Java的注解Annotation应用

╰半夏微凉° 2024-04-23 02:44 1阅读 0赞

**记录**：482

**场景**：Java的注解Annotation应用，包括但不限于注解作用在类上、注解作用在字段上、注解作用在构造函数上、注解作用在方法上、注解作用在方法形式参数上、注解作用在方法的返回参数上，一个注解作用在多个场景上。

**版本**：JDK 1.8,Spring Boot 2.6.3。

**1.基础**

**1.1****Annotation****类**

在JDK的java.lang.annotation.Annotation中对Method的官方说明如下：

The common interface extended by all annotation types. Note that an interface that manually extends this one does not define an annotation type. Also note that this interface does not itself define an annotation type. More information about annotation types can be found in section 9.6 of The Java™ Language Specification. The reflect.AnnotatedElement interface discusses compatibility concerns when evolving an annotation type from being non-repeatable to being repeatable.

**1.2****Java 反射机制**

Java 反射机制是在运行状态中，对于任意一个类，都能够获得这个类的所有属性和方法，对于任意一个对象都能够调用它的任意一个属性和方法。这种在运行时动态的获取信息以及动态调用对象的方法的功能称为Java 的反射机制。

**2.注解作用在类上**

注解作用在类上，在创建类的对象实例时，可以根据注解信息做一些拦截操作。

**2.1定义注解**

//作用在类上的注解
    @Target(ElementType.TYPE)
    @Retention(RetentionPolicy.RUNTIME)
    public @interface Deep01ClassAno {
        String code();
        String value() default "";
    }

**2.2使用注解**

@Deep01ClassAno(code = "deep01", value = "福建")
    public class DeepWorker {
        @Deep02FieldAno(code = "deep02", value = "厦门")
        private String cityInfo;
    
        @Deep04ConstructorAno(code = "deep04", value = "创建城市")
        public DeepWorker() {
            this.cityInfo = "厦门";
        }
    
        @Deep03MethodAno(code = "deep03", value = "海沧")
        public @Deep06ReturnAno(code = "deep06", value = "海沧地铁2号线")
        String getDistrictInfo(@Deep05ParameterAno(code = "deep05", value = "地铁") String code) {
            return "接收信息: " + code;
        }
    }

**2.3获取和解析注解**

Class<?> clz01 = DeepWorker.class;
    //1.从类上获取注解常用方式
    Annotation ann0101 = clz01.getDeclaredAnnotation(Deep01ClassAno.class);
    Annotation[] ann0102 = clz01.getDeclaredAnnotations();
    Annotation ann0103 = clz01.getAnnotation(Deep01ClassAno.class);
    Annotation[] ann0104 = clz01.getAnnotations();
    //2.解析注解值
    Deep01ClassAno deep01 = (Deep01ClassAno) ann0101;
    System.out.println("deep01注解,code=" + deep01.code() + ",value=" + deep01.value());

**3.注解作用在字段上**

注解作用在字段上，可以根据注解信息对字段做些校验等操作。

**3.1定义注解**

//作用在字段上的注解
    @Target(ElementType.FIELD)
    @Retention(RetentionPolicy.RUNTIME)
    public @interface Deep02FieldAno {
        String code();
        String value() default "";
    }

**3.2使用注解**

public class DeepWorker {
        @Deep02FieldAno(code = "deep02", value = "厦门")
        private String cityInfo;
    }

**3.3获取和解析注解**

Class<?> clz02 = DeepWorker.class;
    Field field = clz02.getDeclaredField("cityInfo");
    //1.从字段上获取注解常用方式
    Annotation ann0201 = field.getDeclaredAnnotation(Deep02FieldAno.class);
    Annotation[] ann0202 = field.getDeclaredAnnotations();
    Annotation ann0203 = field.getAnnotation(Deep02FieldAno.class);
    Annotation[] ann0204 = field.getAnnotations();
    //2.解析注解值
    Deep02FieldAno deep02 = (Deep02FieldAno) ann0201;
    System.out.println("deep02注解,code=" + deep02.code() + ",value=" + deep02.value());

**4.注解作用在方法上**

注解作用在方法上，比如：在反射操作时，可以根据注解信息找到需要执行的方法。

**4.1定义注解**

//作用在方法上的注解
    @Target(ElementType.METHOD)
    @Retention(RetentionPolicy.RUNTIME)
    public @interface Deep03MethodAno {
        String code();
        String value() default "";
    }

**4.2使用注解**

public class DeepWorker {
        @Deep03MethodAno(code = "deep03", value = "海沧")
        public @Deep06ReturnAno(code = "deep06", value = "海沧地铁2号线")
        String getDistrictInfo(@Deep05ParameterAno(code = "deep05", value = "地铁") String code) {
            return "接收信息: " + code;
        }
    }

**4.3获取和解析注解**

Class<?> clz03 = DeepWorker.class;
    Method method = clz03.getDeclaredMethod("getDistrictInfo", String.class);
    //1.从方法上获取注解常用方式
    Annotation ann0301 = method.getDeclaredAnnotation(Deep03MethodAno.class);
    Annotation[] ann0302 = method.getDeclaredAnnotations();
    Annotation ann0303 = method.getAnnotation(Deep03MethodAno.class);
    Annotation[] ann0304 = method.getAnnotations();
    //2.解析注解值
    Deep03MethodAno deep03 = (Deep03MethodAno) ann0301;
    System.out.println("deep03注解,code=" + deep03.code() + ",value=" + deep03.value());

