At work, it is necessary to dynamically update the value of the enumeration class. It is unrealistic and inconvenient to manually change it. Now it is recorded for future learning and use.
1. Add a tool class for dynamically updating the enumeration class in the project utils package (according to your own project, you can call it in the specified location)
2. I fell into a misunderstanding at the beginning, thinking that I should modify it according to my own enumeration class. The addEnum and makeEnum methods have two parameters because the online examples are all.
And mine is 5. It took me a long time to find out that it is an array.
package com.genju.ziji.dexiangmu.util;
import sun.reflect.ConstructorAccessor;
import sun.reflect.FieldAccessor;
import sun.reflect.ReflectionFactory;
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;
/**
* Dynamically add enumeration tool class
*/
public class DynamicEnumUtil { 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 1.5/6blankField(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));}
/**
* 判断枚举是否已存在
*
* @param values
* @param enumName
* @param <T>
* @return
*/
public static <T extends Enum<?>> boolean contains(List<T> values, String enumName) { for (T value : values) { if (value.name().equals(enumName)) { return true;}}return false;}
/**
* 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<?>[] 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
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);
if (contains(values, enumName)) { System.out.println("Enum：" + enumName + " 已存在");return;}
// 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);}}
}
3. Test
public static void main(String args[]) {
addEnum("1","2","3","4","5");
for(SubBank e : SunBank.values()){
System.out.println(e.getBankName());
}
}
private static void addEnum(String enumName,String id,String name,String desc,AreaType areaType,List<Class<? extends Enum>> paramTypes){
DynamicEnumUtil.addEnum(Subank.class,enumName,
new Class<?>[]{Stirng.class,String.class,AreaType.class,List.Class},
new object[]{id,nae,desc,areaType,paramTypes});
}
Finally, the effect of printing is ok and can be added. I am writing the enumeration class directly here. If necessary, it can also be made dynamic according to your own needs;
If there is any inaccuracy, please give pointers
Reprinted in: https://www.cnblogs.com/KpGo/p/11527473.html