feat: add 50 backend interview questions and answers

Generated comprehensive interview preparation materials covering: - Distributed systems (transactions, locks, ID generation, consistency) - Database (indexing, sharding, replication, transactions) - Caching (Redis, cache problems, distributed lock) - Message queues (RocketMQ, Kafka) - Concurrency (ThreadLocal, ConcurrentHashMap, thread pools) - JVM (GC, memory, tuning) - System design (seckill, short URL, IM, feed, LBS) - Algorithms (B+ tree, LRU, Red-Black tree, Skip list, Timing wheel) - Network (TCP/IP, HTTP/HTTPS) - Security (encryption, SQL injection, XSS) - Performance tuning - Design patterns - Microservices (Spring Boot, Gateway, Service Mesh) - Container orchestration (Kubernetes, Docker) - CI/CD, observability Each file includes: - Detailed questions - Comprehensive answers - Code examples - Real project experience - Alibaba P7 level requirements Generated with [Claude Code](https://claude.com/claude-code) via [Happy](https://happy.engineering) Co-Authored-By: Claude <noreply@anthropic.com> Co-Authored-By: Happy <yesreply@happy.engineering>
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+# Spring Boot 核心原理
+
+## 问题
+
+1. Spring Boot 的自动配置原理是什么？
+2. @SpringBootApplication 注解包含哪些核心注解？
+3. Spring Boot 的启动流程是怎样的？
+4. 什么是条件注解（@Conditional）？
+5. Spring Boot 如何实现配置文件加载？
+6. Spring Boot 的健康检查和监控如何实现？
+7. Spring Boot 的 Starter 原理是什么？
+
+---
+
+## 标准答案
+
+### 1. @SpringBootApplication 注解
+
+```java
+@SpringBootConfiguration           // 1. 配置类
+@EnableAutoConfiguration           // 2. 自动配置
+@ComponentScan                     // 3. 组件扫描
+public @interface SpringBootApplication {
+}
+```
+
+---
+
+#### **1. @SpringBootConfiguration**
+
+```java
+@Configuration  // 本质上是 @Configuration
+public @interface SpringBootConfiguration {
+}
+```
+
+**作用**：标记为配置类（等同于 XML 配置文件）
+
+---
+
+#### **2. @EnableAutoConfiguration**
+
+```java
+@AutoConfigurationPackage  // 自动配置包
+@Import(AutoConfigurationImportSelector.class)  // 导入自动配置类
+public @interface EnableAutoConfiguration {
+}
+```
+
+**核心**：`AutoConfigurationImportSelector` 
+
+```java
+// 加载 META-INF/spring.factories 中的自动配置类
+String[] configurations = SpringFactoriesLoader.loadFactoryNames(
+    getSpringFactoriesLoaderFactoryClass(),
+    getBeanClassLoader()
+);
+```
+
+---
+
+#### **3. @ComponentScan**
+
+```java
+@ComponentScan(
+    excludeFilters = {           // 排除过滤器
+        @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
+        @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class)
+    }
+)
+```
+
+**作用**：扫描 `@Component`、`@Service`、`@Repository`、`@Controller` 等注解
+
+---
+
+### 2. 自动配置原理
+
+#### **核心流程**
+
+```
+1. @EnableAutoConfiguration
+    ↓
+2. 加载 META-INF/spring.factories
+    ↓
+3. 根据 @Conditional 条件注解决定是否加载配置
+    ↓
+4. 注册 Bean
+```
+
+---
+
+#### **spring.factories 示例**
+
+```properties
+# spring-boot-autoconfigure-2.7.0.jar
+org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
+org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,\
+org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,\
+org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,\
+...
