Java Spring multiple ApplicationContext

The definition of the spring `ApplicationContext` is very ambiguous, I almost finish a whole book of tutorial but still cannot understand what is the `ApplicationContext` stand for.

According the Spring API, `ApplicationContext` is:

> - Central interface to provide configuration for an application. This is read-only while the application is running, but may be reloaded if
> the implementation supports this.
>
> - The root interface for accessing a Spring bean container. This is the basic client view of a bean container.


From above, my questions are:

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?

2) If i instantiate two `ApplicationContext` in one java application (both in `main` body), are these two interfaces to one central container? Or two separate instances? 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 instances or the same instance?


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

You added a "java-ee" tag. Spring is often used in web applications running on a application server. Typically each web application would have it's own application. The web applications are separated and probably that is what is called container in the documentation as you cannot regularly share variables with different applications / containers.

You can have two contexts within an application. If you have two contexts each will have its own singleton.

> 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

ApplicationContext is an implementation of a Spring container. In simple words Spring container manages the entire Spring application via an ApplicationContext. Spring container via ApplicationContext manages the life cycle of a Spring bean i;e from initiation to destruction.

A spring container is nested inside a J2EE container.

> 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?

A container manages the life cycle of an object. Tomcat is a an example of a container. Just like Spring container manages the app via ApplicationContext a J2EE container Tomcat manages the app via web.xml

A container provides communications support. Security in a web application. JSP support, Internationalization, event-propagation & many other features. It supports multithreading, spawns a new thread for every request for a resource. You don't have to explicitly write the code for that. Just like a spring container, a J2ee container manages a servlet life cycle.

> If i instantiate two ApplicationContext in one java application (one
> Main body), are these two interface to one central container?

If you want to instantiate multiple ApplicationContexts in your application. It will be in a parent child hierarchy. There will be one root ApplicationContext & then there will be child ApplicationContext respective to every DispatcherServlet. Beans global to the application will be defined in the root ApplicationContext. All the ApplicationContexts will be managed by only one spring container.

By container they refer to the core spring [Inversion of Control container][1]. The container provides a way to initialize/bootstrap your application (loading the configuration in xml files or annotations), through use of [reflection][2], 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][3], 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][4] 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.


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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.

1. container is a Java object, an instance of one of `ApplicationContext` implementations like `ClassPathXmlApplicationContext`.

2. It is 2 different containers, if `Beans.xml` has a singleton bean B1, then `context1.getBean("B1")` and `context2.getBean("B1")` will return 2 different instances of `B1`

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

**In Spring, the objects that form the backbone of your application and that are managed by the Spring IoC container are called beans. A bean is an object that is instantiated, assembled, and otherwise managed by a Spring IoC container.**

**The interface org.springframework.context.ApplicationContext represents the Spring IoC container. The container gets details or instructions i.e. what objects need to be instantiated, configure and assembly by reading configuring metadata.**

Application classes are combined with configuration metadata so that after the ApplicationContext is created and initialized, you have a fully configured and executable system or application.



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?

As mentioned by others as well you can have multiple application contexts.