Spring Framework

Inversion of control container

The inversion of control (IoC) container is the core container in the Spring Framework.^[2] It provides a consistent means of configuring and managing Java objects^{[2][5]: 127–131} using reflection.^[50] The container is responsible for managing object lifecycles of specific objects:^[5]: 128 creating these objects,^[51] calling their initialization methods,^[50] and configuring these objects by wiring them together.^[52]

In multiple 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^[5]: 122 and integrates with almost all Java environments, from small-scale applications to large enterprise applications.

The programmer does not directly create an object, but describes how it 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.^[62] The motivation for creating a separate AOP framework is to provide basic AOP features without too much complexity in either design, implementation, or configuration. The Spring AOP framework takes full advantage of the Spring container.

The Spring AOP framework is proxy pattern-based.^[63][25] It is configured at run time.^{[citation needed]} This removes the need for a compilation step or load-time weaving.^{[citation needed]} On the other hand, interception only allows for public method-execution on existing objects at a join point.^{[citation needed]}

Compared to the AspectJ framework, Spring AOP is less powerful, but also less complicated.^{[citation needed]} 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.^{[citation needed]} Further, Spring 2.0 added a Spring Aspects library that uses AspectJ to offer common Spring features such as declarative transaction management^[63] and dependency injection via AspectJ compile-time or load-time weaving.^[64] SpringSource uses AspectJ AOP in other Spring projects such as Spring Roo and Spring Insight, with Spring Security offering an AspectJ-based aspect library.^{[citation needed]}

Spring AOP was designed to work with cross-cutting concerns inside the Spring Framework.^[5]: 473 Any object which is created and configured by the container can be enriched using Spring AOP.

Since version 2.0 of the framework, Spring provides two approaches to the AOP configuration:

• schema-based approach^{[65][better source needed]} and
• @AspectJ-based annotation style.^{[66][better source needed]}

<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:mvc="http://www.springframework.org/schema/mvc" 
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:aop="http://www.springframework.org/schema/aop" 
    xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/context
        http://www.springframework.org/schema/context/spring-context.xsd
        http://www.springframework.org/schema/mvc
        http://www.springframework.org/schema/mvc/spring-mvc.xsd
        http://www.springframework.org/schema/aop 
        http://www.springframework.org/schema/aop/spring-aop.xsd">

The Spring team decided not to introduce new AOP-related terminology. Therefore, in the Spring reference documentation and API, terms such as aspect, join point, advice, pointcut, introduction, target object (advised object), AOP proxy, and weaving all have the same meanings^{[citation needed]} as in most other AOP frameworks (particularly AspectJ).

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,^[33] Hibernate,^[33] Java Data Objects (JDO, discontinued since 5.x),^[33] Jakarta Persistence API (JPA),^[33] 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^[67]
• Transaction participation – transparent participation in ongoing transactions^{[5]: 290–291}
• 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.^[68] Critics have said these template classes are intrusive and offer no advantage over using (for example) the Hibernate API directly.^{[69][failed verification]} 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,^[70] and does not support exception translation.^[71]

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.^{[citation needed]} The Spring Framework is the only framework available in Java that offers managed data access environments outside of an application server or container.^{[72][better source needed]}

While using Spring for transaction management with Hibernate, the following beans may have to be configured:

• A Datasource like com.mchange.v2.c3p0.ComboPooledDataSource or org.apache.commons.dbcp.BasicDataSource^[33]
• A SessionFactory like org.springframework.orm.hibernate3.LocalSessionFactoryBean with a DataSource attribute^{[73][5]: 173}
• A HibernateProperties^[5]: 173 like org.springframework.beans.factory.config.PropertiesFactoryBean
• A TransactionManager like org.springframework.orm.hibernate3.HibernateTransactionManager with a SessionFactory attribute^[73]

Other points of configuration include:

• An AOP configuration of cutting points.
• Transaction semantics of AOP advice^{[clarification needed]}.

