Get single field from JSON using Jackson

Question

Given an arbitrary JSON I would like to get value of a single field contentType. How to do it with Jackson?

{   contentType: "foo",   fooField1: ... }  {   contentType: "bar",   barArray: [...] } 

Related

• How to find specified name and its value in JSON-string from Java? (GSON)
• Using gson to deserialize specific JSON field of an object (GSON)

Answer 1

The Jackson Way

Considering that you don't have a POJO describing your data structure, you could simply do:

final String json = "{\"contentType\": \"foo\", \"fooField1\": ... }"; final ObjectNode node = new ObjectMapper().readValue(json, ObjectNode.class); //                              ^  // actually, try and *reuse* a single instance of ObjectMapper  if (node.has("contentType")) {     System.out.println("contentType: " + node.get("contentType")); }     

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Addressing concerns in the comments section

If, however, you wish to not consume the entire source String, but simply access a specific property whose path you know, you'll have to write it yourself, leveraging a Tokeniser.

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Actually, it's the weekend and I got time on my hands, so I could give you a head start: here's a basic one! It can run in strict mode and spew out sensible error messages, or be lenient and return Optional.empty when the request couldn't be fulfilled.

public static class JSONPath {      protected static final JsonFactory JSON_FACTORY = new JsonFactory();      private final List<JSONKey> keys;      public JSONPath(final String from) {         this.keys = Arrays.stream((from.startsWith("[") ? from : String.valueOf("." + from))                 .split("(?=\\[|\\]|\\.)"))                 .filter(x -> !"]".equals(x))                 .map(JSONKey::new)                 .collect(Collectors.toList());     }      public Optional<String> getWithin(final String json) throws IOException {         return this.getWithin(json, false);     }      public Optional<String> getWithin(final String json, final boolean strict) throws IOException {         try (final InputStream stream = new StringInputStream(json)) {             return this.getWithin(stream, strict);         }     }      public Optional<String> getWithin(final InputStream json) throws IOException {         return this.getWithin(json, false);     }      public Optional<String> getWithin(final InputStream json, final boolean strict) throws IOException {         return getValueAt(JSON_FACTORY.createParser(json), 0, strict);     }      protected Optional<String> getValueAt(final JsonParser parser, final int idx, final boolean strict) throws IOException {         try {             if (parser.isClosed()) {                 return Optional.empty();             }              if (idx >= this.keys.size()) {                 parser.nextToken();                 if (null == parser.getValueAsString()) {                     throw new JSONPathException("The selected node is not a leaf");                 }                  return Optional.of(parser.getValueAsString());             }              this.keys.get(idx).advanceCursor(parser);             return getValueAt(parser, idx + 1, strict);         } catch (final JSONPathException e) {             if (strict) {                 throw (null == e.getCause() ? new JSONPathException(e.getMessage() + String.format(", at path: '%s'", this.toString(idx)), e) : e);             }              return Optional.empty();         }     }      @Override     public String toString() {         return ((Function<String, String>) x -> x.startsWith(".") ? x.substring(1) : x)                 .apply(this.keys.stream().map(JSONKey::toString).collect(Collectors.joining()));     }      private String toString(final int idx) {         return ((Function<String, String>) x -> x.startsWith(".") ? x.substring(1) : x)                 .apply(this.keys.subList(0, idx).stream().map(JSONKey::toString).collect(Collectors.joining()));     }      @SuppressWarnings("serial")     public static class JSONPathException extends RuntimeException {          public JSONPathException() {             super();         }          public JSONPathException(final String message) {             super(message);         }          public JSONPathException(final String message, final Throwable cause) {             super(message, cause);         }          public JSONPathException(final Throwable cause) {             super(cause);         }     }      private static class JSONKey {          private final String key;         private final JsonToken startToken;          public JSONKey(final String str) {             this(str.substring(1), str.startsWith("[") ? JsonToken.START_ARRAY : JsonToken.START_OBJECT);         }          private JSONKey(final String key, final JsonToken startToken) {             this.key = key;             this.startToken = startToken;         }          /**          * Advances the cursor until finding the current {@link JSONKey}, or          * having consumed the entirety of the current JSON Object or Array.          */         public void advanceCursor(final JsonParser parser) throws IOException {             final JsonToken token = parser.nextToken();             if (!this.startToken.equals(token)) {                 throw new JSONPathException(String.format("Expected token of type '%s', got: '%s'", this.startToken, token));             }              if (JsonToken.START_ARRAY.equals(this.startToken)) {                 // Moving cursor within a JSON Array                 for (int i = 0; i != Integer.valueOf(this.key).intValue(); i++) {                     JSONKey.skipToNext(parser);                 }             } else {                 // Moving cursor in a JSON Object                 String name;                 for (parser.nextToken(), name = parser.getCurrentName(); !this.key.equals(name); parser.nextToken(), name = parser.getCurrentName()) {                     JSONKey.skipToNext(parser);                 }             }         }          /**          * Advances the cursor to the next entry in the current JSON Object          * or Array.          */         private static void skipToNext(final JsonParser parser) throws IOException {             final JsonToken token = parser.nextToken();             if (JsonToken.START_ARRAY.equals(token) || JsonToken.START_OBJECT.equals(token) || JsonToken.FIELD_NAME.equals(token)) {                 skipToNextImpl(parser, 1);             } else if (JsonToken.END_ARRAY.equals(token) || JsonToken.END_OBJECT.equals(token)) {                 throw new JSONPathException("Could not find requested key");             }         }          /**          * Recursively consumes whatever is next until getting back to the          * same depth level.          */         private static void skipToNextImpl(final JsonParser parser, final int depth) throws IOException {             if (depth == 0) {                 return;             }              final JsonToken token = parser.nextToken();             if (JsonToken.START_ARRAY.equals(token) || JsonToken.START_OBJECT.equals(token) || JsonToken.FIELD_NAME.equals(token)) {                 skipToNextImpl(parser, depth + 1);             } else {                 skipToNextImpl(parser, depth - 1);             }         }          @Override         public String toString() {             return String.format(this.startToken.equals(JsonToken.START_ARRAY) ? "[%s]" : ".%s", this.key);         }     } } 

