What is the point of catching and then also throwing an Exception like below? Is it bad practice to do both?
try{
//something
} catch (Exception e){
throw new RuntimeException("reason for exception");
}
Usually, such code is used to re-wrap exceptions, that means transforming the type of the exception. Typically, you do this when you are limited in what exceptions are allowed out of your method, but internally other types of exceptions can happen. For example:
class MyServiceImplementaiton implements MyService {
void myService() throws MyServiceException { // cannot change the throws clause here
try {
.... // Do something
} catch(IOException e) {
// re-wrap the received IOException as MyServiceException
throw new MyServiceException(e);
}
}
}
This idiom enables to keep propagating exceptions to the caller, while conforming to the throws clause in the interface and hide the details of the internals (the fact that IOExceptions can happen).
In practice, this is always better than just calling e.printStackTrace() which will actually "swallow" the error condition and let the rest of the program run as if nothing had happened. In this respect, behaviour of Eclipse is quite bad as it tends to auto-write such bad-practice constructs if the developer is not careful.
This is called rethrowing an exception, and it is a common pattern.
It allows you to change the class of the exception (such as in this case), or to add more information (also the case here, as long as that error string is meaningful).
It is often a good idea to attach the original exception:
throw new RuntimeException("cause of the problem", e);
Rethrowing as an unchecked exception (a RuntimeException) is sometimes necessary when you still want to throw an exception, but the API of your method does not allow a checked exception.
In your example, an Exception is caught and a RuntimeException is thrown, which effectively replaces a (potentially) checked exception with an unchecked exception that doesn't have to be handled by the caller, nor declared by the throwing method in a throws clause.
Some examples :
This code passes compilation :
public void SomeMethod ()
{
try {
//something
} catch (Exception e){
throw new RuntimeException("reason for exception");
}
}
This code doesn't pass compilation (assuming "something" may throw a checked exception) :
public void SomeMethod ()
{
//something
}
An alternative to catching the Exception and throwing an unchecked exception (i.e. RuntimeException) is to add a throws clause :
public void SomeMethod () throws Exception
{
//something
}
This is one use case of catching one type of exception and throwing another. Another use case is to catch one type of exception and throw another type of checked exception (that your method declares in its throws clause). It is sometimes done in order to group multiple exceptions that may be thrown inside a method, and only throw one type of exception to the caller of the method (which makes it easier for them to write the exception handling code, and makes sense if all those exceptions should be handled in the same manner).
Related
I am having trouble understanding how precise rethrow works in Java 7 and later versions. As pointed out in https://www.theserverside.com/tutorial/OCPJP-Use-more-precise-rethrow-in-exceptions-Objective-Java-7, in Java 7 and later versions we can use the throws clause, in a method declaration, with a comma-separated list of specific exceptions that the method could throw. If all these exceptions are subtypes of the general exception java.lang.Exception, we will be able to catch any of them in a catch block that catches this supertype, while letting client code (eg. a caller method) to know which of the possible subtypes exceptions actually occurred.
Initially, I thought that in order to let know client code which exception actually occurred, we needed to specify the list of specific exceptions in the throws clause. Nevertheless, in the following example the client code (the main() method) seems able to retrieve that information, even if we only specify the exception java.lang.Exception in the throws clause of the called method. Therefore, my question is:
Why the following code outputs the same, regardless of whether the throws clause of the method runException() is throws ExceptionA, ExceptionB or throws Exception ?
I am using Oracle JVM-12 in Eclipse. Thanks in advance!
class ExceptionA extends Exception{}
class ExceptionB extends Exception{}
public class RethrowingAndTypeChecking{
public static void runException(char what) throws Exception{
//public static void runException(char what) throws ExceptionA, ExceptionB{
try{
if(what == 'A')
throw new ExceptionA();
else if (what == 'B')
throw new ExceptionB();
}
catch(Exception e){
throw e;
}
}
public static void main (String args[]){
char ch;
for (int i=0;i<2;i++) {
if(i==0) ch='A';
else ch = 'B';
try{
runException(ch);
}
catch(ExceptionA e){
System.out.print("In main(), 'catch(ExceptionA e){}', caught exception: " + e.getClass());
}
catch(ExceptionB e){
System.out.print("In main(), 'catch(ExceptionB e){}', caught exception: " + e.getClass());
}
catch(Exception e){
System.out.print("In main(), 'catch(Exception e){}', caught exception: " + e.getClass());
}
System.out.println();
}
}
}
output:
In main(), 'catch(ExceptionA e){}', caught exception: class ExceptionA
In main(), 'catch(ExceptionB e){}', caught exception: class ExceptionB
What you're missing is the case where you need to handle those possible exceptions in different ways. Your code is catching individual exceptions, but it is, roughly speaking, performing the same action.
