Read/Write a Raw JSON, Array-Like JSON, and Map-Like JSON File as an Object

In this article, take a look at a tutorial that explains how to read/write a raw JSON, array-like JSON, and map-like JSON file as an object.

Anghel Leonard

CORE ·

Apr. 22, 20 · Tutorial

28.3K Views

In this article, you have a supersonic guide for reading/writing a JSON file via JSON-B, Jackson, and Gson.

Let's start with three text files that represent typical JSON-like mappings:

Raw JSON vs array-like JSON vs map-like JSON

In melons_raw.json , we have a JSON entry per line. Each line is a piece of JSON that's independent of the previous line but has the same schema. In melons_array.json , we have a JSON array, and in melons_map.json , we have a JSON that fits well in a Java Map .

For each of these files, we have a Path , as follows:

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Path pathArray = Paths.get("melons_array.json");
Path pathMap = Paths.get("melons_map.json");
Path pathRaw = Paths.get("melons_raw.json");

Now let's take a look at three dedicated libraries for reading the contents of these files as Melon instances:

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public class Melon {
  private String type;
  private int weight;
      
  // getters and setters omitted for brevity
}

Using JSON-B

Java EE 8 comes with a JAXB-like, declarative JSON binding called JSON-B (JSR-367). JSON-B is consistent with JAXB and other Java EE/SE APIs. Jakarta EE takes Java EE 8 JSON (P and B) to the next level. Its API is exposed via the javax.json.bind.Jsonb and javax.json.bind.JsonbBuilder classes:

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Jsonb jsonb = JsonbBuilder.create();

For deserialization, we use Jsonb.fromJson() , while, for serialization, we use Jsonb.toJson() :

Let's read melons_array.json as an array of Melon :

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Melon[] melonsArray = jsonb.fromJson(Files.newBufferedReader(
  pathArray, StandardCharsets.UTF_8), Melon[].class);

Let's read melons_array.json as a List of Melon :

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List<Melon> melonsList = jsonb.fromJson(Files.newBufferedReader(
    pathArray, StandardCharsets.UTF_8), ArrayList.class);

Let's read melons_map.json as a Map of Melon :

    Java
   
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Map<String, Melon> melonsMap = jsonb.fromJson(Files.newBufferedReader(
  pathMap, StandardCharsets.UTF_8), HashMap.class);

Let's read melons_raw.json line by line into a Map :

    Java
   
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Map<String, String> stringMap = new HashMap<>();
   
try (BufferedReader br = Files.newBufferedReader(pathRaw, StandardCharsets.UTF_8)) {
    
  String line;
  while ((line = br.readLine()) != null) {
    stringMap = jsonb.fromJson(line, HashMap.class);
    System.out.println("Current map is: " + stringMap);
  }
}

Let's read melons_raw.json line by line into a Melon :

    Java
   
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try (BufferedReader br = Files.newBufferedReader(pathRaw, StandardCharsets.UTF_8)) {
  
  String line;
  while ((line = br.readLine()) != null) {
    Melon melon = jsonb.fromJson(line, Melon.class);
    System.out.println("Current melon is: " + melon);
  }
}

Let's write an object into a JSON file ( melons_output.json ):

    Java
   
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Path path = Paths.get("melons_output.json");
jsonb.toJson(melonsMap, Files.newBufferedWriter(path, StandardCharsets.UTF_8,
   StandardOpenOption.CREATE, StandardOpenOption.WRITE));

The complete code is available on GitHub.

