Spring Intro

Application entry point

"Main" class of the Spring app is annotated with the @SpringBootApplication annotation. It has the standard Java main function. Its directory will be scanned automatically to determine the presence of other Spring components (for example, beans or controllers). There is a way to scan other directories.

Take a look at: DemoApplication.java

RestController

All classes, which have requests handling functions, in Spring should be annotated as @Controller or @RestController. The key difference between them is that @RestController is a mixture of the @Controller and the @ResponseBody. Meaning that it is not required to specify @ResponseBody annotation for the inner functions.

Take a look at: ShopController.java, CafeController.java, CashierController.java

Mappings

In this simple applications I used only two types of mapping -- @GetMapping and @PostMapping, which handle get and post requests accordingly. The target path is specified in the round brackets. For example, @PostMapping("/post"). Mapping functions can have arguments, which represent the json request body or @PathVariable arguments, which will be extracted from the target path.

Take a look at: showCat function in ShopController.java

Plain Old Java Object Concept

Spring provides the functionality to work with standard java object instead of non-java ones (jsons, for example). No library is needed to convert json to java class (and in a reversed order). The only requirement is that the Java class should have public fields or public getters (for @RequestBody) and setters (for @ResponseBody) and the fields of the request json should have the same names as the POJO class. Let's consider an example.

Spring parses jsons with fields that don't appear in the POJO representation properly, just by skipping these unrepresented fields.

public class Date {
    public int day;
    public int month;
    public int year;
    public boolean isMonday;
}

{"day": 4, "month": 4, "year": 2020, "isMonday": 0}

Take a look at: sellCat functions in ShopController.java and script.js

Beans and Inversion of Control

It's really hard to explain them so I'll delegate this task to the authors of these two web-pages:

Beans

Beans scopes

In this project I used @ApplicationScope bean. It is something similar to singleton (the object is shared between different servlets of the Spring application) but it could be mocked. More about spring testing.

A bean object is marked as a @Component. A function which returns this components is marked as @Bean("name of the bean"). A class with this function is marked as @Configuration.

@Resource(name = "beanName") annotation is used to access an @ApplicationScope bean with the name "beanName".