Java - 注解使用示例（解析注解格式化类生成字符串）

快来打我* 2022-05-31 11:46 510阅读 0赞

# 目的 #

使用注解标注对象属性，用于格式化输出字符串

# 注解 #

我们主要创建以下两个注解：

1.  @Label：用于定制输出字段的名称
2.  @Format ：用于格式化时间类型的字段

## @Label ##

package test2;
    
    import java.lang.annotation.ElementType;
    import java.lang.annotation.Retention;
    import java.lang.annotation.RetentionPolicy;
    import java.lang.annotation.Target;
    
    /** * Created by Joe on 2018/2/23. * 用于定制输出字段的名称 */
    @Target(ElementType.FIELD)
    @Retention(RetentionPolicy.RUNTIME)
    public @interface Label { 
        String value() default "";
    }

## @Format ##

package test2;
    
    import java.lang.annotation.ElementType;
    import java.lang.annotation.Retention;
    import java.lang.annotation.RetentionPolicy;
    import java.lang.annotation.Target;
    
    /** * Created by Joe on 2018/2/23. * 用于定义日期类型的输出格式 */
    @Target(ElementType.FIELD)
    @Retention(RetentionPolicy.RUNTIME)
    public @interface Format { 
        String pattern() default "yyyy-MM-dd HH:mm:ss";
    
        String timezone() default "GMT+8";
    }

# 定义Student类，使用注解对字段进行标注 #

package test2;
    
    import java.util.Date;
    
    /** * Created by Joe on 2018/2/23. */
    public class Student { 
        @Label("姓名")
        String name;
    
        @Label("出生日期")
        @Format(pattern = "yyyy/MM/dd")
        Date born;
    
        @Label("分数")
        double score;
    
        public Student(String name, Date born, double score) {
            this.name = name;
            this.born = born;
            this.score = score;
        }
    }

# 编写格式化处理 #

package test2;
    
    import java.lang.reflect.Field;
    import java.text.SimpleDateFormat;
    import java.util.Date;
    import java.util.TimeZone;
    
    /** * Created by Joe on 2018/2/23. */
    public class SimpleFormatter { 
        public static String format(Object obj) {
            try {
                Class cls = obj.getClass();
                StringBuilder stringBuilder = new StringBuilder();
    
                for (Field field : cls.getDeclaredFields()) {
                    if (!field.isAccessible()) {
                        field.setAccessible(true);
                    }
    
                    Label label = field.getAnnotation(Label.class);
                    String name = label != null ? label.value() : field.getName();
    
                    Object value = field.get(obj);
                    if (value != null && field.getType() == Date.class) {
                        value = formatDate(field, value);
                    }
    
                    stringBuilder.append(name + ":" + value + "\n");
                }
    
                return stringBuilder.toString();
            } catch (IllegalAccessException e) {
                e.printStackTrace();
            }
    
            return "";
        }
    
        private static Object formatDate(Field field, Object value) {
            Format format = field.getAnnotation(Format.class);
            if (format != null) {
                SimpleDateFormat dateFormat = new SimpleDateFormat(format.pattern());
                dateFormat.setTimeZone(TimeZone.getTimeZone(format.timezone()));
    
                return dateFormat.format(value);
            }
    
            return value;
        }
    }

# 测试 #

package test2;
    
    import java.text.SimpleDateFormat;
    
    /** * Created by Joe on 2018/2/23. */
    public class Main { 
        public static void main(String[] args) throws Exception {
            SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd");
    
            Student student = new Student("张三", simpleDateFormat.parse("1990-12-12"), 80.9);
    
            System.out.println(SimpleFormatter.format(student));
        }
    }

**输出结果：**

> 姓名:张三  
> 出生日期:1990/12/12  
> 分数:80.9