URL decoding: UnsupportedEncodingException in Java

Question

What I understand from the documentation is that UnsupportedEncodingException can only be thrown if I specify a wrong encoding as the second parameter to URLDecoder.decode(String, String) method. Is it so? I need to know cases where this exception can be thrown.

Basically, I have this code segment in one of my functions:

if (keyVal.length == 2) {
    try {
        value = URLDecoder.decode(
            keyVal[1],
            "UTF-8");
    } catch (UnsupportedEncodingException e) {
          // Will it ever be thrown?
    }
}

Since I am explicitly mentioning "UTF-8", is there any way this exception can be thrown? Do I need to do anything in the catch block? Or, if my understanding is completely wrong, please let me know.

Answer 1

It cannot happen, unless there is something fundamentally broken in your JVM. But I think you should write this as:

try {
    value = URLDecoder.decode(keyVal[1], "UTF-8");
} catch (UnsupportedEncodingException e) {
    throw new AssertionError("UTF-8 is unknown");
    // or 'throw new AssertionError("Impossible things are happening today. " +
    //                              "Consider buying a lottery ticket!!");'
}

The cost of doing this is a few bytes of code that will "never" be executed, and one String literal that will never be used. That a small price for the protecting against the possibility that you may have misread / misunderstood the javadocs (you haven't in this case ...) or that the specs might change (they won't in this case ...)

Answer 2

That's because of the odd choice to make UnsupportedEncodingException checked. No, it won't be thrown.

I usually do as follows:

} catch (UnsupportedEncodingException e) {
  throw new AssertionError("UTF-8 not supported");
}

Answer 3

In your special case - no, it won't be thrown. Unless you execute your code in a Java runtime that does not support "UTF-8".

Answer 4

To answer an old question for newer readers:

Java 11 now has URLDecoder.decode(String, Charset); which does not throw. So you don't have to use a try-catch block at all. Just do:

URLDecoder.decode(keyVal[1], StandardCharsets.UTF_8);