If you were to handle ExceptionA in a considerably different way from how you handle ExceptionB, then catching the broad Exception would not allow you to do that specifically:
catch(Exception e){
// something unexpected happened
// e could be an ExceptionA problem
// e could be an ExceptionB problem
// e could be any other unchecked exception
}
When the catch(Exception e){} block is entered, the exception could pretty much be anything, but you have only one generic code block to handle it.
Beside this, if the method you're calling declares specific checked exceptions, then the compiler can help you handle only those exceptions, thus adding to the predictability of the code
try{
runException(ch);
} catch(ExceptionA e){
// code specific to handling ExceptionA problems
} catch(ExceptionB e){
// code specific to handling ExceptionB problems
} catch(ExceptionC e){ //will not compile, because not declared by runException
// code specific to handling ExceptionB problems
}
Quoting #Carlos Heuberger, my code outputs the same, regardless of whether the throws clause of the method runException() is throws ExceptionA, ExceptionB or throws Exception because:
the run-time type of the exception is used to select the catch clause: see 14.20.1. Execution of try - catch
Whatever the exception reference type (in this case ExceptionA, ExceptionB or Exception) used to refer to the exception object thrown by method runException(), such method will throw objects of type either ExceptionA or ExceptionB. These objects will be assignment compatible with the catch parameters of the first two catch of the main() method.
After paragraphs 8.4.6, 11.2.3 and 14.20.1 of the Java Language Specification, I understood that what we actually specify in a throws clause of a method signature is the list of the exception reference types that will be assignment compatible with any possible exception object thrown from the method (given a class reference type we can make it point to instance objects of itself or to instance objects of its subclasses, not superclasses ). That tells any other caller method what exceptions it may have to deal with when invoking the method with the throws clause. In my code example, the advantage of using the clause throws ExceptionA, ExceptionB is that I will not need to catch java.lang.Exception in the main(). In fact, if I choose clause throws Exception in method runException() and delete the cath(Exception) block from the main() I will get a compile-time error. This is because even if we will be throwing ExceptionA or ExceptionB objects at run-time, the compiler will understand that method runException() may throw out an exception object of type Exception, which will not be assignment compatible with any of the catch parameters in the main() (Exception is a superclass of both ExceptionA and ExceptionB).
It's because, you've been throwing the Subclasses at,
try{
if(what == 'A')
throw new ExceptionA();
else if (what == 'B')
throw new ExceptionB();
}
of "Exception class" which are in turn being thrown out at,
catch(Exception e){
throw e;
}
after being assigned to "Exception class( at Exception e)", it will not make a difference if you specify throwing a Superclass type throws objectReference at
public static void runException(char what) throws Exception){
or Subclass type throws objectReferences at
public static void runException(char what) throws ExceptionA, ExceptionB){
As java compiler allows you to specify a throws ObjectReference, if it is of a Superclass of the object being thrown at the try statement.
These throws declarations are so that you list more explicitly what happens out of the method. Otherwise this is ordinary polymorphism: you use base class to combine in multiple subclasses, however you are definitely not changing the instances, this is why at runtime in both cases the exceptions are resolved to their concrete classes.
As a rule, you should never catch (Exception ex). Because this will catch RuntimeExceptions too. It sometimes makes sense to catch (Throwable t) or to use Thread.setDefaultUncaughtExceptionHandler to customize your uncaught exception handler to catch exceptions and then display them to the user. Sometimes I will catch an Exception, wrap it in a RuntimeException (or an Error) and throw that
When it comes to exceptions, you should really only be catching them when you can do something with them, or when you want to make sure that an exception doesn't cause the rest of the method to not process.
