Exception HandlingExceptions are like the "You Are Here" spot on a map. They show you exactly how you got to a particular line of code in your method, by walking you backwards to the method that called you, and then the method that called the caller, and so on, up until your head explodes. They're great! Exceptions get "thrown" in Java, or "raised" in Jython, when something goes wrong. When an Exception is thrown, it's like a bullet that goes speeding back up through all the callers, until it gets to someone who wants to deal with the problem. If you want to deal with the problem, you can "catch" the exception. Some Java and Wyvern API calls require that you catch the Exceptions they throw, or at least declare you're going to throw them up to your caller. Without worrying too much about how to use them, let's see one in action. Here's an example of a fairly helpful Exception stack trace:
java.lang.IllegalArgumentException: Spell.getArtName: Invalid art: 77
at wyvern.lib.classes.magic.Spell.getArtName(Spell.java:523)
at wyvern.lib.classes.magic.Spell.getRequiredMana(Spell.java:467)
at wyvern.lib.commands.CastCommand.adjustCasterMana(CastCommand.java:519)
at wyvern.lib.commands.CastCommand.execute(CastCommand.java:463)
at wyvern.kernel.kernel.Executive.dispatchEvent(Executive.java:852)
at wyvern.kernel.kernel.Executive.workerLoop(Executive.java:750)
at wyvern.kernel.kernel.Executive.access$0(Executive.java:43)
at wyvern.kernel.kernel.Executive$WorkerThread.run(Executive.java:719)
Let's say you're debugging your class, watching log messages on your console, when you suddenly see one of these huge stack traces. What do you do? Well, the one above actually tells you exactly what went wrong, if you look closely at it. The top line is the exception message. It says: java.lang.IllegalArgumentException: Spell.getArtName: Invalid art: 77 That's not terribly useful, but it does tell us a few things:
Let's keep looking further up the stack trace. It's a little backwards - you look down the list to go "up" the trace, where "up" means "tracing back to the caller". Up, down, whatever. Doesn't matter, as long as we find what we need to know. The next thing we see is at wyvern.lib.classes.magic.Spell.getArtName(Spell.java:523) Well, now that's interesting. This is an extremely exact location
in the code base. It tells us that the exception was thrown on
line 523 of Spell.java. The class is
The next three lines of the stack trace look like this: at wyvern.lib.classes.magic.Spell.getRequiredMana(Spell.java:467) at wyvern.lib.commands.CastCommand.adjustCasterMana(CastCommand.java:519) at wyvern.lib.commands.CastCommand.execute(CastCommand.java:463) This is all still in the Wyvern game engine, but it's clear that
the spell system is unhappy about something. First we were
in And in fact, that's exactly what happened. I generated the stack
trace above by changing the
OK, so I contrived this example by deliberately breaking my code. But you can see that Exceptions can often show you how your code (or the Wyvern engine code, in some cases) is misbehaving. Unfortunately, Exceptions aren't always so useful. As another example, I changed my hooktest1.py class, which is covered in detail in the Hooks Tutorial, to throw an Exception, just to see where we were. My changes are the lines highlighted below:
The idea is that I'd drop the Mr. Sensitive object, and in its dropPostHook, it would throw an Exception to show exactly how we got there. However, the result is, well... it looks almost as ugly as Perl code:
java.lang.RuntimeException: I'm in hookEvent!
at sun.reflect.NativeConstructorAccessorImpl.newInstance0\
(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance\
(NativeConstructorAccessorImpl.java:39)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance\
(DelegatingConstructorAccessorImpl.java:27)
at java.lang.reflect.Constructor.newInstance(Constructor.java:274)
at org.python.core.PyReflectedConstructor.__call__\
(PyReflectedConstructor.java)
at org.python.core.PyJavaInstance.__init__(PyJavaInstance.java)
at org.python.core.PyJavaClass.__call__(PyJavaClass.java)
at org.python.core.PyObject.__call__(PyObject.java)
at org.python.pycode._pyx69.hookEvent$6\
(/games/wyvern/wiz/rhialto/python/hooktest1.py:23)
at org.python.pycode._pyx69.call_function\
(/games/wyvern/wiz/rhialto/python/hooktest1.py)
at org.python.core.PyTableCode.call(PyTableCode.java)
at org.python.core.PyTableCode.call(PyTableCode.java)
at org.python.core.PyTableCode.call(PyTableCode.java)
at org.python.core.PyFunction.__call__(PyFunction.java)
at org.python.core.PyMethod.__call__(PyMethod.java)
at org.python.core.PyObject.__call__(PyObject.java)
at org.python.core.PyObject._jcallexc(PyObject.java)
at org.python.core.PyObject._jcall(PyObject.java)
at org.python.proxies.main$hooktest1$35.hookEvent(Unknown Source)
at wyvern.kernel.commands.HookList.callHooks(HookList.java:245)
at wyvern.kernel.commands.HookList.runPostHook(HookList.java:136)
at wyvern.kernel.commands.AbstractCommandable.runPostHook\
(AbstractCommandable.java:397)
at wyvern.kernel.commands.EventImpl.runPostHooks(EventImpl.java:723)
at wyvern.kernel.kernel.Executive.dispatchEvent(Executive.java:855)
at wyvern.kernel.kernel.Executive.workerLoop(Executive.java:750)
at wyvern.kernel.kernel.Executive.access$0(Executive.java:43)
at wyvern.kernel.kernel.Executive$WorkerThread.run(Executive.java:719)
Unfortunately, Jython exceptions can be really long, because Jython is doing a lot of "stuff" under the covers in order to make it so useful. However, if you take out all the lines starting with "sun.reflect" and "org.python", except for the ones that mention my hooktest1.py class, you get something slightly more readable:
java.lang.RuntimeException: I'm in hookEvent!
at org.python.pycode._pyx69.hookEvent$6(/games/wyvern/wiz/rhialto/python/hooktest1.py:23)
at org.python.pycode._pyx69.call_function(/games/wyvern/wiz/rhialto/python/hooktest1.py)
at org.python.proxies.main$hooktest1$35.hookEvent(Unknown Source)
at wyvern.kernel.commands.HookList.callHooks(HookList.java:245)
at wyvern.kernel.commands.HookList.runPostHook(HookList.java:136)
at wyvern.kernel.commands.AbstractCommandable.runPostHook(AbstractCommandable.java:397)
at wyvern.kernel.commands.EventImpl.runPostHooks(EventImpl.java:723)
at wyvern.kernel.kernel.Executive.dispatchEvent(Executive.java:855)
at wyvern.kernel.kernel.Executive.workerLoop(Executive.java:750)
at wyvern.kernel.kernel.Executive.access$0(Executive.java:43)
at wyvern.kernel.kernel.Executive$WorkerThread.run(Executive.java:719)
With this, we can deduce three things:
But at least you've seen Exceptions at their worst, and now you know that you can sometimes extract useful information from the stack traces. In the next lesson, we'll show you how to catch and log Exceptions effectively.
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