Transaction management

Spring's transaction management framework brings an abstraction mechanism to the Java platform.^[74] Its abstraction is capable of:

• working with local and global transactions^[5]: 258 (local transaction does not require an application server)
• working with nested transactions^[75]
• working with savepoints^[75]
• working in almost all environments of the Java platform

In comparison, Java Transaction API (JTA) only supports nested transactions and global transactions, and requires an application server (and in some cases, deployment of applications in an application server).

The Spring Framework ships a PlatformTransactionManager^[76] for a number of transaction management strategies:

• Transactions managed on a JDBC Connection^[74]
• Transactions managed on Object-relational mapping Units of Work^[74]
• Transactions managed via the JTA^[74]JtaTransactionManager^{[77][5]: 255–257} and UserTransaction^[5]: 234
• Transactions managed on other resources, like object databases

Next to this abstraction mechanism the framework provides two ways of adding transaction management to applications:

• Procedurally, by using Spring's TransactionTemplate^[78]
• Declaratively, by using metadata like XML or Java annotations (@Transactional,^[63] etc.)

Together with Spring's data access framework – which integrates the transaction management framework – it is possible to set up a transactional system through configuration without having to rely on JTA or EJB. The transactional framework also integrates with messaging^[79] and caching^[80] engines.

Model–view–controller framework

The Spring Framework features its own model–view–controller (MVC) web application framework,^[36] which was not originally planned. The Spring developers decided to write their own Web framework as a reaction to what they perceived as the poor design of the (then) popular Jakarta Struts Web framework,^{[81][failed verification]} as well as deficiencies in other available frameworks. In particular, they felt there was insufficient separation between the presentation and request handling layers, and between the request handling layer and the model.^[82]

Like Struts, Spring MVC is a request-based framework.^[5]: 375 The framework defines strategy interfaces^[5]: 144 for all of the responsibilities that must be handled by a modern request-based framework. The goal of each interface is to be simple and clear so that it's easy for Spring MVC users to write their own implementations, if they so choose. MVC paves the way for cleaner front end code. All interfaces are tightly coupled to the Servlet API. This tight coupling to the Servlet API is seen by some as a failure on the part of the Spring developers to offer a high level of abstraction for Web-based applications.^{[citation needed]} However, this coupling ensures that the features of the Servlet API remain available to developers while offering a high abstraction framework to ease working with it.

The DispatcherServlet class is the front controller^[83] of the framework and is responsible for delegating control to the various interfaces during the execution phases of an HTTP request.^[84]

The most important interfaces defined by Spring MVC, and their responsibilities, are listed below:^[85]

• Controller: comes between Model and View to manage incoming requests and redirect to proper response.^[86] Controller will map the http request to corresponding methods.^[87] It acts as a gate that directs the incoming information. It switches between going into Model or View.
• HandlerAdapter: responsible for execution of objects that handle incoming requests.^[88]
• HandlerInterceptor: responsible for intercepting incoming requests.^[88] Comparable, but not equal to Servlet filters^[5]: 509 (use is optional^[5]: 511 and not controlled by DispatcherServlet).
• HandlerMapping: responsible for selecting objects that handle incoming requests (handlers) based on any attribute or condition internal or external to those requests^[84]
• LocaleResolver: responsible for resolving and optionally saving of the locale of an individual user.^[89]
• MultipartResolver: facilitate working with file uploads by wrapping incoming requests.^[90]
• View: responsible for returning a response to the client. The View should not contain any business logic and should only present the data encapsulated by the Model.^[36] Some requests may go straight to View without going to the Model part; others may go through all three.
• ViewResolver: responsible for selecting a View based on a logical name for the View^[91][92] (use is not strictly required^[5]: 511).
• Model: responsible for encapsulating business data.^[91] The Model is exposed to the view by the controller.^[5]: 374 (use is not strictly required).

