DJSON-54 Add custom fallback fn for writing unknown types

Signed-off-by: Alex Miller <alex.miller@cognitect.com>

Author: slipset

src/main/clojure/clojure/data/json.clj

@@ -610,7 +610,7 @@
 (defn- write-generic [x out options]
   (if (.isArray (class x))
     (-write (seq x) out options)
-    (throw (Exception. (str "Don't know how to write JSON of " (class x))))))
+    ((:default-write-fn options) x out options)))
 
 (defn- write-ratio [x out options]
   (-write (double x) out options))
@@ -648,13 +648,17 @@
 ;; Maybe a Java array, otherwise fail
 (extend java.lang.Object       JSONWriter {:-write write-generic})
 
+(defn- default-write-fn [x out options]
+  (throw (Exception. (str "Don't know how to write JSON of " (class x)))))
+
 (def default-write-options {:escape-unicode true
                             :escape-js-separators true
                             :escape-slash true
                             :sql-date-converter default-sql-date->instant-fn
                             :date-formatter java.time.format.DateTimeFormatter/ISO_INSTANT
                             :key-fn default-write-key-fn
                             :value-fn default-value-fn
+                            :default-write-fn default-write-fn
                             :indent false
                             :indent-depth 0 ;; internal, to track nesting depth
                             })
@@ -713,6 +717,13 @@
         returns itself, the key-value pair will be omitted from the
         output. This option does not apply to non-map collections.
 
+     :default-write-fn function
+
+        Function to handle types which are unknown to data.json. Defaults
+        to a function which throws an exception. Expects to be called with
+        three args, the value to be serialized, the output stream, and the
+        options map.
+
     :indent boolean
 
         If true, indent json while writing (default = false)."

src/test/clojure/clojure/data/json_test.clj

@@ -423,3 +423,13 @@
      (dotimes [_ 1000]
        (assert (= (json/read-str pass1-string)
                   (json/read-str (json/write-str (json/read-str pass1-string)))))))))
+
+(defn djson-54-default-write-fn [x out options]
+  (#'json/write-string (str x) out options))
+
+(deftest DJSON-54-test
+  (is (thrown? Exception (json/write-str {:foo (java.net.URI. "http://clojure.org")})))
+  (try (json/write-str {:foo (java.net.URI. "http://clojure.org")})
+       (catch Exception e
+         (is (= "Don't know how to write JSON of class java.net.URI" (.getMessage e)))))
+  (is (= "{\"foo\":\"http:\\/\\/clojure.org\"}" (json/write-str {:foo (java.net.URI. "http://clojure.org")} :default-write-fn djson-54-default-write-fn))))