Spring-注解

Spring 注解分类

Spring 注解驱动模型

Spring 元注解

@Documented
@Retention()
@Target()
// 可以继承相关的属性
@Inherited
@Repeatable()

Spirng 模式注解

@ComponentScan 原理
ClassPathScanningCandidateComponentProvider#findCandidateComponents

public Set<BeanDefinition> findCandidateComponents(String basePackage) {if (this.componentsIndex != null && indexSupportsIncludeFilters()) {return addCandidateComponentsFromIndex(this.componentsIndex, basePackage);}else {return scanCandidateComponents(basePackage);}
}

组合注解

比如@RestController,@SpringBootApplication
注解转换为AnnotationAttributes然后进行合并统一

Spring属性别名

显性别名：

隐式别名：
这个组合注解
继承EnableAutoConfiguration的属性，用隐式别名来表达，也就是直接拿过来不需要改变它的值：

如果说我们要补充语义，引用某个属性，自己定义新的属性名称来引用父类的：

举例如果我们要派生这个注解，我们不可能在注解里面写死扫描路径，这个时候就要通过隐式别名来继承注解ComponentScan的属性，这样我们就可以配置scanBasePackages

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@ComponentScan
public @interface MyComponentScan {@AliasFor(annotation = ComponentScan.class, attribute = "basePackages")String scanBasePackages() default "";}

我们也可以这样：

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@ComponentScan
public @interface MyComponentScan {// 传递性别名 basePackages -> value value -> scanBasePackages@AliasFor(annotation = ComponentScan.class, attribute = "value")String scanBasePackages() default "";
}

Spring属性覆盖

注解和原注解出现同名属性的时候会覆盖原注解的属性

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@ComponentScan
public @interface MyComponentScan {@AliasFor(annotation = ComponentScan.class, attribute = "value")String scanBasePackages() default "";
}@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@MyComponentScan
public @interface MyComponentScan2 {// 显示覆盖String[] scanBasePackages() default "";// 隐式覆盖 @AliasFor(attribute = "scanBasePackages")String[] packages() default {};
}
@MyComponentScan2(packages = {"com.yong.annotationpkg"})
// packages 覆盖scanBasePackages scanBasePackages覆盖MyComponentScan scanBasePackages scanBasePackages 引用的是ComponentScan

Spring @Enable 模块驱动

案例1通过@Configration实现

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Import(DemoConfig.class)
public @interface EnableDemo {}
@Configuration
public class DemoConfig {@BeanPerson person() {Person person = new Person();person.setName("liy");person.setId(11L);return person;}
}@EnableDemo
public class EnableAnnotationDemo {public static void main(String[] args) {AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext();context.register(EnableAnnotationDemo.class);context.refresh();Person bean = context.getBean(Person.class);System.out.println(bean);}
}

案例2通过ImportSelector接口实现：

public class DemoImportSelector implements ImportSelector {@Overridepublic String[] selectImports(AnnotationMetadata importingClassMetadata) {return new String[]{"com.yong.annotationpkg.DemoConfig"};}
}
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Import(DemoImportSelector.class)
public @interface EnableDemo {}
@EnableDemo
public class EnableAnnotationDemo {public static void main(String[] args) {AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext();context.register(EnableAnnotationDemo.class);context.refresh();Person bean = context.getBean(Person.class);System.out.println(bean);}
}

案例3通过

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Import(DemoImportBeanDefinitionRegister.class)
public @interface EnableDemo {}
public class DemoImportBeanDefinitionRegister implements ImportBeanDefinitionRegistrar {@Overridepublic void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry, BeanNameGenerator importBeanNameGenerator) {AnnotatedBeanDefinition annotatedBeanDefinition = new AnnotatedGenericBeanDefinition(DemoConfig.class);registry.registerBeanDefinition("demoConfig", annotatedBeanDefinition);}
}

Spring 条件注解

@Configuration
public class ProfileDemo {public static void main(String[] args) {AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext();context.register(ProfileDemo.class);ConfigurableEnvironment environment = context.getEnvironment();// 兜底方案environment.setDefaultProfiles("odd");// 激活方案environment.setActiveProfiles("even");context.refresh();Object odd = context.getBean(Integer.class);System.out.println(odd);context.close();}// 激活不同的环境注册不同的Bean@Bean@Profile("odd")public Integer odd() {return 2;}@Bean@Profile("even")public Integer even() {return 1;}
}

自定义实现：

public class EvenProfileCondition implements Condition {@Overridepublic boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {Environment environment = context.getEnvironment();return environment.acceptsProfiles("even");}
}
@Bean@Conditional(EvenProfileCondition.class)public Integer even() {return 1;}

ConditionEvaluator#shouldSkip

参考资料
小马哥核心编程思想