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 {
    @Bean
    Person 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 {
    @Override
    public 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 {
    @Override
    public 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 {
    @Override
    public 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

原文地址：https://blog.csdn.net/qq_43259860/article/details/139278767

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