I was wondering if there was a way to write a catch statement that is activated whenever any exception is thrown in a program. Thanks!
The problem with the answers you have received so far is that if you include a "catch-all" in your main, it will only catch exceptions thrown on the main thread.
A more robust way to catch all uncaught exceptions is to setup a DefaultUncaughtExceptionHandler for your project, for example, at the start of your main, you can call:
Thread.setDefaultUncaughtExceptionHandler(new Thread.UncaughtExceptionHandler() {
#Override
public void uncaughtException(Thread t, Throwable e) {
//log the exception
}
});
Note that it is generally unreasonable to do anything but logging in there, as the Throwable could be anything, including unrecoverable errors.
You can make the type Exception. That will catch any Exception, but it also doesn't give the programmer much information on what went wrong. It's usually better to try to be more specific about the type of exception that has occurred, mainly for semantic reasons.
The theory
Every type of Java Exception extends the Exception type. Because Exception is a superclass of all exceptions, when an exception is thrown, regardless of it's type, it can still be implicitly casted to an object of type Exception.
Edit
As mentioned in the comments, there is also type Error that can be thrown, so to really catch everything using the Throwable type is best. Throwable is more general than Exception, but it still works on the exact same principle. It is the superclass of anything that can be thrown.
For Example
try{
// Some code
}
catch(Throwable e)
{
// Do something. Edited due to comment!
}
To catch all exceptions some block of code may throw you can do: (This will also catch Exceptions you wrote yourself)
From How can I catch all the exceptions that will be thrown through reading and writing a file?
try {
// exceptional block of code ...
// ...
} catch (Exception e){
// Deal with e as you please.
//e may be any type of exception at all.
}
The reason that works is because Exception is the base class for all runtime exceptions. Thus any runtime exception that may get thrown is an Exception.
The is because all runtime exceptions inherit from Exception. Note that there is a difference between an Error and Exception Unchecked exceptions in Java: Inherit from Error or RuntimeException?, In that case you may need to do:
try {
} catch(Throwable e) {
}
See this When should Throwable be used instead of new Exception?. One user notes:
Error is programmatically unrecoverable in any way and is usually not to be caught, except for logging purposes (which passes it through again). Exception is programmatically recoverable. Its subclass RuntimeException indicates a programming error and is usually not to be catched as well. See comments for additional information.
Yes, any error that falls through will be caught by this:
try {
//do things here
} catch (Exception e) {
//handle exception here
}
Note, however, that it's typically the wrong thing to do.
In your main, put all your code in
try {
//Your code
} catch(Exception e) {
//Handle exceptions
}
This will handle any exception invoked by your code.
Related
Sometimes I would want to throw an exception with more information to the user, so they can easily see why the method failed.
My method would look like this:
public myPublicMethod(...) throws Exception1, Exception2 {
try {
doSomething(); // this throws OtherException
}
catch (OtherException e) {
Exception cause = e.getCause()
if (cause instanceof Exception1) {
throw (Exception1) cause;
}
if (cause instanceof Exception2) {
throw (Exception2) cause;
}
throw new IllegalStateException("some other type of exception occurred", cause);
}
}
Is this design acceptable?
Your thinking is good, however in my humble opinion the way you implemented it is kind of missing the point, I'll try to explain.
You started with the claim that you want to give the user more information about the exception. What you did - striping the cause from its "parent exception" (e.getCause() and then throwing the cause) - actually gives the user less information, since the information in OtherException, that you discarded, might contain useful details about the flow caused the program to crash.
It is a good practice to catch the most specific exception and throw it to the caller, but make sure that you're not omitting details. Throwing the cause without the parent exception is actually omitting details.
It makes sense sometimes to wrap an exception that you caught and provide more details about it (like you intended to do in your last line), but again - make sure that you're only adding details, and not omitting details.
You can read more about exceptions handling best practices here.
I would want to throw an exception with more information
There's nothing wrong with the practice of catching an exception in order to provide additional information by wrapping it with a custom or built-in exception and re-throwing.
I think it's if you want to do it only for a limited number of types of exceptions by wrapping with the IllegalStateException and adding a custom message and preserving the cause of exception. But there's a caveat: a type of OtherException shouldn't be too broad, like general Exception, so that it will encompass only a limited number of exceptions types.
But re-throwing the only a cause like you've done for Exception1 or Exception2 looks suspicious. For some reason, you don't consider them to be the root-cases and going to blind on the information they could carry. It sounds like a design flaw. Also keep in mind that cause could be null (in this case, exception doesn't wrap anything) and this code will fail with NullPointerException. Therefore, in the in example below, the same exception will get re-thrown instead of re-throwing the cause.
From the clean-coding perspective, it's better to use as fewer type-checks as possible. And I think in this case you don't need to resort to instanceof check.