Personally I divide exceptions into 3 types
Problems in your code: This is something for you to fix
Problems with the user: For instance if you tell them to enter a number and they enter 'a', that's the user's error
"Friend" Exceptions: SocketException, for instance, is an example of this. If the socket closes and you have a thread waiting on input on it, it will throw this Exception, releasing the thread and letting you do clean-up on the socket.
public void backendExecute(Map appParams, BackendTaskMetaData metaData) throws Throwable {
try {
PeriodicTaskData ptd = (PeriodicTaskData) appParams.get(PeriodicTaskData.PARAM_KEY);
String bizKey = ptd.getBusinessKey();
} catch (Exception e) {
LogServices.app.error("RPTPeriodicReportGenTask:"+ e.getMessage());
}
}
With regards to the method above, if object pointed to is null, would come across as NullPointerException, I want to know if this exception would be caught or thrown to the invoker method? thanks
Exception is a parent class of NullPointerException, so it will catch it and not throw it to the calling method.
As you are catching Exception class and NullPointerException is its subclass , exception will get catched not throwed.
Regard to above method, if object ptd is null, would come across nullpointexception,
Yes.
i want to know this exception would be catch or throw it to invoker method?
The exception would be caught by the handler. The handler will catch Exception and any exception that is descended from it. NullPointerException is a subclass of RuntimeException which is (in turn) a subclass of Exception. Therefore, it will be caught.
Now, if this may be just an illustrative example ... but it is a bad idea to:
declare a method as throws Throwable, or
catch Exception ... unless you are about to terminate the application.
Declaring a method as throwing Throwable makes it next to impossible for the caller to know what exceptions could be thrown. Instead, the compiler will insist that the caller catches ... or propagates Throwable.
Catching Exception has the problem that you will catch every subtype of Exception ... including various unchecked exceptions that 1) you are not expecting, and 2) are probably symptoms of bugs that you cannot safely recover from.
NullPointerException is a subclass of Exception and thus will be catched, however it is recommended that you don't try and catch runtime exceptions. It is better to avoid them.
For example a null pointer could be avoided by doing the following:
if(ptd != null) {
ptd.getBusinessKey();
} else {
//Notify the user in some way or do something else.
}
catch (Exception e)
means that this will catch any exception (i.e. any subclass of Exception) thrown inside the preceding try block - which, yes, includes NullPointerException. Note that this is generally considered a bad practice, as you almost always will want to handle different sorts of exceptions in different ways, hence the need for multiple catch statements.
For instance, consider a method that could potentially throw an IllegalAccessException at compile time or a NullPointerException at runtime - it's difficult to imagine a situation where you'd want to handle them the same way, so you'll typically want to do something like this:
try {
PeriodicTaskData ptd = (PeriodicTaskData) appParams.get(PeriodicTaskData.PARAM_KEY);
String bizKey = ptd.getBusinessKey();
} catch (NullPointerException e) {
//do something
} catch (IllegalAccessException e) { //for example...
//do something different
}
Often, when implementing a template method or interface method, you can only throw one specific type of exception defined by the method. But your implementation may make class to API's that throw an incompatible exception type, or many different exception types.
Naturally you need to catch them and wrap the exceptions into the type suitable for the implemented method signature. Lets assume we want to implement this interface:
public interface SomeDataGetter {
public long getSomeData() throws IOException;
}
Our implementation makes use of some other API product to implement this, and the API method we are calling may have this signature:
public long loadFromDBOrCache(Object ... params) throws SQLException, IOException, ObjectNotFoundException, RuntimeException, FridayException, NotWeekendException, NumberIs42Exception;
I made this up to demonstrate the case where you can't exactly enumerate all the possibly thrown exceptions by concrete type. Do note that IOException is a type we are allowed to throw from our implementation.
