I have a Java EE application with dozens of web services using the same pattern:
public Response myWebService1() {
try {
// do something different depending on the web service called
} catch (MyCustomException e1) {
return Response.status(409).build();
} catch (UnauthorizedException e2) {
return Response.status(401).build();
} catch (Exception e3) {
return Response.status(500).build();
}
}
Is that possible to factorize this piece of code?
If this is a JAX-RS environment, see Tunaki's answer, handling this is specifically catered for and wonderfully simple.
If not:
You can have a functional interface accepting a function that can throw exceptions and returns a Response:
#FunctionalInterface
public interface Responder {
Response handleRequest() throws Exception;
}
(As Dici points out, you could make that a generic ThrowingSupplier or similar, since you're allowing it to throw Exception.)
Then have a helper method accepting an instance of it:
private static Response respond(Responder responder) {
try {
return responder.handleRequest();
} catch (MyCustomException e1) {
return Response.status(409).build();
} catch (UnauthorizedException e2) {
return Response.status(401).build();
} catch (Exception e3) {
return Response.status(500).build();
}
}
...and use it via a lambda:
public Response myWebService1() {
return respond(() -> {
// Do stuff here, return a Response or throw / allow throws on error
});
}
Since this is in a JAX-RS context, there is a much better way, that does not rely on catching a lot of different exceptions: use an ExceptionMapper. This is a built-in mechanism of JAX-RS 1.0 that translates an exception type into a proper Response object to send to the client.
In your case, you could have the following classes defined once in your application:
#Provider
public class UnauthorizedExceptionMapper implements ExceptionMapper<UnauthorizedException> {
public Response toResponse(UnauthorizedException e) {
return Response.status(401).build();
}
}
#Provider
public class MyCustomExceptionMapper implements ExceptionMapper<MyCustomException> {
public Response toResponse(MyCustomException e) {
return Response.status(409).build();
}
}
#Provider
public class CatchAllExceptionMapper implements ExceptionMapper<Exception> {
public Response toResponse(Exception e) {
return Response.status(500).build();
}
}
The #Provider annotation tells the JAX-RS runtime to discover this class when scanning. This makes sure that, wherever in your code, if a MyCustomException is thrown (and not explicitly catched), a 409 response will be returned. The code in your application would simply become:
public Response myWebService1() {
// do something, and don't catch anything; just care about the happy path
}
The exception hierarchy is correctly taken into account. If the application code throws a MyCustomExceptionMapper, JAX-RS will look for an exception mapper registered with that type, and will go up the super class if it can't find one: this way, there can be a catch-all exception mapper handling every other case.
If all methods handle exceptions the same way, you can extract the exception handling to an external method :
public static Response exceptionHandler (Exception exc)
{
if (exc instanceof MyCustomException) {
return Response.status(409).build();
} else if (exc instanceof UnauthorizedException) {
return Response.status(401).build();
} else {
return Response.status(500).build();
}
}
public Response myWebService1() {
try {
// do something different depending on the web service called
} catch (Exception exc) {
return exceptionHandler(exc);
}
}
Sure, that is possible. We have a solution that looks like:
} catch (Exception exception) {
exceptionConverter.convertAndThrow(exception);
}
to unify re-throwing of exceptions based on the exception caught.
So that exception converter is the one central place where we "switch" over the exception type and do what needs to be done. Of course, the central element here is: all your classes need the exact same handling for the incoming exceptions.
We even go one step further and allow a wild mix of potential "input causes", but we also have extensive unit tests to ensure that the conversion always gives the expected result.
Please note: my answer is just about refactoring those "catch" cascade. You can still turn to TJs solution; but keep in mind: that approach adds a certain bit of complexity by introducing that Runnable aspect.
