AspectJ - Is is possible to extend an enum's value? - reflection

Question

Say I have an enum
public enum E {A,B,C}
Is it possible to add another value, say D, by AspectJ?
After googling around, it seems that there used to be a way to hack the private static field $VALUES, then call the constructor(String, int) by reflection, but seems not working with 1.7 anymore.
Here are several links:
http://www.javaspecialists.eu/archive/Issue161.html (provided by #WimDeblauwe )
and this: http://www.jroller.com/VelkaVrana/entry/modify_enum_with_reflection

Answer 1

Actually, I recommend you to refactor the source code, maybe adding a collection of valid region IDs to each enumeration value. This should be straightforward enough for subsequent merging if you use Git and not some old-school SCM tool like SVN.
Maybe it would even make sense to use a dynamic data structure altogether instead of an enum if it is clear that in the future the list of commands is dynamic. But that should go into the upstream code base. I am sure the devs will accept a good patch or pull request if prepared cleanly.
Remember: Trying to avoid refactoring is usually a bad smell, a symptom of an illness, not a solution. I prefer solutions to symptomatic workarounds. Clean code rules and software craftsmanship attitude demand that.
Having said the above, now here is what you can do. It should work under JDK 7/8 and I found it on Jérôme Kehrli's blog (please be sure to add the bugfix mentioned in one of the comments below the article).
Enum extender utility:
package de.scrum_master.util;
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 DynamicEnumExtender {
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 "$VALUES" holder in enum class and get previous enum
// instances
Field valuesField = null;
Field[] fields = enumType.getDeclaredFields();
for (Field field : fields) {
if (field.getName().contains("$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, // 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);
}
}
}
Sample application & enum:
package de.scrum_master.app;
/** In honour of "The Secret of Monkey Island"... ;-) */
public enum Command {
OPEN, CLOSE, PUSH, PULL, WALK_TO, PICK_UP, TALK_TO, GIVE, USE, LOOK_AT, TURN_ON, TURN_OFF
}
package de.scrum_master.app;
public class Server {
public void executeCommand(Command command) {
System.out.println("Executing command " + command);
}
}
package de.scrum_master.app;
public class Client {
private Server server;
public Client(Server server) {
this.server = server;
}
public void issueCommand(String command) {
server.executeCommand(
Command.valueOf(
command.toUpperCase().replace(' ', '_')
)
);
}
public static void main(String[] args) {
Client client = new Client(new Server());
client.issueCommand("use");
client.issueCommand("walk to");
client.issueCommand("undress");
client.issueCommand("sleep");
}
}
Console output with original enum:
Executing command USE
Executing command WALK_TO
Exception in thread "main" java.lang.IllegalArgumentException: No enum constant de.scrum_master.app.Command.UNDRESS
at java.lang.Enum.valueOf(Enum.java:236)
at de.scrum_master.app.Command.valueOf(Command.java:1)
at de.scrum_master.app.Client.issueCommand(Client.java:12)
at de.scrum_master.app.Client.main(Client.java:22)
Now you can either add an aspect with an advice executed after the enum class was loaded or just call this manually in your application before extended enum values are to be used for the first time. Here I am showing how it can be done in an aspect.
Enum extender aspect:
package de.scrum_master.aspect;
import de.scrum_master.app.Command;
import de.scrum_master.util.DynamicEnumExtender;
public aspect CommandExtender {
after() : staticinitialization(Command) {
System.out.println(thisJoinPoint);
DynamicEnumExtender.addEnum(Command.class, "UNDRESS");
DynamicEnumExtender.addEnum(Command.class, "SLEEP");
DynamicEnumExtender.addEnum(Command.class, "WAKE_UP");
DynamicEnumExtender.addEnum(Command.class, "DRESS");
}
}
Console output with extended enum:
staticinitialization(de.scrum_master.app.Command.<clinit>)
Executing command USE
Executing command WALK_TO
Executing command UNDRESS
Executing command SLEEP
Et voilà! ;-)