Cannot serialize an Enum to GWT - gwt

Question

I am unable to serialize an Enum to GWT if it implements java.io.Serializable. It will GWT compile successfully, but at runtime, I get the dreaded:
Type 'com....security..AdminPrivilege' was not assignable to 'com.google.gwt.user.client.rpc.IsSerializable' and did not have a custom field serializer.For security purposes, this type will not be serialized.: instance = Login to Console
If I implement com.google.gwt.user.client.rpc.IsSerializable it compiles and runs fine. I am trying to avoid IsSerializable, since this Enum is persisted in our DB and is referenced in non-GWT servlets. I do not want to introduce a GWT dependancy, even for that single class.
I've read most of the discussions on this topic here. I have:
added a serialVersionUid (which should not be necessary)
added a no-arg constructor (but this is an Enum, so it must be private - I suspect this may be the problem)
added a callable RPC method that returns the Enum and takes a Set of the Enum as an input argument (trying to get this Enum into the whitelist) -
For all other Enums, I generated a GWT version which implements IsSerializable. But this new Enum is too complex to generate and I need the methods from the Enum in the GWT code.
Thanks for any help on this.
My Enum is below. Notice it has an embedded Enum:
public enum AdminPrivilege implements java.io.Serializable {
// Privileges
MANAGE_XX("Manage XX", PrivilegeCategory.XX),
IMPORT_LICENSE("Import a License", PrivilegeCategory.XX),
SUBMIT_BUG("Submit a Bug", PrivilegeCategory.XX),
TEST_AD("Test AD", PrivilegeCategory.XX),
// Administrator Privileges
LOGIN("Login to XX", PrivilegeCategory.ADMIN),
MANAGE_ADMIN("Manage Administrators", PrivilegeCategory.ADMIN),
MANAGE_ROLE("Manage Roles", PrivilegeCategory.ADMIN),
// Task Privileges
CANCEL_TASK("Cancel Tasks", PrivilegeCategory.TASK), ;
private static final long serialVersionUID = 1L;
/**
* Defines the privilege categories.
*
*/
public enum PrivilegeCategory implements java.io.Serializable {
XX("XX"),
ADMIN("Administrator"),
TASK("Task"), ;
private static final long serialVersionUID = 2L;
private String displayValue;
// This constructor is required for GWT serialization
private PrivilegeCategory() {
}
private PrivilegeCategory(String displayValue) {
this.displayValue = displayValue;
}
#Override
public String toString() {
return displayValue;
}
}
private String displayValue;
private AdminPrivilege parentPrivilege;
private PrivilegeCategory privilegeCategory;
// This constructor is required for GWT serialization
private AdminPrivilege() {
}
private AdminPrivilege(String displayValue, PrivilegeCategory category) {
this.displayValue = displayValue;
this.privilegeCategory = category;
}
private AdminPrivilege(String displayValue, PrivilegeCategory category, AdminPrivilege parent) {
this(displayValue, category);
this.parentPrivilege = parent;
}
/**
* Return the parent privilege for this privilege.
*
* #return
*/
public AdminPrivilege getParentPrivilege() {
return parentPrivilege;
}
/**
* Return the category for this privilege.
*
* #return
*/
public PrivilegeCategory getPrivilegeCategory() {
return privilegeCategory;
}
/**
* Return the set of categories.
*
* #return
*/
public static Set<PrivilegeCategory> getPrivilegeCategories() {
Set<PrivilegeCategory> category = new HashSet<PrivilegeCategory>();
for (PrivilegeCategory c : PrivilegeCategory.values()) {
category.add(c);
}
return category;
}
/**
* Return the set of privileges for a category.
*
* #return
*/
public static Set<AdminPrivilege> getPrivileges(PrivilegeCategory category) {
Set<AdminPrivilege> privileges = new HashSet<AdminPrivilege>();
for (AdminPrivilege p : AdminPrivilege.values()) {
if (category.equals(p.getPrivilegeCategory())) {
privileges.add(p);
}
}
return privileges;
}
/**
* Return the set of child privileges for a specific privilege
*
* #param parent
* #return
*/
public static Set<AdminPrivilege> getChildPrivileges(AdminPrivilege parent) {
Set<AdminPrivilege> children = new HashSet<AdminPrivilege>();
for (AdminPrivilege priv : values()) {
if (parent.equals(priv.getParentPrivilege())) {
children.add(priv);
}
}
return children;
}
/**
* Return the set of privileges that are parent privileges
*
* #return
*/
public static Set<AdminPrivilege> getParentPrivileges() {
Set<AdminPrivilege> parents = new HashSet<AdminPrivilege>();
for (AdminPrivilege priv : values()) {
if (priv.getParentPrivilege() == null) {
parents.add(priv);
}
}
return parents;
}
}
}

Answer 1

Have you specified a parameterized constructor in your enum? If you have, and it has parameters, you need to remember to add a no-parameters constructor as well, even if you don't use it, because GWT will need it. Adding a parameterized constructor and forgetting to add a parameterless one gets me every time, at least with non-enum classes.