Streaming JSON with Jsonista

JSON Lines (aka JSONL)

Sometimes you want to store a stream of JSON objects in a file. This is common for things like logging. This pattern is often called JSON Lines.

Writing

(jsonista.core/write-values (io/output-stream "/tmp/foo.json") [{"foo" 1} {"bar" 1}])

For actual streaming, use a lazy sequence or an eduction instead of a vector. For example:

(jsonista.core/write-values
 (io/output-stream "/tmp/foo.json")
 (eduction (map (fn [i] {:i i})) (range 100)))

Alternatively, you can use Jackson's imperative API directly:

(let [obj-mapper (jsonista.core/object-mapper {:close false})]
  (with-open [out (io/output-stream "/tmp/foo.json")
              wrt (io/writer out)]
    (jsonista.core/write-value wrt {"foo" 1} obj-mapper)
    (.write wrt "\n")
    (jsonista.core/write-value wrt {"bar" 1} obj-mapper)))

Reading

(into [] (jsonista.core/read-values (io/input-stream "/tmp/foo.json")))

Top-level array

Instead of being separated on separate lines, sometimes you just want a big JSON array, but don't want to keep all of the data in memory at once.

Writing

Use jsonista.core/write-values-as-array, which works just like jsonista.core/write-values.

Reading

Use jsonista.core/read-values, it autodetects the format.

An array inside an object

Sometimes you need to stream an array that sits inside an object. For this, it's best to drop down to the Jackson JsonParser API

(let [input "{\"foo\": 1, \"bars\": [{\"bar\": 2},{\"bar\": 3}], \"close\": \"end\"}"
      obj-mapper (jsonista.core/object-mapper)]
   (with-open [rdr (java.io.StringReader. input)]
      (let [p (.. obj-mapper getFactory (createParser rdr))]
        ;; position cursor to start of first entry in "bars"
        (.nextToken p) ; START_OBJECT
        (.nextToken p) ; FIELD_NAME "foo"
        (.nextToken p) ; VALUE_NUMBER_INT 1
        (.nextToken p) ; FIELD_NAME "bar"
        (.nextToken p) ; START_ARRAY
        (.nextToken p) ; START_OBJECT
        ;; grab all entries, ignore rest of input
        (doall (iterator-seq (.readValuesAs p Object))))))