What is a serialVersionUID and why should I use it?
If you're serializing just because you have to serialize for the implementation's sake (who cares if you serialize for an HTTPSession
, for instance...if it's stored or not, you probably don't care about de-serializing
a form object), then you can ignore this.
If you're actually using serialization, it only matters if you plan on storing and retrieving objects using serialization directly. The serialVersionUID
represents your class version, and you should increment it if the current version of your class is not backwards compatible with its previous version.
Most of the time, you will probably not use serialization directly. If this is the case, generate a default SerialVersionUID
by clicking the quick fix option and don't worry about it.
You can tell Eclipse to ignore these serialVersionUID warnings:
Window > Preferences > Java > Compiler > Errors / Warnings > Potential Programming Problems
In case you didn't know, there are a lot of other warnings you can enable in this section (or even have some reported as errors), many are very useful:
- Potential Programming Problems: Possible accidental boolean assignment
- Potential Programming Problems: Null pointer access
- Unnecessary code: Local variable is never read
- Unnecessary code: Redundant null check
- Unnecessary code: Unnecessary cast or 'instanceof'
and many more.
I can't pass up this opportunity to plug Josh Bloch's book Effective Java (2nd Edition). Chapter 10 is an indispensible resource on Java serialization.
Per Josh, the automatically-generated UID is generated based on a class name, implemented interfaces, and all public and protected members. Changing any of these in any way will change the serialVersionUID
. So you don't need to mess with them only if you are certain that no more than one version of the class will ever be serialized (either across processes or retrieved from storage at a later time).
If you ignore them for now, and find later that you need to change the class in some way but maintain compatibility w/ old version of the class, you can use the JDK tool serialver to generate the serialVersionUID
on the old class, and explicitly set that on the new class. (Depending on your changes you may need to also implement custom serialization by adding writeObject
and readObject
methods - see Serializable
javadoc or aforementioned chapter 10.)
The docs for java.io.Serializable
are probably about as good an explanation as you'll get:
The serialization runtime associates with each serializable class a version number, called a
serialVersionUID
, which is used during deserialization to verify that the sender and receiver of a serialized object have loaded classes for that object that are compatible with respect to serialization. If the receiver has loaded a class for the object that has a differentserialVersionUID
than that of the corresponding sender's class, then deserialization will result in anInvalidClassException
. A serializable class can declare its ownserialVersionUID
explicitly by declaring a field namedserialVersionUID
that must be static, final, and of typelong
:
ANY-ACCESS-MODIFIER static final long serialVersionUID = 42L;
If a serializable class does not explicitly declare a
serialVersionUID
, then the serialization runtime will calculate a defaultserialVersionUID
value for that class based on various aspects of the class, as described in the Java(TM) Object Serialization Specification. However, it is strongly recommended that all serializable classes explicitly declareserialVersionUID
values, since the defaultserialVersionUID
computation is highly sensitive to class details that may vary depending on compiler implementations, and can thus result in unexpectedInvalidClassExceptions
during deserialization. Therefore, to guarantee a consistentserialVersionUID
value across different java compiler implementations, a serializable class must declare an explicitserialVersionUID
value. It is also strongly advised that explicitserialVersionUID
declarations use the private modifier where possible, since such declarations apply only to the immediately declaring class —serialVersionUID
fields are not useful as inherited members.