Spring Boot source code interview questions and answers (10,000 words)

1. Introduce what Spring Boot is?

Answer:
Spring Boot is an open source, Java-based framework for creating and deploying standalone, production-grade applications based on the Spring Framework. It simplifies the initialization and development process of Spring applications, providing automated configuration, convention over configuration, and a range of out-of-the-box features that enable developers to quickly build reliable Spring applications.

2. What are the core principles of Spring Boot?

Answer:
The core principles of Spring Boot can be summarized as the following points:

Automatic configuration: Spring Boot automatically applies reasonable default settings by analyzing the dependencies and configurations in the project, reducing the developer's configuration work. It uses conditional configuration and property configuration to provide various features such as database connections, web containers, etc.
Starter dependencies: Spring Boot provides a series of "starter dependencies", which are preconfigured Maven or Gradle dependencies to simplify the integration of specific functions, such as database access, web development, etc. They contain the necessary libraries and automatic configuration to enable developers to quickly get up and running with various features.
Embedded container: Spring Boot provides an embedded Servlet container (such as Tomcat, Jetty), which allows the application to run as an independent executable file without the need for external container deployment.
Simplified configuration: Spring Boot's configuration file adopts the principle of convention over configuration. In most cases, developers do not need to manually write cumbersome XML configuration files and can complete it through simple attribute configuration.
Modular design: Spring Boot divides functionality into multiple modules, with each module focusing on a specific functional area. This modular design allows developers to select appropriate modules to integrate into applications as needed, improving flexibility and scalability.

3. What are the main features of Spring Boot?

Answer:
Spring Boot provides many useful features, including but not limited to the following:

Simplified configuration: Spring Boot uses the principle of convention over configuration and provides a simplified configuration method. Developers can complete most configuration work through attribute configuration or annotation configuration, avoiding cumbersome XML configuration.
Automatic configuration: Spring Boot automatically performs appropriate configuration based on the application's dependencies and configuration, reducing the workload of manual configuration. Developers can override the automatic configuration by modifying the default configuration to achieve personalized configuration.
Starter dependencies: Spring Boot provides a series of starter dependencies, which are preconfigured Maven or Gradle dependencies to simplify the integration of specific functions. The starting dependencies include the required libraries and automatic configuration, allowing developers to quickly integrate and use various functions.
Embedded container: Spring Boot has built-in common Servlet containers (such as Tomcat, Jetty), so that the application can run as an independent executable file without the need for deployment and configuration of external containers.
Health check: Spring Boot provides a health check mechanism, and developers can easily monitor the running status of the application, including the health status of database connections, message queues, caches and other aspects.
Unified log: Spring Boot uses SLF4J and Logback as the log framework by default. Through unified log configuration, developers can easily manage and adjust log output.
Monitoring and management: Spring Boot provides a series of monitoring and management functions, such as application information endpoints (Actuator), through which you can view the application's running status, performance indicators, etc.
Hot deployment: Spring Boot supports hot deployment during the development phase. After modifying the code, you can see the changes without restarting the application, which improves development efficiency.

4. What are the core components of Spring Boot?

Answer:
The core components of Spring Boot mainly include the following parts:

SpringApplication: This is the entry point of the Spring Boot startup class, responsible for starting the application. It can be used to set various application configurations and provides various extension points for customizing the application's behavior.
Auto-configuration: Spring Boot's auto-configuration mechanism is a key component to achieve automated configuration. It is based on conditional configuration, automatically applying appropriate default settings based on the application's dependencies and configuration. Developers can override the automatic configuration by modifying the default configuration.
Starter Dependencies: Starter dependencies are preconfigured Maven or Gradle dependencies used to simplify the integration of specific functions. Each starter dependency contains the required libraries and automatic configuration, allowing developers to quickly start and use various functions.
Externalized Configuration: Spring Boot supports using property configuration files or command line parameters to configure applications. It provides a flexible configuration method that can be configured according to different environments or needs, and supports encryption and decryption of attributes.
Embedded Container: Spring Boot has built-in common Servlet containers (such as Tomcat, Jetty), allowing the application to run as an independent executable file without the need for deployment and configuration of external containers. Developers can customize the behavior of embedded containers through configuration files or programmatically.
Actuator: Spring Boot Actuator provides a series of monitoring and management functions. You can view the running status, performance indicators, health status, etc. of the application by exposing various endpoints. Developers can use Actuator to monitor and manage applications for troubleshooting and performance optimization.
Spring Boot CLI: Spring Boot CLI is a command line tool that helps developers quickly create and run Spring Boot applications. It provides a concise command line interface that supports rapid prototyping and running applications quickly.
Spring Boot Test: Spring Boot provides a testing framework for writing unit tests and integration tests. It simplifies test configuration and writing, and provides various test annotations and tool classes, allowing developers to easily write efficient and reliable test code.

