More on Exceptions

In a previous post, I blogged about why I favour Checked Exceptions over Unchecked ones. Today, I'll talk about how I am circumventing some of the issues with Checked Exceptions.

The big advantage with Checked Exceptions is that the method signature tells you what Exceptions are likely to be thrown. Great as this is, it is also a liability, because any change in the Exception specification can break client code.

SimpleDBM comprises of several modules. In SimpleDBM, the module is the unit of reusability. Each module has an API which is represented by a set of Interfaces, and one or more implementation. An important objective is to make each Module's API stable so that as the code evolves, other modules are not impacted by changes in the API. This is where Exception specification becomes important.

Let us assume there are two modules, A and B, and also assume that B depends upon A. Now, suppose that some methods in A's API throw an exception called ExceptionA and some of B's methods throw ExceptionB. Since B's methods call A's methods, when defining B's methods, we have following options:

1. Allow B's methods to throw ExceptionA.
2. Catch ExceptionA in B's methods and wrap them in ExceptionB.

The problem with first approach is that it makes B's API unstable. What if A's methods start throwing a new Exception type?

The problem with the second approach is that ExceptionA has been wrapped and cannot be caught by a client. A client that invokes B may want to handle ExceptionA in a specific manner. In the first option, the client could write following code, but with the second option, this is not possible:

try {
   // call B's API
   B.someMethod();
}
catch (ExceptionA e) {
   // Catch and handle ExceptionA
}

What we want is to preserve the information that A threw ExceptionA, but still avoid having ExceptionA in B's method signatures.

The solution is to wrap ExceptionA with a specific sub-class of ExceptionB, rather than plan ExceptionB. Let us call this sub-class ExceptionBExpetionA. The trick is that methods in B should only be specified to throw ExceptionB. This is okay because ExceptionBExceptionA is a sub-class of ExceptionB. However, now clients can catch ExceptionBExceptionA and handle this particular exception, while ignoring other instances of ExceptionB.

try {
   // call B's API
   B.someMethod();
}
catch (ExceptionBExceptionA e) {
   // Catch and handle ExceptionBExceptionA
}

Not all exceptions thrown by A need be wrapped in this manner - only those that are specifically useful to the client.

In SimpleDBM, each module defines its own Exception class. Methods of the module can only throw instances of the module Exception. However, where necessary, sub-classes of the Exception are created that represent more specific information, sometimes wrapping Exceptions thrown by other modules.