[Java] How Spring DI works

Features of Spring DI container

• Entry point required when launching application
• In Java, it is the entry point where the Main method is invoked.
• In SpringBoot ** @SpringBootApplication is attached to the class of Main method **
• ** Spring runtime checks all classes under packages with this annotation at startup **
• ** @Bean (and its derived classes, @Component, @RestController, @Service, etc.) is automatically detected and registered in the Spring DI container **
• If the detected method has ** @ GetMapping ** etc., use this as the endpoint and listen to the "/" URL (= 8080).
• Requests coming from there are automatically divided into registered controllers according to the contents of the URL.
• ** When Spring Web is selected, it searches the beans registered in the DI container for the class with @RestController and uses the method with @GetMapping etc. as the endpoint **

About the routing mechanism

• In the example below, DemoApplication and SampleApplication are registered as beans and both are subject to routing.

Project Root
└─src
    └─ main
        └─ java  
            └─ com.example
                └─ demo
                    └─DemoApplication
                    └─SampleApplication

Failure example

• If "/" is specified for both DemoApplication and SampleApplication, SampleApplication has priority.

DemoApplicaton.java

package com.example.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

@SpringBootApplication
@RestController
public class DemoApplication {

    @RequestMapping("/") //From which address do you get it? "/“Is http://localhost:8080/Point to
    String index(){
        return "Hello World!";
    }
    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }
}

SampleApplicaton.java

package com.example.hoge;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;

@Controller
public class SampleController {
    @RequestMapping(value = "/", method = RequestMethod.GET)
    //Receive when a request comes to the root of the URL
    //DemoApplication and root (=8080) is covered
    public String index(Model model) {
        model.addAttribute("message", "Hello World!!");
        return "index";
    }
}

Routing mechanism

• ** If @SpringBootApplication is under com.example.demo, go read the packages below demo **
• Move SampleController.java to com.example.hoge → ** SampleController will not be routed! ** **

Project Root
└─src
    └─ main
        └─ java  
            └─ com.example
                └─ demo
                    └─DemoApplication
                └─ hoge
                    └─SampleApplication

• Change “/” to / hoge → The path is http: // localhost: 8080 / hoge /
• DemoApplication.java runs> ** Hello World! **

DemoApplication.java

package com.example.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

@SpringBootApplication
@RestController
public class DemoApplication {

    @RequestMapping("/hoge")
    String index(){
        return "Hello World!";
    }
    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }
}

• Change "/" of SampleController to "/ bar"
• Go to http: // localhost: 8080 / bar
• ** @SpringBootApplication has not been able to check all the classes under the package! I haven't registered in the DI container! ** **

Whitelabel Error Page

SampleApplicaton.java

package com.example.hoge;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;

@Controller
public class SampleController {
    @RequestMapping(value = "/bar", method = RequestMethod.GET)
    public String index(Model model) {
        model.addAttribute("message", "Hello World!!");
        return "index";
    }
}