Each strategy interface above has an important responsibility in the overall framework. The abstractions offered by these interfaces are powerful, so to allow for a set of variations in their implementations.^[5]: 144 Spring MVC ships with implementations of all these interfaces and offers a feature set on top of the Servlet API. However, developers and vendors are free to write other implementations. Spring MVC uses the Java java.util.Map interface as a data-oriented abstraction for the Model where keys are expected to be String values.^{[citation needed]}

The ease of testing the implementations of these interfaces is one important advantage of the high level of abstraction offered by Spring MVC.^{[93][5]: 324} DispatcherServlet is tightly coupled to the Spring inversion of control container for configuring the web layers of applications. However, web applications can use other parts of the Spring Framework, including the container, and choose not to use Spring MVC.

<servlet>
  <servlet-name>MyServlet</servlet-name>
  <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
</servlet>

<servlet-mapping>
  <servlet-name>MyServlet</servlet-name>
  <url-pattern>/<url-pattern>
</servlet-mapping>

Remote access framework

Spring's Remote Access framework is an abstraction for working with various RPC (remote procedure call)-based technologies available on the Java platform both for client connectivity and marshalling objects on servers.^[97] The most important feature offered by this framework is to ease configuration and usage of these technologies as much as possible by combining inversion of control and AOP.

The framework provides fault-recovery (automatic reconnection after connection failure) and some optimizations for client-side use of EJB remote stateless session beans.

Spring provides support for these protocols and products out of the box

• HTTP-based protocols
  • Hessian: binary serialization protocol,^{[98][5]: 335} open-sourced^[5]: 335 and maintained by CORBA-based protocols^{[citation needed]}. Hessian is maintained by the company Caucho.^[5]: 335 Hessian is suitable for stateless remoting needs, in particular, Java-to-Java communication.^{[5]: 335–336}
  • Burlap: An XML-based binary protocol that is open-sourced and also maintained by the company Caucho.^{[98][5]: 335} The only advantage of using Burlap instead of Hessian is that it is XML-parsable and human readable.^[5]: 335 For Java-to-Java communication, the Hessian is preferred since it is more light-weight and efficient.^[5]: 335
  • RMI (1): method invocations using RMI infrastructure yet specific to Spring^[97]
  • RMI (2): method invocations using RMI interfaces complying with regular RMI usage^[97]
  • RMI-IIOP (CORBA): method invocations using RMI-IIOP/CORBA
• Enterprise JavaBean client integration^[99]
  • Local EJB stateless session bean connectivity: connecting to local stateless session beans
  • Remote EJB stateless session bean connectivity: connecting to remote stateless session beans
• SOAP
  • Integration with the Apache Axis Web services framework^[100]

Apache CXF provides integration with the Spring Framework for RPC-style exporting of objects on the server side.^[100]

Both client and server setup for all RPC-style protocols and products supported by the Spring Remote access framework (except for the Apache Axis support) is configured in the Spring Core container.

There is an alternative open-source implementation (Cluster4Spring) of a remoting subsystem included in the Spring Framework that is intended to support various schemes of remoting (1-1, 1-many, dynamic services discovering).^{[citation needed]}

Convention-over-configuration rapid application development

Batch framework

Spring Batch is a framework for batch processing that provides reusable functions that are essential in processing large volumes of records, including:

• logging/tracing
• transaction management
• job processing statistics^[112]
• job restart

It provides more advanced technical services and features that enables extremely high-volume^[113] and high-performance batch jobs^[112] through optimizations and partitioning^[112] techniques.

Spring Batch executes a series of jobs; a job consists of a number of steps and each step consists of a "READ-PROCESS-WRITE" task or single operation task (tasklet). A "single" operation task is also known as a tasklet.^[114] It means doing a single task only, like cleaning up the resources before or after a step is started or completed.