5. Please explain the automatic configuration principle of Spring Boot?

Answer:
Spring Boot’s automatic configuration is implemented through conditional configuration. It avoids the tedious manual configuration process by automatically applying appropriate default settings based on the application's dependencies and configuration information.

The key to the automatic configuration principle is Spring Boot's startup class SpringApplication, which uses the @EnableAutoConfiguration annotation to enable the automatic configuration function. During the startup process, SpringApplication will scan the application's class path and determine which auto-configuration classes are available based on the agreed naming rules and conditional annotations.

Auto-configuration classes are ordinary Spring configuration classes, and they usually use @Conditional annotations to set conditions. Depending on the conditions, autoconfiguration classes can be selectively loaded or ignored. Spring Boot provides many built-in conditional annotations, such as @ConditionalOnClass, @ConditionalOnBean, @ConditionalOnProperty, etc. Developers can also customize conditional annotations .

When the auto-configuration class meets the conditions, it will register the corresponding bean definition into the Spring container to achieve the effect of auto-configuration. Developers can override the automatic configuration by modifying the default configuration or adding custom configurations.

In general, Spring Boot's automatic configuration principle is to simplify the application configuration process by automatically applying appropriate default settings based on the application's dependencies and configuration information through conditional configuration and convention over configuration.

6. Please explain what the Starter Dependencies of Spring Boot are? What do they do?

Answer:
Spring Boot's starting dependencies are preconfigured Maven or Gradle dependencies that simplify the integration of specific functionality. Each starter dependency contains the required libraries and automatic configuration, allowing developers to quickly start and use various functions.

The role of starting dependencies is as follows:

Simplified configuration: Starter dependencies provide preconfigured dependencies, avoiding the tedious process of manually adding and configuring libraries. Developers only need to introduce the corresponding starting dependencies to obtain the required functions and default configurations.
Avoid version conflicts: Starting dependencies manage the versions of included libraries and resolve conflicts between versions. Developers do not need to manually manage library versions and can safely use the default versions provided by the starting dependencies.
Provide automatic configuration: The starting dependencies not only include the required libraries, but also the corresponding automatic configuration. This means that developers can directly use the functionality provided by the starter dependency without having to configure it manually. Autoconfiguration automatically applies appropriate default settings based on the application's dependencies and configuration information.
Modular integration: Starting dependencies combine related libraries and automatic configuration to form a functional module. Developers can choose appropriate starting dependencies as needed and quickly integrate the required functions. This modular design makes the application's architecture clearer and the code easier to maintain and extend.

Overall, Spring Boot's starter dependencies simplify the application integration and configuration process, providing a way to quickly start and use various features.

7. Please explain what Spring Boot’s externalized configuration is? How can it be configured?

Answer:
Spring Boot's externalized configuration refers to separating the application's configuration information from the code so that it can be configured in different environments. This approach makes the configuration more flexible and can be configured according to different environments or needs without modifying the code.

