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Feature Request for the JLS: Auto-Rethrow

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Feature Request for the JLS: Auto-Rethrow

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Java 7 has eased some pain in the area of exception handling when the new try-with-resources and multi-catch syntaxes were introduced. The latter is very interesting from a syntax perspective because it is the only place in Java where formal union types are allowed, similar to what Ceylon offers. Remember, a union type A | B is a type has all the properties that both types share. For example:

try {
    // ...
}
catch (SQLDataException | SQLWarning e) {
    // Properties that both types share:
    System.out.println(e.getSQLState());
}

The above can be quite useful in cases when we simply want to re-throw a variety of similar but disjoint exceptions in the same way. But often, we want do this:

// Some code
try {
}

// Rethrow
catch (OurException e) {
    throw e;
}

// Wrap other types of exceptions in OurException
catch (Exception e) {
    throw new OurException(e);
}

That’s quite a few lines of code for something that we do all the time and that could be called “rethrow-or-wrap” pattern. How about if we could rewrite the above as such:

// Some code
try {
}

// Re-throw exceptions of type OurException
// ... and wrap all other types in OurException
throws OurException;

Without a catch clause, this would be exactly the same as our previous example. Rethrow or wrap. Java is a language that heavily recycles keywords, so a dedicated keyword is out of question. throws would be a good keyword here as it is somewhat similar to the throws clause of a method signature, i.e. the throws clause of the try block.

If we only want to rethrow some exceptions and write tailor-made catch blocks for other types of exceptions, we could still do the same:

// Some code
try {
}

// Re-throw exceptions of type OurException
throws OurException

// But deal with all other types here
catch(Exception e) {
    throw new OurException("Custom message", e);
}

Java should have a “rethrow Exception” statement.

What are your thoughts on this? Would such a syntax pull its own weight?

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