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Java: Caught exception is immediately rethrown? What's the problem?

I have the following Java code (that I thought was OK) but the IDE is warning me that "IOExcpetion e is immediately rethrown."
I'm new to exception handling in Java, and I wasn't aware there was anything wrong with doing that.
From what I'm reading, it looks like my other options is to simply delete the catch block and replace it with a finally block... but then it warns that the finally block is empty.
I don't have any ideas on what I should do differently.
private InputStream getFlutterAssetAsInputStream(String fromAsset) throws IOException {
String assetPath = binding
.getFlutterAssets()
.getAssetFilePathBySubpath(fromAsset, PACKAGE_NAME);
try {
return binding.getApplicationContext().getAssets().open(assetPath);
} catch (IOException e) {
throw e;
}
}

This
try {
return binding.getApplicationContext().getAssets().open(assetPath);
}
catch (IOException e) {
throw e;
}
is functionally equivalent to this:
return binding.getApplicationContext().getAssets().open(assetPath);
Your IDE is telling you that your try-catch is pointless. It does nothing.
So what do you want to do?
Pass the exception on to your caller? In which case get rid of the try-catch
Signal a different exception on to the caller - maybe one with a more appropriate error message for the specific circumstances? Then make and throw a new exception object with a better error message.
Do something to handle the situation - at the very least, maybe display an error message? Log the event to enable debugging? Then write code in the catch-block to do that. And then you have to decide what happens next.
Fundamentally, this is a design issue. IO errors happen. So how does this method that you're writing want to handle IO errors?

Java know in finally that exception thrown without any variable [duplicate]

In Java, is there an elegant way to detect if an exception occurred prior to running the finally block? When dealing with "close()" statements, it's common to need exception handling within the finally block. Ideally, we'd want to maintain both exceptions and propagate them up (as both of them may contain useful information). The only way I can think of to do this is to have a variable outside the try-catch-finally scope to save a reference to a thrown exception. Then propagate the "saved" exception up with any that occur in the finally block.
Is there a more elegant way of doing this? Perhaps an API call that will reveal this?
Here's some rough code of what I'm talking about:
Throwable t = null;
try {
stream.write(buffer);
} catch(IOException e) {
t = e; //Need to save this exception for finally
throw e;
} finally {
try {
stream.close(); //may throw exception
} catch(IOException e) {
//Is there something better than saving the exception from the exception block?
if(t!=null) {
//propagate the read exception as the "cause"--not great, but you see what I mean.
throw new IOException("Could not close in finally block: " + e.getMessage(),t);
} else {
throw e; //just pass it up
}
}//end close
}
Obviously, there are a number of other similar kludges that might involve saving the exception as an member variable, returning it from a method, etc... but I'm looking for something a bit more elegant.
Maybe something like Thread.getPendingException() or something similar? For that matter, is there an elegant solution in other languages?
This question actually spawned from comments in another question that raised an interesting question.

Your idea about setting a variable outside the scope of the try/catch/finally is correct.
There cannot be more than one exception propagating at once.

Instead of using a Boolean flag, I would store a reference to the Exception object.
That way, you not only have a way to check whether an exception occurred (the object will be null if no exception occurred), but you'll also have access to the exception object itself in your finally block if an exception did occur. You just have to remember to set the error object in all your catch blocks (iff rethrowing the error).
I think this is a missing C# language feature that should be added. The finally block should support a reference to the base Exception class similar to how the catch block supports it, so that a reference to the propagating exception is available to the finally block. This would be an easy task for the compiler, saving us the work of manually creating a local Exception variable and remembering to manually set its value before re-throwing an error, as well as preventing us from making the mistake of setting the Exception variable when not re-throwing an error (remember, it's only the uncaught exceptions we want to make visible to the finally block).
finally (Exception main_exception)
{
try
{
//cleanup that may throw an error (absolutely unpredictably)
}
catch (Exception err)
{
//Instead of throwing another error,
//just add data to main exception mentioning that an error occurred in the finally block!
main_exception.Data.Add( "finally_error", err );
//main exception propagates from finally block normally, with additional data
}
}
As demonstrated above... the reason that I'd like the exception available in the finally block, is that if my finally block did catch an exception of its own, then instead of overwriting the main exception by throwing a new error (bad) or just ignoring the error (also bad), it could add the error as additional data to the original error.

