Basic use of JSONPath in Java

问题:

I have JSON as a string and a JSONPath as a string. I'd like to query the JSON with the JSON path, getting the resulting JSON as a string.

I gather that Jayway's json-path is the standard. The online API, however, doesn't have have much relation to the actual library you get from Maven. GrepCode's version roughly matches up though.

It seems like I ought to be able to do:

String originalJson; //these are initialized to actual data
String jsonPath;
String queriedJson = JsonPath.<String>read(originalJson, jsonPath);

The problem is that read returns whatever it feels most appropriate based on what the JSONPath actually finds (e.g. a List<Object>, String, double, etc.), thus my code throws an exception for certain queries. It seems pretty reasonable to assume that there'd be some way to query JSON and get JSON back; any suggestions?

回答1:

There definitely exists a way to query Json and get Json back using JsonPath. See example below:

 String jsonString = "{\"delivery_codes\": [{\"postal_code\": {\"district\": \"Ghaziabad\", \"pin\": 201001, \"pre_paid\": \"Y\", \"cash\": \"Y\", \"pickup\": \"Y\", \"repl\": \"N\", \"cod\": \"Y\", \"is_oda\": \"N\", \"sort_code\": \"GB\", \"state_code\": \"UP\"}}]}";
 String jsonExp = "$.delivery_codes";
 JsonNode pincodes = JsonPath.read(jsonExp, jsonString, JsonNode.class);
 System.out.println("pincodesJson : "+pincodes);

The output of the above will be inner Json.

[{"postal_code":{"district":"Ghaziabad","pin":201001,"pre_paid":"Y","cash":"Y","pickup":"Y","repl":"N","cod":"Y","is_oda":"N","sort_code":"GB","state_code":"UP"}}]

Now each individual name/value pairs can be parsed by iterating the List (JsonNode) we got above.

for(int i = 0; i< pincodes.size();i++){
    JsonNode node = pincodes.get(i);
    String pin = JsonPath.read("$.postal_code.pin", node, String.class);
    String district = JsonPath.read("$.postal_code.district", node, String.class);
    System.out.println("pin :: " + pin + " district :: " + district );
}

The output will be:

pin :: 201001 district :: Ghaziabad

Depending upon the Json you are trying to parse, you can decide whether to fetch a List or just a single String/Long value.

Hope it helps in solving your problem.

回答2:

Java JsonPath API found at jayway JsonPath might have changed a little since all the above answers/comments. Documentation too. Just follow the above link and read that README.md, it contains some very clear usage documentation IMO.

Basically, as of current latest version 2.2.0 of the library, there are a few different ways of achieving what's been requested here, such as:

Pattern:
--------
String json = "{...your JSON here...}";
String jsonPathExpression = "$...your jsonPath expression here..."; 
J requestedClass = JsonPath.parse(json).read(jsonPathExpression, YouRequestedClass.class);

Example:
--------
// For better readability:  {"store": { "books": [ {"author": "Stephen King", "title": "IT"}, {"author": "Agatha Christie", "title": "The ABC Murders"} ] } }
String json = "{\"store\": { \"books\": [ {\"author\": \"Stephen King\", \"title\": \"IT\"}, {\"author\": \"Agatha Christie\", \"title\": \"The ABC Murders\"} ] } }";
String jsonPathExpression = "$.store.books[?(@.title=='IT')]"; 
JsonNode jsonNode = JsonPath.parse(json).read(jsonPathExpression, JsonNode.class);

And for reference, calling 'JsonPath.parse(..)' will return an object of class 'JsonContent' implementing some interfaces such as 'ReadContext', which contains several different 'read(..)' operations, such as the one demonstrated above:

/**
 * Reads the given path from this context
 *
 * @param path path to apply
 * @param type    expected return type (will try to map)
 * @param <T>
 * @return result
 */
<T> T read(JsonPath path, Class<T> type);

Hope this help anyone.

回答3:

For those of you wondering why some of these years-old answers aren't working, you can learn a lot from the test cases.

As of September 2018, here's how you can get Jackson JsonNode results:

Configuration jacksonConfig = Configuration.builder()
                              .mappingProvider( new JacksonMappingProvider() )
                              .jsonProvider( new JacksonJsonProvider() )
                              .build();

JsonNode node = JsonPath.using( jacksonConfig ).parse(jsonString);

回答4:

Check out the jpath API. It's xpath equivalent for JSON Data. You can read data by providing the jpath which will traverse the JSON data and return the requested value.

This Java class is the implementation as well as it has example codes on how to call the APIs.

https://github.com/satyapaul/jpath/blob/master/JSONDataReader.java

Readme -

https://github.com/satyapaul/jpath/blob/master/README.md