I'm modifying a java program to search through a specific folder and load plug-ins at runtime. The plugin code is working fine. I have created an UncaughtExceptionHandler to catch problems with plugins that weren't coded properly, and for the most part that works. Except for one issue (one so far, anyway):
The plugins need to have a class that implements a specific interface, so that the main program recognises them as plugins. If the developer misses one of the abstract methods, an AbstractMethodError gets thrown. It goes through my ExceptionHandler and I'm able to put up a message to the user indicating that there's a problem with the plugin. After that, though, the program just hangs. What I want is the program to continue going so that I can skip the rest of the plugin stuff, remove it from the plugin list, and let the user run the main program without it. I put a try/catch block directly around the call to the missing method, but the catch doesn't get executed. It just goes to the ExceptionHandler and then... I don't know where it goes.
This is my first attempt at exception handling so I'm sure I'm just missing something obvious. Any help would be greatly appreciated. Thanks so much.
The problem with exceptions that sub-class from the Error class is that most of them are not recoverable (in your case it's AbstractMethodError). As per the Error class java-doc:
An Error is a subclass of Throwable that indicates serious problems that a reasonable application should not try to catch. Most such errors are abnormal conditions. The ThreadDeath error, though a "normal" condition, is also a subclass of Error because most applications should not try to catch it.
I.e. it's not guaranteed that the app is recoverable after an Error is thrown. Depending on how you load/execute that plugin there might be some workarounds. First of all, you could check the loaded plugin class via reflection (some examples) if there is an implementation of the needed methods before actually trying the exectuion - so you could catch/throw an Exception rather than get the Error and hang later.
If it's not an option, you could investigate further on what is actually hanging after getting that error by analyzing thread dump re
Taking thread dumps in production
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I have a GroovyShell instance parsing code inside my Tomcat/Java application. The parsing is very slow, about 1 second for 100 lines. When profiling the application I noticed the parsing throws lots of java.lang.ClassNotFoundException exceptions. I guess something's swallowing them since I don't see them anywhere in the log. Since the script uses a lot of the main application's classes I'm assuming that that's what's slowing down the application.
Is there a way for me to catch those exceptions and get their data? What could be causing them? Is it possible that I'm using the wrong class loader?
Is there a way for me to catch those exceptions and get their data?
It is difficult to answer that without more information that indicates what is causing them. It may be that you are referencing classes that are not available in the appropriate class loader. It might be that the exceptions are expected. More info would be needed to answer that.
What could be causing them?
Code referencing classes that aren't available to be loaded is one thing that can cause them.
Is it possible that I'm using the wrong class loader?
It is. Without seeing your code and knowing more about the which classes cannot be found, it is difficult to say for sure that is the issue though.
I have a legacy java app written up by a previous developer. A bug has recently been found in it, and I've been given the task of fixing it.
Part of the problem is that there were never any error messages reporting from it.
By putting in a lot of logging messages, I finally narrowed it down to a specific line in the code - it's trying to run a method on a null object.
This is something that SHOULD have thrown an error into the log. Yet it hasn't. And even fixing this one, there's a lot of this problem in the code - assuming something will have a value when it doesn't. Every time I put in a data-verification for one, it fails somewhere shortly down the line for a very similar reason, and then I have to go through the hassle of putting in logging commands every other line again to finally narrow it down.
Why would a java program be silently failing instead of throwing errors? I can't seem to find any sort of setting suggesting that this is on purpose, but I'm really not even sure where to look for such a thing.
I used abort() In C to make my program crash. And debugger will trap this.
Maybe equivalent in Java should be exception. But Java also has very strict exception chaining policy, so using of exception for debugging purpose doesn't seem to be a good idea.
What should I use for this in Java? Intentional program crash for debugging purpose which can be trapped by debugger. (I am using Eclipse)
Update
It would be very nice if the code can be optionally opt-out on release build.
In Eclipse you can set debug points, and run in debug mode.
That said, there is a difference between a CheckedException (which must be caught by the calling function or passed up the chain) and an UncheckedException (which is closer to what you want). Throw the latter if you want to force a crash.
However, this is generally a bad way to program. You'll find better results using alternative means to catch the errors you want.
You can use assert(false). Assuming you're using Eclipse for the debugger, you can set Eclipse to break on specific exceptions (in this case, AssertionError). Make sure you enable assertions, otherwise it won't do anything.
