How to convert a JSON string to a Map with Jackson JSON

Question

I'm trying to do something like this but it doesn't work:

Map<String, String> propertyMap = new HashMap<String, String>();

propertyMap = JacksonUtils.fromJSON(properties, Map.class);

But the IDE says:

Unchecked assignment Map to Map<String,String>

What's the right way to do this? I'm only using Jackson because that's what is already available in the project, is there a native Java way of converting to/from JSON?

In PHP I would simply json_decode($str) and I'd get back an array. I need basically the same thing here.

Answer 1

[Update Sept 2020] Although my original answer here, from many years ago, seems to be helpful and is still getting upvotes, I now use the GSON library from Google, which I find to be more intuitive.

I've got the following code:

public void testJackson() throws IOException {  
    ObjectMapper mapper = new ObjectMapper(); 
    File from = new File("albumnList.txt"); 
    TypeReference<HashMap<String,Object>> typeRef 
            = new TypeReference<HashMap<String,Object>>() {};

    HashMap<String,Object> o = mapper.readValue(from, typeRef); 
    System.out.println("Got " + o); 
}   

It's reading from a file, but mapper.readValue() will also accept an InputStream and you can obtain an InputStream from a string by using the following:

new ByteArrayInputStream(astring.getBytes("UTF-8")); 

There's a bit more explanation about the mapper on my blog.

Answer 2

Try TypeFactory. Here's the code for Jackson JSON (2.8.4).

Map<String, String> result;
ObjectMapper mapper;
TypeFactory factory;
MapType type;

factory = TypeFactory.defaultInstance();
type    = factory.constructMapType(HashMap.class, String.class, String.class);
mapper  = new ObjectMapper();
result  = mapper.readValue(data, type);

Here's the code for an older version of Jackson JSON.

Map<String, String> result = new ObjectMapper().readValue(
    data, TypeFactory.mapType(HashMap.class, String.class, String.class));

Answer 3

Warning you get is done by compiler, not by library (or utility method).

Simplest way using Jackson directly would be:

HashMap<String,Object> props;

// src is a File, InputStream, String or such
props = new ObjectMapper().readValue(src, new TypeReference<HashMap<String,Object>>() {});
// or:
props = (HashMap<String,Object>) new ObjectMapper().readValue(src, HashMap.class);
// or even just:
@SuppressWarnings("unchecked") // suppresses typed/untype mismatch warnings, which is harmless
props = new ObjectMapper().readValue(src, HashMap.class);

Utility method you call probably just does something similar to this.

Answer 4

ObjectReader reader = new ObjectMapper().readerFor(Map.class);

Map<String, String> map = reader.readValue("{\"foo\":\"val\"}");

Note that reader instance is Thread Safe.

Answer 5

Converting from String to JSON Map:

Map<String,String> map = new HashMap<String,String>();

ObjectMapper mapper = new ObjectMapper();

map = mapper.readValue(string, HashMap.class);

Answer 6

JavaType javaType = objectMapper.getTypeFactory().constructParameterizedType(Map.class, Key.class, Value.class);
Map<Key, Value> map=objectMapper.readValue(jsonStr, javaType);

i think this will solve your problem.

Answer 7

The following works for me:

Map<String, String> propertyMap = getJsonAsMap(json);

where getJsonAsMap is defined like so:

public HashMap<String, String> getJsonAsMap(String json)
{
    try
    {
        ObjectMapper mapper = new ObjectMapper();
        TypeReference<Map<String,String>> typeRef = new TypeReference<Map<String,String>>() {};
        HashMap<String, String> result = mapper.readValue(json, typeRef);

        return result;
    }
    catch (Exception e)
    {
        throw new RuntimeException("Couldnt parse json:" + json, e);
    }
}

Note that this will fail if you have child objects in your json (because they're not a String, they're another HashMap), but will work if your json is a key value list of properties like so:

{
    "client_id": "my super id",
    "exp": 1481918304,
    "iat": "1450382274",
    "url": "http://www.example.com"
}

Answer 8

Using Google's Gson

Why not use Google's Gson as mentioned in here?

Very straight forward and did the job for me:

HashMap<String,String> map = new Gson().fromJson( yourJsonString, new TypeToken<HashMap<String, String>>(){}.getType());

Answer 9

just wanted to give a Kotlin answer

val propertyMap = objectMapper.readValue<Map<String,String>>(properties, object : TypeReference<Map<String, String>>() {})

Answer 10

Here is the generic solution to this problem.

public static <K extends Object, V extends Object> Map<K, V> getJsonAsMap(String json, K key, V value) {
    try {
      ObjectMapper mapper = new ObjectMapper();
      TypeReference<Map<K, V>> typeRef = new TypeReference<Map<K, V>>() {
      };
      return mapper.readValue(json, typeRef);
    } catch (Exception e) {
      throw new RuntimeException("Couldnt parse json:" + json, e);
    }
  }

Hope someday somebody would think to create a util method to convert to any Key/value type of Map hence this answer :)