import com.alibaba.fastjson.JSON;import java.beans.PropertyDescriptor;import java.lang.annotation.*;import java.lang.reflect.Field;import java.lang.reflect.Method;import java.math.BigDecimal;import java.text.ParseException;import java.text.SimpleDateFormat;import java.time.LocalDateTime;import java.time.ZoneOffset;import java.util.*;/** * mysql 参数 */public interface MysqlConstants {    /**     * 数据库中varchar类型为null时，用该值表示，程序中进行转换     * 字段='NONE' <=> 字段 is null     */    String NULL_VARCHAR = "NONE";    /**     * 数据库中datetime类型时为空，用该值表示，程序中进行转换     * 如果需要查找为空的为，只需要令 字段=NULL_DATE即可走索引，避免IS NULL ,IS NOT NULL的效率问题     * 该日期为 8888-12-31 23:59:59，距离现在还有6000多年，6000年以后的系统就让6000年以后的人去维护就行了     * 由于Date类型不是安全类型，有可能会被程序不小心改变值，所以程序中通过new Date(NULL_DATE_MILLS)来设置默认值，     * 从而避免NULL_DATE的值会被改变     */    long NULL_DATE_MILLS = LocalDateTime.of(8888, 12, 31, 23, 59, 59).toInstant(ZoneOffset.of("+8")).toEpochMilli();    /**     * 时间字符串最大值，避免将空值取出，比空值少一秒     */    String MAX_DATETIME_STR = "8888-12-31 23:59:58";    /**     * tinyint     */    Byte DEF_BYTE = 0;    /**     * short     */    Short DEF_SHORT = 0;    /**     * int     */    Integer DEF_INT = 0;    /**     * bigint     */    Long DEF_LONG = 0L;    /**     * float     */    Float DEF_FLOAT = 0F;    /**     * double     */    Double DEF_DOUBLE = 0D;    /**     * decimal     */    BigDecimal DEF_DECIMAL = BigDecimal.valueOf(0);    /**     * tinyint(1)     */    Boolean DEF_BOOLEAN = Boolean.FALSE;    /**     * mysql 默认值跟空值的转换     * 对应的field必须有可访问的setter getter 方法     */    @Target({ElementType.FIELD})    @Retention(RetentionPolicy.RUNTIME)    @Documented    @interface CastMysqlDefaultNullValue {        /**         * 默认使用MysqlConstants中的默认值         * @return         */        String defaultValue() default "";        /**         * 将默认值字符串转成特定的数据类型         * @return         */        Class
     cast() default MysqlConstantsCast.MysqlConstantsDefaultCast.MysqlConstantsDefaultCast.class;    }    interface MysqlConstantsCast {        /**         * 将默认值转换为特别值         * @param defaultValue         * @param target 目标类型         * @return         */        
    
      T cast(String defaultValue, Class
     
       target);        class MysqlConstantsDefaultCast implements MysqlConstantsCast {            @Override            public 
      
        T cast(String defaultValue, Class
       
         target) {                if (target == String.class) {                    return (T)(defaultValue.isEmpty() ? MysqlConstants.NULL_VARCHAR : defaultValue);                }                if (target == Byte.class) {                    return (T)(defaultValue.isEmpty() ? MysqlConstants.DEF_BYTE : Byte.valueOf(defaultValue));                }                if (target == Short.class) {                    return (T)(defaultValue.isEmpty() ? MysqlConstants.DEF_SHORT : Short.valueOf(defaultValue));                }                if (target == Integer.class) {                    return (T)(defaultValue.isEmpty() ? MysqlConstants.DEF_INT : Integer.valueOf(defaultValue));                }                if (target == Long.class) {                    return (T)(defaultValue.isEmpty() ? MysqlConstants.DEF_LONG : Long.valueOf(defaultValue));                }                if (target == Float.class) {                    return (T)(defaultValue.isEmpty() ? MysqlConstants.DEF_FLOAT : Float.valueOf(defaultValue));                }                if (target == Double.class) {                    return (T)(defaultValue.isEmpty() ? MysqlConstants.DEF_DOUBLE : Double.valueOf(defaultValue));                }                if (target == BigDecimal.class) {                    return (T)(defaultValue.isEmpty() ? MysqlConstants.DEF_DECIMAL : BigDecimal.valueOf(Double.valueOf(defaultValue)));                }                if (target == Boolean.class) {                    return (T)(defaultValue.isEmpty() ? MysqlConstants.DEF_BOOLEAN : Boolean.valueOf(defaultValue));                }                if (target == Date.class) {                    try {                        return (T)(defaultValue.isEmpty() ? new Date(MysqlConstants.NULL_DATE_MILLS) : new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").parse(defaultValue));                    } catch (ParseException e) {                        throw new RuntimeException(e);                    }                }                throw new RuntimeException("无法将默认值[" + defaultValue + "]转换成" + target.getName() + "类型");            }        }    }    /**     * 目前只处理 String Date Integer 的默认值，其他的通过接口提供     */    class MysqlConstantsUtil {        private static final Map
        
