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Can Spring 3 formBackingObject return different command classes?

I have a situation where I need to check a condition in my formBackingObject and, depending on the condition, return one of two classes.
The problem is that, so far as I know, I can only define one commandName and commandClass in the servlet.xml. Anyone know of a way I can handle this? It doesn't seem like a rare use case, but I haven't really found any solution on the net.
Here is the logic block from my controller formBackingObject:
List<FooLoadShed> fooLoadShedList = this.fooLoadShedDao.getActiveSheds();
if(fooLoadShedList.isEmpty()) {
logger.info("LoadShedActive is: " + this.sessionDetailsManager.getSessionDetails().isLoadShedActive());
return new NoAction();
}
else {
this.sessionDetailsManager.getSessionDetails().setLoadShedActive(true);
logger.info("LoadShedActive is : " + this.sessionDetailsManager.getSessionDetails().isLoadShedActive());
logger.info("Number of load sheds: " + nieLoadShedList.size());
return new ModelAndView(new RedirectView("custLookup.htm"));
}
and my servlet.xml config:
<bean name="/index.htm" class="springapp.web.indexController" scope="session">
<property name="sessionForm" value="true"/>
<property name="commandName" value="noAction"/>
<property name="commandClass" value="springapp.service.NoAction"/>
<property name="formView" value="index"/>
<property name="sessionDetailsManager" ref="sessionDetailsManager"/>
<property name="mobiConfigDao" ref="mobiConfigDao"/>
<property name="fooLoadShedDao" ref="fooLoadShed" />
</bean>

This is a very old way to configure controllers Spring MVC! Haven't seen something like this in over 10 years. Why not use more modern Spring MVC configuration, using annotations instead of XML?
In any case, the importance of the command class is in the POST. Spring MVC must be able to construct an instance of the command class. In order to do that, it needs to know the specific class name. Then it will apply the form values to the properties on the command class object that was created. This fully populated command object will be handed to you in the handler method.
The configuration in the XML is for the default command object creation process. You can override this. In modern Spring MVC, this is with a method level #ModelAttribute annotation. In ancient Spring MVC, you need to override the methods that create the command object and create the command object yourself. BaseCommandController has a createCommand method which is protected. If that's the way you created your controller, that's where you would have to start.

How to contribute to a list property of a bean using Spring?

I have 3 projects:
framework
product-a
product-b
Each of the products depends on the framework, but they don't know each other.
I have 3 spring configuration files: one for each project. The configuration file of each product includes (with <import resource="classpath:/...) the configuration file of the framework.
In the framework there is a bean called "manager", which has a property List<AnInterface> theList. The "manager" has a addXxx(anImplementation), which adds elements to the list).
The framework, and each of the product provide implementations of AnInterface, which have to be added to theList.
So in the end, when product-a is running, the manager contains implementations from the framework, and from product-a, idem for product-b
What is the best practice to perform this initialization with Spring ?
The only solution I could think about is to create a dedicated class which contructor will take the manager and a list of contributions, and add them to the manager, but it's ugly because 1/ It manipulate external objects in the constructor, 2/ I have to create a dummy class just to initialize other classes... I don't like that.

I think that code should not know about Spring if it is not really needed. Therefore I would do all initialization in Spring config.
We can use bean definition inheritance and property overriding to do it.
Framework class
public class Manager {
private List<AnInterface> theList;
public void init() {
// here we use list initialized by product
}
}
Framework context
<bean id="manager"
init-method="init"
abstract="true"
class="Manager">
<property name="theList">
<list/> <!-- this will be overriden or extnded -->
</property>
</bean>
Product A context
<bean id="managerA"
parent="manager"
scope="singleton"
lazy-init="false">
<property name="theList">
<list>
<ref bean="impl1"/>
<ref bean="impl2"/>
</list>
</property>
</bean>
Watch out for parent and child properties in such configuration. Not all are inherited from parent. Spring documentation specifies:
The remaining settings are always taken from the child definition: depends on, autowire mode, dependency check, singleton, scope, lazy init.
Moreover, there is also collection merging in Spring so by specifing in child bean
<list merge="true">
you can merge parent and child lists.
I have observed this pattern in a number of projects and some extendable Web frameworks based on Spring.