#T.J.Crowder's response is perfect. But for those who can't use Java 8, this is how to implement it with earlier version of Java:
The Responder interface:
public interface Responder {
Response handleRequest() throws Exception;
}
The helper method:
private static Response respond(Responder responder) {
try {
return responder.handleRequest();
} catch (MyCustomException e1) {
return Response.status(409).build();
} catch (UnauthorizedException e2) {
return Response.status(401).build();
} catch (Exception e3) {
return Response.status(500).build();
}
}
With an anonymous class instead of a lambda expression:
Response response = respond(new Responder() {
#Override
public Response handleRequest() throws Exception {
...
return Response.ok().build();
}
});
Related
I have a Spring Boot application that has the following approximate structure:
project
Api
ApiImpl
Application
Api is an interface that looks like this:
public interface Api {
public String methodOne(...) throws ExceptionOne, ExceptionTwo, ExceptionThree;
...
public int methodN(...) throws ExceptionOne, ExceptionThree, ExceptionFour;
}
ApiImpls is the request controller (in reality there is a second layer, but this should suffice for this example). There, I do something like the following right now:
#Controller
public class ApiImpl {
public String methodOne(...) {
try {
// do stuff that can yield an exception
}
catch(ExceptionOne e) {
// set proper response code and return values
}
catch(ExceptionTwo e) {
// set proper response code and return values
}
catch(ExceptionThree e) {
// set proper response code and return values
}
}
}
Basically, this behaviour yields a lot of repetition (might as well name my exceptions D, R, and Y...), but is otherwise very suited to handling the internal application logic.
My question is: How can I implement a custom Exception Dispatcher that would handle this in Java? Ideally, I would want something like this answer here, but unfortunately simply throwing the current exception like in that C++ code is not possible in Java, as far as I know. For brevity, what I would like to accomplish is something like the following:
#Controller
public class ApiImpl {
public String methodOne(...) {
try {
// do stuff that can yield an exception
}
catch(ExceptionOne e) {
handle()
}
}
private void handle() { // maybe Throwable or Exception subclass as parameter
// handle the correct exception type, set correct response code, etc.
}
}
Are there any good approaches to doing this so as to minimize code repetition?
Here is a preliminary attempt I tried to get this working:
public class Thrower {
public Thrower(int e) throws ExceptionOne, ExceptionTwo, ExceptionThree {
if(e == 0) {
throw new ExceptionOne();
}
if(e == 1) {
throw new ExceptionTwo();
}
if(e == 2) {
throw new ExceptionThree();
}
}
}
class ExceptionOne extends Exception {}
class ExceptionTwo extends Exception {}
class ExceptionThree extends Exception {}
public class ExceptionHandler {
private void handle(Exception ex) throws Exception {
try {
throw ex;
}
catch(ExceptionOne e) {
e.printStackTrace();
System.out.println("Exception one");
}
catch(ExceptionTwo e) {
e.printStackTrace();
System.out.println("Exception two");
}
catch(ExceptionThree e) {
e.printStackTrace();
System.out.println("Exception three");
}
}
public void causesException(int which) throws Throwable {
try {
Thrower t = new Thrower(which);
}
catch(Exception e) {
handle(e);
}
}
public static void main(String[] args) throws Throwable {
ExceptionHandler eh = new ExceptionHandler();
eh.causesException(0);
eh.causesException(1);
eh.causesException(2);
}
}
This works as expected, and I can handle the different exception types as needed (shown here using a constructor, but the principle would be the same). However, this feels extremely clunky.
If you are looking for globally handling all Controller Layer exceptions (in Spring MVC architecture), you can do that at one place for all controllers (option1 below) by using #ExceptionHandler methods which is a ControllerAdvice from Spring.
