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Handling exceptions in Java application

When I throw an exception from the package in which I handle the database, in the package in which I handle the UI, should I throw the same exception or create another?
The UI package should know the exceptions of the package which I handle the database?

We as programmers want to write quality code that solves problems. Unfortunately, exceptions come as side effects of our code. No one likes side effects, so we soon find our own ways to get around them. I have seen some smart programmers deal with exceptions the following way:
public void consumeAndForgetAllExceptions(){
try {
...some code that throws exceptions
} catch (Exception ex){
ex.printStacktrace();
}
}
What is wrong with the code above?
Once an exception is thrown, normal program execution is suspended and control is transferred to the catch block. The catch block catches the exception and just suppresses it. Execution of the program continues after the catch block, as if nothing had happened.
How about the following?
public void someMethod() throws Exception{
}
This method is a blank one; it does not have any code in it. How can a blank method throw exceptions? Java does not stop you from doing this. Recently, I came across similar code where the method was declared to throw exceptions, but there was no code that actually generated that exception. When I asked the programmer, he replied "I know, it is corrupting the API, but I am used to doing it and it works."
Please visit here for more details.

To keep your layers tightly coupled, I recommend to abstract your Exceptions (if there isn't a fitting standard exception). With abstract I mean that you could wrap database-dependent exceptions like SQLException into a PersistenceException or sth. like that. This way you can easily change your layers without having to worry about changing code of the layers above.
You should then only catch and handle the exception if you can handle them, e.g. propagate it to the user in you GUI. Otherwise you should throw them to the next caller, until it is handled on a higher level.
You should avoid recreating new exceptions on their way up, that wouldn't add any information in most cases.

You should throw an Exception that makes sense. More importantly you should not throw an exception that doesn't. Exceptions represent error states that you cannot easily recover from, meaning that if you have code to handle things like missing files, it should not throw an exception. You should only throw an exception when something unexpected happens.
As exceptions propagate up through your stack, they should be diversified, depending on what part of the code they where caught in.
For instance your persistence framework may throw an SqlException, your DAO Layer may re-throwthis as an IllegalArgumentExection, or ObjectNotFoundEception, your Service Layer may throw a MalformedRequestException, an AccessDeniedException or a DeviceNotEnrolledException. Lastly your UI may display this to the user in any number of meaningful ways.

I recommend putting the exceptions where it relates the most. You should have different exceptions that handle the code in different types of situations. The UI is going to have different types of exceptions it should handle than the database.
Use pre-defined exceptions where it makes sense, you could also make your own exceptions and put them in both packages if need be.
Design it so that the code can be handled in a way where the catch statement is easy to logically locate and the catch statement handles the code and can do some sort of recovery after being passed up the hierarchy.
The type of situation we are concerned with the most are checked exceptions. These are problems that we can anticipate and catch and try to make a recovery. Make sure you aren't using too many of these! They are costly to resources!

EJB3.1 System Exceptions vs javax.ejb.EJBException

Just a bit of background on EJB3.1 exceptions before bringing up my question -
Application exceptions comprise of
User defined checked or unchecked exceptions which have
#ApplicationException annotation
All checked exceptions
java.lang.Exception & it's subclass exceptions
except java.rmi.RemoteException & it's subclass exceptions
System exceptions comprise of
java.rmi.RemoteException and it's subclass exceptions
All unchecked exceptions
java.lang.RuntimeException and it's subclass exceptions
java.lang.Error and it's subclass exceptions
Below is a statement I have read in this book
In EJB System exceptions are not excepted by the client, when encountered such exceptions are not passed to the client as is but are wrapaped in a javax.ejb.EJBException.
My questions -
Are all application exceptions described above supposed to be thrown directly by EJB to client?
If System Exceptions are wrapped inside javax.ejb.EJBException before throwing to the client, then is javax.ejb.EJBException considered as System Exception?

