java - Why does my EJB interface need to extend serializable? -

i'm trying figure out going on , can't reason through why.

i've got extremely simple ejb 3 implementation i've been playing (deployed standalone openejb 3.1.2 container).

if setup interface/implementation looking this...

userservice (interface)

public interface userservice {     public string dostuff(string myparam); } 

userservicebean (implementation)

@stateless @remote({userservice.class}) public class userservicebean implements userservice {     public string dostuff(string myparam) {         return "did stuff!";     } } 

...this works great (i.e. can deploy openejb standalone container , run quick test involving jndi lookup , service call returns expected "did stuff!" value).

however, once introduce pojo created method signature, this:

userservice (interface)

public interface userservice {     public user lookupuser(string myparam); } 

userservicebean (implementation)

@stateless @remote({userservice.class}) public class userservicebean implements userservice {     public user lookupuser(string username) {         return new user();     } } 

...where pojo looks like:

public class user implements serializable {     private string username;     private string firstname;     private string lastname;     private string employeeclasscode;     private date termdate;     // getters & setters follow } 

i fatal reported openejb container:

2014-06-26 12:41:32,182 - fatal - couldn't write ejbresponse output stream java.io.notserializableexception: userservicebean 

just error logging lead believe, fix make userservice interface extend serializable - once this, well...but struggling understand why ejb bean needs serializable after i've added pojo method signature. can explain?

by way, question clear, i'm 100% fine answer general question "why things need serialized when working ejbs?" specific point of confusion why bean itself needs serializable because serializable object being used in method signature.

edit: original answer kind of lazy; adding more clarification.

wow, beyond bizarre - wasn't entirely complete in user.java listing. doing was:

return new user() {     {         setfirstname("fernando");         setlastname("alonso");     } }; 

you wouldn't think problem, when changed more orthodox:

user user = new user(); user.setfirstname("fernando"); user.setlastname("alonso"); return user; 

it works charm! issue that, original code, creating anonymous subclass of user pojo. jvm, not same thing instance of user class @ all, rather anonymous class lives within userservicebean itself. illustrate:

user anonymoususer = new user() {     {         setfirstname("fernando");         setlastname("alonso");     } }; 

inspecting anonymoususer reference variable reveals pointing @ instance of userservicebean$1 (with "$1" part being java's way of identifying anonymous inner class)

doing more traditional:

user user = new user(); user.setfirstname("fernando"); user.setlastname("alonso"); 

and inspecting user reference variable reveals to instance of user class (as expect).

so, information in hand, original error indicating userservicebean needed serializable makes perfect sense!