I have accepted the answer of Grzegorz because it's a clean solution to my initial problem, but here as an alternate answer, the a technical solution to contribute to a list property of an existing bean.
<bean class="org.springframework.beans.factory.config.MethodInvokingFactoryBean">
<property name="targetObject" ref="manager"/>
<property name="targetMethod"><value>addXxx</value></property>
<property name="arguments"><list value-type="com.xxx.AnInterface">
<value ref="impl1" />
<value ref="impl2" />
...
</list></property>
</bean>

spring 3: inject a dependency from a library?

I am developing a project and using 3rd party libraries. Let's say I use a library which gives me the object ExtObj. In my project I have a class MyObj, which uses ExtObj. How can I configure spring 3 to inject ExtObj in MyObj?
I tried to research the topic on the internet, but I didn't find a straight answer. I would like to use xml configuration and maybe (?) #Autowired, not #EJB or #Inject
Thanks in advance!
UPDATE
my guess was:
<bean id="myObj" value="me.MyObj">
<property name="extObj" value=" ... ??? ...">
</bean>
So, I don't know what I should put into value. I guess that's where the reference to the external object goes. But spring can only reference objects that have been already defined/configured in spring. So:
<bean id="extObj" value="ext.lib.ExtObj">
<bean id="myObj" value="me.MyObj">
<property name="extObj" value="extObj">
</bean>
Is that configuration right?

First you need to define a bean for your ExtObj in your application context (an xml file or a #Configuration class). Eg. if ExtObj has a constructor taking a String you can define the bean this way:
<bean id="extObj" class="ext.lib.ExtObj">
<constructor-arg value="SomeString"/>
</bean>
To define MyObj you can use constructor injection:
<bean id="myObj" class="me.MyObj">
<constructor-arg ref="extObj"/>
</bean>
or setter injection:
<bean name="myObj" class="me.MyObj">
<property name="extObj" ref="extObj"/>
</beans>
If you use setter injection then MyObj needs to have a setter setExtObj. If you use constructor injection MyObj needs to have a constructor taking an instance of the ExtObj class.

Of course you can inject a 3rd party library, as long as it has constructors that Spring can access.
You can use either XML or annotations - your choice.
You need to ask Spring to instantiate the instance(s) of the library class and then inject that into your objects that need them.
You do this every time you create a Spring data source that uses a JDBC driver. That's a 3rd party library.

Spring Config file consisting of List of Implementations

I Am very new to Spring. I have an Interface (MessageHandler ) which has a get method, this method returns a list of Implementations of another interface (messageChecker).
public interface MessageHandler {
public void process(BufferedReader br);
public void setMessageCheckerList(List mcList);
[B]public List getMessageCheckerList();[/B]
}
In my Spring XML configuration , i have something like this ,along with other beans
<bean id="messageHandler" class="com.XXX.messagereceiver.MessageHandlerImpl">
<property name="messageCheckerList" ref="checkerList"/>
</bean>
<bean id="checkerList" class="java.util.ArrayList">
<constructor-arg>
<list>
<ref bean="HL7Checker"/>
</list>
</constructor-arg>
</bean>
<bean id="HL7Checker" class="com.XXX.messagereceiver.HL7CheckerImpl">
<property name="messageExecutor" ref="kahootzExecutor"/>
</bean>
Here i am passing a checkerlist - which is a list of Implementations ( For now i have only 1) of the Interface (messageChecker)
Checkerlist is containing references to Bean Id's which are actual implementaions.
HL7Checker is an implementation of an Interface messageChecker.
But when i run the main program, When i inject the bean "messageHandler" and call the getMessageCheckerList, It returns a null value. These getter and setter methods are working fine without using spring.
I am not sure what seems to be the problem.

