JAVA动态增加枚举项

前言

在项目中,会存在很多枚举,比如下面颜色和水果的枚举,COLOR[RED(1),GREEN(2),BLUE(3)],FRUIT[APPLE(4),BANANA(5),ORANGE(6)],但是问题是,如果要增加枚举类型,就会涉及修改代码的问题,一个比较常见的例子是,枚举生成一个列表,供前台选择,如果增加类型,那么前后台都需要进行修改。
一个思路,是将枚举项保存在数据库里,项目启动或定时刷新枚举项。在项目中,只定义枚举类型,比如

public enum COLOR{}
public enum FRUIT{}

在接口中,使用COLOR.valueOf()就可以获取枚举,并传入后面的处理层了。为什么要使用枚举呢?类型安全么,只要可以获取枚举,就说明前台传的值是正确的,即进行了范围校验。

代码

代码是参考了一篇文章https://www.niceideas.ch/roll…
有兴趣的可以看一下。他的例子,是在一个数据库表里面有一张类似业务流水表,上面有一个CODE字段,代码了对记录的不同处理,CODE很多,他不想手写大量的if…else语句,想转成enum进行处理(这个我还没有相同,动态的enum也没办法直接在代码上switch case,那么是不是他在生成的枚举上,包含了调用的方法)。
下面是在他代码之上,进行了些许的修改,便于适应我自己的业务场景。

public enum CodeInfoEnum {
    LOCK(1L,1L,"LOCK_TYPE","LOCK"),UNLOCK(1L,2L,"LOCK_TYPE","LOCK");
    public Long classId;
    public Long infoId;
    public String classCode;
    public String infoCode;
    CodeInfoEnum(Long classId,Long infoId,String classCode,String infoCode){
        this.classId = classId;
        this.infoId = infoId;
        this.classCode = classCode;
        this.infoCode = infoCode;
    }
    public static CodeInfoEnum getByInfoId(Long infoId){
        return CodeInfoEnum.valueOf(infoId+"");
    }
    public static List<CodeInfoEnum> getByClassId(Long classId){
        return Arrays.stream(CodeInfoEnum.values()).filter(item->item.classId.equals(classId)).collect(Collectors.toList());
    }
    public static CodeInfoEnum getByClassCodeAndInfoCode(String classCode,String infoCode){
        Optional<CodeInfoEnum> opt = Arrays.stream(CodeInfoEnum.values()).filter(item->item.classCode.equals(classCode)&&item.infoCode.equals(infoCode)).findFirst();
        return opt.orElse(null);
    }

    @Override
    public String toString() {
        return "CodeInfoEnum{" +
                "classId=" + classId +
                ", infoId=" + infoId +
                ", classCode='" + classCode + '\'' +
                ", infoCode='" + infoCode + '\'' +
                '}';
    }
}
public class DynamicEnumTest {
    private static ReflectionFactory reflectionFactory = ReflectionFactory.getReflectionFactory();
    private static void setFailsafeFieldValue(Field field, Object target, Object value) throws NoSuchFieldException,
            IllegalAccessException {

        // let's make the field accessible
        field.setAccessible(true);
        // next we change the modifier in the Field instance to
        // not be final anymore, thus tricking reflection into
        // letting us modify the static final field
        Field modifiersField = Field.class.getDeclaredField("modifiers");
        modifiersField.setAccessible(true);
        int modifiers = modifiersField.getInt(field);

        // blank out the final bit in the modifiers int
        modifiers &= ~Modifier.FINAL;
        modifiersField.setInt(field, modifiers);

        FieldAccessor fa = reflectionFactory.newFieldAccessor(field, false);
        fa.set(target, value);
    }
    private static void blankField(Class<?> enumClass, String fieldName) throws NoSuchFieldException,
            IllegalAccessException {
        for (Field field : Class.class.getDeclaredFields()) {
            if (field.getName().contains(fieldName)) {
                AccessibleObject.setAccessible(new Field[] { field }, true);
                setFailsafeFieldValue(field, enumClass, null);
                break;
            }
        }
    }

    private static void cleanEnumCache(Class<?> enumClass) throws NoSuchFieldException, IllegalAccessException {
        blankField(enumClass, "enumConstantDirectory"); // Sun (Oracle?!?) JDK 1.5/6
        blankField(enumClass, "enumConstants"); // IBM JDK
    }
    private static ConstructorAccessor getConstructorAccessor(Class<?> enumClass, Class<?>[] additionalParameterTypes)
            throws NoSuchMethodException {
        Class<?>[] parameterTypes = new Class[additionalParameterTypes.length + 2];
        parameterTypes[0] = String.class;
        parameterTypes[1] = int.class;
        System.arraycopy(additionalParameterTypes, 0, parameterTypes, 2, additionalParameterTypes.length);
        return reflectionFactory.newConstructorAccessor(enumClass.getDeclaredConstructor(parameterTypes));
    }

    private static Object makeEnum(Class<?> enumClass, String value, int ordinal, Class<?>[] additionalTypes,
                                   Object[] additionalValues) throws Exception {
        Object[] parms = new Object[additionalValues.length + 2];
        parms[0] = value;
        parms[1] = Integer.valueOf(ordinal);
        System.arraycopy(additionalValues, 0, parms, 2, additionalValues.length);
        return enumClass.cast(getConstructorAccessor(enumClass, additionalTypes).newInstance(parms));
    }

    /**
     * Add an enum instance to the enum class given as argument
     *
     * @param <T> the type of the enum (implicit)
     * @param enumType the class of the enum to be modified
     * @param enumName the name of the new enum instance to be added to the class.
     */
    @SuppressWarnings("unchecked")
    public static <T extends Enum<?>> void addEnum(Class<T> enumType, String enumName,Class<?>[] paramClass,Object[] paramValue) {

