2019年java中高级java面试题（四）springBoot

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;
	}

}