java动态枚举类_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 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 > void addEnum(Class enumType, String enumName) {
// 0. Sanity checks
if (!Enum.class.isAssignableFrom(enumType)) {
throw new RuntimeException("class " + enumType + " is not an instance of Enum");
}
// 1. Lookup "$VALUES" holder in enum class and get previous enum instances
Field valuesField = null;
Field[] fields = DeclaredFields();
for (Field field : fields) {
if (Name().contains("$VALUES")) {
valuesField = field;
break;
}
}
AccessibleObject.setAccessible(new Field[] { valuesField }, true);
try {
// 2. Copy it
T[] previousValues = (T[]) (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<>
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,
// 6. Clean enum cache
cleanEnumCache(enumType);
} catch (Exception e) {
e.printStackTrace();
throw new Message(), 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);
wConstructorAccessor(
}
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 = DeclaredField("modifiers");
modifiersField.setAccessible(true);
int modifiers = Int(field);
// blank out the final bit in the modifiers int
modifiers &= ~Modifier.FINAL;
modifiersField.setInt(field, modifiers);
FieldAccessor fa = wFieldAccessor(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 : DeclaredFields()) {
if (Name().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");
enum类型如何使用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]
}
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