Using Jackson

Jackson is a popular and fast library dedicated to processing (serializing/deserializing) JSON data. The Jackson API relies on com.fasterxml.jackson.databind.ObjectMapper . Let's go over the preceding examples again, but this time using Jackson:

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ObjectMapper mapper = new ObjectMapper();

For deserialization, we use ObjectMapper.readValue() , while for serialization, we use ObjectMapper.writeValue() :

Let's read melons_array.json as an array of Melon :

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Melon[] melonsArray = mapper.readValue(Files.newBufferedReader(
  pathArray, StandardCharsets.UTF_8), Melon[].class);

Let's read melons_array.json as a List of Melon :

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List<Melon> melonsList = mapper.readValue(Files.newBufferedReader(
  pathArray, StandardCharsets.UTF_8), ArrayList.class);

Let's read melons_map.json as a Map of Melon :

    Java
   
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Map<String, Melon> melonsMap = mapper.readValue(Files.newBufferedReader(
  pathMap, StandardCharsets.UTF_8), HashMap.class);

Let's read melons_raw.json line by line into a Map :

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Map<String, String> stringMap = new HashMap<>();
  
try (BufferedReader br = Files.newBufferedReader(pathRaw, StandardCharsets.UTF_8)) {
   
  String line;
  while ((line = br.readLine()) != null) {
    stringMap = mapper.readValue(line, HashMap.class);
    System.out.println("Current map is: " + stringMap);
  }
}

Let's read melons_raw.json line by line into a Melon :

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try (BufferedReader br = Files.newBufferedReader(pathRaw, StandardCharsets.UTF_8)) {
     
  String line;
  while ((line = br.readLine()) != null) {
    Melon melon = mapper.readValue(line, Melon.class);
    System.out.println("Current melon is: " + melon);
  }
}

Let's write an object into a JSON file ( melons_output.json ):

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Path path = Paths.get("melons_output.json");
   
mapper.writeValue(Files.newBufferedWriter(path, StandardCharsets.UTF_8,
  StandardOpenOption.CREATE, StandardOpenOption.WRITE), melonsMap);

The complete code is available on GitHub.

Using Gson

Gson is another fast library dedicated to processing (serializing/deserializing) JSON data. In a Maven project, it can be added as a dependency in pom.xml . Its API relies on a class name, com.google.gson.Gson :

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Gson gson = new Gson();

Let's read melons_array.json as an array of Melon :

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Melon[] melonsArray = gson.fromJson(
  Files.newBufferedReader(pathArray, StandardCharsets.UTF_8), Melon[].class);

Let's read melons_array.json as a List of Melon :

    Java
   
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List<Melon> melonsList = gson.fromJson(Files.newBufferedReader(
  pathArray, StandardCharsets.UTF_8), ArrayList.class);

Let's read melons_map.json as a Map of Melon :

    Java
   
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Map<String, Melon> melonsMap = gson.fromJson(Files.newBufferedReader(
  pathMap, StandardCharsets.UTF_8), HashMap.class);
    
// or, as TypeTolen, new TypeToken<HashMap<String, Melon>>() {}.getType()

Let's read melons_raw.json line by line into a Map :

    Java
   
xxxxxxxxxx

Map<String, String> stringMap = new HashMap<>();
  
try (BufferedReader br = Files.newBufferedReader(pathRaw, StandardCharsets.UTF_8)) {
    
  String line;
  while ((line = br.readLine()) != null) {
    stringMap = gson.fromJson(line, HashMap.class);
    System.out.println("Current map is: " + stringMap);
  }
}

Let's read melons_raw.json line by line into a Melon :

    Java
   
xxxxxxxxxx

try (BufferedReader br = Files.newBufferedReader(pathRaw, StandardCharsets.UTF_8)) {
    
  String line;
  while ((line = br.readLine()) != null) {
    Melon melon = gson.fromJson(line, Melon.class);
    System.out.println("Current melon is: " + melon);
  }
}

Let's write an object into a JSON file ( melons_output.json ):

    Java
   
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Path path = Paths.get("melons_output.json");
    
gson.toJson(melonsMap, Files.newBufferedWriter(path, StandardCharsets.UTF_8,
   StandardOpenOption.CREATE, StandardOpenOption.WRITE));

The complete code is available on GitHub.

If you enjoyed this article, then I'm sure you will love my book, Java Coding Problems, which has an entire chapter dedicated to Java I/O - Paths, files, buffers, scanning, and formatting. Check it out!

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Read/Write a Raw JSON, Array-Like JSON, and Map-Like JSON File as an Object

Using JSON-B

Using Jackson

Using Gson

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