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运行时动态增加枚举类型

最近在使用一个内部框架的时候，希望能够在运行时指定枚举类型，却发现这是一件挺麻烦的事情（不找别的替代方式，就是要动态增加enum的类型），方法也不正统，不过作为有趣的尝试，研究研究也无妨，下面的内容主要来自于《Java – create enum instances dynamically》这样一篇文章。在一切开始前，如果你想问，为什么非要增加/改变enum类型？其实这是一个非常好的问题，多数情况下这是应对被避免的，但是这不在今天我的讨论范围内。

一、方法 void addEnum(Class<T> enumType, String enumName)，用于增加一个枚举类型：

/**      * Add an enum instance to the enum class given as argument      *      * @param 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) {          // 0. Sanity checks         if (!Enum.class.isAssignableFrom(enumType)) {             throw new RuntimeException("class " + enumType + " is not an instance of Enum");         }         // 1. Lookup "$VALUES" holder in enum class and get previous enum instances         Field valuesField = null;         Field[] fields = TestEnum.class.getDeclaredFields();         for (Field field : fields) {             if (field.getName().contains("$VALUES")) {                 valuesField = field;                 break;             }         }         AccessibleObject.setAccessible(new Field[] { valuesField }, true);          try {              // 2. Copy it             T[] previousValues = (T[]) valuesField.get(enumType);             List values = new ArrayList(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<><[] {}, // can be used to pass values to the enum constuctor                     new Object[] {}); // can be used to pass values to the enum constuctor              // 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);          } catch (Exception e) {             e.printStackTrace();             throw new RuntimeException(e.getMessage(), e);         }     }

基于反射，把枚举类的属性列表全部取出来，增加一个新的枚举类型以后再放回去。上面的第3、5、6步下面分别说明。

二、在其中使用到了 makeEnum(Class<?> enumClass, String value, int ordinal, Class<?>[] additionalTypes, Object[] additionalValues) 这样一个方法：

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)); }  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)); }

这是真正使用newInstance方法来创建新的枚举对象的方法，注意其中的构造器参数类型，第一个是String，第二个是int，这是需要符合enum内部实现的。

三、setFailsafeFieldValue(Field field, Object target, Object value) throws NoSuchFieldException, IllegalAccessException，这个修改方法modifier并设值的方法：

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); }

四、清除枚举缓存：cleanEnumCache(Class<?> enumClass) throws NoSuchFieldException, IllegalAccessException：

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 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;         }     } }

最终的运行时测试代码：

private static enum TestEnum {     a,     b,     c; };  public static void main(String[] args) {      // Dynamically add 3 new enum instances d, e, f to TestEnum     addEnum(TestEnum.class , "d");     addEnum(TestEnum.class , "e");     addEnum(TestEnum.class , "f");      // Run a few tests just to show it works OK.     System.out.println(Arrays.deepToString(TestEnum.values()));     // Shows : [a, b, c, d, e, f] }

原文 http://www.importnew.com/19549.html

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