You could always set a boolean flag in your catch(es). I don't know of any "slick" way to do it, but then I'm more of a .Net guy.

Use logging...
try {
stream.write(buffer);
} catch(IOException ex) {
if (LOG.isErrorEnabled()) { // You can use log level whatever you want
LOG.error("Something wrong: " + ex.getMessage(), ex);
}
throw ex;
} finally {
if (stream != null) {
try {
stream.close();
} catch (IOException ex) {
if (LOG.isWarnEnabled()) {
LOG.warn("Could not close in finally block", ex);
}
}
}
}

In vb.net, it's possible to use a "Catch...When" statement to grab an exception to a local variable without having to actually catch it. This has a number of advantages. Among them:
If nothing is going to 'ultimately' catch the exception, an unhandled exception trap will be fired from the spot of the original exception. Much nicer than having the debugger trap at the last rethrow, especially since information that might be needed for debugging won't yet have gone out of scope or been swept up by 'finally' statements.
Although a rethrow won't clear the stack trace the way "Throw Ex" would, it will still often jinx the stack trace. If the exception isn't caught, the stack trace will be clean.
Because this feature is unsupported in vb, it may be helpful to write a vb wrapper to implement the code in C (e.g. given a MethodInvoker and an Action(Of Exception), perform the MethodInvoker within a "Try" and the Action in a "Finally".
One interesting quirk: it's possible for the Catch-When to see an exception which will end up getting overwritten by a Finally-clause exception. In some cases, this may be a good thing; in other cases it may be confusing. In any event, it's something to be aware of.

Exception - why even bother throwing?

So, I know this is going to sound a little extreme but I'm trying to make a point:
OK. so my code doesn't care what kind of exception is being thrown, 99.9% of the time if an exception is being thrown it handles it the same no matter what.
Now, Why should i even bother throwing \ creating new exception in my code?
apparently all the libraries i use throw them already and the are very informative as well.
a null object passed for my method? who cares, a null pointer exception will be thrown automatically for me.
Can you make a good argument why should i create new exceptions and throw them?
EDIT what i mean:
why bother with this:
public myMethod() {
try {
doStuff1();
}
catch(Exception e) {
throw new Exception("Error in doStuff1");
}
try {
doStuff2();
}
catch(Exception e) {
throw new Exception("Error in doStuff2");
}
when i can use this:
public myMethod() {
doStuff1();
doStuff2();
}

Your client wants detailed error reporting. What went wrong, and where did it go wrong.
They don't have a clue what a null pointer exception is. And even then, that in itself is not particularly helpful without the stack trace. So a null pointer gets passed into your RetrieveReportX method? Check if it's null and throw a ReportXNoIdentifierException rather than letting it auto-throw a null pointer exception. Then you'll have a error handler somewhere that, based on these custom exceptions, can report exactly what went wrong in what (human) process, and your client is happy because rather than NullPointerException at [stacktrace], they can see "An identifier was not supplied when attempting to retrieve report X."

why should i create new exceptions and throw them?
In some cases it is common and good practice to throw Exception deliberately.
Take
IllegalArgumentException
If your method only accepts arguments within a particular range, e.g. only positive numbers, then you should check for invalid parameters and throw an IllegalArgumentException.
For example:
public int calculateFactorial(int n) {
if (n < 0)
throw new IllegalArgumentException("n must be positive");
if (n >= 60)
throw new IllegalArgumentException("n must be < 60");
...
}
public void findFactorial(int n)
{
try{
calculateFactorial(n)
}catch(IllegalArgumentException ie){
System.out.println(ie.getMessage());
}
}

From Java tutorials - Advantages of Exceptions
Advantage 1: Separating Error-Handling Code from "Regular" Code
Advantage 2: Propagating Errors Up the Call Stack
Advantage 3: Grouping and Differentiating Error Types
Answer
Exception handlers that are too general can make code more error-prone by catching and handling exceptions that weren't anticipated by the programmer and for which the handler was not intended.
As noted, you can create groups of exceptions and handle exceptions in a general fashion, or you can use the specific exception type to differentiate exceptions and handle exceptions in an exact fashion.

