Spring boot in this article is 2.3 Version 4

1. Spring boot features, understand the principle of automatic configuration

1.1. Dependency management

• Dependency management for parent project:

<!-- Dependency management -->
<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.3.4.RELEASE</version>
</parent>

		
<!-- ↓↓↓ spring-boot-starter-parent Parent project of ↓↓↓ -->
<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-dependencies</artifactId>
    <version>2.3.4.RELEASE</version>
</parent>

<!-- spring-boot-dependencies Dependency, which declares almost all dependencies and version numbers commonly used in development,Automatic version arbitration mechanism -->

• Develop and import starter scenario launcher:

  1. See a lot of spring boot starter - *: * represents a certain scenario
  2. As long as the starter is introduced, we will automatically introduce all the conventional dependencies of this scenario
  3. All supported scenarios of SpringBoot, portal: https://docs.spring.io/spring-boot/docs/current/reference/html/using-spring-boot.html#using-boot-starter
  4. * - Spring boot starter: * is the dependency provided by the third party for us to integrate with Spring.
  5. The lowest level dependency of all scenario initiators

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter</artifactId>
    <version>2.3.4.RELEASE</version>
    <scope>compile</scope>
</dependency>

• No need to pay attention to the version number, automatic version arbitration

1,By default, no version can be written when importing dependencies, for example:
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>

2,Introducing non version arbitrations jar，To write the version number.

• You can modify the default version number

1,see spring-boot-dependencies It specifies the current dependent version key. 
2,Rewrite the configuration in the current project
<properties>
    <mysql.version>5.1.43</mysql.version>
</properties>

1.2. Automatically configure Web dependencies

• Automatically configure Tomcat

  • The spring boot starter web coordinate is introduced. The pom coordinate contains the dependency of spring boot starter tomcat, so Tomcat can be selected as the web server when starting

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>

• Default package structure:

  • The components in the package where the main program is located and all its sub packages will be scanned by default
  • No previous package scan configuration is required
  • To change the scanning path, @ SpringBootApplication(scanBasePackages="com.atguigu")

• Or use the @ ComponentScan annotation in the main startup class to specify the scan path.

@SpringBootApplication annotation
 Equivalent to
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan("com.atguigu.boot")

• Various configurations have default values

• • The default configuration is ultimately mapped to a class, such as MultipartProperties
  • The value of the configuration file will eventually be bound to each class, which will create objects in the container

• Load all auto configuration items on demand

• • Very many starter s
  • When the dependency of a scenario is introduced, the automatic configuration of the scenario will be enabled, which does not mean that all configuration classes will take effect.
  • All spring boot autoconfigure functions are in the spring boot autoconfigure package

Simulate third-party dependencies integrated with SpringBoot

Overall structure

• Create new modules named XXX starter and XXX autoconfigure
  • XXX starter project is responsible for POM Various dependencies are introduced into XML. If others want to use a function that the third party depends on, they can directly introduce the coordinates of XXX starter. The XXX starter project introduces the XXX autoconfigure project dependency.
  • The XXX autoconfigure project is responsible for initializing the automatic configuration of this third-party dependency (such as registering the configuration class, xxxProperties... Into the container)

starter project

• In POM Introducing autoconfigure coordinates into XML

<dependencies>
    <dependency>
        <groupId>com.calm</groupId>
        <artifactId>calm-hello-spring-boot-autoconfigure</artifactId>
        <version>1.0-SNAPSHOT</version>
    </dependency>
</dependencies>

autoconfigure project

• In POM Add required dependencies to XML

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.3.4.RELEASE</version>
    <relativePath/>
</parent>

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter</artifactId>
    </dependency>
</dependencies>

• Properties

// Find the attributes prefixed with "prefix attribute value" in the configuration file and bind their values to this class
@ConfigurationProperties(prefix = "calm.hello")
public class HelloProperties {

    private String prefix;
    private String suffix;

    public String getPrefix() {
        return prefix;
    }

    public void setPrefix(String prefix) {
        this.prefix = prefix;
    }

    public String getSuffix() {
        return suffix;
    }

    public void setSuffix(String suffix) {
        this.suffix = suffix;
    }
}

• service

public class HelloService {
    
    // Injection properties
    @Autowired
    private HelloProperties helloProperties;

    public String sayHello(String name){
        return helloProperties.getPrefix() + "--" + name + "--" + helloProperties.getSuffix();
    }
    
}

• config configuration class

@Configuration
// Enable the configuration binding function of HelloProperties class, and put it into the IOC container after binding.
@EnableConfigurationProperties(HelloProperties.class) 
public class HelloServiceAutoConfiguration {

    // Judge that if there is no Bean of HelloService type in the IOC container, HelloService will be registered in the container.
    // The purpose is to use the HelloService customized by the user if the user has customized it. If there is no customization, use ours.
    @ConditionalOnMissingBean(HelloService.class)
    @Bean
    public HelloService helloService(HelloProperties helloProperties){
        // Get HelloProperties from the container and set the value.
        helloProperties.setPrefix("register Bean Into the container.");
        return new HelloService();
    }

}

• Create a new meta-inf / spring Factories file

  • Where the value of EnableAutoConfiguration represents the autoconfiguration class to be loaded when the project starts

  org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
  com.calm.config.HelloServiceAutoConfiguration
  

test

Create a new project and introduce a user-defined starter dependency. autoconfigure will automatically introduce it and pass the dependency.

<dependency>
    <groupId>com.calm</groupId>
    <artifactId>calm-hello-spring-boot-starter</artifactId>
    <version>1.0-SNAPSHOT</version>
</dependency>

• Create another applicaiton Yaml profile

calm:
  hello:
    prefix: Configure prefix
    suffix: Configuration suffix

• Write a test class under the test package

@SpringBootTest
@RunWith(SpringRunner.class)
public class SpringBootStarterAutoConfiguration {

    @Autowired
    private HelloService helloService;

    @Test
    public void sayHello(){
        System.out.println(helloService.sayHello("Zhang San"));
    }

}