Assuming the following JSON content:

{   "people": [{     "name": "Eric",     "age": 28   }, {     "name": "Karin",     "age": 26   }],   "company": {     "name": "Elm Farm",     "address": "3756 Preston Street Wichita, KS 67213",     "phone": "857-778-1265"   } } 

... you could use my JSONPath class as follows:

    final String json = "{\"people\":[],\"company\":{}}"; // refer to JSON above     System.out.println(new JSONPath("people[0].name").getWithin(json)); // Optional[Eric]     System.out.println(new JSONPath("people[1].name").getWithin(json)); // Optional[Karin]     System.out.println(new JSONPath("people[2].name").getWithin(json)); // Optional.empty     System.out.println(new JSONPath("people[0].age").getWithin(json));  // Optional[28]     System.out.println(new JSONPath("company").getWithin(json));        // Optional.empty     System.out.println(new JSONPath("company.name").getWithin(json));   // Optional[Elm Farm] 

Keep in mind that it's basic. It doesn't coerce data types (every value it returns is a String) and only returns leaf nodes.

Actual test case

It handles InputStreams, so you can test it against some giant JSON document and see that it's much faster than it would take your browser to download and display its contents:

System.out.println(new JSONPath("info.contact.email")             .getWithin(new URL("http://test-api.rescuegroups.org/v5/public/swagger.php").openStream())); // Optional[[email protected]] 

Quick test

Note I'm not re-using any already existing JSONPath or ObjectMapper so the results are inaccurate -- this is just a very rough comparison anyways:

public static Long time(final Callable<?> r) throws Exception {     final long start = System.currentTimeMillis();     r.call();     return Long.valueOf(System.currentTimeMillis() - start); }  public static void main(final String[] args) throws Exception {     final URL url = new URL("http://test-api.rescuegroups.org/v5/public/swagger.php");     System.out.println(String.format(   "%dms to get 'info.contact.email' with JSONPath",                                         time(() -> new JSONPath("info.contact.email").getWithin(url.openStream()))));     System.out.println(String.format(   "%dms to just download the entire document otherwise",                                         time(() -> new Scanner(url.openStream()).useDelimiter("\\A").next())));     System.out.println(String.format(   "%dms to bluntly map it entirely with Jackson and access a specific field",                                         time(() -> new ObjectMapper()                                                 .readValue(url.openStream(), ObjectNode.class)                                                 .get("info").get("contact").get("email")))); } 

378ms to get 'info.contact.email' with JSONPath
756ms to just download the entire document otherwise
896ms to bluntly map it entirely with Jackson and access a specific field

Answer 2

Just want to update for 2019. I found the following easiest to impl:

//json can be file or String JsonNode parent= new ObjectMapper().readTree(json); String content = parent.path("contentType").asText(); 

I would suggest to use path instead of get as get throws a NPE, where path returns with a default 0 or "", which is safer to work with if setting up the parsing correctly for 1st time.

My $0.02