RESTEasy integration with Spring – Tutorial Part 2 – Exeption handling

Second part will describe exception handling in REST scope – a few design patterns and way of implementing them. I will give also some examples of anti patters – just for a contrast.

In general, JEE projects are dividing exceptions into three parts – checked, unchecked exceptions (runtime), and throwable.

  • Checked exceptions are visible in method signature, and therefore they must be handled by the developer. When method declares such exception, it means, that someone can possibly process this exception, and do something useful with it. For example, it can display message to the user, that entered password is incorrect, or given account was not found. It is very important to NOT overuse checked exceptions – do not define every possible event as checked exception, this will decrease method readability. When defining check exception, ask yourself always a question: it is possible to react in a special way on this particular event? Can it be handled ? And for the record – creating long entry does not count! Also when you notice frequent catch(Exception e){….} blocks around your code, it means, that something went wrong during design phase – people just does not care about defined exceptions, so maybe they should be runtime? I am assuming, that generic catch blocks are rear in correctly written code, if existing, they are well documented so the others can understand the reason.
  • Unchecked exceptions are not visible in method signature – those are all possible events, which break current process and nothing can be done about it. In the most cases, log entry is created, monitoring framework is being notified, and GUI displays generic error message.
  • Throwables shoud never be handled by developer –  catch(Throwable e){….} block in not acceptable. Errors like LinkageError or OutOfMemoryError must be handled by container.

Remember, that decision whenever given exception is runtime or checed depends on given context. For example:

  • DBConnectionError on high level business interface is definitely runtime – here we even do not notice, that database is part of our transaction – this is being encapsulated behind service interface, which as every proper high level interface hides implementation details. On the other hand, connection pool maybe can handle such exception by reestablishing connection to another DB instance – on this level DBConnectionError would be checked exception, because it is clearly connection pool responsibility to react on this error.
  • Null-Pointer-Like-Exceptions are runtime in every scope – it breaks given transaction and this is all – no one will catch this exception, so it is not visible in method signature, and as usual not expected.
  • AccountNotFoundException is always checked – it does not matter on which level you are. Account was not found, and therefore certain action cannot be executed, in that case developer can create missing account, or in the worst case notify user, that he actually does not exists.

Enough theory – lets concentrate on exception handling in REST scope. We will extend project from previous post by few error use cases:

  • Checked exception: MessageForbidenException, it will be thrown, when hi-message begins with “jo!”. REST interface in this case returns status code: “400 Bad Request” and sets response header “ERR_CODE=MESSAGE_FORBIDDEN”
  • Checked exception: IncorrectLengthException, it will be thrown, when message size in not between 3 and 10 characters. REST interface in this case returns status code: “400 Bad Request” and sets response header “ERR_CODE=INCORRECT_LENGTH”
  • We would like also to log all possible exceptions (runtimes) – there are several reasons for that – to create dedicated log entry, or to notify monitoring framework. In case of any exception, we would like to set status code: “500 Internal Server Error”, and HTTP Header: EX_CLASS, which contains exception’s class name, and message body should contain stack trace (this might be security issue for some systems – be cheerful here).

There are two approaches to implement exception handling – traditional by defining try-catch blocks, or exception mappers as providers.

Project update

We will extend project from previous tutorial, by adding new exceptions,  exception mappers, add new REST Resource: HelloRestServiceCatch.
The existing REST resource HelloRestService will remain almost unchanged, its methods will throw new exceptions, and those will be handled by exception mappers.
HelloRestServiceCatch in contrast will contain try-catch exception handling, so both approaches can be easily compared.

This is the updated project structure:

and this simple validation logic responsible for throwing exceptions (build into existing service):

Curl examples triggering new exceptions:

>> GET REQUEST <<
curl -iX POST -H “Content-Type: application/json” -d ‘{“msg”:”Jo!”,”gender”:”MALE”}’
http://localhost:8080/resteasy_spring_p2/rest/Hello/catch/javabean

HTTP/1.1 400 Bad Request
Server: Apache-Coyote/1.1
ERR_CODE: MESSAGE_FORBIDDEN
Content-Type: application/json
Content-Length: 18
Connection: close

>> RESPONSE <<
Jo! is not allowed

========================================================================

>> GET REQUEST <<
curl -iX POST -H “Content-Type: application/json” -d ‘{“msg”:”Hi”,”gender”:”MALE”}’
http://localhost:8080/resteasy_spring_p2/rest/Hello/catch/javabean

HTTP/1.1 400 Bad Request
Server: Apache-Coyote/1.1
ERR_CODE: INCORRECT_LENGTH
Content-Type: application/json
Content-Length: 46
Connection: close

>> RESPONSE <<
Message length not between 3 and 10 characters

========================================================================

> GET REQUEST <<
curl -iX POST -H “Content-Type: application/json” -d ‘{“msg”:”Hi”,”gender”:”FRED”}’
http://localhost:8080/resteasy_spring_p2/rest/Hello/javabean

HTTP/1.1 500 Internal Server Error
Server: Apache-Coyote/1.1
EX_CLASS: org.codehaus.jackson.map.JsonMappingException
Content-Type: application/json
Content-Length: 4689
Connection: close

>> RESPONSE <<
Can not construct instance of org.mmiklas.resttutorial.model.Gender
from String value ‘FRED’: value not one of declared Enum instance names…..

Traditional try-catch Approach

Traditional try-catch approach has several problems, and well …. no advantages:

  • Different methods throw the same exception, and have similar catch block, to handle it. This code could be extracted as separate method, but still you need to relay on developer, to call it. On the end there is no guarantee, that the same exception will be always handled in the same way. The goal of consistent error handling is, to map given exception to the same REST representation – MessageVorbidenException results always in 400.
  • Catching Exception is mostly bad practice. If we define new checked exception, we also expect that it will be handled by developers in dedicated way – at the begining code should not compile, since there is no catch block for this new exception. But in case of catch(Exception e){….} code will compile and our new exception will be treated as general error, and not in a special way as expected. We might even not notice that.

This is the REST service implementation using try-catch statements:

Exception Mappers

The approach with exception mappers has few nice futures:

  • Clan code – REST implementation is not overflowed with try-catch blocks
  • No code repetition
  • Centralized and automatic mapping from Java exceptions to REST representation
  • Each exception will always result in the same REST response – this improves interface stability and integrity
  • No Exception catch blocks – check style will be happy about that

How it works?
REST resource class do not catch exceptions, they are simply declared in method signature. By default RESTEasy would convert such exception to “500 Internal Server Error”, since it was not handled by application logic.
In order to handle exceptions we need to register exception mappers. Each one can handle single exception, and also its subclasses – if there is no dedicated handler for particular subclass. For example: we can register handler for Exception and it will get called for every possible exception inclusive runtimes, but we can still register handler for IncorrectLengthException (subclass of Exception), and in this handler will be called when IncorrectLengthException occurs. This was not always the case – in the older RESTEasy versions, exception mapper registered on parent class would be called for all child exceptions, so it was not possible to register exception mapper on Exception because other mappers would not be called.

RESTEasy looks for classes marked @Provider and implementing ExceptionMapper<E>, where E declares exception class which will be handled by this mapper. In the normal case, RESTEasy would scann class path, but since we are using Spring integration, it will ask Spring instead. This is the reason, why mapper classes are annotated with @Named and @Provider.

Updated REST Resource (only exceptions in method signature):

Exception mappers:

Project Source Download

resteasy_spring_p2_src.zip

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