我参考了以下关于反射和枚举的文章：
https://www.niceideas.ch/roller2/badtrash/entry/java_create_enum_instances_dynamically
相应的源代码：
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 ReflectionUtils {
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) {
// 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<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
new Object[]{}); // could be used to pass values to the enum constuctor if needed
// 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);
}
}
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]
}
}
我不知何故需要从方法中返回一个新的枚举值：
public TestEnum theValue() {
return TestEnum.f;
}
当然，这不会编译。如何从上述方法返回说f（这是添加的枚举值之一）？
修改：
我在考虑以下几点：
private TestEnum testEnum;
@Override
public TestEnum theValue() {
ReflectionUtils.addEnum(TestEnum.class, "f");
//How can I set the testEnum field to have 'f' as a value?
return this.testEnum;
}
答案 0 :(得分：1)
从
修改您的方法public static <T extends Enum<?>> void addEnum(Class<T> enumType, String enumName)
到
public static <T extends Enum<?>> T addEnum(Class<T> enumType, String enumName)
并从此方法返回值。
// 6. Clean enum cache
cleanEnumCache(enumType);
// 7. Clean enum cache
return newvalue;
并从异常块返回newvalue。
然而，正如大多数其他评论者所指出的那样，对我进行这样的反思似乎并不是一个好主意。如果不是因为不可修改的第三方来源，您应该重新设计您的问题，以便在没有此类枚举的情况下工作。