**5.注解作用在构造函数上**

注解作用在构造函数上，可以根据注解信息做一些需求判断。

**5.1定义注解**

@Target(ElementType.CONSTRUCTOR)
    @Retention(RetentionPolicy.RUNTIME)
    public @interface Deep04ConstructorAno {
        String code();
        String value() default "";
    }

**5.2使用注解**

@Deep01ClassAno(code = "deep01", value = "福建")
    public class DeepWorker {
        @Deep04ConstructorAno(code = "deep04", value = "创建城市")
        public DeepWorker() {
            this.cityInfo = "厦门";
        }
    }

**5.3获取和解析注解**

Class<?> clz04 = DeepWorker.class;
    Constructor<?> constructor = clz04.getConstructor();
    //1.从构造函数上获取注解常用方式
    Annotation ann0401 = constructor.getDeclaredAnnotation(Deep04ConstructorAno.class);
    Annotation[] ann0402 = constructor.getDeclaredAnnotations();
    Annotation ann0403 = constructor.getAnnotation(Deep04ConstructorAno.class);
    Annotation[] ann0404 = constructor.getAnnotations();
    //2.解析注解值
    Deep04ConstructorAno deep04 = (Deep04ConstructorAno) ann0401;
    System.out.println("deep04注解,code=" + deep04.code() + ",value=" + deep04.value());

**6.注解作用在方法形式参数上**

注解作用在方法形式参数上，可以根据注解信息做些校验等操作。

**6.1定义注解**

//作用在形式参数上的注解
    @Target(ElementType.PARAMETER)
    @Retention(RetentionPolicy.RUNTIME)
    public @interface Deep05ParameterAno {
        String code();
        String value() default "";
    }

**6.2使用注解**

**6.3获取和解析注解**

Class<?> clz05 = DeepWorker.class;
    Method method02 = clz05.getDeclaredMethod("getDistrictInfo", String.class);
    //1.从方法上获取形式参数的注解常用方式
    Annotation[][] ann0501 = method02.getParameterAnnotations();
    //2.解析注解值-遍历二维数组
    for (int i = 0; i < ann0501.length; i++) {
        if (ann0501[i] == null) continue;
        for (int j = 0; j < ann0501[i].length; j++) {
            Deep05ParameterAno deep05 = (Deep05ParameterAno) ann0501[i][j];
            System.out.println("deep05注解,code=" + deep05.code() + ",value=" + deep05.value());
        }
    }

**7.注解作用在方法的返回参数上**

注解作用在方法返回参数上，可以根据注解信息规范返回值格式等操作。

**7.1定义注解**

//作用在返回值上的注解
    @Target(ElementType.METHOD)
    @Retention(RetentionPolicy.RUNTIME)
    public @interface Deep06ReturnAno {
        String code();
        String value() default "";
    }

**7.2使用注解**

**7.3获取和解析注解**

Class<?> clz06 = DeepWorker.class;
    Method method03 = clz06.getDeclaredMethod("getDistrictInfo", String.class);
    //1.从方法上获取返回值注解常用方式
    Annotation ann0601 = method.getDeclaredAnnotation(Deep06ReturnAno.class);
    Annotation[] ann0602 = method.getDeclaredAnnotations();
    Annotation ann0603 = method.getAnnotation(Deep06ReturnAno.class);
    Annotation[] ann0604 = method.getAnnotations();
    //2.解析注解值
    Deep06ReturnAno deep06 = (Deep06ReturnAno) ann0601;
    System.out.println("deep06注解,code=" + deep06.code() + ",value=" + deep06.value());

**8.注解作用在多个场景上**

一个注解根据ElementType配置，可以作用在不同场景上。

**8.1定义注解**

//作用在多中场合的注解
    @Target({ElementType.TYPE, ElementType.METHOD})
    @Retention(RetentionPolicy.RUNTIME)
    public @interface Deep07MultipleAno {
        String code();
        String value() default "";
    }

**8.2使用注解**

@Deep07MultipleAno(code = "deep07-01", value = "福建")
    public class DeepWorker02 {
    
        @Deep07MultipleAno(code = "deep07-02", value = "厦门")
        public String getCity01(String code) {
            return "厦门";
        }
        public @Deep07MultipleAno(code = "deep07-03", value = "海沧") String getCity02() {
            return "海沧";
        }
    }

**8.3获取和解析注解**

Class<?> clz07 = DeepWorker02.class;
    //1.注解作用在类上
    Annotation ann0701 = clz07.getDeclaredAnnotation(Deep07MultipleAno.class);
    //2.注解作用在方法上
    Method method04 = clz07.getDeclaredMethod("getCity01", String.class);
    Annotation ann0702 = method04.getDeclaredAnnotation(Deep07MultipleAno.class);
    //3.注解作用在方法的返回值上
    Method method05 = clz07.getDeclaredMethod("getCity02");
    Annotation ann0703 = method05.getDeclaredAnnotation(Deep07MultipleAno.class);
    Deep07MultipleAno deep0701 = (Deep07MultipleAno) ann0701;
    Deep07MultipleAno deep0702 = (Deep07MultipleAno) ann0702;
    Deep07MultipleAno deep0703 = (Deep07MultipleAno) ann0703;
    System.out.println("deep0701注解,code=" + deep0701.code() + ",value=" + deep0701.value());
    System.out.println("deep0702注解,code=" + deep0702.code() + ",value=" + deep0702.value());
    System.out.println("deep0703注解,code=" + deep0703.code() + ",value=" + deep0703.value());

以上，感谢。

2024年3月1日