Spring Boot的注解，你知道或者不知道的都在这里！

1.1 定义

Annotation(注解)，用于为Java代码提供元数据。简单理解注解可以看做是一个个标签，用来标记代码。是一种应用于类、方法、参数、变量、构造器及包的一种特殊修饰符。

1.2 注解的声明

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
public @interface MyAnnotation{
​
}

　　

1.3 元注解

元注解就是注解到注解上的注解，或者说元注解是一种基本注解，它能用来注解其他注解。

我们可以将元注解看成一种特殊的修饰符，用来解释说明注解，它是注解的元数据。

• @Documented

被@Documented修饰的Annotation类将会被javadoc工具提取成文档。

• @Inherited

被@Inherited修改的Annotation将具有继承性，如果某个类使用了@MyAnnotation注解(定义该Annotation时使用了@Inherited修饰)修饰，则其子类将自动被@MyAnnotation修饰。

• @Retention

被@Retention修改的注解，结合RetentionPolicy.XXX可以指定该注解存在的声明周期。

SOURCE:仅存在Java源码文件，经过编译器后便丢弃

CLASS:存在Java源文件，以及经过编译器后生成的Class字节码文件，但在运行时JVM中不再保留

RUNTIME:存在源文件、变异生成的Class字节码文件，以及保留在运行时JVM中，可以通过反射读取注解信息

• @Target

表示该注解类型所使用的程序元素类型，结合ElementType.XXX来使用。

• @Repeatable

Java8新增的可重复注解。

1.4 JDK中常见注解

• @Override

用于告知编译器，我们需要覆写超类的当前方法。

• @Deprecated

使用这个注解，用于告知编译器，某一程序元素(比如方法，成员变量)不建议使用了(即过时了)。

• @SuppressWarnings

用于告知编译器忽略特定的警告信息，例在泛型中使用原生数据类型，编译器会发出警告，当使用该注解后，则不会发出警告。

• @FunctionalInterface

用户告知编译器，检查这个接口，保证该接口是函数式接口，即只能包含一个抽象方法，否则就会编译出错。

1.5 自定义注解使用

• 格式

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
public @interface MyAnnotation{
​
}

• 成员变量

定义：用无形参的方法形式来声明，注解方法不能带参数，比如name(),age()

类型：前面可以用数据类型来修饰

默认值：成员变量可以有默认值，比如default "spring boot"

• 注解使用

@MyAnnotation(name="Jack",age=16)
public class Person {
}

• 反射获取类上的注解

//1.获取需要解析注解的类
Class<Person> clazz=Person.class;
//2.判断该类上是否有注解
if(clazz.isAnnotationPresent(MyAnnotation.class)){
    //3.获取该类上的注解
    MyAnnotation myAnnotation=clazz.getAnnotation(MyAnnotation.class);
    //4.打印出注解上的内容
    System.out.println(myAnnotation.name()+":"+myAnnotation.age());
}

2 @SpringBootApplication

官网见：18. Using the @SpringBootApplication Annotation

The @SpringBootApplication annotation is equivalent to using @Configuration, @EnableAutoConfiguration, and @ComponentScan with their default attributes

@SpringBootConfiguration等同于@Configuration

3 @SpringBootConfiguration

@SpringBootConfiguration等同于@Configuration，@Configuration等同于@Component

3.1 作用

@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，实例名就是方法名。

3.2 使用

• @Configuration: allow to register extra beans in the context or import additional configuration classes

@SpringBootConfiguration
public class Config{
    @Bean
    public Map getMap(){
        Map map=new HashMap();
        map.put("username","Jack");
        return map;
    }
}

可以直接通过context.getBean("getMap")的方式获取。

3.3 扩展

@Configuration等同与@Component

官网见[Spring Framework Core]：1.10.1. @Component and Further Stereotype Annotations

conclusion:@Component includes @Configuration,@Repository,@Service and @Controller

4 @ComponentScan

• @ComponentScan: enable @Component scan on the package where the application is located (see the best practices)

官网见[Spring Framework Core]：1.10.3. Automatically Detecting Classes and Registering Bean Definitions

To autodetect these classes and register the corresponding beans, you need to add @ComponentScan to your @Configuration class, where the basePackages attribute is a common parent package for the two classes. (Alternatively, you can specify a comma- or semicolon- or space-separated list that includes the parent package of each class.)

@ComponentScan主要就是定义扫描的路径以及子路径中，找出标识了需要装配的类自动装配到Spring的bean容器中。

官网见：17. Spring Beans and Dependency Injection

If you structure your code as suggested above (locating your application class in a root package), you can add @ComponentScan without any arguments. All of your application components (@Component, @Service, @Repository, @Controller etc.) are automatically registered as Spring Beans.