Option(1): Configure Exceptions in Separate Class
#ControllerAdvice
class MyProjectExceptionHandler {
#ExceptionHandler(value = ExceptionOne.class)
public R exceptionOne(ExceptionOne exe) {
//set proper response code and return values
}
#ExceptionHandler(value = ExceptionTwo.class)
public R exceptionTwo(ExceptionTwo exe) {
//set proper response code and return values
}
}
Option(2): Configure Exceptions in Controller Class itself
If you are looking for handling the exceptions within the Controller class itself, then you can do that as below:
#Controller
public class ApiImpl {
public String methodOne(...) {
}
#ExceptionHandler(ExceptionOne.class)
public R exceptionTwo(ExceptionOne exe) {
//set proper response code and return values
}
//other exceptions
}
You can look more on this at here
I have a feign client with this external call:
#RequestMapping(method = RequestMethod.GET, value = "GetResourceA", consumes = "application/json")
#Cacheable("ResourceA")
List<Stop> getResourceA() throws MyOwnException;
And in my application.yml I have this setting:
hystrix:
command:
default:
execution.isolation.thread.timeoutInMilliseconds: 1000
fallback.enabled: false
Now if getResourceA times out, i.e. it takes more than one second to complete, I either get this:
com.netflix.hystrix.exception.HystrixRuntimeException: getResourceA timed-out and no fallback available
Or, if I define a fallback from which I throw my own exception, I get this:
com.netflix.hystrix.exception.HystrixRuntimeException: getResourceA timed-out and fallback failed.
Can I not throw my own exception from the fallback?
What if I wish to throw my own exception when the service is down? I would like to not have a fallback (because I have no reasonable value to return from the fallback), but instead throw my own error that I can catch and let the program resume. Can someone help me out with this?
Update after the answer from Ben:
So I tried the approach with catching HysterixRuntimeException and checking what caused it, but ended up with this ugly code:
try {
getResourceA();
} catch (HystrixRuntimeException e) {
if (e.getFailureType().name().equals("TIMEOUT")) {
throw new MyOwnException("Service timed out");
}
throw e;
}
All that to be able to throw MyOwnException on a timeout. Surely there must be another way?
You should be able to get the exception you throw from your fallback by getting the cause of the HystrixRuntimeException
So, to handle your custom exception, you can do this:
try {
getResourceA();
} catch (HystrixRuntimeException e) {
if (e.getCause() instanceof MyException) {
handleException((MyException)e.getCause());
}
}
You can use an ErrorDecoder and return your own exception from there and then use an exception handler. I had a similar problem and solved it like this:
public class ExceptionHandlerControllerAdvice extends ResponseEntityExceptionHandler
...
#ResponseStatus(BAD_REQUEST)
#ExceptionHandler(HystrixRuntimeException.class)
public ExceptionResponse handleHystrixRuntimeException(HystrixRuntimeException exception) {
if(exception.getCause() instanceof MyException) {
return handleMyException((MyException) exception.getCause());
...
And then in my Configuration class for my FeignClient:
#Bean
public ErrorDecoder feignErrorDecoder() {
return new ErrorDecoder() {
#Override
public Exception decode(String methodKey, Response response) {
return new MyException("timeout");
}
};
}
That way you don't need multiple nested getCause()
If you want replace timeout exception throw by hystrix，you can write like this:
try {
testClient.timeoutTest();
} catch (HystrixRuntimeException e) {
Throwable fallbackException = e.getFallbackException();
if (fallbackException.getCause().getCause() instanceof CustomException) {
log.info("customer exception!");
}
}
I would like to annotate some of my test cases with KnownFault - which would do pretty much what expectedException does plus some magic using YouTrack's REST API. I would also like to have an IntermittentFailure attribute which would mean that I'm aware that the test might occasionally fail with [exception] [message] but I wouldn't want this to block the rest of my build chain.
After some research I found that my test class should implement IHookable, then I could have something like this:
#Override
public void run(IHookCallBack callBack, ITestResult result) {
callBack.runTestMethod(result);
if (result.getThrowable().getCause() instanceof IllegalArgumentException){
System.out.println("This is expected.");
result.setThrowable(null);
}
else{
System.out.println("Unexpected exception");
}
}
The problem with this is the actual implementation of invokeHookable:
final Throwable[] error = new Throwable[1];
IHookCallBack callback = new IHookCallBack() {
#Override
public void runTestMethod(ITestResult tr) {
try {
invokeMethod(thisMethod, testInstance, parameters);
} catch (Throwable t) {
error[0] = t;
tr.setThrowable(t); // make Throwable available to IHookable
}
}
#Override
public Object[] getParameters() {
return parameters;
}
};
hookable.run(callback, testResult);
if (error[0] != null) {
throw error[0];
}
Unfortunately that last line means that my test case is going to throw an exception no matter what as the error array is completely out of my hands in the run method.
