How to handle a Findbugs "Non-transient non-serializable instance field in serializable class"?

Question

Consider the class below. If I run Findbugs against it it will give me an error ("Non-transient non-serializable instance field in serializable class") on line 5 but not on line 7.

1 public class TestClass implements Serializable { 2 3  private static final long serialVersionUID = 1905162041950251407L; 4 5  private Set<Integer> mySet;      // Findbugs error 6 7  private HashSet<Integer> myOtherSet; 8 9 } 

That's correct because java.util.Set never implements Serializable in its hierarchy and java.util.HashSet does. However it is best practice to code against interfaces instead of concrete implementations.

How can I best handle this?

I can add a @Suppresswarnings(justification="No bug", values="SE_BAD_FIELD") on line 3. I have quite a lot of Sets and Lists in my actual code and I'm afraid it will litter my code too much.

Are there better ways?

Answer 1

However it is best practice to code against interfaces instead of concrete implementations.

I submit that no, in this case it is not. Findbugs quite correctly tells you that you risk running into a NotSerializableException as soon as you have a non-serializable Set implementation in that field. This is something you should deal with. How, that depends on the design of your classes.

• If those collections are initialized within the class and never set from outside, then I see absolutely nothing wrong with declaring the concrete type for the field, since fields are implementation details anyway. Do use the interface type in the public interface.
• If the collection are passed into the class via a public interface, you have to ensure that they are in fact Serializable. To do that, create an interface SerializableSet extends Set, Serializable and use it for your field. Then, either:
  • Use SerializableSet in the public interface and provide implementation classes that implement it.
  • Check collections passed to the class via instanceof Serializable and if they're not, copy them into something that is.

Answer 2

I know this is an old question that's already answered but just so others know is that you can set the Set<Integer> field as transient if you have no interest in serializing that particular field which will fix your FindBugs error.

public class TestClass implements Serializable {      private static final long serialVersionUID = 1905162041950251407L;     private transient Set<Integer> mySet;  } 

I prefer this method instead of forcing users of your API to cast to your concrete type, unless it's just internal, then Michael Borgwardt's answer makes more sense.