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Exception Handling with Ring

[metosin/reitit-middleware "0.3.9"]

Exceptions thrown in router creation can be handled with custom exception handler. By default, exceptions thrown at runtime from a handler or a middleware are not caught by the reitit.ring/ring-handler. A good practise is a have an top-level exception handler to log and format the errors for clients.

(require '[reitit.ring.middleware.exception :as exception])

exception/exception-middleware

A preconfigured middleware using exception/default-handlers. Catches:

  • Request & response Coercion exceptions
  • Muuntaja decode exceptions
  • Exceptions with :type of :reitit.ring/response, returning :response key from ex-data.
  • Safely all other exceptions
(require '[reitit.ring :as ring])

(def app
  (ring/ring-handler
    (ring/router
      ["/fail" (fn [_] (throw (Exception. "fail")))]
      {:data {:middleware [exception/exception-middleware]}})))

(app {:request-method :get, :uri "/fail"})
;{:status 500
; :body {:type "exception"
;        :class "java.lang.Exception"}}

exception/create-exception-middleware

Creates the exception-middleware with custom options. Takes a map of identifier => exception request => response that is used to select the exception handler for the thrown/raised exception identifier. Exception identifier is either a Keyword or a Exception Class.

The following handlers are available by default:

key description
:reitit.ring/response value in ex-data key :response will be returned
:muuntaja/decode handle Muuntaja decoding exceptions
:reitit.coercion/request-coercion request coercion errors (http 400 response)
:reitit.coercion/response-coercion response coercion errors (http 500 response)
::exception/default a default exception handler if nothing else matched (default exception/default-handler).
::exception/wrap a 3-arity handler to wrap the actual handler handler exception request => response (no default).

The handler is selected from the options map by exception identifier in the following lookup order:

  1. :type of exception ex-data
  2. Class of exception
  3. :type ancestors of exception ex-data
  4. Super Classes of exception
  5. The ::default handler
;; type hierarchy
(derive ::error ::exception)
(derive ::failure ::exception)
(derive ::horror ::exception)

(defn handler [message exception request]
  {:status 500
   :body {:message message
          :exception (.getClass exception)
          :data (ex-data exception)
          :uri (:uri request)}})

(def exception-middleware
  (exception/create-exception-middleware
    (merge
      exception/default-handlers
      {;; ex-data with :type ::error
       ::error (partial handler "error")

       ;; ex-data with ::exception or ::failure
       ::exception (partial handler "exception")

       ;; SQLException and all it's child classes
       java.sql.SQLException (partial handler "sql-exception")

       ;; override the default handler
       ::exception/default (partial handler "default")

       ;; print stack-traces for all exceptions
       ::exception/wrap (fn [handler e request]
                          (println "ERROR" (pr-str (:uri request)))
                          (handler e request))})))

(def app
  (ring/ring-handler
    (ring/router
      ["/fail" (fn [_] (throw (ex-info "fail" {:type ::failue})))]
      {:data {:middleware [exception-middleware]}})))

(app {:request-method :get, :uri "/fail"})
; ERROR "/fail"
; => {:status 500,
;     :body {:message "default"
;            :exception clojure.lang.ExceptionInfo
;            :data {:type :user/failue}
;            :uri "/fail"}}

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