So, what would be the proper way of intercepting an exception and handling it the way I want to?
What you are trying to do is really interesting. You should try to propose changes on https://github.com/cbeust/testng/pull/
But maybe IHookable is not the best listener you can use. Did you try IInvokedMethodListener?
void afterInvocation(IInvokedMethod method, ITestResult result) {
if (result.getThrowable().getCause() instanceof IllegalArgumentException) {
System.out.println("This is expected.");
result.setThrowable(null);
result.setStatus(SUCCESS); // If you want to change the status
} else {
System.out.println("Unexpected exception");
}
}
I'm implementing an Iterator and in order to deal with the Exceptions I'm using the following pattern: The actual work is done in the private hasNextPriv() method whereas the hasNext() method deals with the Exceptions. The reason for doing it this way is because I don't want to litter hasNextPriv() with try-catch blocks.
#Override
public boolean hasNext()
{
try {
return hasNextPriv();
} catch (XMLStreamException e) {
e.printStackTrace();
try {
reader.close();
} catch (XMLStreamException e1) {
e1.printStackTrace();
}
}
return false;
}
Questions:
Is there a better way to do this?
What would be a good name for the private method hasNextPriv()?
Another way to handle exceptions would be to extract each part that throws exception in a small pure function that properly handles each exception. And then construct final result composing those functions.
Optional<Resource> open() {
try{
//...
return Optional.of(resource);
} catch {
//....
return Optional.empty();
}
}
Optional<Value> read(Resource resource) {
try{
//...
return Optional.of(resource.value);
} catch {
//....
return Optional.empty();
}
}
boolean hasNext() {
open().flatMap(this::read).isPresent();
}
There is no need to return Optional everywhere. Usually there is some dummy value like in Null Object Pattern
Another pattern is to wrap a function execution in object that produces either result or error value. In library javaslang it looks like
return Try.of(this::hasNextPriv)
.recover(x -> Match(x).of(
Case(instanceOf(Exception_1.class), /*handle exception*/),
Case(instanceOf(Exception_2.class), ...)))
.getOrElse(false);
Try object is similar to java 8 Optional but instead of holding present value or missing value Try contains value of either success or failure.
Regarding naming hasNextPriv in your case there is specific domain of data structure. Probably you could come up with more specific name like hasMoreNodes or notEmpty etc.
There are some task that should't be done in parallel, (for example opening a file, reading, writing, and closing, there is an order on that...)
But... Some task are more like a shoping list, I mean they could have a desirable order but it's not a must..example in communication or loading independient drivers etc..
For that kind of tasks,
I would like to know a java best practice or pattern for manage exceptions..
The java simple way is:
getUFO {
try {
loadSoundDriver();
loadUsbDriver();
loadAlienDetectorDriver();
loadKeyboardDriver();
} catch (loadSoundDriverFailed) {
doSomethingA;
} catch (loadUsbDriverFailed) {
doSomethingB;
} catch (loadAlienDetectorDriverFailed) {
doSomethingC;
} catch (loadKeyboardDriverFailed) {
doSomethingD;
}
}
But what about having an exception in one of the actions but wanting to
try with the next ones??