The "READ-PROCESS-WRITE" process consists of these steps: "read" data from a resource (comma-separated values (CSV), XML, or database), "process" it, then "write" it to other resources (CSV, XML, or database). For example, a step may read data from a CSV file,^[114] process it, and write it into the database. Spring Batch provides a number of classes to read/write CSV, XML, and database.^[115]

The steps can be chained together to run as a job.^[114]

Integration framework

Spring Integration is a framework for Enterprise application integration that provides reusable functions essential to messaging or event-driven architectures.

• routers – routes a message to a message channel based on conditions^[116]
• transformers – converts/transforms/changes the message payload and creates a new message with transformed payload^[117]
• adapters – integrates with other technologies and systems (HTTP, AMQP (Advanced Message Queuing Protocol),^[118] JMS (Java Message Service), XMPP (Extensible Messaging and Presence Protocol), SMTP (Simple Mail Transfer Protocol),^[119] IMAP (Internet Message Access Protocol), FTP (File Transfer Protocol) as well as FTPS/SFTP, file systems, etc.)
• filters – filters a message based on criteria. If the criteria are not met, the message is dropped.^[120]
• service activators – invoke an operation on a service object. Spring supports the use of the annotation @ServiceActivator to declare the component that requires this functionality.^[121]
• management and auditing
• gateways - exposes an interface to the client for the requested services. A messaging middleware is responsible for provisioning this interface. This interface decouples the messaging middleware from the client by hiding the underlying JMS or Spring Integration APIs. Gateways are related to the Facade pattern. Spring's Integration class, SimpleMessagingGateway, provides essential support for gateways. SimpleMessagingGateway enables the Spring application to specify the channel that sends requests, and the channel that expects to receive responses. The primary focus of SimpleMessagingGateway is to deal with payloads, which spares the client from the intricate details of the transmitted and received messages. SimpleMessagingGateway is used along with channels to enable integration with file systems, JMS, e-mail, or any other systems that require payloads and channels.^[122]
• splitter - Separates a large payload into smaller payloads to support different processing flows. The splitter is achieved in Spring using the splitter component. The splitter component usually forwards the messages to classes with more specialized functionality. Spring supports the @Splitter annotation to declare the component that requires this functionality.^[123]
• aggregator - Used for combining multiple messages into a single result. Loosely speaking, the aggregator is the reverse of the splitter. The aggregator publishes a single message for all components downstream. Spring supports the @Aggregator annotation to declare the component that requires this functionality.^[123]

Spring Integration supports pipe-and-filter based architectures.

Spring WebSocket

An essential rule for dealing with data streams effectively is to never block.^[124] The WebSocket is a viable solution to this problem.^[124] The WebSocket Protocol is a low-level transport protocol that allows full-duplex communication channels over a TCP connection. The WebSocket acts as an alternative to HTTP to enable two-way communication between the client and the server. The WebSocket is especially useful for applications that require frequent and fast exchanges of small data chunks, at a high speed and volume.^[124]

Spring supports the WebSocket protocol by providing the WebSocket API for the reactive application. The @EnableWebSocket annotation gives Websocket request processing functionality when placed in a Spring configuration class. A mandatory interface is the WebSocketConfigurer which grants access to the WebSocketConfigurer. Then, the Websocket URL is mapped to the relevant handlers by implementing the registerWebSocketHandlers(WebSocketHandlerRegistry) method.^[125]

Spring WebFlux

Spring WebFlux is a framework following the functional programming paradigm, designed for building reactive Spring applications. This framework uses functional programming and Reactive Streams extensively. A good use case for Spring WebFlux is for applications that require sending and receiving instantaneous information, such as a web application with chatting capabilities.^[126]

Although applications using Spring WebFlux technology is usually less readable than their MVC counterparts, they are more resilient, and simpler to extend.^[127] Spring WebFlux reduces the need to deal with the complications associated with synchronizing thread access.^[127]

Spring WebFlux supports server-sent events (SSE), which is a server push technology that allows the client to get automatic updates from a server through an HTTP connection. This communication is unidirectional, and shares a number of similarities with the publish/subscribe model found in JMS.^[124]