println( "Print black and white text: " .concat(txt));
    }
}

5. Read the bean configuration file and create an instance

package com.example.demo.printer;

import java.io.IOException;
import java.util.Properties;

public class GetBeans {
    private static Properties p = new Properties();
    static{
        try{
            //读取bean配置文件
            p.load(TestComputer.class.getResourceAsStream("/bean.properties"));
        }catch(IOException e){
            System.out.println( "Could not find configuration file!" );
        }
    }
    public static Object getBean(String keyName){
        Object o = null ;
         try {
             // Create an instance according to the keyword defined in the property file 
            o = Class.forName(p.get(keyName).toString()).newInstance();
        }catch (Exception e){
            System.out.println( "Unable to instantiate object!" );
        }
        return o;
    }
}

6. Computer

package com.example.demo.printer;

public class Computer {

    Printer p;
    public Computer(Printer p) {
        this.p = p;
    }

    public Printer getP() {
        return p;
    }

    public void setP(Printer p) {
        this.p = p;
    }
}

7. Test class

package com.example.demo.printer;

public class TestComputer {
    public static void main(String[] args) {
        Printer p = (Printer) GetBeans.getBean("printer");
        Computer pcl = new Computer(p);
         // The implementation does not use the new keyword 
        pcl.getP().print("Print test page..." );
    }
}

8. Configuration file

printer = com.example.demo.printer.ColorPrinter

9. Effect: