EXC04-J. Do not exit abruptly from a finally block

In the presence of a finally block, irrespective of whether the try or catch blocks execute to completion or not, the finally block is executed. Consequently, statements that cause the finally block to terminate abruptly may mask any thrown exceptions. Keywords such as return, break, continue and throw should never be used within a finally block.

Noncompliant Code Example

In this noncompliant code example, the finally block completes abruptly because a return statement occurs within its body.

class TryFinally {              
  private static boolean doLogic() {
    try {
      throw new IllegalStateException(); 
    } finally {
      System.out.println("Uncaught Exception");
      return true;
    }
  }

  public static void main(String[] args) {
    doLogic();	
  }
}

Consequently, when the IllegalStateException is thrown, it does not propagate all the way up through the call stack. This is because of the abrupt termination of the finally block that suppresses any useful exception information from being displayed as a result of overriding the exception thrown in the try block.

Note that this example would not be insecure if only the try block were to return some value; the finally block always executes.

Compliant Solution

This compliant solution removes the return statement from the finally block. Any return statements must occur after this block.

class TryFinally {         
  private static boolean doLogic() {
    try {
      throw new IllegalStateException(); 
    } finally {
      System.out.println("Caught Exception");
    }
    // Any return statements must go here; applicable only when exception is thrown conditionally
  }

  public static void main(String[] args) {
    doLogic();	
  }
}

If this is adopted, the compiler throws an error as the return statement is unreachable because of the explicit, unavoidable throwing of IllegalStateException. If the exception is thrown conditionally, the return statement can be used without any compilation errors.

Risk Assessment

Exiting abruptly from a finally block may result in the masking of thrown exceptions.

Automated Detection

TODO

Related Vulnerabilities

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

Bibliography

[[JLS 2005]] Section 14.20.2, Execution of try-catch-finally
[[Bloch 2005]] Puzzle 36: Indecision
[[Chess 2007]] 8.2 Managing Exceptions, "The Vanishing Exception"
[[MITRE 2009]] CWE ID 705 "Incorrect Control Flow Scoping", CWE ID 584 "Return Inside Finally Block"

EXC03-J. Use a logging API to log critical security exceptions      06. Exceptional Behavior (EXC)      EXC05-J. Handle checked exceptions that can be thrown within a finally block