Spring Boot supports multiple external configuration methods:

Property configuration files: Configuration information can be stored in property configuration files (such as application.properties or application.yml). These files can be placed at a specific location under the classpath or specified via command line arguments. Developers can configure applications using standard property key-value pairs or YAML format.
Command line parameters: Configuration information can be passed to the application through command line parameters. Spring Boot supports command line parameters prefixed with --, for example --server.port=8080. This method is suitable for temporarily adjusting the configuration or overwriting the default configuration.
Environment variables: You can use environment variables to configure the application. Spring Boot uses certain naming rules to map environment variables to corresponding configuration properties. For example, SPRING_SERVER_PORT=8080 can be mapped to the server.port attribute.
Java system properties: You can configure your application by setting Java system properties. This can be achieved via the command line argument -D or programmatically. For example, -Dserver.port=8080 can set the server.port attribute.
Configuration server: Spring Boot supports storing configuration information on a remote configuration server, such as Spring Cloud Config. Applications can dynamically obtain configuration information by interacting with the configuration server.
Configuration annotations: Spring Boot provides some annotations for configuring properties directly in code. For example, the @Value annotation can be used to inject property values, and the @ConfigurationProperties annotation can be used to bind properties to a Java object.

Through these external configuration methods, developers can easily configure and manage application properties and behaviors according to different environments and needs.

8. Please explain what Spring Boot’s health check is? How do I do a health check?

Answer:
Spring Boot's health check refers to the process of monitoring and evaluating the running status of the application. It determines whether the application is running properly by checking various aspects of the application, such as database connections, message queues, caches, disk space, etc.

Spring Boot provides the Actuator module to implement health checks. Actuator exposes a series of endpoints, including the /health endpoint for health checks. When the /health endpoint is accessed, the application returns a response containing health information.

The results of the health examination may have the following possibilities:

Healthy (UP): Indicates that all aspects of the application are functioning properly.
Unhealthy (DOWN): Indicates that some aspect of the application is faulty or abnormal.
UNKNOWN: Indicates that the health status of the application cannot be determined, usually because an error occurred during the check.

Actuator also provides some other endpoints, such as the /info endpoint for getting application information, and the /metrics endpoint for getting application performance metrics. wait.

Developers can control the exposure and security of Actuator endpoints through configuration files or annotations. You can selectively expose different endpoints and configure the permissions and authentication required to access the endpoints.

Through health checks, developers can monitor the health of the application and detect and handle failures in a timely manner. Health checks can be integrated into operation and maintenance tools or monitoring systems to achieve automated troubleshooting and alarms.

Please explain what hot deployment is in Spring Boot? How to implement hot deployment?

Answer:
Spring Boot's hot deployment means that during the development phase, you can see the changes without restarting the application after modifying the code. This method can improve development efficiency and avoid the time consumption of repeated startup and loading.

Spring Boot supports multiple hot deployment methods:

Development tool support: Many integrated development environments (IDEs) and development tools provide hot deployment support for Spring Boot. For example, using an IDE such as IntelliJ IDEA or Eclipse, you can enable hot-deployment plug-ins and automatically reload the application after modifying the code.
Spring Boot DevTools: Spring Boot DevTools is a development toolkit that provides hot deployment and automatic restart functions. It monitors files under the classpath for changes and reloads the application when changes are found.
Spring Loaded: Spring Loaded is a hot-deployed JVM agent that can load modified class files at runtime. It can be integrated with Spring Boot for fast class loading and hot deployment.
JRebel: JRebel is a commercial hot deployment tool that allows you to modify the code without restarting the application. It supports multiple development frameworks and servers, including Spring Boot.

By using these tools and technologies, developers can hot-deploy Spring Boot applications and quickly modify and debug code.

10. Please explain what is Spring Boot monitoring and management? How to monitor and manage?

Answer:
Spring Boot provides a series of monitoring and management functions for monitoring and managing the running status, performance indicators and health status of the application. These functions can help developers understand the running status of applications in real time and conduct troubleshooting, performance optimization, and operation and maintenance management.

Monitoring and management functions are mainly implemented through the Spring Boot Actuator module. Actuator provides a series of endpoints that can be accessed through HTTP requests to obtain application information and metrics.

