Introducing the Spring framework

Transcription

1 Chair of Software Engineering Languages in Depth Series: Java Programming Prof. Dr. Bertrand Meyer Introducing the Spring framework Marco Piccioni

2 Yet another framework? It addresses areas that other frameworks don t Does not reinvent the wheel It is both comprehensive and modular Offers a simple programming model (based on POJOs) Open source project since February 2003 > 2.4 million downloads 2

3 What is it about? Helps in writing code that is easy to unit test Effectively organizes middle tier objects Centralizes configuration A light weight alternative to EJB s (most of the time) Eliminates Singleton proliferation Consistent framework for data access 3

4 The overall picture Picture from 4

5 A deceivingly simple example public class HelloWorld public static void main(string[] aargh) System.out.println( Hello again?!"); } } 5

6 Requirements happen to change What if we want to change the message? What if we want to output the message differently? To stderr instead of stdout Enclosed in HTML tags rather than as plain text New requirements Our application should support a simple, flexible mechanism for changing the message It should be simple to change the rendering behavior. 6

7 Quiz Which is the simplest way to change the message without changing the code? How to implement a more flexible rendedering behavior? 7

8 Quiz solution Which is the simplest way to change the message without changing the code? Using it as a command line argument How to implement a more flexible rendedering behavior? Separating the component that renders the message from the component that obtains the message 8

9 A more flexible design <<interface>> MessageRenderer +setmessageprovider() +getmessageprovider() <<interface>> MessageProvider +getmessage() +render() StandardOutMessageRenderer +setmessageprovider() +getmessageprovider() HelloWorldMessageProvider +getmessage() +render() 9

10 Decoupling hello world public class DecoupledHelloWorld } public static void main(string[] aargh) MessageRenderer mr = new StandardOutMessageRenderer(); MessageProvider mp = new StandardOutMessageRenderer(); mr.setmessageprovider(mp); mr.render(); } 10

11 Improving the design using a Factory public class DecoupledHelloWorldWithFactory public static void main(string[] aargh) MessageRenderer mr = MessageSupportFactory.getInstance().getMes sagerenderer(); MessageProvider mp = MessageSupportFactory.getInstance().getMes sageprovider(); mr.setmessageprovider(mp); mr.render(); }} 11

12 Flexibility comes at a price The factory will be able to create a message renderer and a message provider by name, reading a configuration file We have to write lots of glue code We still have to provide the MessageRenderer with an instance of MessageProvider manually 12

13 Spring teaser public class XmlConfigHelloSpringDI public static void main(string[] aargh) BeanFactory factory = getbeanfactory() MessageRenderer mr = (MessageRenderer) factory.getbean( renderer ); mr.render(); } //see next page for getbeanfactory() } 13

14 More Spring teaser... private static BeanFactory getbeanfactory() throws Exception... XmlBeanFactory factory = new XmlBeanFactory(new FileSystemResource( src/ch/ethz/inf/java/ hello/msgconf.xml )); return factory; } 14

15 How Spring config file looks like <!DOCTYPE beans PUBLIC "-//SPRING/DTD BEAN//EN" " <beans> <bean id="renderer class= ch.ethz.inf.java.hello.standardoutmessagerende rer"> </bean> <property name="messageprovider"> </property> <ref local="provider"/> <bean id="provider class="ch.ethz.inf.java.hello.helloworldmessageprovid er"/> </bean> </beans> 15

16 Hello Spring Here is where the Spring Framework comes into play Provides lots of well written plumbing code Sets the dependencies automatically Your application interacts with the Spring DI container using the BeanFactory (or the more powerful ApplicationContext) interface We can also set values directly in the configuration file, or using constructor args (e.g. injecting hello world ) 16

17 References [1] [2] Rob Harrop, Jan Machacek. Pro Spring: Apress 2005 [3] Rod Johnson. Expert One-on-One J2EE Design and Development, Wrox press 2001 [4] Craig Walls, Ryan Breidenbach. Spring in Action Manning