Exceptions in Java

At the moment I am developing a website while using the Playframework2. I am just a beginner in programming. I read some books about exceptions but now in the real world , exception handling is really strange.
To be honest I don't really care what exceptions are thrown I handle all exceptions the same way.
return badrequest(); . I only use exceptions for logging.
try{
...
}
catch(Exeption e){
//log
return badrequest();
}
But this is so much boilerplate and it's really annoying to write, because every method throws the same exceptions.
Any tips , hints or resources that you could give me?
edit:
An example would be my "global" config file. Because I need to connect to the db every time I thought i could write a singleton for this problem.
private Datastore connect() throws UnknownHostException, MongoException,
DbAuthException {
Mongo m = new Mongo(dbUrl, dbPort);
Datastore ds = new Morphia().createDatastore(m, dbName);
boolean con = ds.getDB().authenticate(username, password.toCharArray());
if (!con)
throw new DbAuthException();
return ds;
}
This also results in a try and catch every time I want to connect to the db. My problem is I don't think I can handle them diffidently.
A code example :
public static Result addComment(String title) {
try {
Datastore ds = DatabaseConnect.getInstance().getDatastore();
Form<Comment> filledForm = commentForm.bindFromRequest();
Comment userComment = filledForm.get();
userComment.setUsername(Util.getUsernameFromSession(ctx()));
User.increasePointsBy(ctx(), 1);
UserGuides.addComment(title, userComment);
} catch (Exception e) {
return badRequest();
}
return redirect(routes.Guides.blank());
}
In this case I was to lazy to write the same try and catch over and over again, and this is duplicated code.
Maybe there is a book that explains how to design a big application with exception handling?

When you invoke a method, you do not necessarily have to catch the exceptions right there. You can let your callers handle them (declaring a throws clause if it is a checked exception). In fact, the ability to pass them on to the callers without any additional work is the distinguishing feature of exceptions.
My team has adopted the following coding standard: We throw checked exceptions for those rare cases when we want to recover from a failure, and unchecked exceptions for anything else. There is only a single catch block for the unchecked exceptions in a method so high in the call stack that all requests pass through it (for instance in a ServletFilter). This catch block logs the exception, and forwards the user to the "Sorry, this shouldn't have happened" page.

Have you looked at your code to examine why you're throwing all these exceptions? Exceptions are there for a reason- to tell you that something went wrong. If you're writing too much "boilerplate" try-catch code and you're not in a thousand line application, you have to refactor.
try-catch can be irritating when you have a complex block and can become very monotonous and boilerplate (Marc Gravell even said he usually uses try-finally) but as a new programmer, it would be helpful for you to examine the code that you write and figure out how to either handle or avoid those exceptions.
As akf mentions, ignoring exceptions can also be hazardous to debugging. It will be harder to track down where something catastrophic went wrong if you're missing exceptions leading up to it.

Is it a bad practice to catch Throwable?

Is it a bad practice to catch Throwable?
For example something like this:
try {
// Some code
} catch(Throwable e) {
// handle the exception
}
Is this a bad practice or we should be as specific as possible?

You need to be as specific as possible. Otherwise unforeseen bugs might creep away this way.
Besides, Throwable covers Error as well and that's usually no point of return. You don't want to catch/handle that, you want your program to die immediately so that you can fix it properly.

