EXC30-J. Do not exit abruptly from a finally block

If a finally clause is specified, irrespective of whether the try or catch block executes to completion or not, the finally block is executed. Consequently statements that abruptly exit from the finally block may cause thrown exceptions to be masked. Therefore, keywords like return, break, continue and throw should never be used within a finally block.

Noncompliant Code Example

Here, the finally block completes abruptly since a return statement occurs within it. As a result, when the IllegalStateException is thrown, it does not propagate all the way up through the call stack. This is due to the abrupt termination of the finally block that suppresses any useful exception information from being displayed in the try block by overriding it with its own message. Note that even if the try block returns some value, the finally block is executed.

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();	
  }
}

Compliant Solution

This compliant solution removes the return statement from the finally block. Any return statements must occur after this block. In this example, the compiler will throw an error as the return statement is unreachable due to the explicit, unavoidable throwing of IllegalStateException. If the exception is thrown conditionally, the return statement can be used without any compilation errors.

class TryFinally {         
  private static boolean doLogic() {
    try {
      throw new IllegalStateException(); 
    }  
    finally {
      System.out.println("Caught Exception");
    }
    // any return statements must go here
  }

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

Risk Assessment

Exiting abruptly from a finally block may lead to masking of thrown exceptions.

Automated Detection

TODO

Related Vulnerabilities

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

References

[[JLS 05]] Section 14.20.2, Execution of try-catch-finally
[[Bloch 05]] Puzzle 36: Indecision
[[Chess 07]] 8.2 Managing Exceptions, "The Vanishing Exception"
[[MITRE 09]] CWE ID 705 "Incorrect Control Flow Scoping", CWE ID 584 "Return Inside Finally Block"

EXC04-J. Prevent against inadvertent calls to System.exit() or forced shutdown      10. Exceptional Behavior (EXC)      EXC31-J. Handle checked exceptions that can be thrown within a finally block