依赖注入之setter注入---只需修改配置，电脑就可以安装不同的打印机；读取properties配置文件并创建实例；实现不采用new的方式直接实例化对象

package com.example.demo.printer;

public class GrayPrinter implements Printer{

    @Override
    public void init() {
        System.out.println("启动打印机");
    }

    @Override
    public void print(String txt) {
        System.out.println("打印黑白文字：".concat(txt));
    }
}

package com.example.demo.printer;

public class ColorPrinter implements Printer {
    @Override
    public void init() {
        System.out.println("启动彩色打印机！");
    }

    @Override
    public void print(String txt) {
        System.out.println("打印彩色文字：".concat(txt));
    }
}

package com.example.demo.printer;

/**
 * Created by Admin on 2018/5/8.
 */
public class Computer {

    Printer p;

    public void printTxt(String txt){
        p.init();
        p.print(txt);
    }

    public Printer getP() {
        return p;
    }

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

printer = com.example.demo.printer.ColorPrinter

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("无法找到配置文件！");
        }
    }
    public static Object getBean(String keyName){
        Object o = null;
        try{
            //根据属性文件中定义的关键字创建实例
            o = Class.forName(p.get(keyName).toString()).newInstance();
        }catch (Exception e){
            System.out.println("无法实例化对象！");
        }
        return o;
    }
}

package com.example.demo.printer;
public interface Printer {
    void init();

    void print(String txt);
}

package com.example.demo.printer;

public class TestComputer {
    public static void main(String[] args) {
        Computer pcl = new Computer();
        //实现不用new关键字
        Printer p = (Printer) GetBeans.getBean("printer");
        pcl.setP(p);
        pcl.getP().print("打印测试页...");
    }
}

printer = com.example.demo.printer.GrayPrinter