[[overview]]
= Spring Framework Overview
:toc: left
:toclevels: 1
:docinfo1:

Spring makes it easy to create Java enterprise applications. It provides everything you
need to embrace the Java language in an enterprise environment, with support for Groovy
and Kotlin as alternative languages on the JVM, and with the flexibility to create many
kinds of architectures depending on an application's needs. As of Spring Framework 5.0,
Spring requires JDK 8+ (Java SE 8+) and provides out-of-the-box support for JDK 9 already.

Spring supports a wide range of application scenarios. In a large enterprise, applications
often exist for a long time and have to run on a JDK and application server whose upgrade
cycle is beyond developer control. Others may run as a single jar with the server embedded,
possibly in a cloud environment. Yet others may be standalone applications (such as batch
or integration workloads) that do not need a server.

Spring is open source. It has a large and active community that provides continuous feedback
based on a diverse range of real-world use cases. This has helped Spring to successfully
evolve over a very long time.




[[overview-spring]]
== What We Mean by "Spring"

The term "Spring" means different things in different contexts. It can be used to refer to
the Spring Framework project itself, which is where it all started. Over time, other Spring
projects have been built on top of the Spring Framework. Most often, when people say
"Spring", they mean the entire family of projects. This reference documentation focuses on
the foundation: the Spring Framework itself.

The Spring Framework is divided into modules. Applications can choose which modules they need.
At the heart are the modules of the core container, including a configuration model and a
dependency injection mechanism. Beyond that, the Spring Framework provides foundational
support for different application architectures, including messaging, transactional data and
persistence, and web. It also includes the Servlet-based Spring MVC web framework and, in
parallel, the Spring WebFlux reactive web framework.

A note about modules: Spring's framework jars allow for deployment to JDK 9's module path
("Jigsaw"). For use in Jigsaw-enabled applications, the Spring Framework 5 jars come with
"Automatic-Module-Name" manifest entries which define stable language-level module names
("spring.core", "spring.context" etc) independent from jar artifact names (the jars follow
the same naming pattern with "-" instead of ".", e.g. "spring-core" and "spring-context").
Of course, Spring's framework jars keep working fine on the classpath on both JDK 8 and 9.




[[overview-history]]
== History of Spring and the Spring Framework

Spring came into being in 2003 as a response to the complexity of the early
https://en.wikipedia.org/wiki/Java_Platform,_Enterprise_Edition[J2EE] specifications.
While some consider Java EE and Spring to be in competition, Spring is, in fact, complementary
to Java EE. The Spring programming model does not embrace the Java EE platform specification;
rather, it integrates with carefully selected individual specifications from the EE umbrella:

* Servlet API (https://jcp.org/en/jsr/detail?id=340[JSR 340])
* WebSocket API (https://www.jcp.org/en/jsr/detail?id=356[JSR 356])
* Concurrency Utilities (https://www.jcp.org/en/jsr/detail?id=236[JSR 236])
* JSON Binding API (https://jcp.org/en/jsr/detail?id=367[JSR 367])
* Bean Validation (https://jcp.org/en/jsr/detail?id=303[JSR 303])
* JPA (https://jcp.org/en/jsr/detail?id=338[JSR 338])
* JMS (https://jcp.org/en/jsr/detail?id=914[JSR 914])
* as well as JTA/JCA setups for transaction coordination, if necessary.

The Spring Framework also supports the Dependency Injection
(https://www.jcp.org/en/jsr/detail?id=330[JSR 330]) and Common Annotations
(https://jcp.org/en/jsr/detail?id=250[JSR 250]) specifications, which application developers
may choose to use instead of the Spring-specific mechanisms provided by the Spring Framework.

As of Spring Framework 5.0, Spring requires the Java EE 7 level (e.g. Servlet 3.1+, JPA 2.1+)
as a minimum - while at the same time providing out-of-the-box integration with newer APIs
at the Java EE 8 level (e.g. Servlet 4.0, JSON Binding API) when encountered at runtime.
This keeps Spring fully compatible with e.g. Tomcat 8 and 9, WebSphere 9, and JBoss EAP 7.

Over time, the role of Java EE in application development has evolved. In the early days of
Java EE and Spring, applications were created to be deployed to an application server.
Today, with the help of Spring Boot, applications are created in a devops- and
cloud-friendly way, with the Servlet container embedded and trivial to change.
As of Spring Framework 5, a WebFlux application does not even use the Servlet API directly
and can run on servers (such as Netty) that are not Servlet containers.

Spring continues to innovate and to evolve. Beyond the Spring Framework, there are other
projects, such as Spring Boot, Spring Security, Spring Data, Spring Cloud, Spring Batch,
among others. It’s important to remember that each project has its own source code repository,
issue tracker, and release cadence. See https://spring.io/projects[spring.io/projects] for
the complete list of Spring projects.




[[overview-philosophy]]
== Design Philosophy

When you learn about a framework, it’s important to know not only what it does but what
principles it follows. Here are the guiding principles of the Spring Framework:

* Provide choice at every level. Spring lets you defer design decisions as late as possible.
For example, you can switch persistence providers through configuration without changing
your code. The same is true for many other infrastructure concerns and integration with
third-party APIs.
* Accommodate diverse perspectives. Spring embraces flexibility and is not opinionated
about how things should be done. It supports a wide range of application needs with
different perspectives.
* Maintain strong backward compatibility. Spring’s evolution has been carefully managed
to force few breaking changes between versions. Spring supports a carefully chosen range
of JDK versions and third-party libraries to facilitate maintenance of applications and
libraries that depend on Spring.
* Care about API design. The Spring team puts a lot of thought and time into making APIs
that are intuitive and that hold up across many versions and many years.
* Set high standards for code quality. The Spring Framework puts a strong emphasis on
meaningful, current, and accurate javadoc. It is one of very few projects that can claim
clean code structure with  no circular dependencies between packages.




[[overview-feedback]]
== Feedback and Contributions

For how-to questions or diagnosing or debugging issues, we suggest using StackOverflow,
and we have a https://spring.io/questions[questions page] that lists the suggested tags to use.
If you're fairly certain that there is a problem in the Spring Framework or would like
to suggest a feature, please use the https://github.com/spring-projects/spring-framework/issues[GitHub Issues].

If you have a solution in mind or a suggested fix, you can submit a pull request on
https://github.com/spring-projects/spring-framework[Github]. However, please keep in mind
that, for all but the most trivial issues, we expect a ticket to be filed in the issue
tracker, where discussions take place and leave a record for future reference.

For more details see the guidelines at the
https://github.com/spring-projects/spring-framework/blob/master/CONTRIBUTING.md[CONTRIBUTING],
top-level project page.




[[overview-getting-started]]
== Getting Started

If you are just getting started with Spring, you may want to begin using the Spring
Framework by creating a http://projects.spring.io/spring-boot/[Spring Boot]-based
application. Spring Boot provides a quick (and opinionated) way to create a
production-ready Spring-based application. It is based on the Spring Framework, favors
convention over configuration, and is designed to get you up and running as quickly
as possible.

You can use http://start.spring.io/[start.spring.io] to generate a basic project or follow
one of the https://spring.io/guides["Getting Started" guides], such as
https://spring.io/guides/gs/rest-service/[Getting Started Building a RESTful Web Service].
As well as being easier to digest, these guides are very task focused, and most of them
are based on Spring Boot. They also cover other projects from the Spring portfolio that
you might want to consider when solving a particular problem.
