The core container is the basis of the whole Spring ecosystem and comprehends four components—core, beans, context, and expression language.

Core and beans are responsible for providing the fundamentals of the framework and dependency injection. These modules are responsible for managing the IoC container, and the principal functions are the instantiation, configuration, and destruction of the object residents in the Spring container.

There are two critical interfaces inside these modules—BeanFactory and ApplicationContext. The BeanFactory takes care of the bean lifecycle, instantiating, configuring, managing, and destroying, and the ApplicationContext helps developers to work with files resources in a generic way, enable to publish events to registered listeners. Also, the ApplicationContext supports internationalization and has the ability to work with messages in different Locales.  

These modules help the context component to provide a way to access the objects inside the container. The context component has the ApplicationContext interface with the essential class for the container.

Spring Framework supports a wide range of messaging systems. The Java platform is recognized as providing excellent support for messaging applications, and Spring Framework follows this approach and offers a variety of projects to help developers to write powerful applications with more productivity and fewer lines of infrastructure code. The basic idea of these projects is to provide some template classes that have the convenience methods to interact with the messaging systems.

Also, the project supplies some listener annotations to provide support for listening to messages from the brokers. The framework maintains the standard for different projects. In general, the prefix of the annotations is the name of the messaging system, for example, @KafkaListener.

The framework supplies many abstractions to create messaging applications in a generic way. This is interesting stuff because the application requirements change during the application lifecycle and the message broker solution may change as well. Then, with small changes, the application built with the Spring message module can work in different brokers. This is the goal.

This module is the first one built by the Spring Team to support the web applications in Spring Framework. This module uses the Servlet API as its foundation, and then these web applications must follow the Servlet Specification and be deployed into servlet containers. In version 5.0, the Spring Team created a Reactive web framework, which will be covered later in this book.

The Spring Web MVC module was developed using the front controller pattern. When the framework was created, this pattern was a common choice for many frameworks, such as Struts and JSF, among others. Under the hood, there is the main servlet in Spring called DispatcherServlet. This servlet will redirect through an algorithm to do the desired work.

It enables developers to create amazing web applications on the Java platform. This portion of the framework provides full support to develop this kind of application. There are some interesting features for this purpose, such as support for internationalization and support for handling cookies. Also, multipart requests are an exciting feature for when the application needs to handle upload files and support routing requests. 

These characteristics are common for most web applications, and the framework has excellent support for these features. This support makes the framework a good choice for this kind of application. In Chapter 2, Starting in the Spring World - The CMS Application, we will create an application using this module and the main features will be explored in depth.

The module has full support for annotation programming since to declare HTTP endpoints until to wrap the request attribute in an HTTP request. It makes the application extremely readable without the boilerplate code to get the request parameter, for example.

Web application-wise, it enables developers to work with robust template engines such as Thymeleaf and Freemarker. It is entirely integrated with routing features and bean validation.

Also, the framework allows developers to build REST APIs with this module. Given all of this support, the module has become a favorite in the Spring ecosystem. Developers have started to create APIs with this stack, and some important companies have started to use it, especially given that the framework provides an easy way to navigate through the annotations. Because of this, the Spring Team added the new annotation @RestController in version 4.0.

We will work a lot with this module. Chapter by chapter, we will learn interesting things about this part of the framework.

A new module introduced in Spring 5.0, Spring WebFlux, can be used to implement web applications built with Reactive Streams. These systems have nonblocking characteristics and are deployed in servers built on top of Netty, such as Undertown and servlet containers that support + 3.1.

This module supports annotations based on Spring MVC modules, such as @GetMapping, @PostMapping, and others. This is an important feature that enables us to migrate to this new version. Of course, some adjustments are necessary, such as adding Reactor classes (Mono or Flux).

This module meets the modern web requirements to handle a lot of concurrent channels where the thread-per-request model is not an option.

We will learn about this module in Chapter 3, Adding Persistence with Spring Data and Putting it into Reactive Fashion and implement a fully Reactive application based on Reactive Streams.

Spring Data is an interesting module that provides the easiest way to manage application data with Spring-based programming. The project is an umbrella project, with subprojects to support different databases technologies, even relational and nonrelational databases. The Spring Team supports some databases technologies, such as Apache Cassandra, Apache Solr, Redis, and JPA Specification, and the community maintains the other exciting projects, such as ElasticSearch, Aerospike, DynamoDb, and Couchbase. The full list of projects can be found at http://projects.spring.io/spring-data.

The goal is to remove the boilerplate code from the persistence code. In general, the data access layer is quite similar, even in different projects, differing only in the project model, and Spring Data provides a powerful way to map the domain model and repository abstraction.

There are some central interfaces; they're a kind of marker to instruct the framework to choose the correct implementation. Under the hood, Spring will create a proxy and delegate the correct implementation. The amazing thing here is that developers don't have to write any persistence code and then take care of this code; they simply choose the required technology and Spring takes care of the rest.

The central interfaces are CrudRepository and PagingAndSortingRepository, and their names are self-explanatory. CrudRepository implements the CRUD behaviors, such as create, retrieval, update, and delete. PagingAndSortingRepository is an extension of CrudRepository and adds some features such as paging and sorting. Usually, we will find derivations of these interfaces such as MongoRepository, which interacts with MongoDB database technology.

Security for Java applications was always a pain for developers, especially in Java Enterprise Edition. There was a lot of boilerplate code to look up objects in the application servers, and the security layer was often heavily customized for the application.

In that chaotic scenario, the Spring Team decided to create a Spring Security project to help developers handle the security layer on the Java application.

