ring.middleware.format_params.cjl throws on malformed JSON

Question

I use luminus template with ring/compojure/swagger setup.

When I feed the REST API i created malformed JSON i get 500 and:

java.lang.IllegalArgumentException: No value supplied for key: {:formats (:json-kw :yaml-kw :edn :transit-json :transit-msgpack), :handle-error #<middleware$handle_req_error compojure.api.middleware$handle_req_error@3130fc88>}, compiling:(pythonapi.clj:14:1)
    at clojure.lang.Compiler$InvokeExpr.eval(Compiler.java:3558)
    at clojure.lang.Compiler$DefExpr.eval(Compiler.java:417)
    at clojure.lang.Compiler.eval(Compiler.java:6708)
    at clojure.lang.Compiler.load(Compiler.java:7130)
    at clojure.lang.RT.loadResourceScript(RT.java:370)
    at clojure.lang.RT.loadResourceScript(RT.java:361)
    at clojure.lang.RT.load(RT.java:440)
    at clojure.lang.RT.load(RT.java:411)
    at clojure.core$load$fn__5066.invoke(core.clj:5641)
    at clojure.core$load.doInvoke(core.clj:5640)
    at clojure.lang.RestFn.invoke(RestFn.java:408)
    at clojure.core$load_one.invoke(core.clj:5446)
    at clojure.core$load_lib$fn__5015.invoke(core.clj:5486)
    at clojure.core$load_lib.doInvoke(core.clj:5485)
    at clojure.lang.RestFn.applyTo(RestFn.java:142)
    at clojure.core$apply.invoke(core.clj:626)
    at clojure.core$load_libs.doInvoke(core.clj:5524)
    at clojure.lang.RestFn.applyTo(RestFn.java:137)
    at clojure.core$apply.invoke(core.clj:626)
    at clojure.core$require.doInvoke(core.clj:5607)
    at clojure.lang.RestFn.invoke(RestFn.java:1289)
    at voicepin_collector.handler$eval23$loading__4958__auto____24.invoke(handler.clj:1)
    at voicepin_collector.handler$eval23.invoke(handler.clj:1)
    at clojure.lang.Compiler.eval(Compiler.java:6703)
    at clojure.lang.Compiler.eval(Compiler.java:6692)
    at clojure.lang.Compiler.load(Compiler.java:7130)
    at clojure.lang.RT.loadResourceScript(RT.java:370)
    at clojure.lang.RT.loadResourceScript(RT.java:361)
    at clojure.lang.RT.load(RT.java:440)
    at clojure.lang.RT.load(RT.java:411)
    at clojure.core$load$fn__5066.invoke(core.clj:5641)
    at clojure.core$load.doInvoke(core.clj:5640)
    at clojure.lang.RestFn.invoke(RestFn.java:408)
    at clojure.core$load_one.invoke(core.clj:5446)
    at clojure.core$load_lib$fn__5015.invoke(core.clj:5486)
    at clojure.core$load_lib.doInvoke(core.clj:5485)
    at clojure.lang.RestFn.applyTo(RestFn.java:142)
    at clojure.core$apply.invoke(core.clj:626)
    at clojure.core$load_libs.doInvoke(core.clj:5524)
    at clojure.lang.RestFn.applyTo(RestFn.java:137)
    at clojure.core$apply.invoke(core.clj:626)
    at clojure.core$require.doInvoke(core.clj:5607)
    at clojure.lang.RestFn.invoke(RestFn.java:408)
    at voicepin_collector.listener$_contextInitialized.invoke(listener.clj:1)
    at voicepin_collector.listener.contextInitialized(Unknown Source)
    at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:4973)
    at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5467)
    at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150)
    at org.apache.catalina.manager.ManagerServlet.start(ManagerServlet.java:1291)
    at org.apache.catalina.manager.HTMLManagerServlet.start(HTMLManagerServlet.java:694)
    at org.apache.catalina.manager.HTMLManagerServlet.doPost(HTMLManagerServlet.java:217)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:646)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
    at org.apache.catalina.filters.CsrfPreventionFilter.doFilter(CsrfPreventionFilter.java:213)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
    at org.apache.catalina.filters.SetCharacterEncodingFilter.doFilter(SetCharacterEncodingFilter.java:108)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:610)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:171)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:950)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:408)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1040)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:607)
    at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:314)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.IllegalArgumentException: No value supplied for key: {:formats (:json-kw :yaml-kw :edn :transit-json :transit-msgpack), :handle-error #<middleware$handle_req_error compojure.api.middleware$handle_req_error@3130fc88>}
    at clojure.lang.PersistentHashMap.create(PersistentHashMap.java:77)
    at ring.middleware.format_params$wrap_restful_params.doInvoke(format_params.clj:251)
    at clojure.lang.RestFn.invoke(RestFn.java:423)
    at compojure.api.middleware$api_middleware.doInvoke(middleware.clj:214)
    at clojure.lang.RestFn.invoke(RestFn.java:423)
    at compojure.api.core$api_middleware_with_routes.invoke(core.clj:21)
    at clojure.lang.AFn.applyToHelper(AFn.java:156)
    at clojure.lang.AFn.applyTo(AFn.java:144)
    at clojure.lang.Compiler$InvokeExpr.eval(Compiler.java:3553)
    ... 78 more

I believe I have to intercept this exception in order to inform user that malformed JSON has been posted and return 400.

Where is the best place to perform try/catch?

Answer 1

The usual way I handle this is to make a new middleware, wrapping the existing middleware, that catches the expected error and returns your custom message and status.

(defn safe-restful-params
  [handler]
  (fn [request]
    (try ((wrap-restful-params handler) request)
         (catch IllegalArgumentException e
            {:status 400 ; bad request
             :body "malformed JSON"}))))

In practice you likely want to set some headers, etc. and you could also write a more general version that wraps arbitrary middleware instead of special-casing the wrapping of this one, but this is the general concept. A ring middleware always takes a handler as its argument, and returns a function you can call on a ring request, and in practice should usually call the handler on the request as part of its execution. This middleware also manually inserts another middleware between itself and the handler, and is meant to replace that other middleware.