Instead, you can utilize the features that exception handling mechanism provides.
Since you need to perform the same action (re-throw the exception that was caught) if Exception1 or Exception2 occurs, if types Exception1 or Exception2 are siblings (i.e. they don't extend each other), these exceptions can be handled with a multi-catch clause to avoid code duplication:
catch (Exception1 | Exception2 e) {
throw e;
}
Otherwise, if let's say Exception2 is a subtype of Exception1 then you should handle them in the separate catch blocks starting from more specific type, i.e. subtype should be handled before supertype:
catch (Exception2 e) {
throw e;
}
catch(Exception1 e) {
throw e;
}
And after that add a catch clause with the least specific type (more wide than all exception types defined previously), to handle all other exception types that might take place by throwing a new instance of IllegalStateException.
Example:
public void myPublicMethod(Path source, Path destination)
throws FileAlreadyExistsException, DirectoryNotEmptyException {
try {
Files.copy(source, destination);
// do some other actions
}
catch (FileAlreadyExistsException | DirectoryNotEmptyException e) {
throw e;
}
catch (IOException e) {
throw new IllegalStateException("some other type of exception occurred", e);
}
}
New Java programmers frequently encounter errors phrased like this:
"error: unreported exception <XXX>; must be caught or declared to be thrown"
where XXX is the name of some exception class.
Please explain:
What the compilation error message is saying,
the Java concepts behind this error, and
how to fix it.
First things first. This a compilation error not a exception. You should see it at compile time.
If you see it in a runtime exception message, that's probably because you are running some code with compilation errors in it. Go back and fix the compilation errors. Then find and set the setting in your IDE that prevents it generating ".class" files for source code with compilation errors. (Save yourself future pain.)
The short answer to the question is:
The error message is saying that the statement with this error is throwing (or propagating) a checked exception, and the exception (the XXX) is not being dealt with properly.
The solution is to deal with the exception by either:
catching and handling it with a try ... catch statement, or
declaring that the enclosing method or constructor throws it1.
1 - There are some edge-cases where you can't do that. Read the rest of the answer!
Checked versus unchecked exceptions
In Java, exceptions are represented by classes that descend from the java.lang.Throwable class. Exceptions are divided into two categories:
Checked exceptions are Throwable, and Exception and its subclasses, apart from RuntimeException and its subclasses.
Unchecked exceptions are all other exceptions; i.e. Error and its subclasses, and RuntimeException and its subclasses.
(In the above, "subclasses" includes by direct and indirect subclasses.)
The distinction between checked and unchecked exceptions is that checked exceptions must be "dealt with" within the enclosing method or constructor that they occur, but unchecked exceptions need not be dealt with.
(Q: How do you know if an exception is checked or not? A: Find the javadoc for the exception's class, and look at its parent classes.)
How do you deal with a (checked) exception
From the Java language perspective, there are two ways to deal with an exception that will "satisfy" the compiler:
You can catch the exception in a try ... catch statement. For example:
public void doThings() {
try {
// do some things
if (someFlag) {
throw new IOException("cannot read something");
}
// do more things
} catch (IOException ex) {
// deal with it <<<=== HERE
}
}
In the above, we put the statement that throws the (checked) IOException in the body of the try. Then we wrote a catch clause to catch the exception. (We could catch a superclass of IOException ... but in this case that would be Exception and catching Exception is a bad idea.)
You can declare that the enclosing method or constructor throws the exception
public void doThings() throws IOException {
// do some things
if (someFlag) {
throw new IOException("cannot read something");
}
// do more things
}
In the above we have declared that doThings() throws IOException. That means that any code that calls the doThings() method has to deal with the exception. In short, we are passing the problem of dealing with the exception to the caller.
Which of these things is the correct thing to do?
It depends on the context. However, a general principle is that you should deal with exceptions at a level in the code where you are able to deal with them appropriately. And that in turn depends on what the exception handling code is going to do (at HERE). Can it recover? Can it abandon the current request? Should it halt the application?
Solving the problem
To recap. The compilation error means that:
your code has thrown a checked exception, or called some method or constructor that throws the checked exception, and
it has not dealt with the exception by catching it or by declaring it as required by the Java language.
Your solution process should be:
Understand what the exception means, and why it could be thrown.
Based on 1, decide on the correct way to deal with it.
Based on 2, make the relevant changes to your code.
Example: throwing and catching in the same method
Consider the following example from this Q&A
public class Main {
static void t() throws IllegalAccessException {
try {
throw new IllegalAccessException("demo");
} catch (IllegalAccessException e){
System.out.println(e);
}
}
public static void main(String[] args){
t();
System.out.println("hello");
}
}
If you have been following what we have said so far, you will realise that the t() will give the "unreported exception" compilation error. In this case, the mistake is that t has been declared as throws IllegalAccessException. In fact the exception does not propagate, because it has been caught within the method that threw it.
