Checked exception

From APIDesign

Revision as of 10:24, 4 April 2016

Checked exceptions are Java invention and many like to argue that they are the worst invention ever. I like exceptions and I like Checked exceptions. One day I'll explain why.

There is a really nice thing on checked exceptions: if a method declares that it throws a checked exception, the caller of the method has to handle it. This is a really nice language feature, if used at the appropriate place. What is such appropriate place? If one reads a file one shall be ready for an input/output error - e.g. forcing people to catch IOException seems like the right thing to do.

Thus in certain situations having checked exceptions is beneficial. On the other hand, throwing checked exceptions in cases where the recovery is unlikely - a frequently mentioned example is ParserConfigurationException - is just going to pollute the client code with useless catch statements.

There is however one more example: Imagine an exception that needs to be caught when thrown from certain methods, but when it is thrown from other methods, it should behave as as RuntimeException - e.g. propagate silently. We have seen an example of this recently in our Truffle project. The InteropException should smoothly propagate through many calls, but if invoked via the ForeignAccess's send method, we want every caller to handle it. What are our API design options?

Duplicate the Exceptions

We could have one RuntimeException subclass and one Exception subclass. Sometimes that might work, but in this case there are four subclasses of InteropException currently (and the number is expected to grow in the future), and having a duplicated set of classes is clearly annoying. Such solution would only support the argument of some that checked exceptions are the worst invention ever!

Wrap the Exceptions

We could introduce one RuntimeException subclass and let it carry the real checked InteropException exception. That would probably work on the catch-side:

try {
   // some interop code
} catch (InteropRuntimeException ex) {
  throw (InteropException)ex.getCause();
}

and even on the throw side the code wouldn't be that bad, imagine throwing UnknownIdentifierException (a subclass of InteropException) exception:

throw new InteropRuntimeException(new UnknownIdentifier("name"));
// which could be simplified and hidden into a factory method:
throw UnknownIdentifierException.raise("name");

However this still suffers from the duality of exceptions. In situations where needs to be sure, one needs to catch both exceptions InteropRuntimeException as well as InteropException, which is again a reason for few to claim that checked exceptions are bad.

Unchecking Checked Exception

Luckily, if one knows the difference between source compatibility and binary compatibility one can realize that the JVM doesn't know anything about the difference between RuntimeException and checked Exception - it is all just a Java language construct. Other languages built on top of JVM may ignore it. And that is what we decided to do.

We designed a checked exception InteropException (and its subclasses) and added a raise method to throw the exception as unchecked one. The usage is simple:

throw UnknownIdentifierException.raise("name");

E.g. the usage on the throwing side is exactly the same as in case of wrapping of the exceptions. There is just one difference - there is no wrapping. The checked exception UnknownIdentifierException is really being thrown and one can use

try {
  // the interop code
} catch (UnknownIdentifierException ex) {
  System.out.println("Error " + ex.getUnknownIdentifier());
}

Drawbacks

Of course, throwing checked exceptions in such a hidden way may yield some surprises. However, at least from my perspective, the benefits seem to outweigh them. One problem is the return type of the raise method. The typical recommended usage is

void silentCheckedThrow() {
  throw UnknownIdentifierException.raise("name");
}

and that means the raise method must return a RuntimeException or Error. But there is no such object - there is just an instance of checked exception. That means the method raise mustn't return - it must throw the exception before returning. Thus one could also write:

void silentCheckedThrow() {
  UnknownIdentifierException.raise("name");
}

and the effect would be the same. Yes, it would be unless the method returns some value. The following only compiles with throw and as such the throw style is the recommended one:

int silentCheckedThrowWithReturn() {
  throw UnknownIdentifierException.raise("name");
}

Other problems may appear when catching the exception. Per Java specification it is not allowed to write

void uselessCatch() {
  try {
    Integer.valueOf("10);
  } catch (IOException ex) {
    // never generated
  }
}

as the compiler knows that the IOException is not generated in the body of the Integer parse method and refuses to compile such code. This isn't a problem in Truffle API, as we want people to only catch the InteropException when one of ForeignAccess sendXYZ methods is called - and all of them declare they throw the InteropException. Still, there is a way to fool the compiler when necessary:

void uselessCatch() {
  try {
    Integer.valueOf("10);
    fakeThrow();
  } catch (IOException ex) {
    // never generated
  }
}
void fakeThrow() throws IOException {
}

and with this little change everything compiles without problems.

TBD