EXC13-J. Throw specific exceptions rather than the more general RuntimeException or Exception

Methods are forbidden to throw RuntimeException or Exception; handling these exceptions requires catching RuntimeException, which is forbidden in guideline EXC14-J. Catch specific exceptions rather than the more general RuntimeException or Exception. Moreover, throwing a RuntimeException can lead to subtle errors, for instance, a caller cannot examine the exception to determine why it was thrown, and consequently cannot attempt recovery.

Instead, prefer throwing a more specific exception, subclassed from Exception. Note that it is permissible to construct an exception class specifically for a single throw statement.

Noncompliant Code Example

This noncompliant code example accepts a string and returns true when it consists of a capital letter followed by lowercase letters. To handle corner cases, it checks for various exceptional conditions and throws exceptions when they are likely to disrupt normal operation.

boolean isCapitalized(String s) {
  if (s == null) {
    throw new RuntimeException("Null String");
  }
  if (s.equals("")) {
    return true;
  }
  String first = s.substring(0, 1);
  String rest = s.substring(1);
  return (first.equals(first.toUpperCase()) &&
          rest.equals(rest.toLowerCase()));
}

To handle the case of passing in a null string parameter, code calling this method may require catching RuntimeException, which is a violation of guideline EXC14-J. Catch specific exceptions rather than the more general RuntimeException or Exception.

Compliant Solution

This compliant solution throws a specific exception (NullPointerException) to denote the particular exceptional condition.

boolean isCapitalized(String s) {
  if (s == null) {                   // redundant; shown for clarity
    throw new NullPointerException();// redundant; shown for clarity
  }                                  // redundant; shown for clarity
  if (s.equals("")) {
    return true;
  }
  String first = s.substring(0, 1);
  String rest = s.substring(1);
  return (first.equals(first.toUpperCase()) &&
          rest.equals(rest.toLowerCase()));
}

Although the explicit throw statement is redundant in this small example, more complex code may require explicit testing of invariants and appropriate throw statements.

Noncompliant Code Example

This noncompliant code example uses a broad Exception class in the throws declaration of the method.

private void doSomething() throws Exception {
  //...
}

Compliant Solution

To be compliant, be as specific as possible when declaring exceptions while continuing to respect the required abstraction level.

private void doSomething() throws IOException {
  //...
}

Using instanceof to check for narrower exceptions in a general catch block is often insufficient; it is usually impossible to enumerate all possible exceptions that the code could throw.

Exceptions

EXC13-EX0: Classes that sanitize exceptions to comply with a security policy are permitted to translate specific exceptions into more general exceptions. This translation could potentially result in throwing RuntimeException or Exception in some cases, depending on the details of the security policy.

EXC13-EX1: Widely used pre-existing APIs whose previous versions contain methods that throw RuntimeException or Exception may continue to do so, to preserve backwards compatibility. We strongly encourage maintainers of such APIs to consider deprecating such methods, replacing them with methods that throw more specific exceptions.

Risk Assessment

Throwing RuntimeException and Exception prevents classes from catching the intended exceptions without catching other unintended exceptions as well.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this guideline on the CERT website.

Bibliography

[[Goetz 2004b]]
[[MITRE 2009]] CWE ID 397 "Declaration of Throws for Generic Exception", CWE ID 537 "Information Leak Through Java Runtime Error Message"
[[Tutorials 2008]] Unchecked Exceptions — The Controversy

EXC12-J. Do not log unsanitized user input      06. Exceptional Behavior (EXC)      EXC14-J. Catch specific exceptions rather than the more general RuntimeException or Exception