MicroSpring A Spring Framework (mainly) compatible IOC container by J.Gibbons, Feb 2005 http://www.tallsoftware.com/microspring/

MicroSpring Guide

MicroSpring was written in a weekend, as a standalone IOC container which is compatible with the Spring Framework's XML config format.

This means you can play with IOC, take the MicroSpring code and very easily change it - it's a tiny code base. It also means you can switch to the real Spring Framework with very little pain if you need to.

What do you need to read?

This html page, and the one page guide, and that's all. It would help if you understood Inversion Of Control as well!

What does MicroSpring do?

It is tiny compared with Spring as a whole, and it doesn't even do all the IOC things, so what does it do?

• It supports beans, properties, values as sets, maps, lists, properties, primitives and idref's to other beans.
• It supports singletons and non singletons, and also lazy-load="true"
• It supports ApplicationListeners for close and reset events
• It supports bean templating with the abstract and parent attributes.
• It supports parent ApplicationContexts.
• It supports the standard bean locator functions that any BeanFactory does.
• It support init-method, destroy-method.
• It supports application context aware.
• Currently all lists, sets, maps and properties support either beans (i.e. other beans in the config), or strings as members. You cannot currently specify a list of Integer, or Double types.

Refresh behaviour

Calling refresh on the FileSystemXmlApplicationContext will read the config file. Compare class names with new class names on all singletons, and if different, destroy the old singleton. It will then call all the property setters again on existing singletons, followed by the onApplicationEvent (if its an application listener). Finally it tells any child contexts to refresh. i.e. it doesn't call the init method again, so if you want common behavior on start up and on refresh do it yourself. The reset behaviour is the only concrete advantage of setter injection over constructor injection that I have seen (the big downside being the risk of null ptrs if your config is wrong).

The refresh function is also called on startup. This in turn calls broadcasts the ContextRefreshedEvent to all singleton=true, lazy-init=false, where the class implements ApplicationListener.

What doesn't it do?

• Doesn't support bean alliases.
• Doesn't do load time validation - you can get runtime failures. Obviously singletons by default are created at load time, so there is validation providing lazy-init=false.
• Doesn't support bean name lookup - you have to use <ref bean=""> rather than value to locate other beans.
• Doesn't support constructor injection.

In fact the simplest thing to do is read the 1 page Spring crib sheet, MicroSpring does everything on this sheet ! And everything on that sheet is Spring best practice (as stated in their own documentation).

What is incompatible?

The class paths for the interfaces are tallsoft rather than spring. And you should comment out the doctype line of the xml config file.

The import's not matching are annoying, but this is simply solved by refactoring, or by renaming MicroSpring to be compatible. I do not presume to do this in my source code. After all MicroSpring is not endorsed or supported by the Spring Framework team.

An example

There are two things going on here:

• the cityFinder singleton is being injected into the region object, and we are using maps and lists for this.
• the templateGeography is a base definition with two 'derived' definitions, one of which overrides a property.

The XML

<?xml version="1.0" encoding="UTF-8"?>
<!-- !DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN//EN" "http://www.springframework.org/dtd/spring-beans.dtd"-->

<beans>
    <!-- example of list and map and ref and property values -->
    <bean id="cityFinder" class="com.tallsoft.springeg.CityScape"    singleton="true"> 
        <property name="cityMap">
            <map>
                <entry key="LDN"><value>London</value></entry>
                <entry key="FFT"><value>Frankfurt</value></entry>
            </map>
        </property>
    </bean>
    <!-- example of list and depends on for creation ordering-->
    <bean id="region" class="com.tallsoft.springeg.RegionInfo"    depends-on="cityFinder">
        <property name="cityFinder"><ref bean="cityFinder"/></property>
        <property name="regions">
            <list>
                <value>Europe</value>
                <value>America</value>
            </list>
        </property>
    </bean>
    <!-- example of template, and the init method -->
    <bean id="templateGeography" abstract="true"
            class="com.tallsoft.springeg.BaseGeography">
        <property name="planetName"><value>Earth</value></property>
        <property name="system"><value>sol</value></property>
    </bean> 
    <bean id="earth" class="com.tallsoft.springeg.PlanetInfo"
                    parent="templateGeography" />
    <bean id="mars" class="com.tallsoft.springeg.PlanetInfo"    
                    parent="templateGeography" init-method="initialize">
        <!-- overriding the value here -->
        <property name="planetName"><value>Mars</value></property>
    </bean>
</beans>

The Code

try {
	FileSystemXmlApplicationContext factory = new FileSystemXmlApplicationContext("example.xml");
   RegionInfo r = (RegionInfo)factory.getBean("region");
} catch (Exception ex) {
	ex.printStackTrace(System.out);
}