In the beginning, the project had extensive support for Java Enterprise Edition and integration with EJB 3 security annotations. Nowadays, the project supports many different ways to handle authorization and authentication for Java applications.

Spring Security provides a comprehensive model to add authorization and authentication for Java applications. The framework can be configured with a couple of annotations, which makes the task of adding a security layer extremely easy. The other important characteristics concern how the framework can be extended. There are some interfaces that enable developers to customize the default framework behaviors, and it makes the framework customized for different application requirements.

It is an umbrella project, and it is subdivided into these modules:

These are the main modules, and there are many other projects to support a wide range of types of authentication. The module covers the following authentication and authorization types:

The module also offers a domain-specific language (DSL) to provide an easy configuration. Let's see a simple example:

http
  .formLogin()
    .loginPage("/login")
     .failureUrl("/login?error")
      .and()
    .authorizeRequests()
      .antMatchers("/signup","/about").permitAll()
      .antMatchers("/admin/**").hasRole("ADMIN")
      .anyRequest().authenticated();

As we can see, the DSL makes the configuration task extremely easy and very understandable.

Spring Security's main features are as follows:

We will learn more about Spring Security in Chapter 8, Circuit Breakers and Security. We will also put it into practice.

Spring Cloud Netflix is perhaps the most popular Spring module nowadays. This fantastic project allows us to integrate the Spring ecosystem with the Netflix OSS via Spring Boot AutoConfiguration features. The supported Netflix OSS libraries are Eureka for service discovery, Ribbon to enable client-side load balancing, circuit breaker via Hystrix to protect our application from external outages and make the system resilient, the Zuul component provides an intelligent routing and can act as an edge service. Finally, the Feign component can help developers to create HTTP clients for REST APIs with a couple of annotations.

Let's look at each of these:

This exciting project provides an easy way to manage system configurations for distributed systems, and this is a critical issue in cloud environments because the file system is ephemeral. It also helps us to maintain different stages of the deployment pipeline. Spring profiles are fully integrated with this module.

We will need an application that will provide the configuration for other applications. We can understand its workings by thinking of the concepts of the server and the client, the server will provide some configurations through HTTP and the client will look up the configuration on the server. Also, it is possible to encrypt and decrypt property values.

There are some storage implementations to provide these property files, and the default implementation is Git. It enables us to store our property files in Git, or we can use the file system as well. The important thing here is that the source does not matter.

There is an interesting integration between Spring Cloud Bus and this module. If they are integrated, it is possible to broadcast the configuration changes on the cluster. This is an important feature if the application configuration changes with frequency. There are two annotations that tell Spring to apply changes at runtime: @RefreshScope and @ConfigurationProperties.

In Chapter 7, Airline Ticket System, we will implement an exciting service to provide external configurations for our microservices using this module. Server concepts will be explained in more detail. The client details will be presented as well.

Spring Cloud Consul provides integrations with Hashicorp's Consul. This tool addresses problems in the same way as service discovery, a distributed configuration, and control bus. This module allows us to configure Spring applications and Consul with a few annotations in a Spring-based programming model. Autoconfiguration is supported as well. The amazing thing here is that this module can be integrated with some Netflix OSS libraries, such as Zuul and Ribbon, via Spring Cloud Zuul and Spring Cloud Ribbon respectively (for example).

This module is like an extension from Spring Security. However, distributed systems have different requirements for security. Normally, they have central identity management, or the authentication lies with the clients in the case of REST APIs. Normally, in distributed systems, we have microservices, and these services might have more than one instance in the runtime environment whose characteristics make the authentication module slightly different from monolithic applications. The module can be used together with Spring Boot applications and makes the OAuth2 implementation very easy with a couple of annotations and a few configurations. Also, some common patterns are supported, such as single sign-on, token relay, and token exchange.

For the microservice applications based on the Spring Cloud Netflix, it is particularly interesting because it enables downstream authentication to work with a Zuul proxy and offers support from Feign clients. An interceptor is used to fetch tokens.

The main goal of this project is to provide an easy way to broadcast changes spread throughout the cluster. The applications can connect the distributed system nodes through the message broker.

It provides an easy way for developers to create a publish and subscribe mechanism using the ApplicationContext provided by Spring Container. It enables the possibility to create applications using the event-driven architecture style with the Spring Ecosystem.

To create custom events, we need to create a child class from RemoteApplicationEvent and mark the class to be scanned via @RemoteApplicationEventScan.

The projects support three message brokers as the transport layer:

The idea behind this module is to provide an easy way to build message-driven microservices. The module has an opinionated way of configuration. It means we need to follow some rules to create these configurations. In general, the application is configured by the yaml|properties file.

The module supports annotations as well. This means that a couple of annotations are enough to create consumers, producers, and bindings; it decouples the application and makes it easy to understand. It supplies some abstractions around the message brokers and channels, and it makes the developer's life more comfortable and productive as well.

Spring Cloud Stream has Binder implementations for RabbitMQ and Kafka.

This module supports a lot of Enterprise Application patterns and brings the Spring programming model to this topic. The Spring programming model enables extensive dependence injection support and is annotations programming-centric. The annotations instruct us as to how the framework needs to be configured and defines framework behaviors.

The POJO model is suggested because it is simple and widely known in the Java development world.

This project has some intersections with the other modules. Some other projects use these module concepts to do their work. There is a project called Spring Cloud Stream, for instance.

The Enterprise Integration patterns are based on a wide range of communication channels, protocols, and patterns. This project supports some of these.

The modules support a variety of features and channels, such as the following:

There are three main concepts of Enterprise application integration:

Finally, the Spring Integration module offers a comprehensive way to create application integration and enables developers to do it using amazing support.