I've thought this approach, but don't seem to be a good use for exceptions
I don't know if it works, doesn't matter, it's really awful!!
getUFO {
Exception ex=null;
try {
try{ loadSoundDriver();
}catch (Exception e) { ex=e; }
try{ loadUsbDriver();
}catch (Exception e) { ex=e; }
try{ loadAlienDetectorDriver();
}catch (Exception e) { ex=e; }
try{ loadKeyboardDriver()
}catch (Exception e) { ex=e; }
if(ex!=null)
{ throw ex;
}
} catch (loadSoundDriverFailed) {
doSomethingA;
} catch (loadUsbDriverFailed) {
doSomethingB;
} catch (loadAlienDetectorDriverFailed) {
doSomethingC;
} catch (loadKeyboardDriverFailed) {
doSomethingD;
}
}
seems not complicated to find a better practice for doing that.. I still didn't
thanks for any advice
Consider the execute around idiom.
Another option (which isn't really all that different, it just decouples them more) is to do each task in a separate thread.
Edit:
Here is the kind of thing I have in mind:
public interface LoadableDriver {
public String getName();
public void loadDriver() throws DriverException;
public void onError(Throwable e);
}
public class DriverLoader {
private Map<String, Exception> errors = new HashMap<String, Exception>();
public void load(LoadableDriver driver) {
try {
driver.loadDriver();
} catch (DriverException e) {
errors.put(driver.getName(), e);
driver.onError(e);
}
}
public Map<String, Exception> getErrors() { return errors; }
}
public class Main {
public void loadDrivers() {
DriverLoader loader = new DriverLoader();
loader.loadDriver(new LoadableDriver(){
public String getName() { return "SoundDriver"; }
public void loadDriver() { loadSoundDriver(); }
public void onError(Throwable e) { doSomethingA(); }
});
//etc. Or in the alternative make a real class that implements the interface for each driver.
Map<String, Exception> errors = loader.getErrors();
//react to any specific drivers that were not loaded and try again.
}
}
Edit: This is what a clean Java version would ultimately look like if you implemented the drivers as classes (which is what the Java OO paradigm would expect here IMHO). The Main.loadDrivers() method would change like this:
public void loadDrivers(LoadableDriver... drivers) {
DriverLoader loader = ...
for(LoadableDriver driver : drivers) {
loader.load(driver);
}
//retry code if you want.
Set<LoadableDriver> failures = loader.getErrors();
if(failures.size() > 0 && tries++ > MAX_TRIES) {
//log retrying and then:
loadDrivers(drivers.toArray(new LoadableDriver[0]));
}
}
Of course I no longer use a map because the objects would be self-sufficient (you could get rid of the getName() method as well, but probably should override toString()), so the errors are just returned in a set to retry. You could make the retry code even simpler if each driver was responsible for knowing how often it should it retry.
Java won't look as nice as a well done C++ template, but that is the Java language design choice - prefer simplicity over complex language features that can make code hard to maintain over time if not done properly.
Try this:
protected void loadDrivers() {
loadSoundDriver();
loadUsbDriver();
loadAlienDetectorDriver();
loadKeyboardDriver();
}
Then:
protected void loadSoundDriver() {
try {
// original code ...
}
catch( Exception e ) {
soundDriverFailed( e );
}
}
protected void soundDriverFailed( Exception e ) {
log( e );
}
This gives subclasses a chance to change the behaviour. For example, a subclass could implement loading each driver in a separate thread. The main class need not care about how the drivers are loaded, nor should any users of the main class.
IMO, for your case, if the exception is "ignorable" it's best if the "loadSoundDriver" method catches the exception and simply returns an error.
Then in the function that loads stuff, you can record all the errors and at the end of the sequence, decide what to do with them.
[edit]
Something like this:
// init
MyError soundErr = loadSoundDriver();
MyError otherErr = loadOtherDriver();
if(soundErr!=null || otherErr !=null){
// handle the error(s)
}
Just surround every single load operation with its own try / catch block.
try {
loadSoundDriver();
} catch (loadSoundDriverFailed) {
doSomethingA;
}
try {
loadUsbDriver();
} catch (loadUsbDriverFailed) {
doSomethingB;
}
// ...
So you can handle every exception by itself and continue processing the oder operations.