2019年java中高级java面试题（四）springBoot

野性酷女 2022-01-22 09:55 255阅读 0赞

### **1、如何在 Spring Boot 启动的时候运行一些特定的代码？** ###

可以实现接口 **ApplicationRunner** 或者 **CommandLineRunner**，这两个接口实现方式一样，它们都只提供了一个 run 方法

@Component
    @Order(1)
    public class Mytest implements ApplicationRunner {
    	@Override
    	public void run(ApplicationArguments args) throws Exception {
    		System.out.println("启动测试");
    	}
    
    }

@Component
    @Order(1)
    public class Mytest implements CommandLineRunner {
    
    	@Override
    	public void run(String... args) throws Exception {
    		System.out.println("启动测试");
    	}
    	
    }

### 2、Spring Boot 有哪几种读取配置的方式？ ###

Spring Boot 可以通过 @PropertySource,@Value,@Environment, @ConfigurationProperties 来绑定变量

@PropertySource(value={"classpath:/user.properties"})
    public class Configs {
        @Value("${u.name}")
        private String userName;
     
        @Value("${u.age}")
        private Integer age;
    }

mail.host=localhost
    mail.port=25
    mail.smtp.auth=false
    mail.smtp.starttls-enable=false
    mail.from=me@localhost
    mail.username=duan
    mail.password=duan123456

import org.springframework.boot.context.properties.ConfigurationProperties;
     
    @ConfigurationProperties(locations = "classpath:mail.properties", ignoreUnknownFields = false, prefix = "mail")
    public class MailProperties {
        private String host;
        private int port;
        private String from;
        private String username;
        private String password;
        private Smtp smtp;
     
    }

### 3、springBoot自动化配置的原理 ###

启动类的@SpringBootApplication 是一个复合注解

@Target(ElementType.TYPE)
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @Inherited
    @SpringBootConfiguration
    @EnableAutoConfiguration
    @ComponentScan(excludeFilters = {
    		@Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
    		@Filter(type = FilterType.CUSTOM,
    				classes = AutoConfigurationExcludeFilter.class) })
    public @interface SpringBootApplication {

@SpringBootConfiguration继承自@Configuration，二者功能也一致，标注当前类是配置类，  
并会将当前类内声明的一个或多个以@Bean注解标记的方法的实例纳入到spring容器中，并且实例名就是方法名。

@EnableAutoConfiguration里通过@Import导入了`EnableAutoConfigurationImportSelector`

会查询`META-INF/spring.factories`文件中包含的`JAR`文件。

RedisAutoConfiguration

@Configuration
    @ConditionalOnClass(RedisOperations.class)
    @EnableConfigurationProperties(RedisProperties.class)
    @Import({ LettuceConnectionConfiguration.class, JedisConnectionConfiguration.class })
    public class RedisAutoConfiguration {
    
    	@Bean
    	@ConditionalOnMissingBean(name = "redisTemplate")
    	public RedisTemplate<Object, Object> redisTemplate(
    			RedisConnectionFactory redisConnectionFactory) throws UnknownHostException {
    		RedisTemplate<Object, Object> template = new RedisTemplate<>();
    		template.setConnectionFactory(redisConnectionFactory);
    		return template;
    	}
    
    	@Bean
    	@ConditionalOnMissingBean
    	public StringRedisTemplate stringRedisTemplate(
    			RedisConnectionFactory redisConnectionFactory) throws UnknownHostException {
    		StringRedisTemplate template = new StringRedisTemplate();
    		template.setConnectionFactory(redisConnectionFactory);
    		return template;
    	}
    
    }