IndoKnight, what a perfect wrap up you gave of how Exception semantics within the Java EE framework.. work.
Here's the two only lines a "bean provider", that is you and me, need to know about exceptions in Java EE:
Your bean is totally free to recover from any type of exception or error as you see fit, with respect to other constraints the bean might be under. If you recovered from an exception, then congratulations - there's no problem anymore =)
A relevant constraint could for example be that "enterprise beans that use container-managed transaction demarcation must not use any transaction-management methods that interfere with the container's transaction demarcation boundaries" to quote the Java EE 7 tutorial page 48-2 (would you like to programmatically set the rollback of a container managed transaction, use EJBContext.setRollbackOnly()).
You are also discouraged, as with any type of Java application, to handle Throwable or Error thrown from a really low level. RuntimeException is theoretically counted in this category, as it so famously signals a "developer error" that's like "totally totally unexpected" - yet we all know it isn't.
Basically, an unexpected exception (runtime exceptions + other shit that we assume originate from someone else) is assumed to be unhandleable by your code and should be handled by the server instead. The server is required to handle the "unhandleable" exception (see the EJB 3.2 specification, page 204) by printing something about it in the log (I'll get into the details a bit later!).
More specifically..
You asked (and here is what I believe or hold true):
Are all application exceptions described above supposed to be thrown
directly by EJB to client?
Yes. And the transaction (if one is active) shall not be rolled back unless you explicitly state that it should by using the rollback attribute of the ApplicationException. Java code throwing exceptions is a very natural thing. Java EE has no intention of demolishing this programming model. So go ahead, throw checked exceptions however you like, forcing the clients to try-catch those or mark runtime exceptions as being application exceptions and throw those too. Happy throwing!
If System Exceptions are wrapped inside javax.ejb.EJBException before
throwing to the client, then is javax.ejb.EJBException considered as
System Exception?
Yes, but not for the reason you provide. To be clear, there is no type called SystemException. It is just fancy wording to describe those kind of exceptions that the majority of beans out there didn't figure could happen, and most likely, they originate from the EJB container. Your code should definitely not throw an EJBException. Doing so would probably only thwart the mind of the Server. Nor can you annotate the exception as being #ApplicationException since you don't own the code, it is provided by the Java EE API. You could extend EJBException, but it wouldn't make any sense to disguise your code as being part of the server's codebase. Most importantly, since EJBException extends RuntimeException, I think it is safe to categorize the EJBException as a "system-level exception".
Watch out for the Devil
Some remote clients will receive the RemoteException instead of the EJBException.
Interceptors that you might not even be aware of in a huge enterprise project, could swallow exceptions thrown from your method making an active transaction commit even though you never had plans to let him do that.
I bet you think that a suppressed exception is always retrievable using Throwable.getCause(). Do note that the EJB 3.2 specification does not say that the container has to or should spare a reference to the suppressed cause. In fact, the only thing the container has to do is log the exception. Then, if the bean is not a singleton, the bean instance must be discarded and never used again. Also, if the bean is a message-driven bean, only then is the original exception required to be "wrapped". How-to "wrap" the exception is not specified. Super portable cool code should probably have a look at both the Throwable.getCause() method and Throwable.getSuppressed() too. Don't blindly expect in your handling code to always find the original exception.
Asynchronous methods (public session bean methods annotated #Asynchronous) that return void cannot propagate an exception to his client. Thus they must not declare or throw application exceptions (see the EJB 3.2 specification page 82). Do note that when you invoke an asynchronous method, it might be the case that the server cannot provide threading resources to service your request and if so, he is required to throw an.. EJBException (page 48)!

Yes, more or less that's how they work. For further details on EJB's behaviors check out the following blog post: Link
Also from this question:
An application exception shall be thrown when there is a
business-logic error, as opposed to a system error.
There is an important difference: application exceptions do not
automatically cause a transaction to rollback. The client has an
opportunity to recover after an application exception is thrown.
Application exceptions are sent to the client without being repackaged
as an EJBException. Therefore, you can use them to report validation
errors or business logic problems, and they will reach the client.
javax.ejb.EJBException extends RuntimeException so yes, it is a System Exception.
Common scenario associated with this: if you've got an uncaught RuntimeException in your application code it will roll-back. It's pretty useful default behavior.

Is it a good practice to use nested exceptions?

This is probably a broad question, not quite SO style, but I'd still like to get some hints or guidelines if possible.
I've been looking through some legacy code and found a part of it that has methods with exceptions nested 3 or 4 levels down.
Is this considered to be a normal practice or should one avoid such codestyle where possible? If it should be avoided, what are the negative effects besides the increasing costs of exception handling and decreasing readability? Are there common ways of refactoring the code to avoid this?