This is a bad idea. In fact, even catching Exception is usually a bad idea. Let's consider an example:
try {
inputNumber = NumberFormat.getInstance().formatNumber( getUserInput() );
} catch(Throwable e) {
inputNumber = 10; //Default, user did not enter valid number
}
Now, let's say that getUserInput() blocks for a while, and another thread stops your thread in the worst possible way ( it calls thread.stop() ). Your catch block will catch a ThreadDeath Error. This is super bad. The behavior of your code after catching that Exception is largely undefined.
A similar problem occurs with catching Exception. Maybe getUserInput() failed because of an InterruptException, or a permission denied exception while trying to log the results, or all sorts of other failures. You have no idea what went wrong, as because of that, you also have no idea how to fix the problem.
You have three better options:
1 -- Catch exactly the Exception(s) you know how to handle:
try {
inputNumber = NumberFormat.getInstance().formatNumber( getUserInput() );
} catch(ParseException e) {
inputNumber = 10; //Default, user did not enter valid number
}
2 -- Rethrow any exception you run into and don't know how to handle:
try {
doSomethingMysterious();
} catch(Exception e) {
log.error("Oh man, something bad and mysterious happened",e);
throw e;
}
3 -- Use a finally block so you don't have to remember to rethrow:
Resources r = null;
try {
r = allocateSomeResources();
doSomething(r);
} finally {
if(r!=null) cleanUpResources(r);
}

Also be aware that when you catch Throwable, you can also catch InterruptedException which requires a special treatment. See Dealing with InterruptedException for more details.
If you only want to catch unchecked exceptions, you might also consider this pattern
try {
...
} catch (RuntimeException exception) {
//do something
} catch (Error error) {
//do something
}
This way, when you modify your code and add a method call that can throw a checked exception, the compiler will remind you of that and then you can decide what to do for this case.

straight from the javadoc of the Error class (which recommends not to catch these):
* An <code>Error</code> is a subclass of <code>Throwable</code>
* that indicates serious problems that a reasonable application
* should not try to catch. Most such errors are abnormal conditions.
* The <code>ThreadDeath</code> error, though a "normal" condition,
* is also a subclass of <code>Error</code> because most applications
* should not try to catch it.
* A method is not required to declare in its <code>throws</code>
* clause any subclasses of <code>Error</code> that might be thrown
* during the execution of the method but not caught, since these
* errors are abnormal conditions that should never occur.
*
* #author Frank Yellin
* #version %I%, %G%
* #see java.lang.ThreadDeath
* #since JDK1.0

It's not a bad practice if you absolutely cannot have an exception bubble out of a method.
It's a bad practice if you really can't handle the exception. Better to add "throws" to the method signature than just catch and re-throw or, worse, wrap it in a RuntimeException and re-throw.

Catching Throwable is sometimes necessary if you are using libraries that throw Errors over-enthusiastically, otherwise your library may kill your application.
However, it would be best under these circumstances to specify only the specific errors thrown by the library, rather than all Throwables.

The question is a bit vague; are you asking "is it OK to catch Throwable", or "is it OK to catch a Throwable and not do anything"? Many people here answered the latter, but that's a side issue; 99% of the time you should not "consume" or discard the exception, whether you are catching Throwable or IOException or whatever.
If you propagate the exception, the answer (like the answer to so many questions) is "it depends". It depends on what you're doing with the exception—why you're catching it.
A good example of why you would want to catch Throwable is to provide some sort of cleanup if there is any error. For example in JDBC, if an error occurs during a transaction, you would want to roll back the transaction:
try {
…
} catch(final Throwable throwable) {
connection.rollback();
throw throwable;
}
Note that the exception is not discarded, but propagated.
But as a general policy, catching Throwable because you don't have a reason and are too lazy to see which specific exceptions are being thrown is poor form and a bad idea.

Throwable is the base class for all classes than can be thrown (not only exceptions). There is little you can do if you catch an OutOfMemoryError or KernelError (see When to catch java.lang.Error?)
catching Exceptions should be enough.

it depends on your logic or to be more specific to your options / possibilities. If there is any specific exception that you can possibly react on in a meaningful way, you could catch it first and do so.
If there isn't and you're sure you will do the same thing for all exceptions and errors (for example exit with an error-message), than it is not problem to catch the throwable.
Usually the first case holds and you wouldn't catch the throwable. But there still are plenty of cases where catching it works fine.

