摘要：注解分析注解定義注解，用于為代碼提供元數據。我們可以將元注解看成一種特殊的修飾符，用來解釋說明注解，它是注解的元數據。被修改的注解，結合可以指定該注解存在的聲明周期。新增的可重復注解。

Spring Boot 注解分析

1 注解
1.1 定義
Annotation(注解)，用于為Java代碼提供元數據。簡單理解注解可以看做是一個個標簽，用來標記代碼。是一種應用于類、方法、參數、變量、構造器及包的一種特殊修飾符。

1.2 注解的聲明

@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，或者說元注解是一種基本注解，它能用來注解其他注解。 
我們可以將元注解看成一種特殊的修飾符，用來解釋說明注解，它是注解的元數據。

@Documented

被@Documented修飾的Annotation類將會被javadoc工具提取成文檔。

@Inherited

被@Inherited修改的Annotation將具有繼承性，如果某個類使用了@MyAnnotation注解(定義該Annotation時使用了@Inherited修飾)修飾，則其子類將自動被@MyAnnotation修飾。

@Retention

被@Retention修改的注解，結合RetentionPolicy.XXX可以指定該注解存在的聲明周期。

SOURCE:僅存在Java源碼文件，經過編譯器后便丟棄

CLASS:存在Java源文件，以及經過編譯器后生成的Class字節碼文件，但在運行時JVM中不再保留

RUNTIME:存在源文件、變異生成的Class字節碼文件，以及保留在運行時JVM中，可以通過反射讀取注解信息

@Target

表示該注解類型所使用的程序元素類型，結合ElementType.XXX來使用。

@Repeatable

Java8新增的可重復注解。

1.4 JDK中常見注解

@Override

用于告知編譯器，我們需要覆寫超類的當前方法。

@Deprecated

使用這個注解，用于告知編譯器，某一程序元素(比如方法，成員變量)不建議使用了(即過時了)。

@SuppressWarnings

用于告知編譯器忽略特定的警告信息，例在泛型中使用原生數據類型，編譯器會發出警告，當使用該注解后，則不會發出警告。

@FunctionalInterface

用戶告知編譯器，檢查這個接口，保證該接口是函數式接口，即只能包含一個抽象方法，否則就會編譯出錯。

1.5 自定義注解使用

格式

@Documented
@Target(ElementType.METHOD)
@Inherited
@Retention(RetentionPolicy.RUNTIME)
public @interface MyAnnotataion{

   String name();
   int age() default 17;
   String hello() default "spring boot";

}

成員變量

定義：用無形參的方法形式來聲明，注解方法不能帶參數，比如name(),age()

類型：前面可以用數據類型來修飾

默認值：成員變量可以有默認值，比如default "spring boot"

注解使用

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

反射獲取類上的注解

//1.獲取需要解析注解的類
Class clazz=Person.class;
//2.判斷該類上是否有注解
if(clazz.isAnnotationPresent(MyAnnotation.class)){

   //3.獲取該類上的注解
   MyAnnotation myAnnotation=clazz.getAnnotation(MyAnnotation.class);
   //4.打印出注解上的內容
   System.out.println(myAnnotation.name()+":"+myAnnotation.age());

}

2 @SpringBootApplication
官網見：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繼承自@Configuration，二者功能也一致，標注當前類是配置類。

并會將當前類內聲明的一個或多個以@Bean注解標記的方法的實例納入到Spring容器中，實例名就是方法名。

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;
   }

}

可以直接通過context.getBean("getMap")的方式獲取。

3.3 擴展

@Configuration等同與@Component

官網見[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)

官網見[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主要就是定義掃描的路徑以及子路徑中，找出標識了需要裝配的類自動裝配到Spring的bean容器中。

官網見：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

官網見： 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可以幫助Spring Boot應用將所有符合條件的@Configuration配置都加載到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());
}

讀取候選裝配組件getCandidateConfigurations

protected AutoConfigurationEntry getAutoConfigurationEntry(

     AutoConfigurationMetadata autoConfigurationMetadata,
     AnnotationMetadata annotationMetadata) {
  if (!isEnabled(annotationMetadata)) {
     return EMPTY_ENTRY;
  }
  AnnotationAttributes attributes = getAttributes(annotationMetadata);
  List configurations = getCandidateConfigurations(annotationMetadata,
        attributes);
  configurations = removeDuplicates(configurations);
  Set exclusions = getExclusions(annotationMetadata, attributes);
  checkExcludedClasses(configurations, exclusions);
  configurations.removeAll(exclusions);
  configurations = filter(configurations, autoConfigurationMetadata);
  fireAutoConfigurationImportEvents(configurations, exclusions);
  return new AutoConfigurationEntry(configurations, exclusions);

}

protected List getCandidateConfigurations(AnnotationMetadata metadata,

     AnnotationAttributes attributes) {
  List 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 loadFactoryNames(Class factoryClass, @Nullable ClassLoader classLoader) {

  String factoryClassName = factoryClass.getName();
  return loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());

}

點開loadSpringFactories

Enumeration urls = (classLoader != null ?

  classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
  ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));

點開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
官網見：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
官網見：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”

翻譯:不同類型的Conditional

（1）ConditionalOnClass：當且僅當ClassPath存在指定的Class時，才創建標記上該注解的類的實例

（2）ConditionalOnBean: 當且僅當指定的bean classes and/or bean names在當前容器中,才創建標記上該注解的類的實例

（3）ConditionalOnProperty：當且僅當Application.properties存在指定的配置項時，創建標記上了該注解的類的實例

（4）ConditionalOnResource：在classpath下存在指定的resource時創建

（5）ConditionalOnWebApplication：在web環境下創建