Since you have to explicitly tell the JVM to enable assertions, I think this would count as "code [that] can be optionally opt-out on release build."
I do not know how to handle it in Eclipse. However, under JDB, this can be done by catching the Exceptions. Eclipse should have such an interface.
Using command line like this
"catch java.io.FileNotFoundException"
For details, please refer to here
I have a large program, that i modified in java. I used the intelliJ idea (community edition) IDE for compiling. When i go to run the program, it starts up the GUI and then proceeds to do everthing i want from it, with very few problems (of which are unrelated to the exceptions). But the code always generates class not found exceptions (even the original unmodified code does this once you extract it from the .jar file. Despite these errors, it executes within the IDE perfectly, while still noting the errors, but they don't appear to have an effect on the program. However, when i execute them from within the virtual machine (with java filename) the exceptions which are usually ignored prevent the ultimate execution of the program. The errors are exactly the same as the ones that the iDE shows, but the IDE ignores them! How could i get a virtual machine to ignore the errors and execute the program (is there an option to pass to java - for example java -ignoreerrors filename).
Is this possible, or will i have to alter the code?
There's no way to ignore ClassNotFoundExceptions unless that class isn't actually needed by the code. Some frameworks do that by trying to load a class to discover whether some feature is available. However, if a CNFE is preventing your app from running, you'll just have to fix it. If you show some stack traces, someone might be able to steer you in the right direction.
If you are having trouble with ClassNotFoundExceptions then you can always localize the problem and catch and log using try { ... } catch (...) { ... }.
If you are instead getting ClassNotFoundErrors then it's not a localizable problem with reflection, but a failure to initialize code that's needed. You should try to prune unneeded dependencies but you really shouldn't use classes that haven't initialized properly.
If you absolutely have to, you can always load your program using a custom ClassLoader that generates bogus empty classes for any name that is not resolvable using the system classloader and use that to load your main class. That will replicate, to some degree, what your IDE is doing, though your IDE probably goes the extra step to make sure that partially well-defined classes have the right interface even if some methods are stubbed out because their bodies don't compile.
You can only ignore compiler warnings. You cannot ignore errors.
The errors that IntelliJ shows are coming from the same compiler.
ClassNotFoundException would indicate that your code failed to dynamically load a class at runtime.
This could mean that a required dependency (jar) is missing from your classpath. Try to consult your code documentation and make sure you've resolved all runtime dependencies. Also make sure that the dependent jars are in the classpath otherwise the runtime won't be able to find them.
I'm running Eclipse for Java. I created a DOM version of an XML file. Now I want to change an attribute of an element in the file. I called a method that called a method in the class that controls the DOM, and I got a dialog box saying "JDI Thread Evaluations has encountered a problem. Exception processing async thread queue" while debugging.
I'm a relative newbie at Java and have not come across such an error, and I have no idea what's causing it.
If anyone has any suggestions as to the cause of the problem and/or ways to fix it ...
Thanks so much!
Keep an eye on your "Watch" expressions - if you don't need them, remove them all. Sometimes certain watch expressions have caused this error message for me.
This might also explain why it happens to intermittently for people (sometimes they have watch assignments that cause the error, however restarting or removing the right watch can solve the issue without them knowing it.)
The Java Debug Interface (JDI) is part of the Java Platform Debugger Architecture. One apparent way to trigger this exception occurs when an object's toString() method carelessly returns null. You might want to try a different debugger, and it wouldn't hurt to validate your XML. As noted in comments, the problem may appear intermittently, suggesting a thread synchronization problem.
This error occurs quiet a few times when you debugging.
You can clean this up by removing all watch statements and all breakpoints and restarting Eclipse.
The problem can also occur if source lookup is incorrectly configured. For example lets say one wants to watch the expression foo.bar(). Now if source lookup is incorrectly configured, the debugger is unable to resolve the binding for expression foo (see this eclipse bug). Then the invocation of .bar() will fail with a NullPointerException.
This problem may occur if you have "Detail Formatters" configured for specific classes (in Eclipse: Java -> Debug -> Detail Formatters). Try to disable them.
This was replicated in eclipse when I was debugging the expression which reads: "", which is just an empty expression.
So, if you're getting this problem, one solution may be to check to see if one of your expressions is empty or similar, then aptly delete it.