The fix in this example will be to remove the throws IllegalAccessException.
The mini-lesson here is that throws IllegalAccessException is the method saying that the caller should expect the exception to propagate. It doesn't actually mean that it will propagate. And the flip-side is that if you don't expect the exception to propagate (e.g. because it wasn't thrown, or because it was caught and not rethrown) then the method's signature shouldn't say it is thrown!
Bad practice with exceptions
There are a couple of things that you should avoid doing:
Don't catch Exception (or Throwable) as a short cut for catching a list of exceptions. If you do that, you are liable catch things that you don't expect (like an unchecked NullPointerException) and then attempt to recover when you shouldn't.
Don't declare a method as throws Exception. That forces the called to deal with (potentially) any checked exception ... which is a nightmare.
Don't squash exceptions. For example
try {
...
} catch (NullPointerException ex) {
// It never happens ... ignoring this
}
If you squash exceptions, you are liable to make the runtime errors that triggered them much harder to diagnose. You are destroying the evidence.
Note: just believing that the exception never happens (per the comment) doesn't necessarily make it a fact.
Edge case: static initializers
There some situations where dealing with checked exceptions is a problem. One particular case is checked exceptions in static initializers. For example:
private static final FileInputStream input = new FileInputStream("foo.txt");
The FileInputStream is declared as throws FileNotFoundException ... which is a checked exception. But since the above is a field declaration, the syntax of the Java language, won't let us put the declaration inside a try ... catch. And there is no appropriate (enclosing) method or constructor ... because this code is run when the class is initialized.
One solution is to use a static block; for example:
private static final FileInputStream input;
static {
FileInputStream temp = null;
try {
temp = new FileInputStream("foo.txt");
} catch (FileNotFoundException ex) {
// log the error rather than squashing it
}
input = temp; // Note that we need a single point of assignment to 'input'
}
(There are better ways to handle the above scenario in practical code, but that's not the point of this example.)
Edge case: static blocks
As noted above, you can catch exceptions in static blocks. But what we didn't mention is that you must catch checked exceptions within the block. There is no enclosing context for a static block where checked exceptions could be caught.
Edge case: lambdas
A lambda expression (typically) should not throw an unchecked exception. This is not a restriction on lambdas per se. Rather it is a consequence of the function interface that is used for the argument where you are supplying the argument. Unless the function declares a checked exception, the lambda cannot throw one. For example:
List<Path> paths = ...
try {
paths.forEach(p -> Files.delete(p));
} catch (IOException ex) {
// log it ...
}
Even though we appear to have caught IOException, the compiler will complain that:
there is an uncaught exception in the lambda, AND
the catch is catching an exception that is never thrown!
In fact, the exception needs to be caught in the lambda itself:
List<Path> paths = ...
paths.forEach(p -> {
try {
Files.delete(p);
} catch (IOException ex) {
// log it ...
}
}
);
(The astute reader will notice that the two versions behave differently in the case that a delete throws an exception ...)
More Information
The Oracle Java Tutorial:
The catch or specify requirement
... also covers checked vs unchecked exceptions.
Catching and handling exceptions
Specifying the exceptions thrown by a method
In IntelliJ I'm working on some code that retrieves entities from Google Cloud DataStore, using a try catch block like this:
try {
T dataAccessObject = class.newInstance();
entity = datastore.get(KeyFactory.createKey(dataAccessObject.GetEntityName().toString(), id));
dataModel = (T)dataAccessObject.ToModel(entity);
return dataModel;
} catch (EntityNotFoundException e) {
} catch (InstantiationException e) {
} catch (IllegalAccessException e) {
}
To me, the empty catch statement for the EntityNotFoundException is a code smell, and I'd rather remove it and allow the exception to be thrown.
When I remove that catch statement however, it causes a compiler error, and I'm not seeing any explanation or rationale as to why removing the statement is invalid.
The datastore.get is calling something that implements the com.google.appengine.api.datastore.DatastoreService interface, which means it's possible for an EntityNotFoundException to be thrown, which can be seen if we look at the constructor as defined in the interface:
com.google.appengine.api.datastore.DatastoreService
public interface DatastoreService extends BaseDatastoreService {
Entity get(Key var1) throws EntityNotFoundException;
Why would I need to catch the exception though? Why do I get the compile error?
Java has two different kinds of exceptions: checked and unchecked.
Checked Exceptions:
Any java.lang.Throwable that does not extend from java.lang.Error or java.lang.RuntimeException.
Must be explicitly handled where they can be thrown. Not doing so results in a compilation error. A checked exception is handled if it is caught in a try-catch block or if the containing method is declared to throws the checked exception.