Now I can go the lazy route when implementing this and wrap anything to fit my signature:
#Override
public long getSomeData() throws IOException {
try {
return loadFromDB(...);
} catch (Exception e) {
throw new IOException(e.getMessage(), e);
}
}
This will obviously wrap any exception into an IOException (even an IOException) and it works out ok. But I'd like to not wrap IOExceptions, since I am allowed to throw those without wrapping them:
#Override
public long getSomeData() throws IOException {
try {
return loadFromDB(...);
} catch (IOException e) {
throw e;
} catch (Exception e) {
throw new IOException(e.getMessage(), e);
}
}
You can imagine this gets cumbersome quickly if there are multiple possible exception in the implementation and multiple exceptions you are allowed from the implementation. I need an extra catch for each exception I want to pass throgh.
Whats the best idiom to keep that readable (also, I'm lazy, and don't want to write all these extra catches) and still avoid unneccessary exception nesting? Or shoud I not bother and just wrap everything?
One approach would be making a method that wraps all "prohibited" exceptions in an allowed one, while returning all the allowed ones unwrapped, like this:
private static void throwIoException(Exception e)
throws IOException // <<= Add other "allowed" exceptions here
{
if (e instanceof IOException) {
throw (IOException)e;
}
... // <<= Add checks for other "allowed" exceptions here
throw new IOException(e.getMessage(), e);
}
Now you can use a single catch block, and do the wrapping as needed:
try {
return loadFromDB(...);
} catch (Exception e) {
throwIoException(e);
}
One unpleasant consequence of this is that the stack trace shows the utility method at the top of the newly created IOException, but that's not important, because the real exception is the wrapped one, not the IOException wrapper. If the exception that you caught happens to be IOException, the correct stack trace should remain in place.
I would consider the lazy route of wrapping all exceptions you get into IOExceptions (or another checked exception) to be a bad practice. Instead I would consider wrapping the exceptions in runtime exceptions, thereby bypassing the catch or specify requirement. E.g.
#Override
public long getSomeData() throws IOException {
try {
return loadFromDB(...);
} catch (Exception e) {
throw new RuntimeException(e.getMessage(), e);
}
}
The reason why this is better is that checked exceptions carry a certian meaning. If you catch for instance a ParseException in your code and rethrow that as a IOException you are lying. As a user of your code I might be able to do something about certain types of checked exceptions, but if you obfuscate the true cause of an exception it will be more difficult to debug the code when an error occurs.
In general I think you should minimize the use of checked exceptions since it litters error handling code throughout your application. Also if you are using someone else's code you have no guarantee that a RuntimeException won't be thrown anyway (unless you carefully read through it all). Therefore you have to consider that possibility anyway and handle it somewhere so your application don't crash. The virtues of unchecked exception vs checked exceptions has been discussed quite a lot elsewhere here and here for instance.
In Java, is it possible to make a method that has a throws statement to be not checked.
For example:
public class TestClass {
public static void throwAnException() throws Exception {
throw new Exception();
}
public static void makeNullPointer() {
Object o = null;
o.equals(0);//NullPointerException
}
public static void exceptionTest() {
makeNullPointer(); //The compiler allows me not to check this
throwAnException(); //I'm forced to handle the exception, but I don't want to
}
}
You can try and do nothing about it:
public static void exceptionTest() {
makeNullPointer(); //The compiler allows me not to check this
try {
throwAnException(); //I'm forced to handle the exception, but I don't want to
} catch (Exception e) { /* do nothing */ }
}
Bear in mind, in real life this is extemely ill-advised. That can hide an error and keep you searching for dogs a whole week while the problem was really a cat(ch). (Come on, put at least a System.err.println() there - Logging is the best practice here, as suggested by #BaileyS.)
Unchecked exceptions in Java extend the RuntimeException class. Throwing them will not demand a catch from their clients:
// notice there's no "throws RuntimeException" at the signature of this method
public static void someMethodThatThrowsRuntimeException() /* no need for throws here */ {
throw new RuntimeException();
}
Classes that extend RuntimeException won't require a throws declaration as well.
And a word from Oracle about it:
Here's the bottom line guideline: If a client can reasonably be expected to recover from an exception, make it a checked exception. If a client cannot do anything to recover from the exception, make it an unchecked exception.
There are 3 things you can do :
Throw a RuntimeException (or something extending a RuntimeException, like NullPointerException, IllegalArgumentException,...), you don't have to catch these as they are unchecked exceptions.
