+
+ +
+
+ +
+ +

Exception Handling with Ring

+
[metosin/reitit-middleware "0.4.2"]
+
+

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:

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
keydescription
:reitit.ring/responsevalue in ex-data key :response will be returned
:muuntaja/decodehandle Muuntaja decoding exceptions
:reitit.coercion/request-coercionrequest coercion errors (http 400 response)
:reitit.coercion/response-coercionresponse coercion errors (http 500 response)
::exception/defaulta default exception handler if nothing else matched (default exception/default-handler).
::exception/wrapa 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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