         , Member[]> classMembers = new HashMap<>();        /**         * 将目标方法中的空值换成默认值         */        public static void castNullToDefaultValue(Object dest) {            try {                Member[] members = getMembers(dest.getClass());                if (members != null && members.length > 0) {                    for (Member member : members) {                        if (member.getMethod.invoke(dest) == null) {                            member.setMethod.invoke(dest, new Object[]{member.defaultValue});//设置默认值                        }                    }                }            } catch (Throwable e) {                throw new RuntimeException(e);            }        }        /**         * 将默认值转成null，主要给前端展示用         * @param dest         */        public static void castDefaultValueToNull(Object dest) {            try {                Member[] members = getMembers(dest.getClass());                if (members != null && members.length > 0) {                    for (Member member : members) {                        Object value = member.getMethod.invoke(dest);                        //等于默认值                        if (value != null && value.equals(member.defaultValue)) {                            member.setMethod.invoke(dest, new Object[]{null});                        }                    }                }            } catch (Throwable e) {                throw new RuntimeException(e);            }        }        private static Member[] getMembers(Class
          target) {            Member[] members = classMembers.get(target);            if (members == null) {                synchronized (target) {                    members = classMembers.get(target);                    if (members == null) {                        try {                            members = parseMembers(target);                        } catch (Exception e) {                            throw new RuntimeException(e);                        }                        classMembers.put(target, members);                    }                }            }            return members;        }        private static Member[] parseMembers(Class
          target) throws Exception {            List
         
           members = new ArrayList<>(); List
          
            fieldList = new ArrayList<>(); Class
            clazz = target; while (clazz != null && clazz != Object.class) { fieldList.addAll(Arrays.asList(clazz.getDeclaredFields())); clazz = clazz.getSuperclass(); } for (Field field : fieldList) { field.setAccessible(true); CastMysqlDefaultNullValue castMysqlDefaultNullValue = field.getAnnotation(CastMysqlDefaultNullValue.class); if (castMysqlDefaultNullValue != null) { PropertyDescriptor pd = new PropertyDescriptor(field.getName(), target); Method getMethod = pd.getReadMethod(); Method setMethod = pd.getWriteMethod(); if (getMethod == null || setMethod == null) { throw new RuntimeException(target.getName() + "." + field.getName() + "没有相应的setter/getter方法"); } if (getMethod.getReturnType().isPrimitive()) { throw new RuntimeException("注解" + CastMysqlDefaultNullValue.class.getName() + "不支持基本类型"); } Object defaultValue = castMysqlDefaultNullValue.cast().newInstance().cast(castMysqlDefaultNullValue.defaultValue(), field.getType()); members.add(new Member(defaultValue, field, getMethod, setMethod)); } } return members.toArray(new Member[members.size()]); } //测试效果用的 static class CastTest { @CastMysqlDefaultNullValue Byte byte1; @CastMysqlDefaultNullValue Short short1; @CastMysqlDefaultNullValue Integer integer; @CastMysqlDefaultNullValue Long long1; @CastMysqlDefaultNullValue String string; @CastMysqlDefaultNullValue Float float1; @CastMysqlDefaultNullValue Double double1; @CastMysqlDefaultNullValue Date date; @CastMysqlDefaultNullValue BigDecimal bigDecimal; @CastMysqlDefaultNullValue Boolean boolean1; public Byte getByte1() { return byte1; } public void setByte1(Byte byte1) { this.byte1 = byte1; } public Short getShort1() { return short1; } public void setShort1(Short short1) { this.short1 = short1; } public Integer getInteger() { return integer; } public void setInteger(Integer integer) { this.integer = integer; } public Long getLong1() { return long1; } public void setLong1(Long long1) { this.long1 = long1; } public String getString() { return string; } public void setString(String string) { this.string = string; } public Float getFloat1() { return float1; } public void setFloat1(Float float1) { this.float1 = float1; } public Double getDouble1() { return double1; } public void setDouble1(Double double1) { this.double1 = double1; } public Date getDate() { return date; } public void setDate(Date date) { this.date = date; } public BigDecimal getBigDecimal() { return bigDecimal; } public void setBigDecimal(BigDecimal bigDecimal) { this.bigDecimal = bigDecimal; } public Boolean getBoolean1() { return boolean1; } public void setBoolean1(Boolean boolean1) { this.boolean1 = boolean1; } } public static void main(String[] args) { CastTest castTest = new CastTest(); castNullToDefaultValue(castTest); System.out.println(JSON.toJSONString(castTest)); castDefaultValueToNull(castTest); System.out.println(JSON.toJSONString(castTest)); } } class Member { Object defaultValue; Field field; Method getMethod; Method setMethod; Member(Object defaultValue, Field field, Method getMethod, Method setMethod) { this.defaultValue = defaultValue; this.field = field; this.getMethod = getMethod; this.setMethod = setMethod; } }}