Java Spring multiple ApplicationContext

By container they refer to the core spring Inversion of Control container. The container provides a way to initialize/bootstrap your application (loading the configuration in xml files or annotations), through use of reflection, and to manage the lifecycle of Java objects, which are called beans or managed objects.

During this initial phase, you do not have (normally, yet it is possible) control in your application, instead you will get a completely initialized state for the application when the bootstrapping is done (or nothing, in case it fails).

It is a replacement, or possibly an addition, to what is called an EJB3 container; yet, the spring provided one fails to adhere to the EJB defined standard. Historically, adoption of EJB has been limited by the complexity of that specification, with spring being a newly created project for having EJB3 comparable features running on a J2SE jvm and without an EJB container, and with much easier configuration.

ApplicationContext (as an interface, and by the direct implementation flavours) is the mean of implementing this IoC container, as opposed to the BeanFactory, which is now (a sparsely used and) more direct way of managing beans, which by the way provides the base implementation features for the ApplicationContext.

As per your second question, you can have multiple instances of ApplicationContexts, in that case, they will be completely isolated, each with its own configuration.

First you questions :

1) I keep seeing the book mentioned "container", what is the container refer to? One container does it mean one java process? or one container refer to one ApplicationContext object?

The ApplicationContext is the central interface, but the underlying container is a BeanFactory. More exactly, BeanFactory is a lower level interface implemented by all Application contextes from which you get the Beans. In that sense, I think that the word container stands here for the BeanFactory - one per ApplicationContext.

2) If i instantiate two ApplicationContext in one java application (one Main body), are these two interface to one central container? Or two separate different instance? See the code below, what is the difference between context1 and context2? If there is a Singleton in Beans.xml, it is invoked by context1 and context2, are they two separated instance or same instance?

ApplicationContext context1 = new ClassPathXmlApplicationContext("Beans.xml"); ApplicationContext context2 = new ClassPathXmlApplicationContext("Beans.xml");>

With that instanciations, you will get 2 totally independent application contexts. One bean declared in first will not be found in the other.

BUT

It is common to have more than one application context in a Web application, because Spring has a notion of hierachies of ApplicationContext. You could declare them as :

ApplicationContext context1 = new ClassPathXmlApplicationContext("Beans.xml");
ApplicationContext context2 = new ClassPathXmlApplicationContext("Beans.xml", context1);>

Here you can retrieve from context1 only beans declared in it, but from context2 you will retrieve beans from context2 and context1. Specifically, beans are first looked for in context2 and if not found then looked for in context1.

This is used in Spring MVC where you normally have one root context (for all beans not directly related to the MVC DispatcherServlet) and one child context dedicated to the DispatcherServlet that will contain the beans for controllers, views, interceptors, etc.

I keep seeing the book mentioned "container", what is the container refer to? One container does it mean one java process? or one container refer to one ApplicationContext object?

Here container means spring container which is nothing but ApplicationContext. This internally reads spring configuration and loads the classes based on configuration. You may think it as SpringProcessor which provides the various functionality like bean initialization, injection, i18n, bean Post processing etc off the shelf

with

ApplicationContext context1 = new ClassPathXmlApplicationContext("Beans.xml"); ApplicationContext context2 = new ClassPathXmlApplicationContext("Beans.xml");

There will be two conatiners , hence two singleton beans. Here singleton means singleton instance per container. Ideally you should have only one container until and unless you have a reason for two. For learning purpose it makes sense to understand the concepts