Some commonly used monitoring and management endpoints include:

/healthEndpoint: Used to view the health status of the application, including checking the health of database connections, message queues, caches, etc.
/infoEndpoint: used to view the basic information of the application, such as the application name, version number, description, etc.
/metricsEndpoint: Used to view application performance indicators, such as the number of requests, response time, memory usage, etc.
/loggersEndpoint: used to view and modify the application's log configuration, and can dynamically modify the log level and output destination.
/traceEndpoint: used to view request tracking information, including request path, parameters, response code, etc.

In addition to the above endpoints, custom endpoints can also be added in a customized manner to meet specific monitoring and management needs.

Developers can control the exposure and security of Actuator endpoints through configuration files or annotations. Specific endpoints can be selectively exposed, limiting the permissions and authentication required to access the endpoints.

Monitoring and management functions can be integrated with operation and maintenance tools, monitoring systems or log analysis tools to achieve real-time monitoring and management of applications.

11. Please explain what is Spring Boot’s event model? How to use event model?

Answer:
Spring Boot's event model is an observer pattern for publishing and processing events in an application. Events can be state changes within the application, user operations, system events, etc.

Spring Boot’s event model is mainly implemented through the Spring Framework’s event mechanism. The core classes are ApplicationEvent and ApplicationEventPublisher.

ApplicationEvent: Represents an event. Developers can inherit this class to create custom event classes.
ApplicationEventPublisher: Used to publish events. Developers can publish events through the implementation class of this interface.

Developers can use the event model by following these steps:

Create a custom event class and inherit ApplicationEvent. For example, you can create an event class named UserCreatedEvent.
Call the method of ApplicationEventPublisher at the appropriate place (such as business logic code) to publish the event. For example, you can publish a event where user creation is successful. publishEvent()UserCreatedEvent
Create an event listener (observer), implement the ApplicationListener interface, and use the @EventListener annotation on the class. Event listeners can handle specific types of events.
When a published event is triggered, the event listener will automatically call the corresponding processing method for processing.

Through the event model, developers can achieve decoupling within the application, separate business logic and event processing logic, and improve the maintainability and scalability of the code.

12. Please explain what Spring Boot’s testing support is? How to do unit testing and integration testing?

Answer:
Spring Boot provides comprehensive testing support for writing unit and integration tests. These testing tools and frameworks help developers write high-quality test code and ensure the proper functioning of the application.

Spring Boot’s testing support includes the following aspects:

Unit testing: Spring Boot integrates popular unit testing frameworks such as JUnit and Mockito, and you can test individual components in the application by writing unit test classes. Unit testing can verify that each component is functioning properly and provide quick feedback and verification.
Integration testing: Spring Boot provides the @SpringBootTest annotation for writing integration test classes. Integration test classes can simulate the entire application context, including starting the Spring Boot application, loading configuration, initializing beans, etc. This allows you to test how the entire application works together, ensuring proper integration between components.
Web testing: Spring Boot provides the @WebMvcTest annotation for writing test classes for the Web layer. This annotation automatically configures Spring MVC-related components, can simulate HTTP requests and responses, and test the behavior of the controller. This allows you to test RESTful API requests and responses and verify that the controller logic is correct.
Database testing: Spring Boot supports using an in-memory database (such as H2 or HSQLDB) in a test environment for database-related testing. Developers can use the @DataJpaTest annotation or @SpringBootTest annotation to perform integration testing with the in-memory database to verify the correctness of the database operation.
Automated testing: Spring Boot integrates tools such as Selenium and Cucumber to perform automated user interface testing and behavior-driven development (BDD) testing. These tools can simulate user actions and browser behavior for comprehensive end-to-end testing.

By using these testing support tools and frameworks, developers can write comprehensive test code, covering different layers and components of the application, ensuring the quality and stability of the application.

The above is part of the interview questions and answers about Spring Boot source code, covering the core concepts, features and related tools of Spring Boot. Hope these questions and answers are helpful. Remember that in addition to theoretical knowledge, it is also important to demonstrate your practical project experience and problem-solving skills during the interview. Good luck with your interview!