构建后端第3篇之---springb @Alias注解使用

write by 张艳涛 in 202210210,after today i will  write aritles by english,because english is so diffent from chiness that i can't 

understand english in java,spring doc notice ,so i begin to use english for a lifelong time. atthought  is hard for me ,but i am a man would like learn little by little , yes  is the only way to learn language by use it in common daily time

Tody I would lile recode usage of spring annotion @Aliasfor

Capture One

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ToIntroduce @Aliasfor , you master kown the  @SpringBootApplication   ,it uses start Main Class, then show the context 

@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 {

    /**
     * Exclude specific auto-configuration classes such that they will never be applied.
     * @return the classes to exclude
     */
    @AliasFor(annotation = EnableAutoConfiguration.class)
    Class<?>[] exclude() default {};

    /**
     * Exclude specific auto-configuration class names such that they will never be
     * applied.
     * @return the class names to exclude
     * @since 1.3.0
     */
    @AliasFor(annotation = EnableAutoConfiguration.class)
    String[] excludeName() default {};

    /**
     * Base packages to scan for annotated components. Use {@link #scanBasePackageClasses}
     * for a type-safe alternative to String-based package names.
     * @return base packages to scan
     * @since 1.3.0
     */
    @AliasFor(annotation = ComponentScan.class, attribute = "basePackages")
    String[] scanBasePackages() default {};

    /**
     * Type-safe alternative to {@link #scanBasePackages} for specifying the packages to
     * scan for annotated components. The package of each class specified will be scanned.
     * <p>
     * Consider creating a special no-op marker class or interface in each package that
     * serves no purpose other than being referenced by this attribute.
     * @return base packages to scan
     * @since 1.3.0
     */
    @AliasFor(annotation = ComponentScan.class, attribute = "basePackageClasses")
    Class<?>[] scanBasePackageClasses() default {};

}

and  with 

@SpringBootApplication(scanBasePackages ={ "com.zyt" } )//这个是rouyi的方案
public class ZytAuthApplication {
    public static void main(String[] args) {
        SpringApplication.run(ZytAuthApplication.class, args);
        System.out.println("(♥◠‿◠)ﾉﾞ  授权认证中心启动成功   ლ(´ڡ`ლ)ﾞ  
" );
    }
}

you can know the attibute  (scanBasePackages ={ "com.zyt" } ) is for 

    @AliasFor(annotation = ComponentScan.class, attribute = "basePackages")
    String[] scanBasePackages() default {};

then the  SpringBootApplication Annotation attibute scanBasePackages ={ "com.zyt" } , so we can undestand 

@AliasFor isfor give scanBasePackages ={ "com.zyt" } to ComponentScan.class ,

ComponentScan overrite its attibute  with {"com.zyt"}

In One World like  meta-annotation get attibute value from Top Annotation 's attibute
this is call Explicit alias for attribute in meta-annotation:

Capture Two

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Explicit aliases within an annotation: 

 public @interface ContextConfiguration {
  
      @AliasFor("locations")
      String[] value() default {};
  
      @AliasFor("value")
      String[] locations() default {};
  
      // ...
   }

for more

Capture Three

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Example: Implicit Aliases within an Annotation

 @ContextConfiguration
   public @interface MyTestConfig {
  
      @AliasFor(annotation = ContextConfiguration.class, attribute = "locations")
      String[] value() default {};
  
      @AliasFor(annotation = ContextConfiguration.class, attribute = "locations")
      String[] groovyScripts() default {};
  
      @AliasFor(annotation = ContextConfiguration.class, attribute = "locations")
      String[] xmlFiles() default {};
   }

 @MyTestConfig
   public @interface GroovyOrXmlTestConfig {
  
      @AliasFor(annotation = MyTestConfig.class, attribute = "groovyScripts")
      String[] groovy() default {};
  
      @AliasFor(annotation = ContextConfiguration.class, attribute = "locations")
      String[] xml() default {};
   }

for more