Unchecked Exceptions:
Any instance of java.lang.Error or java.lang.RuntimeException.
Can be caught and handled, but doesn't need to be. Can also be used in the throws clause of a method signature but doing so is typically considered bad practice. If one wants to document the possibility of an unchecked exception being thrown by a method they should do so in the Javadoc, via #throws.
Based on the compilation error, I can only assume EntityNotFoundException is a checked exception and thus must be handled. For more information, see Java: checked vs unchecked exception explanation.
I agree that an empty catch block is smelly. At the very least, you should log the exception. If you end up doing the same thing for each possible exception, you could rewrite the try-catch like so:
try {
/* Do stuff... */
} catch (EntityNotFoundException | IntantiationException | IllegalAccessException ex) {
// log ex...
}
The above syntax requires Java 7+, I believe.
Lately, I have been seeing a lot of (commercial) code that throws and catches the exact same exception, similar to the code below:
public class Foo {
public void bar() throws AnException {
// do something (1)
try {
// do something (2) -- able to throw AnException
} catch (AnException ex) {
throw ex;
}
// do something (3)
}
}
Contrast this to:
public class Foo {
public void bar() throws AnException {
// do something (1)
// do something (2) -- able to throw AnException
// do something (3)
}
}
Does the former actually accomplish anything, in either behavior or semantics or else? Will it have any effect on code performance (time and space) or does it help readability?
P.S. this question is very different from this question.
EDIT: The catch block in the former doesn't do anything else other than rethrowing the caught exception.
This is a violation of a basic rule:
The code should catch only the exceptions that it knows how to handle.
Oracle's guidelines suggests that each time you throw an exception, you should supply something useful for the code that catches your exception (see page 3 of Effective Java Exceptions). Catching an exception simply to re-throw it serves no practical purpose, because there is no additional information added to it for the code above it in the invocation chain, and no information extracted by the method itself.
It makes no sense for sure assuming code you provided.
Rethrowing exceptions makes sense when you want to handle this exception somehow (e.g. log it) but want it to be handled further in a stack trace.
Regarding commercial code, maybe the reason you saw it was that there was some handling logic initially that was removed but exceptions rethrowing wasn't removed. I saw such situations several times.
There's no reason for the try/catch in the code you posted. As with #nickolay.laptev's answer, I believe that if you find this in commercial code, it's probably left over cruft from something that used to be meaningful.
Aside from some sort of partial error handling, there's another use case for simply catching and rethrowing an exception. This would be to prevent the exception from being caught by a subsequent, more general, catch clause. So, for instance, if you wanted to handle every exception other than instances of AnException, you would either have to do a type check:
try {
// stuff
} catch (Exception ex) {
if (ex instanceof AnException) {
throw ex; // rethrow since we're not handling it
} else {
// handle all other exceptions
}
}
or use something like this (which, in my view, is cleaner and more readable):
try {
// stuff
} catch (AnException) {
throw ex;
} catch (Exception ex) {
// handle all other exceptions
}
Then if you later decided to not handle all those other exceptions, you'd delete the second catch clause and end up with the code you posted.
I'm implementing the Accumulo BatchWriter interface to perform some additional computation over records before inserting. That computation may throw a checked exception if something goes wrong, but the only exception thrown by BatchWriter is the MutationsRejectedException. As such, I'm unable to throw the necessary checked exception when an error occurs in the pre-processing I'm trying to do.
Now, I could catch the checked exception and simply throw another exception in its place: either an unchecked exception like some RuntimeException or a MutationsRejectedException. Neither option seems great - an unchecked exception is a poor simulacrum of the exception I'd like to actually throw while throwing a MutationsRejectedException wouldn't allow me to see the actual cause of the error.
What is the best practice here?
"MutationsRejectedException wouldn't allow me to see the actual cause of the error."
Yes MutationsRejectedException would allow you to see the actual cause via chained exceptions.
Note the "Throwable cause" in the constructor.
Code for version 1.7;
try{
//...
} catch (Exception e) {
throw new MutationsRejectedException(null, null, (Map<TabletId,Set<SecurityErrorCode>>)null, null, 1, e);
}
.
try{
//...
} catch (MutationsRejectedException e) {
Throwable c = e.getCause();
if(c instanceof MyException){
//...
}else{
throw e;
}
}
The BatchWriter interface actually covers exactly your situation by expecting you to wrap whatever root cause you got into its MutationsRejectedException. That is much better design than declaring a general Exception and exception translation has been the recommended Java idiom since version 1.3 or so.
With throws Exception hopefully crossed out as an antipattern, all the choice you have is between exception translation and the "sneaky throw" idiom. I believe your preference would be the former, especially considering that the latter is very similar to having throws Exception, but more obscure.