Completely understand the process of automatic configuration of spring boot in one article (recommended)

Spring Boot's automatic configuration mechanism is one of its important features, greatly simplifying the configuration of Spring applications. The core idea of ​​automatic configuration is to intelligently configure based on dependencies, environment configuration and custom code in the class path, avoiding developers manually writing a large amount of boilerplate code.

Next, I will introduce the process of automatic configuration of Spring Boot, the core principles and the key components involved in detail, and conduct in-depth analysis based on the source code.

1. Spring Boot automatic configuration workflow

• @SpringBootApplicationNoteThe starting point of automatic configuration is usually@SpringBootApplicationAnnotation, it is a combination annotation, which contains three important annotations:

  in@EnableAutoConfigurationis the core of automatic configuration, which guides the automatic configuration mechanism.

  • @SpringBootConfiguration: Marked as a Spring configuration class, similar to@Configuration。
  • @EnableAutoConfiguration: Enable Spring Boot's automatic configuration mechanism.
  • @ComponentScan: Scan all Spring components under the current package and its subpackages.

@EnableAutoConfigurationandAutoConfigurationImportSelector@EnableAutoConfigurationThe purpose of the annotation is to tell Spring Boot to automatically configure the Spring application context when it is started. This annotation introducesAutoConfigurationImportSelector, this is the core processor that is automatically configured.

@Target()
@Retention()
@Documented
@Inherited
@AutoConfigurationPackage
@Import()
public @interface EnableAutoConfiguration {
}

AutoConfigurationImportSelectorThe class will be from the configuration file (usually) Read all the autoconfiguration classes and import them into the application context.

documentAutomatic configuration class is passedspring-boot-autoconfigureModularMETA-INF/File to configure. This file lists all configuration classes that can be loaded automatically:

=\
,\
,\
,\
...

These configuration classes are selectively loaded according to the current environment conditions when Spring Boot is started.

• Conditional assembly (@ConditionalSeries of notes)Spring Boot does not blindly load all autoconfiguration classes. Each automatic configuration class is usually used@ConditionalSeries of annotations for conditional loading. The most common conditional annotations are:

  For example,DataSourceAutoConfigurationOnly data source-related dependencies exist in the project (such aswill be loaded only when class).

  • @ConditionalOnClass: Effective only when a certain class exists in the classpath.
  • @ConditionalOnMissingBean: Effective when there is no bean in the Spring context.
  • @ConditionalOnProperty: Take effect only when a configuration property meets a specific condition.
  • @ConditionalOnBean: Effective when a bean exists in the Spring context.

Automatic configuration class example:DataSourceAutoConfiguration

Spring BootDataSourceAutoConfigurationIt is an automatic configuration class for configuring data sources, and its source code is as follows:

@Configuration
@ConditionalOnClass()
@EnableConfigurationProperties()
@Import({,
         })
public class DataSourceAutoConfiguration {
    @Bean
    @ConditionalOnMissingBean
    public DataSource dataSource(DataSourceProperties properties) {
        // Create and return the DataSource object        return ().build();
    }
}

This condition-based configuration ensures flexibility in Spring Boot, allowing users to skip certain automatic configurations by overwriting the default bean or not meeting the conditions.

• @ConditionalOnClass(): Only if it exists under the classpathDataSourceAutomatic configuration of the data source is only carried out when the class is used.
• @ConditionalOnMissingBean: If there is no other in the Spring contextDataSourceBean, then automatically configure one.

2. The core steps of Spring Boot automatic configuration

• Collect automatic configuration classesAt startup,AutoConfigurationImportSelectorfromRead all automatic configuration classes in the file and pass@ImportImport these classes.
• Condition checkThe loading of the automatic configuration class is not unconditional, Spring Boot will use it according to@ConditionalAnnotation performs condition checks to ensure that only automatic configuration classes that meet the conditions will take effect.
• Inject the required beansOnce the automatic configuration class passes the conditional check, Spring Boot registers the required beans based on these configuration classes. For example,DataSourceAutoConfigurationThe data source-related beans will be automatically configured.
• Allow users to override automatic configurationAutomatic configuration is not mandatory. Users can override the default behavior of auto-configuration by explicitly declaring their own beans. For example, if the user defines theDataSource, then Spring Boot will no longer automatically configure the data source.

3. Actual cases of automatic configuration of Spring Boot

• Automatic configuration of web applicationsIn Spring Boot Web Applications,DispatcherServletAutoConfigurationResponsible for automatically configuring the core components of Spring MVC, such asDispatcherServlet、RequestMappingHandlerMappingwait.

  • If there is a projectspring-webDependency, thenDispatcherServletAutoConfigurationIt will be loaded automatically.
  • If not manually definedDispatcherServlet, Spring Boot will automatically create aDispatcherServletand configure it into Spring container.

Automatic configuration of database connection poolSpring Boot will also automatically configure database connection pools (such as HikariCP, Tomcat JDBC, etc.), which depends on the projectspring-boot-starter-data-jpaorspring-boot-starter-jdbcDependence.

At the same time, users can useFile custom connection pool configuration:

=jdbc:mysql://localhost:3306/mydb
=root
=secret
-pool-size=10

• DataSourceAutoConfigurationandDataSourcePropertiesTogether are responsible for automatically configuring the data source.
• If there is a connection pooling class in the classpath (such asHikariDataSource), then Spring Boot will automatically configure the connection pool.

• Spring Security Automatic configurationWhen introducedspring-boot-starter-securityWhen dependency is made, Spring Boot will automatically configure the security mechanism and provide the HTTP Basic authentication mechanism by default.

  • SecurityAutoConfigurationResponsible for automatically configuring Spring Security's infrastructure.
  • If you need to customize your security policy, you can customize itWebSecurityConfigurerAdapterto override the default configuration.

4. The benefits of Spring Boot automatic configuration

• Greatly simplify configuration work: Developers no longer need to write configuration code for each infrastructure component, and the automatic configuration mechanism automatically injects the required beans based on project dependencies.
• flexibility: Automatic configuration does not restrict developers. Developers can easily override the default automatic configuration through custom configurations.
• Conventions are better than configuration: Spring Boot follows the principle of "convention is better than configuration". Spring Boot can complete complex initialization work with just a small amount of configuration.

5. The difference between Spring Boot automatic configuration and Spring automatic assembly

• Spring automatic assembly: refers to passing@Autowiredand other annotations, automatically inject dependency beans according to the type. It focuses on injecting already configured beans.
• Spring Boot Automatic Configuration: It is a process of automatically configuring Spring components based on dependencies and environment information in the classpath. It is responsible for creating and configuring the required infrastructure beans.

In summary, Spring Boot's automatic configuration mechanism passes@EnableAutoConfigurationStart, based onConfiguration and@ConditionalConditional judgment, automatic injection of required beans simplifies the developer's configuration work while retaining flexible customization capabilities.

This is the article about thoroughly understanding the process of spring boot automatic configuration in this article. For more related spring boot automatic configuration content, please search for my previous article or continue browsing the related articles below. I hope everyone will support me in the future!

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