The Spring Framework is an application framework and inversion of control container for the Java platform. The framework’s core features can be used by any Java application, but there are extensions for building web applications on top of the Java EE (Enterprise Edition) platform. Although the framework does not impose any specific programming model, it has become popular in the Java community as an addition to the Enterprise JavaBeans (EJB) model. The Spring Framework is open source.
The Spring Framework includes several modules that provide a range of services:
- Spring Core Container: this is the base module of Spring and provides spring containers (Bean Factory and Application Context).
- Aspect-oriented programming: enables implementing cross-cutting concerns.
- Authentication and authorization: configurable security processes that support a range of standards, protocols, tools and practices via the Spring Security sub-project (formerly Acegi Security System for Spring).
- Convention over configuration: a rapid application development solution for Spring-based enterprise applications is offered in the Spring Roo module
- Data access: working with relational database management systems on the Java platform using Java Database Connectivity (JDBC) and object-relational mapping tools and with NoSQL databases
- Inversion of control container: configuration of application components and lifecycle management of Java objects, done mainly via dependency injection
- Messaging: configurative registration of message listener objects for transparent message-consumption from message queues via Java Message Service (JMS), improvement of message sending over standard JMS APIs
- Model–view–controller: an HTTP- and servlet-based framework providing hooks for extension and customization for web applications and RESTful (representational state transfer) Web services.
- Remote access framework: configurative remote procedure call (RPC)-style marshalling of Java objects over networks supporting Java remote method invocation (RMI), CORBA (Common Object Request Broker Architecture) and HTTP-based protocols including Web services (SOAP (Simple Object Access Protocol))
- Transaction management: unifies several transaction management APIs and coordinates transactions for Java objects
- Remote management: configurative exposure and management of Java objects for local or remote configuration via Java Management Extensions (JMX)
- Testing: support classes for writing unit tests and integration tests
Inversion of control container (dependency injection)
Central to the Spring Framework is its inversion of control (IoC) container, which provides a consistent means of configuring and managing Java objects using reflection. The container is responsible for managing object lifecycles of specific objects: creating these objects, calling their initialization methods, and configuring these objects by wiring them together.
Objects created by the container are also called managed objects or beans. The container can be configured by loading XML (Extensible Markup Language) files or detecting specific Java annotations on configuration classes. These data sources contain the bean definitions that provide the information required to create the beans.
Objects can be obtained by means of either dependency lookup or dependency injection. Dependency lookup is a pattern where a caller asks the container object for an object with a specific name or of a specific type. Dependency injection is a pattern where the container passes objects by name to other objects, via either constructors, properties, or factory methods.
In many cases one need not use the container when using other parts of the Spring Framework, although using it will likely make an application easier to configure and customize. The Spring container provides a consistent mechanism to configure applications and integrates with almost all Java environments, from small-scale applications to large enterprise applications.
The container can be turned into a partially compliant EJB (Enterprise JavaBeans) 3.0 container by means of the Pitchfork project. Some criticize the Spring Framework for not complying with standards. However, SpringSource doesn’t see EJB 3 compliance as a major goal, and claims that the Spring Framework and the container allow for more powerful programming models. The programmer does not directly create an object, but describe how they should be created, by defining it in the Spring configuration file. Similarly services and components are not called directly; instead a Spring configuration file defines which services and components must be called. This IoC is intended to increase the ease of maintenance and testing.
Aspect-oriented programming framework
The Spring Framework has its own Aspect-oriented programming (AOP) framework that modularizes cross-cutting concerns in aspects. The motivation for creating a separate AOP framework comes from the belief that it should be possible to provide basic AOP features without too much complexity in either design, implementation, or configuration. The Spring AOP framework also takes full advantage of the Spring container.
The Spring AOP framework is proxy pattern-based, and is configured at run time. This removes the need for a compilation step or load-time weaving. On the other hand, interception only allows for public method-execution on existing objects at a join point.
Compared to the AspectJ framework, Spring AOP is less powerful, but also less complicated. Spring 1.2 includes support to configure AspectJ aspects in the container. Spring 2.0 added more integration with AspectJ; for example, the pointcut language is reused and can be mixed with Spring AOP-based aspects. Further, Spring 2.0 added a Spring Aspects library that uses AspectJ to offer common Spring features such as declarative transaction management and dependency injection via AspectJ compile-time or load-time weaving. Spring Source also uses AspectJ AOP in other Spring projects such as Spring Roo and Spring Insight, with Spring Security also offering an AspectJ-based aspect library.
Spring AOP has been designed to make it able to work with cross-cutting concerns inside the Spring Framework. Any object which is created and configured by the container can be enriched using Spring AOP.
The Spring Framework uses Spring AOP internally for transaction management, security, remote access, and JMX.
Data access framework
Spring’s data access framework addresses common difficulties developers face when working with databases in applications. Support is provided for all popular data access frameworks in Java: JDBC, iBatis/MyBatis, Hibernate, Java Data Objects (JDO), Java Persistence API (JPA), Oracle TopLink, Apache OJB, and Apache Cayenne, among others.
For all of these supported frameworks, Spring provides these features
- Resource management – automatically acquiring and releasing database resources
- Exception handling – translating data access related exception to a Spring data access hierarchy
- Transaction participation – transparent participation in ongoing transactions
- Resource unwrapping – retrieving database objects from connection pool wrappers
- Abstraction for binary large object (BLOB) and character large object (CLOB) handling
All these features become available when using template classes provided by Spring for each supported framework. Critics have said these template classes are intrusive and offer no advantage over using (for example) the Hibernate API directly. In response, the Spring developers have made it possible to use the Hibernate and JPA APIs directly. This however requires transparent transaction management, as application code no longer assumes the responsibility to obtain and close database resources, and does not support exception translation.
Together with Spring’s transaction management, its data access framework offers a flexible abstraction for working with data access frameworks. The Spring Framework doesn’t offer a common data access API; instead, the full power of the supported APIs is kept intact. The Spring Framework is the only framework available in Java that offers managed data access environments outside of an application server or container.
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