        // 0. Sanity checks
        if (!Enum.class.isAssignableFrom(enumType)) {
            throw new RuntimeException("class " + enumType + " is not an instance of Enum");
        }

        // 1. Lookup "$VALUES" holder in enum class and get previous enum instances
        Field valuesField = null;
        Field[] fields = CodeInfoEnum.class.getDeclaredFields();
        for (Field field : fields) {
            if (field.getName().contains("$VALUES")) {
                valuesField = field;
                break;
            }
        }
        AccessibleObject.setAccessible(new Field[] { valuesField }, true);

        try {

            // 2. Copy it
            T[] previousValues = (T[]) valuesField.get(enumType);
            List<T> values = new ArrayList<T>(Arrays.asList(previousValues));

            // 3. build new enum
            T newValue = (T) makeEnum(enumType, // The target enum class
                    enumName, // THE NEW ENUM INSTANCE TO BE DYNAMICALLY ADDED
                    values.size(),
                    //new Class<?>[] {}, // could be used to pass values to the enum constuctor if needed
                    paramClass,
                    //new Object[] {}
                    paramValue
            ); // could be used to pass values to the enum constuctor if needed

            // 4. add new value
            values.add(newValue);
            Object object=values.toArray((T[]) Array.newInstance(enumType, 0));
            // 5. Set new values field
            setFailsafeFieldValue(valuesField, null, object);

            // 6. Clean enum cache
            cleanEnumCache(enumType);

        } catch (Exception e) {
            e.printStackTrace();
            throw new RuntimeException(e.getMessage(), e);
        }
    }

    public static void main(String[] args) {

        //
        synchronized (CodeInfoEnum.class) {
            addEnum(CodeInfoEnum.class, "3", new Class<?>[]{Long.class, Long.class, String.class, String.class}, new Object[]{2L, 3L, "ActiveStatus", "Active"});
            addEnum(CodeInfoEnum.class, "4", new Class<?>[]{Long.class, Long.class, String.class, String.class}, new Object[]{2L, 4L, "ActiveStatus", "Inactive"});
            addEnum(CodeInfoEnum.class, "5", new Class<?>[]{Long.class, Long.class, String.class, String.class}, new Object[]{3L, 5L, "Optype", "OP1"});
            addEnum(CodeInfoEnum.class, "6", new Class<?>[]{Long.class, Long.class, String.class, String.class}, new Object[]{3L, 6L, "Optype", "OP2"});
            addEnum(CodeInfoEnum.class, "7", new Class<?>[]{Long.class, Long.class, String.class, String.class}, new Object[]{3L, 7L, "Optype", "OP3"});
            addEnum(CodeInfoEnum.class, "8", new Class<?>[]{Long.class, Long.class, String.class, String.class}, new Object[]{3L, 8L, "Optype", "OP4"});
        }
        CodeInfoEnum codeInfoEnum =CodeInfoEnum.valueOf("5");
        System.out.println(codeInfoEnum);
        // Run a few tests just to show it works OK.
        System.out.println(Arrays.deepToString(CodeInfoEnum.values()));
        System.out.println("============================打印所有枚举(包括固定的和动态的),可以将数据库中保存的CIC以枚举的形式加载到JVM");
        for(CodeInfoEnum codeInfo:CodeInfoEnum.values()){
            System.out.println(codeInfo.toString());
        }

        System.out.println("============================通过codeId找到的枚举,用于PO转VO的处理");
        CodeInfoEnum activeStatus_Active = CodeInfoEnum.getByInfoId(3L);
        System.out.println(activeStatus_Active);

        System.out.println("============================通过ClassId找到的枚举列表");
        List<CodeInfoEnum> activeStatusEnumList = CodeInfoEnum.getByClassId(3L);
        for(CodeInfoEnum codeInfo : activeStatusEnumList){
            System.out.println(codeInfo);
        }

        System.out.println("============================通过ClassCode和InfoCode获取枚举,用于导入验证CIC合法性");
        CodeInfoEnum toGetActiveStatus_Active = CodeInfoEnum.getByClassCodeAndInfoCode("ActiveStatus","Active");
        System.out.println(toGetActiveStatus_Active);

        System.out.println("============================通过ClassCode和InfoCode获取枚举,输入不存在的Code,则返回NULL");
        CodeInfoEnum toGetActiveStatus_miss = CodeInfoEnum.getByClassCodeAndInfoCode("ActiveStatus","MISS");
        System.out.println(toGetActiveStatus_miss);


    }
}

我将项目中所有的枚举,都定义在了CodeInfoEnum中,其中包含两部分,一部分是固定的枚举,我是预定义的,比如记录状态(有效|删除),锁定状态(可用|锁定)等,这些的枚举字面值是英文大写单词;另一部分是动态的,具有比较强的业务含义,比如仓库管理中的库位类型,包装类型这些,这些的枚举字面值是数字,即数据库中的ID。

在使用的使用,注意:

  1. 在和前台交互的时候,统一使用字面值
  2. 在校验的时候,使用class_code和字面值进行校验。
  3. 在controller进行参数组装的时候,将前台传入的字面值转成CodeInfoEnum
  4. 在持久化的时候使用字面值
  5. 查询的时候,将数据库中的字面值转化为CodeInfoEnum

上面写的‘使用’,目前还是设想,还没有动手实现。也是我下面准备做的。希望有看到本文的大神,如能提供宝贵意见,将不胜感激。

    原文作者:沈子平
    原文地址: https://segmentfault.com/a/1190000013251802
    本文转自网络文章,转载此文章仅为分享知识,如有侵权,请联系博主进行删除。
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