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


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.

12/1/2017 4:13:13 PM

Accepted Answer

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.

8/8/2016 10:00:55 AM

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));

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.


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

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

Note that reader instance is Thread Safe.


Converting from String to JSON Map:

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

ObjectMapper mapper = new ObjectMapper();

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

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.


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