Catch the exception and do nothing (not recommended) :
public static void exceptionTest() {
makeNullPointer(); //The compiler allows me not to check this
try {
throwAnException(); //I'm forced to handle the exception, but I don't want to
} catch (Exception e) {
// Do nothing
}
}
Change exceptionTest () declaration to say that it throws an Exception, and let the method calling it catch the Exception and do what is appropriate :
public static void exceptionTest() throws Exception {
makeNullPointer(); //The compiler allows me not to check this
throwAnException(); //I'm no more forced to handle the exception
}
In Java there is two kinds of Exceptions, Checked Exceptions and Unchecked Exceptions.
Exception is a checked exception, must caught or thrown.
NullPointerException is a RuntimeException, (the compiler doesn’t forces them to be declared in the throws claus) you can ignore it, ,but it still may occur in the Runtime, and your application will crash.
From Exception documentation:
The class Exception and any subclasses that are not also subclasses of
RuntimeException are checked exceptions. Checked exceptions need to be
declared in a method or constructor's throws clause if they can be
thrown by the execution of the method or constructor and propagate
outside the method or constructor boundary.
From the RuntimeException documentation:
RuntimeException is the superclass of those exceptions that can be
thrown during the normal operation of the Java Virtual Machine.
RuntimeException and its subclasses are unchecked exceptions.
Unchecked exceptions do not need to be declared in a method or
constructor's throws clause if they can be thrown by the execution of
the method or constructor and propagate outside the method or
constructor boundary.
No, it raises a compiler error. Being a checked exception, you must either catch it or propagate it by declaring your method as potentially throwing it.
Check this and this.
Throw a RuntimeException or an exception which is derived from RuntimeException. Then the compiler will not force you to catch it.
The other answers are right, in that they correctly tell you what you should do, but it is actually possible to throw a undeclared checked exception. There are a few ways this can be done; the simplest is:
public void methodThatSecretlyThrowsAnException() {
Thread.currentThread().stop(new Exception());
}
or if your goal is to wrap an existing method that does declare its exception
public void methodThatSecretlyThrowsAnException() {
try {
methodThatAdmitsItThrowsAnException();
} catch(final Exception e) {
Thread.currentThread().stop(e);
}
}
(Needless to say, you should never do this.)
Just catch an exception and dont do any thing with it, leave it as it is and catch the generic exception in case you are not aware of the specific exception
try{
//Your logic goes here
}
catch(Exception e)//Exception is generic
{
//do nothing
}
AS I know, it's impossible in the case. Only unchecked exception, compiler can skip to check. such as RuntimeException.
You can use a loophole in the Java Compiler. Add the following code:
public RuntimeException hideThrow(Throwable e) {
if (e == null)
throw new NullPointerException("e");
this.<RuntimeException>hideThrow0(e);
return null;
}
#SuppressWarnings("unchecked")
private <GenericThrowable extends Throwable> void hideThrow0(Throwable e) throws GenericThrowable {
throw (GenericThrowable) e;
}
You can catch the exception, then invoke hideThrow with the exception to throw it without the compiler noticing. This works because of type erasure. At compile time, GenericThrowable represents RuntimeException because that is what we are passing. At run time, GenericThrowable represents Throwable because that is the basic type in the type parameter specification.
It is not advisable to avoid an exception with an empty catch block even though you are completely sure that is not going to fail under any circumstance. Sometimes, we are not aware of the human factor.
If you are sure that an exception is very unlikely to happen (if not impossible) you should create your own Exception and and wrap the unexpected exception in it.
For example:
private class UnlikelyException extends RuntimeException {
public UnlikelyException (Exception e){
super (e);
}
}
Then wrap your code with a try-catch block and throw your exception, which you don't have to catch
try {
// Your code
} catch (Exception e) {
throw new UnlikelyException(e);
}
I'd like to catch an exception, log it, set a flag, and the rethrow the same exception
I have this code:
public Boolean doJobWithResult() {
boolean result = true;
final Feed feed = Feed.findById(feedId);
try {
feed.fetchContents();
} catch (Exception ex) {
result = false;
Logger.info("fetching feed(%d) failed", feedId);
throw ex;
}
return result;
}
But eclipse complains at throw ex, telling that "Unhandled exception type Exception", and suggests me to add a try-catch block around it.
