Just Spring

Chapter 4. Advanced Concepts

The Spring framework comes with lots of bells and whistles. There are at times we wish to inject Java Collections such as Maps or Lists. There are also instances we wish to alter the bean configurations for our own customizations . Configuring such collections outside of the application in a properties file or customizing the beans to our own needs is easily done in Spring. While we learned the basics in the first three chapters, we discuss the more mature and advanced concepts in this chapter.

Bean Scopes

Did you wonder how many instances will be created when the Spring’s container is loads up the bean config file during the application startup? Do we get the same instance whenever you query the container for the same bean? If we have a case where one and only one bean (such as a service or a factory) is to be created irrespective of the number of times you call the container, how is this achieved? Or for every call, how can we fetch a brand new instance? How does Spring achieve this?

Well, it turns out to be a simple config tag that dictates these types—the scope tag with values of singleton and prototype.

Singleton Scope

When you need one and only one instance of a bean (TrainFactory in this case), you should set the scope tag to singleton, as shown here:

<bean name="trainFactory" scope="singleton"
     class="com.madhusudhan.jscore.fundamentals.scope.TrainFactory">
</bean>

However, the default scope is always singleton. Hence, you can ignore the setting of ...