Convert JSON String to Java Map with Jackson

Convert JSON String to Java Map with Jackson

In this tutorial, we'll be taking a look at how to convert a JSON String into a Java Map using Jackson, an extremely popular data-binding library for Java.

Specifically, we'll be working with this JSON object:

{
"Task 1" : "In_Progress",
"Task 2" : "Done",
"Task 3" : "Planned"
}

Since we're working with an external library, let's add the required dependency. If you're using Maven, you can add it to your project with:

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.11.3</version>
</dependency>

Or if you're using Gradle, you can add:

implementation group: 'com.fasterxml.jackson.core', name: 'jackson-databind', version: '2.11.3'

Convert JSON String to Java Map

For our task status labels, let's define an Enum. We'll have a Map<String, TASK_STATUS> pair, though, you can go with any type here, really:

enum TASK_STATUS {
    In_Progress, Done, Planned
}

Naturally, Jackson's key class is the ObjectMapper class - the main API for object-related data-binding of the library.

Much like you'd map JSON values to other types, to convert JSON contents into a Java Map, you'll use the readValue() method of the ObjectMapper instance, which deserializes it into the provided class reference:

String json = "{\n" +
               "\"Task 1\" : \"In_Progress\",\n" +
               "\"Task 2\" : \"Done\",\n" +
               "\"Task 3\" : \"Planned\"\n" +
               "}";

// ObjectMapper instantiation
ObjectMapper objectMapper = new ObjectMapper();

// Deserialization into a Map
Map<String, TASK_STATUS> result = objectMapper.readValue(json, HashMap.class);
        
// Printing the results
System.out.println(result.entrySet());

We've chucked the json contents into the readValue() method, and since it contains JSON that can be deserialized into a Map, given the key-value pairings, told Jackson to deserialize into a HashMap. Running this code results in:

[Task 2=Done, Task 1=In_Progress, Task 3=Planned]

Now, since HashMaps do not preserve the order of insertion, you might want to use a LinkedHashMap instead, if the order of insertion is important to you:

Map<String, TASK_STATUS> result = objectMapper.readValue(json, LinkedHashMap.class);

This results in:

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[Task 1=In_Progress, Task 2=Done, Task 3=Planned]

An alternative to specifying the JavaType directly would be to use the TypeReference class from Jackson:

Map<String, TASK_STATUS> result = objectMapper.readValue(json, 
            new TypeReference<LinkedHashMap>(){});

Now, printing this map will also result in:

[Task 1=In_Progress, Task 2=Done, Task 3=Planned]

Both of these construct an object by calling the exact same deserialization process. So the only difference between these two calls is whether you're making a static (JavaType) or dynamic (TypeReference) reference to the type.

Last Updated: November 28th, 2021
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David LandupAuthor

Entrepreneur, Software and Machine Learning Engineer, with a deep fascination towards the application of Computation and Deep Learning in Life Sciences (Bioinformatics, Drug Discovery, Genomics), Neuroscience (Computational Neuroscience), robotics and BCIs.

Great passion for accessible education and promotion of reason, science, humanism, and progress.

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