I personally prefer the following ideology
Wrap Alien Exceptions
An "alien" exception is an exception thrown by a Java API or a third party library. In other words, an exception you do not control.
Its better to catch all alien exceptions and wrap them in an appropriate application specific exception. Once the alien exception is converted to your own exception, you can propagate that exception any way you like.
Rethrowing Checked Exceptions can get Messy
If your application uses checked exceptions, rethrowing the original exception means that the method rethrowing it must also declare it.
The closer you get to the top of the call hierarchy, the more exceptions will be declared thrown. Unless you just declare all your methods to throw Exception. However, if you do so you might as well use unchecked exceptions, since you are not really getting any benefit from the compiler exception checking anyways.
This is why I prefer to catch non-application specific exceptions and wrap them in an application specific exception, before propagating them up the call stack.
Guidelines For Wrapping : The context in which an exception occurs may be just as important as the location of the exception itself. A given location in the application may be reachable via different execution paths, and the execution path may influence the severity and cause of the error, if it occurs.
If you need to add context information to an exception as you propagate it up the call stack, you need to use active propagation. In other words, you need to catch the exception in various relevant locations on the way up the call stack, and add the relevant context information to it, before rethrowing or wrapping it.
public void doSomething() throws SomeException{
try{
doSomethingThatCanThrowException();
} catch (SomeException e){
e.addContextInformation(“more info”);
throw e; //throw e, or wrap it – see next line.
//throw new WrappingException(e, “more information”);
} finally {
//clean up – close open resources etc.
}
}

Checked Exceptions should not be propagated up the stack or chained if possible. If a method is throwing a checked Exception its caller is supposed to handle it, if caller is not handling it and propagating it to its caller, then overall complexity increases.
In a three layered example : Dao , Service , Controller
DAO layer will throw DAOException
Service layer should not expose DAOException to Controller , instead it should be throwing relevant BuinessExceptions, which the Controller should be handling.

Exception handling tends to be an expensive way to handle flow control (certainly for C# and Java).
The runtime does quite a lot of work when an exception object is constructed - getting the stack trace together, figuring out where the exception is handled and more.
All this costs in memory and CPU resources that do not need to be expanded if flow control statements are used for flow control.
Additionally, there is a semantic issue. Exceptions are for exceptional situations, not for normal flow control. One should use exception handling for handling unanticipated/exceptional situations, not as normal program flow, because otherwise, an uncaught exception will tell you much less.
Apart from these two, there is the matter of others reading the code. Using exceptions in such a manner is not something most programmers will expect, so readability and how understandable your code is suffer. When one sees "Exception", one thinks - something bad happened, something that is not supposed to happen normally. So, using exceptions in this manner is just plain confusing.
Please take a look at below links
Exception Handling: Common Problems and Best Practice with Java 1.4 - pdf
Why not use exceptions as regular flow of control?
Best Practices for Exception Handling
Error Handling
Mr. Google Links

I've been looking through some legacy code and found a part of it that has methods with exceptions nested 3 or 4 levels down.
Is this considered to be a normal practice or should one avoid such codestyle where possible?
This is not a necessary process to handle your exception in this way, as it will increase your application overhead, until you really need to handle very specific exception(checked or Alien Exceptions) and you can ignore overhead to get specific information to handle that exception.
If it should be avoided, what are the negative effects besides the increasing costs of exception handling and decreasing readability?
As I mentioned you will not get specific information about the exception, if you are not going to use nested exception handling(throws with some added information to the upper handler) you may/may'not do specific action on behalf of some tough exception, but in nested case you can do action by handling that specific situation.
Are there common ways of refactoring the code to avoid this?
If you have a poorly factored program that does what the you want and has no serious bugs, for god sake leave it alone! When you need to fix a bug or add a feature, you Refactor Mercilessly the code that you encounter in your efforts. Override the Exception Class in your custom Exception Handler and add some added features to handle your problem.
The overriding method must NOT throw checked exceptions that are new or broader than those declared by the overridden method. For example, a method that declares a FileNotFoundException cannot be overridden by a method that declares a SQLException, Exception, or any other non-runtime exception unless it's a subclass of FileNotFoundException.
Hop this will help you.

You should do away with the exception nesting. You should either avoid chaining the exceptions in the first place, or (selectively) unwrap and then rethrow the nested exceptions further up the stack.

