33. ERR51-JG. Prefer user-defined exceptions over more general exception types

Because an exception is caught by its type, it is better to define exceptions for specific purposes than to use the general exception types for multiple purposes. Throwing the general exception types makes code hard to understand and maintain, and defeats much of the advantage of the Java exception-handling mechanism.

Noncompliant Code Example

This noncompliant code example attempts to distinguish between different exceptional behaviors by looking at the exception's message:

try {
    doSomething();
} catch (Throwable e) {
  String msg = e.getMessage();
  switch (msg) {
    case "file not found":
      // Handle error
      break;
    case "connection timeout":
      // Handle error
      break;
    case "security violation":
      // Handle error
      break;
    default: throw e;
  }
}

If doSomething() throws an exception or error whose type is a subclass of Throwable, the switch statement allows selection of a specific case to execute. For example, if the exception message is "file not found," the appropriate action is taken in the exception handling code.

However, any change to the exception message literals involved will break the code. For example, if a maintainer were to edit the throw expression to read throw new Exception("file not found");, the exception would be rethrown by the code of the noncompliant code example rather than handled. Finally, exceptions may be thrown without a message.

This noncompliant code example falls under ERR08-EX0 of rule ERR08-J. Do not catch NullPointerException or any of its ancestors.

Compliant Solution

This compliant solution uses specific exception types and defines new special-purpose exception types where required.

public class TimeoutException extends Exception {
  TimeoutException () {
    super();
  }
  TimeoutException (String msg) {
    super(msg);
  }
}

// ...

try {
    doSomething();
} catch (FileNotFoundException e) {
  // Handle error
} catch (TimeoutException te) {
  // Handle error
} catch (SecurityException se) {
  // Handle error
}

Applicability

Exceptions are used to handle exceptional conditions. If an exception is not caught, the program will be terminated. An exception that is incorrectly caught or is caught at the wrong level of recovery will often cause incorrect behavior.

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