内容简介: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。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。
1 注解
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
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@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)。
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@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。
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@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
@Documented @Target(ElementType.METHOD) @Inherited @Retention(RetentionPolicy.RUNTIME) public @interface MyAnnotataion{ String name(); int age() default 17; String hello() default "spring boot"; }
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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 { }
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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
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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; }
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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.factories
file).
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 @Bean
methods. 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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