About handling legacy code I would recommend you have a look at the book covering the topic:
http://www.amazon.com/Working-Effectively-Legacy-Michael-Feathers/dp/0131177052
You dont even have to go through the whole book, just look at the things that concern you at the moment.
Also a good book regarding good practices is:
http://www.amazon.com/Clean-Code-Handbook-Software-Craftsmanship/dp/0132350882/ref=sr_1_1?s=books&ie=UTF8&qid=1356340809&sr=1-1&keywords=clean+code
The best approach when handling nested exceptions is refactoring the code and using runtime instead of checked exceptions, and handling those where needed. This way the code is more readable and easier to maintain.

Its depend on the Business logic. You may take action on the exception there itself or you may propogate it all the way upto caller and leave it to the caller for what action he want.
e.g. There are lot of third party API where they don't handle the exception but they throw it from method and hence facilitate API users to take actions as per their need.
e.q. oracle JDBC driver. Driver.getConnection() throws exception. Now caller/API user can handle it as per their need. One may just print stack trace, one may notify admin asking for his attention or one may choose just silently exit the application.

There are two approaches:
To generate a separate exception for each event.
To create a generic exception and describe what caused it
The first approach allows you to write different code for handling the different events, but it requires you to write lot of Exception classes and in some case it could be just too much.
The second approach is more concise, but it makes it difficult to handle the different situations.
As it happens very often in programming the best solution is in the middle, where you balance generating separate exceptions and using one exception for other cases.
The rule of the thumb in this case could be to generate a separate Exception class for the exceptions you want to handle specifically with separate code.
Similarly to the what to throw, we should also have control on what to catch. We can use two approaches for our catch blocks:
A single catch block for all. For example:
catch (Throwable e) {
throw new CommandExecutorException(e);
}
many catch blocks one for each Exception. For example:
} catch (ClassCastException e1) {
...
} catch (FileNotFoundException e) {
...
} catch (IOException e) {
...
}
The first approach is very compact but basically groups every exception in the same case, it's useful only in the situation where all the exceptions are managed equally and do the same thing.
This approach is generally discouraged as it gives no control on the exceptions being catched, leading sometimes to tough bugs, which are also hard to find.
The second approach requires more lines of code but you can execute different operations depending on the exception occurred. This approach is more flexible but in some cases leads your methods to be very long due to exception handling.

Java Asynchronous Exceptions: Can I catch them?

I have been reading the JLS and I encountered the section 11.1.3. Asynchronous Exceptions from which I quote:
Most exceptions occur synchronously as a result of an action by the
thread in which they occur, and at a point in the program that is
specified to possibly result in such an exception. An asynchronous
exception is, by contrast, an exception that can potentially occur at
any point in the execution of a program.
And
Asynchronous exceptions occur only as a result of:
[...]
An internal error or resource limitation in the Java virtual machine that prevents it from implementing the semantics of the
Java programming language. In this case, the asynchronous exception
that is thrown is an instance of a subclass of VirtualMachineError.
Is it possible to catch such exceptions for logging purposes or notification (because I believe such thing is unrecoverable)? How can I achieve such thing?

You can catch such exceptions just like any other exception. The only problem is that they may occur at any place in your program, so catching them reliably is hard. You would basically have to wrap the run method of all threads and the main method in a try..catch block, but you can't do that for threads you don't control (like the Swing EDT, or threads for timers etc.).
Also catching any subclass of Error is usually not recommended, because the JVM may be in an unstable state, which might lead to a further failure (for example in the case of OutOfMemoryError, you might not even have enough memory to to the exception handling). However, logging would be a valid reason for catching Errors in my eyes.
My suggested solution would be to use an uncaught exception handler for this by setting it as the default exception handler. In this handler you will get all exceptions and errors, if they are not caught anywhere in the code, and you can try to log them.

There is no point of catching these exceptions (Subclasses of VirtualMachineError) as you have no indecattion in which state the pogram is at the point, the Doc saies about Virtual Machine Errors:
A Java virtual machine implementation throws an object that is an
instance of a subclass of the class VirtualMethodError when an
internal error or resource limitation prevents it from implementing
the semantics described in this chapter. This specification cannot
predict where internal errors or resource limitations may be
encountered and does not mandate precisely when they can be reported.
so assuming you get in an OutOfMemoryError or an UnknownError there isnt much you can do about it, and once your vritualmashine doesnt work properly you cant provide the user anyhelp as your program isnt working properly as well, besides you have no idea at what time, point, and reason it happends since its not a code error that was caused from your program.

