动态增加枚举值

    之前做东西遇到了这么一个需求，希望动态的增加枚举类中的枚举值。我们正常情况下都是先定义枚举类，然后写好静态的枚举值，然后在代码中使用，那可不可以动态的增加枚举呢？研究后通过反射是可以实现的。工具类如下：

package com.huifu.uttf.util.utils;

import java.lang.reflect.AccessibleObject;
import java.lang.reflect.Array;
import java.lang.reflect.Field;
import java.lang.reflect.Modifier;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

import sun.reflect.ConstructorAccessor;
import sun.reflect.FieldAccessor;
import sun.reflect.ReflectionFactory;

public class DynamicEnumUtils {

    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<?>> T addEnum(Class<T> enumType, String enumName, Class<?>[] additionalTypes, Object[] additionalValues) {

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

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

        try {

            // 2. Copy it
            if (valuesField == null) {
                throw new RuntimeException("valuesField is null");
            }
            T[] previousValues = (T[]) valuesField.get(enumType);
            List<T> values = new ArrayList<T>(Arrays.asList(previousValues));

            // 3. build new enum
            T newValue = (T) makeEnum(enumType, enumName, values.size(), additionalTypes, additionalValues);

            // 4. add new value
            values.add(newValue);

            // 5. Set new values field
            setFailsafeFieldValue(valuesField, null, values.toArray((T[]) Array.newInstance(enumType, 0)));

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

            return newValue;

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


    }
}

    使用 DynamicEnumUtils.addEnum(……)这个方法即可动态增加枚举类的枚举值，各个参数的解析如下：

• Class enumType：需要增加枚举值的枚举类
• String enumName：枚举值名称
• Class<?>[] additionalTypes：枚举值中每个属性的类
• Object[] additionalValues：枚举值中的每个属性的值

需要注意的是，后两项枚举值属性的类和枚举值属性的值需要和枚举类中构造方法里的参数个数和顺序保持一致。