5 @EnableAutoConfiguration

• @EnableAutoConfiguration: enable Spring Boot’s auto-configuration mechanism

官网见： 11.3.2 The @EnableAutoConfiguration Annotation

The second class-level annotation is @EnableAutoConfiguration. This annotation tells Spring Boot to “guess” how you want to configure Spring, based on the jar dependencies that you have added. Since spring-boot-starter-web added Tomcat and Spring MVC, the auto-configuration assumes that you are developing a web application and sets up Spring accordingly.

5.1 @Import(XXX)

借助AutoConfigurationImportSelector，@EnableAutoConfiguration可以帮助Spring Boot应用将所有符合条件的@Configuration配置都加载到IoC容器中

5.2 SpringFactoriesLoader

• selectImports方法

@Override
public String[] selectImports(AnnotationMetadata annotationMetadata) {
   if (!isEnabled(annotationMetadata)) {
      return NO_IMPORTS;
   }
   AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
         .loadMetadata(this.beanClassLoader);
   AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(
         autoConfigurationMetadata, annotationMetadata);
   return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
}

• 读取候选装配组件getCandidateConfigurations

protected AutoConfigurationEntry getAutoConfigurationEntry(
      AutoConfigurationMetadata autoConfigurationMetadata,
      AnnotationMetadata annotationMetadata) {
   if (!isEnabled(annotationMetadata)) {
      return EMPTY_ENTRY;
   }
   AnnotationAttributes attributes = getAttributes(annotationMetadata);
   List<String> configurations = getCandidateConfigurations(annotationMetadata,
         attributes);
   configurations = removeDuplicates(configurations);
   Set<String> exclusions = getExclusions(annotationMetadata, attributes);
   checkExcludedClasses(configurations, exclusions);
   configurations.removeAll(exclusions);
   configurations = filter(configurations, autoConfigurationMetadata);
   fireAutoConfigurationImportEvents(configurations, exclusions);
   return new AutoConfigurationEntry(configurations, exclusions);
}
protected List<String> getCandidateConfigurations(AnnotationMetadata metadata,
      AnnotationAttributes attributes) {
   List<String> configurations = SpringFactoriesLoader.loadFactoryNames(
         getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader());
   Assert.notEmpty(configurations,
         "No auto configuration classes found in META-INF/spring.factories. If you "
               + "are using a custom packaging, make sure that file is correct.");
   return configurations;
}

• SpringFacotriesLoader.loadFactoryNames

public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) {
   String factoryClassName = factoryClass.getName();
   return loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());
}

点开loadSpringFactories

Enumeration<URL> urls = (classLoader != null ?
      classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
      ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));

点开FACTORIES_RESOURCE_LOCATION

public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";

5.3 autoconfigure.jar

# Initializers
org.springframework.context.ApplicationContextInitializer=
org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer,
org.springframework.boot.autoconfigure.logging.ConditionEvaluationReportLoggingListener

6 @Conditional

* Indicates that a component is only eligible for registration when all
* {@linkplain #value specified conditions} match.

6.1 Understanding Auto-Configured Beans

官网见：49.1 Understanding Auto-configured Beans

Under the hood, auto-configuration is implemented with standard @Configuration classes. Additional @Conditional annotations are used to constrain when the auto-configuration should apply. Usually, auto-configuration classes use @ConditionalOnClass and @ConditionalOnMissingBean annotations. This ensures that auto-configuration applies only when relevant classes are found and when you have not declared your own @Configuration.

You can browse the source code of spring-boot-autoconfigure to see the @Configuration classes that Spring provides (see the META-INF/spring.factoriesfile).

6.2 Condition Annotations

官网见：49.3 Condition Annotations

You almost always want to include one or more @Conditional annotations on your auto-configuration class. The @ConditionalOnMissingBean annotation is one common example that is used to allow developers to override auto-configuration if they are not happy with your defaults.

Spring Boot includes a number of @Conditional annotations that you can reuse in your own code by annotating @Configuration classes or individual @Beanmethods. These annotations include:

• Section 49.3.1, “Class Conditions”

• Section 49.3.2, “Bean Conditions”

• Section 49.3.3, “Property Conditions”

• Section 49.3.4, “Resource Conditions”

• Section 49.3.5, “Web Application Conditions”

• Section 49.3.6, “SpEL Expression Conditions”

翻译:不同类型的Conditional

（1）ConditionalOnClass：当且仅当ClassPath存在指定的Class时，才创建标记上该注解的类的实例

（2）ConditionalOnBean: 当且仅当指定的bean classes and/or bean names在当前容器中,才创建标记上该注解的类的实例

（3）ConditionalOnProperty：当且仅当Application.properties存在指定的配置项时，创建标记上了该注解的类的实例

（4）ConditionalOnResource：在classpath下存在指定的resource时创建

（5）ConditionalOnWebApplication：在web环境下创建

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