Although it is described as a very bad practice, you may sometimes find rare cases that it not only useful but also mandatory. Here are two examples.
In a web application where you must show a meaning full error page to user.
This code make sure this happens as it is a big try/catch around all your request handelers ( servlets, struts actions, or any controller ....)
try{
//run the code which handles user request.
}catch(Throwable ex){
LOG.error("Exception was thrown: {}", ex);
//redirect request to a error page.
}
}
As another example, consider you have a service class which serves fund transfer business. This method returns a TransferReceipt if transfer is done or NULL if it couldn't.
String FoundtransferService.doTransfer( fundtransferVO);
Now imaging you get a List of fund transfers from user and you must use above service to do them all.
for(FundTransferVO fundTransferVO : fundTransferVOList){
FoundtransferService.doTransfer( foundtransferVO);
}
But what will happen if any exception happens? You should not stop, as one transfer may have been success and one may not, you should keep go on through all user List, and show the result to each transfer. So you end up with this code.
for(FundTransferVO fundTransferVO : fundTransferVOList){
FoundtransferService.doTransfer( foundtransferVO);
}catch(Throwable ex){
LOG.error("The transfer for {} failed due the error {}", foundtransferVO, ex);
}
}
You can browse lots of open source projects to see that the throwable is really cached and handled. For example here is a search of tomcat,struts2 and primefaces:
https://github.com/apache/tomcat/search?utf8=%E2%9C%93&q=catch%28Throwable
https://github.com/apache/struts/search?utf8=%E2%9C%93&q=catch%28Throwable
https://github.com/primefaces/primefaces/search?utf8=%E2%9C%93&q=catch%28Throwable

Generally speaking you want to avoid catching Errors but I can think of (at least) two specific cases where it's appropriate to do so:
You want to shut down the application in response to errors, especially AssertionError which is otherwise harmless.
Are you implementing a thread-pooling mechanism similar to ExecutorService.submit() that requires you to forward exceptions back to the user so they can handle it.

Throwable is the superclass of all the errors and excetions.
If you use Throwable in a catch clause, it will not only catch all exceptions, it will also catch all errors. Errors are thrown by the JVM to indicate serious problems that are not intended to be handled by an application. Typical examples for that are the OutOfMemoryError or the StackOverflowError. Both are caused by situations that are outside of the control of the application and can’t be handled. So you shouldn't catch Throwables unless your are pretty confident that it will only be an exception reside inside Throwable.

If we use throwable, then it covers Error as well and that's it.
Example.
public class ExceptionTest {
/**
* #param args
*/
public static void m1() {
int i = 10;
int j = 0;
try {
int k = i / j;
System.out.println(k);
} catch (Throwable th) {
th.printStackTrace();
}
}
public static void main(String[] args) {
m1();
}
}
Output:
java.lang.ArithmeticException: / by zero
at com.infy.test.ExceptionTest.m1(ExceptionTest.java:12)
at com.infy.test.ExceptionTest.main(ExceptionTest.java:25)

A more differentiated answer would be: it depends.
The difference between an Exception and an Error is that an Exception is a state that has to be expected, while an Error is an unexpected state, which is usually fatal. Errors usually cannot be recovered from and require resetting major parts of the program or even the whole JVM.
Catching Exceptions is something you should always do to handle states that are likely to happen, which is why it is enforced by the JVM. I.E. opening a file can cause a FileNotFoundException, calling a web resource can result in a TimeoutException, and so on. Your code needs to be prepared to handle those situations as they can commonly occur. How you handle those is up to you, there is no need to recover from everything, but your application should not boot back to desktop just because a web-server took a little longer to answer.
Catching Errors is something you should do only if it is really necessary. Generally you cannot recover from Errors and should not try to, unless you have a good reason to. Reasons to catch Errors are to close critical resources that would otherwise be left open, or if you i.E. have a server that runs plugins, which can then stop or restart the plugin that caused the error. Other reasons are to log additional information that might help to debug that error later, in which case you of course should rethrow it to make sure the application terminates properly.
Rule of thumb: Unless you have an important reason to catch Errors, don't.
Therefore use catch (Throwable t) only in such really important situation, otherwise stick to catch (Exception e)