Design tips: Exception types

Expanding on a few slides from my JavaOne talk last year, here are a few tips to keep in mind when designing exception types.

First, all exceptions are serializable since Throwable implements Serializable; therefore, like all other serializable classes, exception types should declare a serialVersionUID field to ease evolving the type in the future. Using's javac's -Xlint:serial option will warn about missing serialVersionUID fields on serializable classes, amongst other possible issues.

Second, when adding a new exception class, consider providing more information beyond just a distinct name, such as methods to return information about what specific situation triggered the exception and possibly how to recover from it. Providing this additional information can interact with being serializable; when the additional information is not logically serializable, the specification may need to allow the information to be unavailable after deserialization.

Third, when multiple related exceptions types are added, a common direct superclass allows a single catch block to handle the related exceptions uniformly. (Having a common super-exception would still be useful even if multi-catch is added to the language in JDK 7.)

Looking at the exceptions in the JSR 269 API, various methods note the possible impact of serialization-deserialization on the returned values. JSR 269 provided a trio of similar exceptions for the situation of encountering a kind of object unknown in an earlier version of the language, such as a JDK 6 era annotation processor coming across a module from JDK 7:

However, the original JSR 269 API does not have a common direct superclass to group these related conditions. That deficiency was addressed in JDK 7 build 48 with the addition of javax.lang.model.UnknownEntityException as the direct superclass of these three exceptions (6794071). Retrofitting this change is binary compatible because UnknownEntityException directly extends RuntimeException as did the old exceptions and serialization compatibility is preserved by the existing serialVersionUID fields in the old exceptions.

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