關於微服務你不得不知道的事——Spring Boot註解分析

1 註解

1.1 定義

Annotation(註解)，用於爲Java代碼提供元數據。簡單理解註解可以看做是一個個標籤，用來標記代碼。是一種應用於類、方法、參數、變量、構造器及包的一種特殊修飾符。

1.2 註解的聲明

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

1.3 元註解

元註解就是註解到註解上的註解，或者說元註解是一種基本註解，它能用來註解其他註解。

我們可以將元註解看成一種特殊的修飾符，用來解釋說明註解，它是註解的元數據。

• @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註解(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修改的註解，結合RetentionPolicy.XXX可以指定該註解存在的聲明週期。

SOURCE:僅存在Java源碼文件，經過編譯器後便丟棄

CLASS:存在Java源文件，以及經過編譯器後生成的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，可以通過反射讀取註解信息

• @Target

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

• @Repeatable

Java8新增的可重複註解。

1.4 JDK中常見註解

• @Override

用於告知編譯器，我們需要覆寫超類的當前方法。

• @Deprecated

使用這個註解，用於告知編譯器，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

用於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，當使用該註解後，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 自定義註解使用

• 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，註解方法不能帶參數，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"

• 註解使用

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

• 反射獲取類上的註解

1.獲取需要解析註解的類
Class<Person> clazz=Person.class;
//2.判斷該類上是否有註解
if(clazz.isAnnotationPresent(MyAnnotation.class)){
    //3.獲取該類上的註解
    MyAnnotation myAnnotation=clazz.getAnnotation(MyAnnotation.class);
    //4.打印出註解上的內容
    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等同於@Configuration

3 @SpringBootConfiguration

@SpringBootConfiguration等同於@Configuration，@Configuration等同於@Component

3.1 作用

@SpringBootConfiguration繼承自@Configuration，二者功能也一致，標註當前類是配置類。

並會將當前類內聲明的一個或多個以@Bean註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.)</pre>

@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-webadded Tomcat and Spring MVC, the auto-configuration assumes that you are developing a web application and sets up Spring accordingly.

5.1 @Import(XXX)

藉助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<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());
}

點開loadSpringFactories

Enumeration<URL> 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</pre>

6 @Conditional

• Indicates that a component is only eligible for registration when all
• {@linkplain #value specified conditions} match.</pre>

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，才創建標記上該註解的類的實例

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

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

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

（5）ConditionalOnWebApplication：在web環境下創建