In fact, I want the process calling this method to handle the exception, and not handle it myself... I just want to return true if everything goes ok, and log it if there's an exception
On the other hand, I can wrap the exception inside another exception, but I can't throw the same one..
any idea?
I think there are various things to mention here:
You either want doJobWithResult() to return true on success and false on failure, or return nothing on success and throw an exception on failure.
Both at the same time is not possible. In the first case, catch the Exception, log it and return false, in the second case change your signature to return void and throw an exception and handle it in the caller.
It's a Don't to catch an exception, log it and rethrow it. Why? Because a potential caller of your method does not know that you are already logging it, and migh log it as well.
Either throw an exception (in which case the caller has to deal with it) or catch it and handle it (log it).
Note that throwing Exception does not give the caller of your method any clue about what might potentially go wrong in your method, it's always better to throw more specific exceptions, or to wrap an exception in a user-defined one and rethrow it.
Moreover, if you throw Exception, a caller might be tempted to catch Exception without noticing that this will also catch every RuntimeException (since its derived from Exception), which might not be desired behavior.
Your doJobWithResult method needs to declare that it can throw Exception:
public Boolean doJobWithResult() {
becomes
public Boolean doJobWithResult() throws Exception {
You can throw the same exception if you add throws Exception to your method signature.
Otherwise you can throw a RuntimeException.
public Boolean doJobWithResult() {
boolean result = true;
final Feed feed = Feed.findById(feedId);
try {
feed.fetchContents();
} catch (Exception ex) {
result = false;
Logger.info("fetching feed(%d) failed", feedId);
throw new RuntimeException(ex);
}
return result;
}
In such a case, you won't need to indicate that public Boolean doJobWithResult() throws something but make sure you handle it properly later on (catch or expect your thread to stop... it's a RuntimeException afterall).
Since Exception is checked, an alternative to catching the Exception is to declare your method as throwing it:
public Boolean doJobWithResult() throws Exception {
// ...
}
If doJobWithResult doesn't have to handle the exception, then remove the catch block and add "throws Exception" to the method signature. The exception logging can be done in the class/method that have to deal with the Exception in a corresponding try/catch block.
There is no need to set the result as false in the catch block, as the value won't be returned(as we are throwing an exception).
Your method should also declare that it throws an exception and so the client will be forced to handle it.
Also consider using a more specific exception which will be thrown in this particular case.
Add throws Exception to your method. You also don't need to add result = false; in your catch block.
I think the way you handle this exception is really appropriate if any failure of feed.fetchContents() method cannot be recovered. (Idea is better to halt rather than continuing)
Apart from that I would suggest you to use more specific exception hierarchy.
And another thing I got from effective java book is if you write such a method you must document with #throw (in comments) with the reason.
You could throw an unchecked exception
Logger.info("fetching feed(%d) failed", feedId);
throw new RuntimeException(ex);
I spent the last hour looking for it since not even the Complete Reference book mentions this explicitly: unhandled throw ThrowableInstance works only with unchecked exceptions.. And only runtime exceptions are unchecked. By unhandled I mean something like this:
class ThrowDemo {
static void demoproc() {
try {
throw new NullPointerException("demo");
} catch(NullPointerException e) {
System.out.println("Caught inside demoproc.");
throw e; // re-throw the exception
}
}
public static void main(String args[]) {
try {
demoproc();
} catch(NullPointerException e) {
System.out.println("Recaught: " + e);
}
}
}
This example is taken verbatim from the Complete Reference book (9th edition).
The first throw statement i.e throw new NullPointerException("demo"); is handled by the following catch block, but the second throw statement i.e. throw e; is unhandled by the demoproc() method. Now this works here and the above code compiles successfully because NullPointerException is a runtime/ unchecked exception. If the e instance were a checked exception or even an Exception class instance then you'd get an error saying the exception e is unhandled and you'd either have to handle it within demoproc() or you'd have to explicitly declare that demoproc() throws an exception using throws in the method signature.