I don't know the answer for you troubles, but I would check:
is the setter setMessageCheckerList(List) in messageHandler bean called? (either using some debugger or some trace output like System.out...). If it's not, there's probably something wrong with your Spring XML configuration setup. The bean definition you posted requires the property to be set and Spring wouldn't create the messageHandler bean without setting the property.
who calls the setMessageCheckerList(List) setter? Or even more precise, what code writes to the field which stores the value of the property? Maybe the field is initialized properly by Spring but gets overwritten to null later on?
are you sure you call the getMessageCheckerList on the very same object Spring has configured for you (that is, the messageHandler bean). The definition you have posted clearly states an instance of MessageHandlerImpl is created by Spring, but it doesn't prevent other instances to be created in other ways. So maybe the instance created by Spring holds the proper value, but you run the get... on a wrong instance?

How to register handler interceptors with spring mvc 3.0?

It should be easy:
<bean id="handlerMapping"
class="org.springframework.web.servlet.mvc.annotation.DefaultAnnotationHandlerMapping">
<property name="interceptors">
<list>
<ref bean="myInterceptor" />
</list>
</property>
</bean>
but this way the interceptor isn't called.

By default, Spring will register a BeanNameUrlHandlerMapping, and a DefaultAnnotationHandlerMapping, without any explicit config required.
If you define your own HandlerMapping beans, then the default ones will not be registered, and you'll just get the explicitly declared ones.
So far, so good.
The problem comes when you add <mvc:annotation-driven/> to the mix. This also declares its own DefaultAnnotationHandlerMapping, which replaces the defaults. However, if you also declare your own one, then you end up with two. Since they are consulted in order of declaration, this usually means the one registered by <mvc:annotation-driven/> gets called first, and your own one gets ignored.
It would be better if the DefaultAnnotationHandlerMapping registered by <mvc:annotation-driven/> acted like the default one, i.e. if explicitly declared ones took precedence, but that's not the way they wrote it.
My current preference is to not use <mvc:annotation-driven/> at all, it's too confusing, and too unpredictable when mixed with other config options. It doesn't really do anything especially complex, it's not difficult or verbose to explicitly add the stuff that it does for you, and the end result is easier to follow.

Problem I faced: Spring MVC tag doesn't go well with custom definition of DefaultAnnotationHandlerMapping.
Why..? the reason is very well explained in the answers above.
Why i wanted to use DefaultAnnotationHandlerMapping? I want to define an interceptor for my every request. a Spring-Mobile interceptor to determine the USER AGENT..mobile or a browser?
Now Due to this clash of mvc-annotation and DefaultAnnotationHandlerMapping, I cant use DefaultAnnotationHandlerMapping anymore.
The problem comes down to how can i register my interceptors with tag.
The solution was simple...but hard to find. Posting it so it can be helpful to the other solution seekers..
Use tag and register the interceptor bean in your dispathcer-servlet.xml
example :
<mvc:interceptors>
<!-- This runs for all mappings -->
<bean class="main.com.XXX.MobileDeviceResolverHanlderInterceptor"/>
</mvc:interceptors>

The reason for this behaviour is that two beans of type org.springframework.web.servlet.mvc.annotation.DefaultAnnotationHandlerMapping exist in the application context. Spring resolves the two, but asks only the first for interceptors. To fix this, the following init parameter should be set to the DispatcherServlet
<init-param>
<param-name>detectAllHandlerMappings</param-name>
<param-value>false</param-value>
</init-param>
This makes the dispatcher servlet use only the handlerMapping defined in the x-servlet.xml
It is beyond me why this is the default behaviour. I'm expecting an answer from the spring community.

In my case I can NOT get rid of <mvc:annotation-driven/> as I am using jackson for json support using annotation.
What I tried, moved my all interceptors <mvc:interceptors> in separate "xml" file (interceptor-config.xml) and imported it from my x-dispatcher-servlet.xml
<import resource="interceptor-config.xml"/>
It solve my issue and avoid default 'DefaultAnnotationHandlerMapping' beans my application context.
Rather than creating separate 'xml', you can copy/paste interceptor contents directly in 'x-dispatcher-servlet.xml'.
Following is my interceptor:
<mvc:interceptors>
<mvc:interceptor>
<!-- Intercepting specific URL -->
<mvc:mapping path="/abc/**" />
<bean id= "myInterceptor"
class="xx.xxx.xxx.MyInterceptor" />
</mvc:interceptor>
<mvc:interceptors>

In Spring MVC 3.0 you can use <mvc:interceptors> instead of manual defining the handler mapping.