Why Would you catch Throwable and do nothing but print a stack trace?

I have tried looking for an answer to this on other threads, but so far I have only seen threads that state that catching Throwable is bad. My question is, is there ever a reason why you would WANT to do this and then do nothing in the catch block except print out the stack trace?
I was recently brought onto a project and given the task of cleaning up the error handling of an existing set of classes for a RESTful service. Several of the helper service classes have try/catch blocks that only catch Throwable and print out the stack trace, as shown below:
class MainService {
SubService1 s1;
SubService2 s2;
public doMainService() {
}
}
class SubService1 {
public int findSomething() {
try {
// Do Something
} catch (Throwable t) {
t.printStackTrace();
}
}
}
class SubService2 {
public int findSomethingElse() {
try {
// Do Something
} catch (Throwable t) {
t.printStackTrace();
}
}
}
Is there a case that this is acceptable? Would it be better for the methods to throw Exception and not have the try/catch blocks?

This is almost never a good practice for a variety of well known reasons.
In particular, it doesn't distinguish between checked and unchecked exceptions and errors. More importantly, one of the effects of this code is allowing the application to execute beyond the exception handler which may result in all kinds of strange behavior due to violated invariants. In other words, since the exception caught may be really anything including violated assertions, programming errors, thread interruptions, missing classes, I/O errors, OOM conditions and even library and VM bugs, the program state is practically unpredictable beyond the exception handler.
In some rare situations broad exception handling may make sense. Imagine a server handling multiple independent requests. You may not want to crash due to a problem encountered while serving one of the requests. Since you do not rely on the state left after the exception handler, you can simply print the stack trace and let someone investigate while the server continues serving other requests.
Even in these situations one should carefully consider whether errors, e.g. VirtualMachineError should really be caught.

One reason that I think people do this is just because Java forces you to either surround calls that throw with a try/catch block, or add throws to the method declaration.
If you "know" that you're not going to have an exception occur, then it's kind of a way to prevent it from going up the chain (cause if you do throws, who ever calls your code needs to surround with a try/catch and so on), but if something does occur it'll dump it without crashing.

You might do that if you don't know any other way to get the stack trace and you want to know how you got to your current location. Not a very good reason, but a possible one. This doesn't seem to fit what you're looking at though; you seem to have been given code which doesn't crash but also doesn't do a good job with error handling.

I use it to see exactly where my program is crashing. So basically just for debugging. Also just to see if it's flowing in the way expected.

It is possible that they wanted to see the stack trace without crashing the program.
For example it can be appropriate for code executed by a thread to log exceptions but otherwise do nothing when it is unacceptable for the thread to crash since the next iteration (say where threads are taking items from a queue) may work as expected. This depends on the use case, but for a server you generally want the threads to be bullet proof and log any errors, instead of halting any further processing (but use cases may vary).

Most simplest example we come across in your problem is when it comes to close an input stream.Follwoing is the method declaration in InputStream class.
public void close() throws IOException
Althoug there is a possiblity to throw an exception when we call this method it would be no harm for the program execution.(Since we will not use that InputStream further) In that case we should only log the error and continue the program. But if you find out an exception which change the states of an object and then you must think about recovery code or halt execution.

Never do this (e.printStackTrace()). many IDE's default to it, but its horrible (many apps run with stderr redirected in ways that are not accessible, and thus, never seen). rules of thumb:
if you truly don't care about the exception (never, ever, ever, ever will care), catch it and do nothing with a big comment which says "we really, really don't ever care if this exception gets thrown"
if you never expect an exception to actually be thrown (the interface defines a checked exception, but you know the impl doesn't actually throw it), rethrow the exception as something like new UnsupportedOperationException("this should never happen", t) (that way if/when the underlying impl gets changed you find out about it sooner rather than later)
if you expect that the exception is unlikely, and you usually will not care about it, log it with a proper logging infrastructure (log4j, commons logging, or java.util.logging). that way it will most likely make it somewhere that you can see later if you decide that you actually do care about the exception after all.