第2章 入门

1.经典的helloworld案例

 

 

package spring;
 
public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello  world");
    }
}

该例非常简单， 它可以完成相关工作，但不具有可扩展性。 

 

public class HelloWorldWithCommandLine {
 
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        if (args.length > 0) {
            System.out.println(args[0]);
        } else {
            System.out.println("Hello world");
        }
    }
}

 

该例完成了所需的功能，现在可以在不更改代码的情况下更改消息。但是 ， 这个应用程序仍然存在问题： 负责渲染消息的组件同时也负责获取消息。 

 

package spring;
 
public interface MessageProvider {
    String getMessage();
}

package spring;
 
public interface MessageRenderer {
    void render();
 
    void setMessageProvider(MessageProvider provider);
 
    MessageProvider getMessageProvider();
}

 

package spring;
 
import javax.management.RuntimeErrorException;
 
public class StandardOutMessageRenderer implements MessageRenderer {
    private MessageProvider messageProvider;
 
    @Override
    public void render() {
        // TODO Auto-generated method stub
        if (messageProvider == null) {
            throw new RuntimeException("You must set the property messageProvider of class :"
                    + StandardOutMessageRenderer.class.getName());
        }
        System.out.println(messageProvider.getMessage());
    }
 
    @Override
    public void setMessageProvider(MessageProvider provider) {
        // TODO Auto-generated method stub
        this.messageProvider = provider;
    }
 
    @Override
    public MessageProvider getMessageProvider() {
        // TODO Auto-generated method stub
        return this.messageProvider;
    }
 
}
 

package spring;
 
public class HelloWorldDecoupled {
    public static void main(String... args) {
        MessageRenderer mr = new StandardOutMessageRenderer();
        MessageProvider mp = new HelloWorldMessageProvider();
        mr.setMessageProvider(mp);
        mr.render();
    }
}
 

 

 

此时的代码非常简单。我们实例化了 HelloWorld.MessageProvider 和 StandardOutMessageRenderer 的实例，虽然声明的类型分别是 MessageProvider 和 MessageRenderer 。这是因为只需要在编程逻辑中与接口提供的方法进行交互，并且 HelloWorld.MessageProvider 和 StandardOutMessageReriderer 已经分别实现了这些接口 。 然后，将 MessageProvider传递给 MessageRenderer 并调用 MessageRenderer.render（）。 如果编译并运行该程序，将会得到预期的 “Hello World !”输出。 

现在， 该例更能满足我们的要求，但存在一个小问题。 更改 MessageRenderer 或 MessageProvider 接口的实现意味着对代码进行更改。 为了解决这个问题，可以创建一个简单的工厂类，它从属性文件中读取实现类的名称，并代表应用程序对它们进行实例化。

 

 

package spring;
 
import java.util.Properties;
import java.util.Set;
 
public class MessageSupportFactory {
    private static MessageSupportFactory instance;
    private Properties props;
    private MessageRenderer renderer;
    private MessageProvider provider;
 
    private MessageSupportFactory() {
        // TODO Auto-generated constructor stub
        props = new Properties();
        try {
            props.load(this.getClass().getResourceAsStream("/msf.properties"));
 
            Set set  =  props.keySet();
            String rendererClass = props.getProperty("renderer.class");
            String providerClass = props.getProperty("provider.class");
            renderer = (MessageRenderer) Class.forName(rendererClass).newInstance();
            provider = (MessageProvider) Class.forName(providerClass).newInstance();
        } catch (Exception e) {
            // TODO: handle exception
            e.printStackTrace();
        }
    }
 
    static {
        instance = new MessageSupportFactory();
    }
 
    public static MessageSupportFactory getinstance() {
        return instance;
 
    }
 
    public MessageRenderer getMessageRenderer() {
        return renderer;
 
    }
 
    public MessageProvider getMessageProvider() {
        return provider;
    }
}

 

renderer.class=spring.StandardOutMessageRenderer
provider.class=spring.HelloWorldMessageProvider

package spring;
 
public class HelloWorldDecoupled {
    public static void main(String... args) {
        MessageRenderer mr = MessageSupportFactory.getinstance().getMessageRenderer();
        MessageProvider mp = MessageSupportFactory.getinstance().getMessageProvider();
        mr.setMessageProvider(mp);
        mr.render();
    }
}

 

 

 

 

 

用 Spring 重构 

 

xml方式进行重构

 

package spring;
 
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
 
public class HelloWorldSpringDI {
 
    public static void main(String[] args) {
        ApplicationContext ctx = new ClassPathXmlApplicationContext("test/spring/app-context.xml ");
        MessageRenderer mr = ctx.getBean("renderer", MessageRenderer.class);
        mr.render();
    }
 
}
 

 

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xmlns="http://www.springframework.org/schema/beans"
 xmlns:p="http://www.springframework.org/schema/p"
 xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-4.2.xsd ">
<bean id="provider"  class="test.spring.HelloWorldMessageProvider" />
<bean id="renderer"  class="test.spring.StandardOutMessageRenderer"  p:messageProvider-ref="provider"/>
</beans>

注解方式进行重构

 

package spring;
 
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
 
@Configuration
public class HelloWorldConfiguration {

    @Bean 
    public MessageProvider provider() {
        return new HelloWorldMessageProvider();
    }

    @Bean
    public MessageRenderer renderer() {
        MessageRenderer renderer = new StandardOutMessageRenderer();
        renderer.setMessageProvider(provider());
        return renderer;
    }
}

 

package spring;
 
import org.springframework.context.ApplicationContext;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
 
public class HelloWorldSpringDI {
 
    public static void main(String[] args) {
        ApplicationContext ctx = new AnnotationConfigApplicationContext(HelloWorldConfiguration.class);
        MessageRenderer mr = ctx.getBean("renderer", MessageRenderer.class);
        mr.render();
    }
 
}