Thursday, June 13, 2013

Spring HelloWorld Example in Java using Dependency Injection

In this Spring framework tutorial, we will learn how to write hello world example. This should be the first tutorial to start learning Spring framework, as it gets ball rolling and while coding and running this example, you learn a lot about Spring framework, Spring XSD files, required JAR file and more importantly how spring works.This Helloworld program in Spring framework is an example of classical Java hello world program, written using dependency Injection design pattern by using Spring Framework's IOC container. Spring is so far one of the most popular Java application framework, which promotes some best practices while writing Java application e.g. dependency Injection. Spring provides an IOC container to manage life-cycle of Spring beans and provides support to get beans any time from IOC container. Apart from Spring's IOC container, it also provide rich API to simply many of Java task e.g. JdbcTemplate help you to write JDBC code without taking care of boiler plate things like closing connection, statement, result-set etc. Similarly Spring framework also provides JmsTemplate to simply JMS related task e.g. sending and receiving messages in Java. I first learned about usefulness of Spring framework, while reading Expert Spring MVC and Web Flow, it first few chapters, which explains about how dependency injection provides improved testing, loose coupling and help in clean code. Those two chapters are so far my best read on Spring, and I also suggest to take a look. By the way in this Spring tutorial, we will  see one of the most simple example of Dependency Injection e.g. Hello Example. Message to Hello class is provided by Spring framework using Dependency Injection.


Spring and Java HelloWorld Example

In this example we have created a bean or a Java class called Hello, which accepts a String message as dependency. This spring bean is initialized using spring configuration file i.e. spring-config.xml. All beans declared in spring configuration file is created and managed by Spring IOC container. If you look Spring configuration file, than you will find that id of bean is "hello", which will be further used to get reference of this bean from Spring framework using getBean() method of ApplicationContext or BeanFactory class. In order to test this Spring HelloWorld example, we have created a Main class, which has classical public static void main(String args[]) method.  In main method, we are creating an instance of ClassPathXMLApplicationContext by providing spring-config.xml, which must be available in classpath, in order for Spring to initialize beans. Next two lines of code is self explanatory, where we are getting reference of Hello bean by using "id" attribute of Hello bean declaration. For printing HelloWorld in console, Instead of using System.out.println, we have used log4j. As its important to setup logging in every Java application, I have configured log4j using log4j.xml which is also available in classpath. By the way, there are mainly two ways to inject dependency using Spring, Constructor injection and Setter Injection, and this example uses setter injection to pass message to Hello object.

So this Spring hello world example contains following configuration files and Java classes.

Hello.java  - Hello Bean which prints message provided to it by using spring dependency Injection
Main.java - Test class for this Spring Helloworld example
spring-config.xml  - Spring configuration file which contains bean declaration
log4j.xml - log4j configuration file


Required dependency JAR files
This Spring hello world example uses Spring 3.1.2.RELEASE.jar files e.g.

spring-core-3.1.2.RELEASE.jar
commons-logging-1.1.1.jar
log4j-1.2.16.jar
spring-context-3.1.2.RELEASE.jar
spring-aop-3.1.2.RELEASE.jar
aopalliance-1.0.jar
spring-beans-3.1.2.RELEASE.jar

In order to run this Spring hello world example, just run Main class as Java application from command line or Eclipse IDE.

Spring HelloWorld Java Class

import org.apache.log4j.Logger;

/*
 * Java class which accept message as dependency injected
 */
public class Hello {

        private static final Logger logger = Logger.getLogger(Hello.class);
        private String message;

        public void setMessage(String message) {
                this.message = message;
        }

        public String getMessage() {
                return message;
        }
      
        public void sayHello(){
                logger.info(“Hello world Spring message is :+ message);        }

}

Main Class to Test HelloWorld Bean

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

/*
 * Main class to start and test this Java application
 */
public class Main {

        public static void main(String args[]){
               ApplicationContext context = new ClassPathXmlApplicationContext(
                                                "spring-config.xml");
                Hello hello = (Hello) context.getBean("hello");
                hello.sayHello();
        }
}

log4j.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE log4j:configuration SYSTEM "log4j.dtd">
<log4j:configuration xmlns:log4j='http://jakarta.apache.org/log4j/'>

        <appender name="console" class="org.apache.log4j.ConsoleAppender" >
            <layout class="org.apache.log4j.PatternLayout">
                 <param name="ConversionPattern" value="%d %-4r [%t] %-5p %c %x - %m%n" />
            </layout>
        </appender>

        <logger name="org.springframework" >
             <level value="ERROR" />
             <appender-ref ref="console" />
        </logger>

        <logger name="org.apache">
             <level value="DEBUG" />
             <appender-ref ref="console" />
        </logger>

        <root>
             <level value="DEBUG" />
             <appender-ref ref="console" />
        </root>
</log4j:configuration>

Spring Configuration File spring-config.xml

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:p="http://www.springframework.org/schema/p"
        xmlns:context="http://www.springframework.org/schema/context"
        xsi:schemaLocation="
http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.0.xsd">

        <bean id="hello" class="Hello">
                <property name="message" value="Good Afternoon" />
        </bean>

</beans>

Output
Hello world Spring message is : Good Afternoon



Further Reading on Spring Framework
If you are beginner in Spring and just started learning it, you may want to take a look at these books to further learning on this popular Java framework                        
  1. Expert Spring MVC
  2. Spring Recipes
  3. Pro Spring 3
  4. Spring in Action   
 
That's all on this Spring and Java HelloWorld Example using dependency Injection. For a Java programmer, knowledge of Spring framework is quickly becoming from good to have, to must have and it's high time to learn and use Spring framework while writing Java applications. Initially it take some time to get used of XML and Spring configuration file, various spring releases and JAR files and Spring API. Once you get hold of basics of Spring framework and dependency Injection design principle, writing Java program in Spring framework is real fun.

1 comment :

Gauri said...

Hello, Can you please also post Spring hello world example using Annotation? I am familiar with XML config file but really struggle with using annotation in Spring. please help.

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