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This article is about how to quickly get started with SpringIoC and SpringMVC. The editor thinks it is very practical, so I share it with you. I hope you can get something after reading this article.

For more information, you are welcome to follow the official account of Wechat: Xiao Hui, a whole dish engineer.

What are the advantages of Spring?

The coupling between components is reduced, and the decoupling between software layers is realized.

You can use many services that are easy to provide, such as transaction management, message services, etc.

The container provides singleton mode support

The container provides AOP technology, which makes it easy to implement functions such as permission interception, runtime monitoring and so on.

Spring's IoC mechanism reduces the complexity of business object replacement

Containers provide a large number of auxiliary classes, which can speed up the development of applications

Spring provides integration support for mainstream application frameworks, such as Hibernate,JPA,Mybatis.

Spring is a low-intrusive design with extremely low code pollution and is independent of various application servers.

The highly open nature of Spring does not force applications to rely entirely on Spring, and developers are free to choose part or all of spring.

Why not talk about AOP in this article?

IoC and AOP are the two major features of the Spring framework. The processes of IoC and MVC are inseparable and can be regarded as the implementation of object-oriented programming; while the AOP feature is the embodiment of aspect-oriented programming and a supplement to the former, so it can be separated and explained later.

> this article provides a simple explanation of the process of SpringIoC and SpringMVC. For a more detailed process, please take a look at the author's code base, where almost every line of code is commented in detail. A request debug follows, and the process is clear at a glance.

Poke me, let's go learn ~

What is SpringIoC?

Control inversion IoC (Inversion of Control) is a design idea. In a program without IoC, we use the creation of object-oriented programming objects and the dependency relationship between objects to be completely hard-coded in the program, the creation of objects is controlled by the program itself, and after using control inversion, the creation of objects is transferred to a third party.

IoC is the core content of the Spring framework, using a variety of ways to achieve IoC perfectly, you can use XML configuration, you can also use annotations, the new version of Spring can also implement IoC with zero configuration. The Spring container first reads the configuration file during initialization, creates and organizes objects according to the configuration file or metadata and stores them in the container, and then takes out the needed objects from the IoC container when the program uses it.

> when configuring Bean in XML mode, the definition information of Bean is separated from the implementation, and the two can be combined by annotations. The definition information of Bean is directly defined in the implementation class in the form of annotations, thus achieving the goal of zero configuration.

The process of SpringIoC?

Basic package scan. Read the configuration file, then scan the full package according to the configuration path, and finally add the scanned class name to the array collection after adding the fully qualified name.

Instantiate all annotated classes. Iterate through the array collection in the previous step, judge the comments on the class and instantiate the class. Put it in the hash table with the class name key and the instance value

Dependency injection. Iterate through the hash table in the previous step, get all the Fields on the class, traverse the Fields, determine whether the annotation on the Fields is of type @ Autowired, and if so, assign the instance in the Map to the field

Url address and method mapping. Traverse the hash table in the second step. If it is a Controller instance, get Methods and traverse it to determine whether the method contains @ RequestMapping annotation. If so, get its value. The @ Controller annotation value is spliced with @ RequestMapping value to form key. The current method is loaded into hanlerMap as value.

Request reflection call. (this step already belongs to the category of MVC) A url request comes over, gets its address, splits, gets the Controller instance, gets the Method instance, and the reflection executes Method.

What is SpringMVC?

The SpringMVC framework is request-driven, designed around Servlet, sends the request to the controller, and then displays the view of the request result through the model object and dispatcher. The core class is DispatcherServlet, which is a Servlet, and the top level is the implemented Servlet interface.

Why use SpringMVC?

The role of the framework has always been to simplify programming:

Servlet has only doGet and doPost, and a servlet class can handle only one url-pattern.

The SpringMVC class can handle many requests through RequestMapping, and supports Rest-style requests, such as DELETE/PUT, etc. The mapping of SpringMVC parameters can be directly encapsulated into entity classes.

The process of SpringMVC

The flowchart shows:

The user sends a request to the front-end controller DispatcherServlet.

The front-end controller DispatcherServlet invokes the processor mapper HandlerMapping after receiving the request.

The processor mapper HandlerMapping finds the specific processor according to the requested Url, and generates the processor object Handler and the processor interceptor HandlerIntercepter (if any) back to the front-end controller DispatcherServlet.

The front-end controller DispatcherServlet calls the processor Controller through the processor adapter HandlerAdapter.

Execution processor (Controller, also known as back-end controller)

The processor Controller returns ModelAndView after execution.

The processor mapper HandlerAdapter returns the result ModelAndView returned by the processor Controller execution to the front-end controller DispatcherServlet.

The front-end controller DispatcherServlet passes the ModelAnView to the view parser ViewResolver.

The view parser ViewResolver returns the specific view View after parsing.

The front-end controller DispatcherServlet renders the view View (that is, populating the model data into the view)

The front controller DispatcherServlet responds to the user.

Modules in the MVC process:

DispatcherServlet: front-end controller (without programmer development) user requests arrive at the front-end controller, which is equivalent to C (Controller) in MVC mode. DispatcherServlet is the center of the whole process control, which calls other components to deal with user requests. The existence of DispatcherServlet reduces the coupling between components.

Function: as a result of accepting the request, it is equivalent to the transponder and CPU, reducing the coupling between other components.

HandlerMapping: processor mapper (no programmer development required)

HandlerMapping is responsible for finding Handler (that is, processor) according to user's request. SpringMVC provides different mappers to implement different mapping methods, such as configuration file, implementation interface, annotation and so on.

Function: to find the Handler based on the requested Url

Handler: processor (to be developed by programmers)

Handler is the back-end controller following the DispatcherServlet front-end controller. Under the control of DispatcherServlet, Handler processes specific user requests. Because Handler is designed to specific user business requests, programmers are generally required to develop Handler according to business requirements.

HandlerAdapter: the processor adapter executes on the processor through HandlerAdapter, which is an application of the adapter pattern, and can be executed on more types of processors by extending the adapter.

Function: execute Handler according to specific rules (rules required by HandlerAdapter)

ViewResolver: view parser (does not require programmer development) ViewResolver is responsible for generating View view of the processing result. ViewResolver first parses the logical view name into a physical view name, that is, a specific page address, and then generates a View view object. Finally, the View is rendered and the processing result is displayed to the user through the page. The SpringMVC framework provides many types of View views, including JSTLView, freemarkerView, pdfView, and so on.

Function: perform view parsing, parsing into a real view (view) according to the logical view name.

View view (requires programmers to develop jsp)

View is an interface, and the implementation class supports different View types (jsp, freemarker, etc.). In general, the model data needs to be displayed to the user through the page tag or page template technology, and the programmer needs to develop specific pages according to business requirements.

The above is how to quickly get started with SpringIoC and SpringMVC. The editor believes that there are some knowledge points that we may see or use in our daily work. I hope you can learn more from this article. For more details, please follow the industry information channel.

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