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.
(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)))
(into [] (jsonista.core/read-values (io/input-stream "/tmp/foo.json")))
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.
Use jsonista.core/write-values-as-array
, which works just like jsonista.core/write-values
.
Use jsonista.core/read-values
, it autodetects the format.
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))))))
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