+```
+
+---
+
+#### **条件注解示例**
+
+```java
+@Configuration
+@ConditionalOnClass(DataSource.class)           // 类路径存在 DataSource
+@ConditionalOnMissingBean(DataSource.class)      // 容器中不存在 DataSource Bean
+@EnableConfigurationProperties(DataSourceProperties.class)
+public class DataSourceAutoConfiguration {
+    
+    @Bean
+    public DataSource dataSource(DataSourceProperties properties) {
+        // 创建 DataSource
+        return properties.initializeDataSourceBuilder().build();
+    }
+}
+```
+
+---
+
+### 3. Spring Boot 启动流程
+
+```java
+@SpringBootApplication
+public class Application {
+    public static void main(String[] args) {
+        SpringApplication.run(Application.class, args);
+    }
+}
+```
+
+---
+
+#### **核心步骤**
+
+```
+1. 创建 SpringApplication
+    ↓
+2. 准备 Environment
+    ↓
+3. 打印 Banner
+    ↓
+4. 创建 ApplicationContext
+    ↓
+5. 刷新 Context（加载 Bean）
+    ↓
+6. 调用 Runner（ApplicationRunner、CommandLineRunner）
+```
+
+---
+
+#### **源码分析**
+
+```java
+public SpringApplication(Class<?>... primarySources) {
+    // 1. 保存主配置类
+    this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
+    
+    // 2. 推断 Web 应用类型（SERVLET、REACTIVE、NONE）
+    this.webApplicationType = WebApplicationType.deduceFromClasspath();
+    
+    // 3. 加载 ApplicationContextInitializer
+    setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
+    
+    // 4. 加载 ApplicationListener
+    setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
+    
+    // 5. 推断主类（main 方法所在类）
+    this.mainApplicationClass = deduceMainApplicationClass();
+}
+```
+
+---
+
+### 4. 配置文件加载
+
+#### **加载顺序**
+
+```
+1. 命令行参数（--server.port=8081）
+2. Java 系统属性（System.getProperties()）
+3. 操作系统环境变量
+4. JAR 外部的 application-{profile}.properties
+5. JAR 内部的 application-{profile}.properties
+6. JAR 外部的 application.properties
+7. JAR 内部的 application.properties
+8. @PropertySource
+9. 默认属性（SpringApplication.setDefaultProperties）
+```
+
+---
+
+#### **配置文件示例**
+
+```yaml
+# application.yml
+server:
+  port: 8080
+
+spring:
+  profiles:
+    active: dev
+---
+spring:
+  config:
+    activate:
+      on-profile: dev
+  datasource:
+    url: jdbc:mysql://localhost:3306/dev
+---
+spring:
+  config:
+    activate:
+      on-profile: prod
+  datasource:
+    url: jdbc:mysql://prod-db:3306/prod
+```
+
+---
+
+### 5. 自定义 Starter
+
+#### **步骤**
+
+**1. 创建 autoconfigure 模块**：
+
+```java
+@Configuration
+@ConditionalOnClass(HelloService.class)
+@EnableConfigurationProperties(HelloProperties.class)
+public class HelloAutoConfiguration {
+    
+    @Bean
+    @ConditionalOnMissingBean
+    public HelloService helloService(HelloProperties properties) {
+        return new HelloService(properties.getMessage());
+    }
+}
+```
+
+**2. 创建配置类**：
+
+```java
+@ConfigurationProperties(prefix = "hello")
+public class HelloProperties {
+    private String message = "Hello World";
+    
+    // getter/setter
+}
+```
+
+**3. 创建 spring.factories**：
+
+```properties
+# META-INF/spring.factories
+org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
+com.example.hello.HelloAutoConfiguration
+```
+
+**4. 使用**：
+
+```yaml
+# application.yml
+hello:
+  message: Hello Spring Boot
+```
+
+```java
+@Autowired
+private HelloService helloService;
+
+helloService.sayHello();  // 输出：Hello Spring Boot
+```
+
+---
+
+### 6. Actuator 监控
+
+#### **依赖**
+
+```xml
+<dependency>
+    <groupId>org.springframework.boot</groupId>
+    <artifactId>spring-boot-starter-actuator</artifactId>
+</dependency>
+```
+
+---
+
+#### **配置**
+
+```yaml
+management:
+  endpoints:
+    web:
+      exposure:
+        include: health,info,metrics,prometheus  # 暴露端点
+  endpoint:
+    health:
+      show-details: always  # 显示详细信息
+```
+
+---
+
+#### **常用端点**
+
+| 端点 | 说明 | 示例 |
+|------|------|------|
+| **/actuator/health** | 健康检查 | `{"status":"UP"}` |
+| **/actuator/info** | 应用信息 | 版本、描述等 |
+| **/actuator/metrics** | 指标 | JVM、HTTP 请求等 |
+| **/actuator/prometheus** | Prometheus 格式 | 监控数据 |
+| **/actuator/env** | 环境变量 | 配置属性 |
+| **/actuator/loggers** | 日志配置 | 日志级别 |
+
+---
+
+#### **自定义健康检查**
+
+```java
+@Component
+public class DatabaseHealthIndicator implements HealthIndicator {
+    @Autowired
+    private DataSource dataSource;
+
+    @Override
+    public Health health() {
+        try (Connection conn = dataSource.getConnection()) {
+            if (conn.isValid(1)) {
+                return Health.up()
+                    .withDetail("database", "MySQL")
+                    .build();
+            }
+        } catch (SQLException e) {
+            return Health.down()
+                .withException(e)
+                .build();
+        }
+    }
+}
+```
+
+---
+
+### 7. 阿里 P7 加分项
+
+**深度理解**：
+- 理解 Spring Boot 的条件装配机制
+- 理解 SpringApplication 的启动流程
+- 理解 AutoConfigurationImportSelector 的工作原理
+
+**实战经验**：
+- 有自定义 Starter 的经验
+- 有解决自动配置冲突的经验
+- 有 Spring Boot 性能优化的经验
+
+**架构能力**：
+- 能设计可复用的 Spring Boot Starter
+- 能设计多环境配置方案
+- 能设计微服务监控体系
+
+**技术选型**：
+- 了解 Spring Boot 2.x vs 3.x 的区别
+- 了解 GraalVM Native Image 的支持
+- 了解 Quarkus、Micronaut 等替代框架