Jackson JSON parser Quick Examples

Followings are quick getting started examples of using Jackson API for JSON processing. The examples shows the basic data-binding capabilities of Jackson's ObjectMapper class.

Maven dependency

pom.xml

<dependency>
   <groupId>com.fasterxml.jackson.core</groupId>
   <artifactId>jackson-databind</artifactId>
   <version>2.9.2</version>
</dependency>

• Simple Java Object Conversion

  To Json:

     ObjectMapper om = new ObjectMapper();
     Map<String, Integer> map = Map.of("one", 1, "two", 2);
     String s = om.writeValueAsString(map);
     System.out.println(s);
  

  {"one":1,"two":2}
  

  To Java Object:

     ObjectMapper om = new ObjectMapper();
     String jsonString = "{\"bananas\":2,\"apple\":5}";
     Map<String, Integer> map = om.readValue(jsonString, Map.class);
     System.out.println(map);
     System.out.println(map.getClass());
  

  {bananas=2, apple=5}
  class java.util.LinkedHashMap
  

  
  
  

• POJO Conversion

  public class MyObject {
    private int intVal;
    private String StringVal;
    private List<String> list;
      .............
  }

  To Json:

     MyObject pojo = new MyObject();
     pojo.setIntVal(3);
     pojo.setStringVal("test string");
     pojo.setList(List.of("item1", "item2"));
     ObjectMapper om = new ObjectMapper();
     String s = om.writeValueAsString(pojo);
     System.out.println(s);
  

  {"intVal":3,"list":["item1","item2"],"stringVal":"test string"}
  

  To Pojo:

     String s = "{\"list\":[\"item3\",\"item4\"],\"stringVal\":\"test string2\",\"intVal\":5}";
     ObjectMapper om = new ObjectMapper();
     MyObject obj = om.readValue(s, MyObject.class);
     System.out.println(obj);
  

  MyObject{intVal=3, StringVal='test string', list=[item1, item2]}
  

  
  
  

• Conversion Involving Java Generics

  As seen in the first example above, Jackson can handle generics for simple types. Let's try generics of a POJO:

  To Json:

     MyObject pojo = new MyObject();
     pojo.setIntVal(3);
     pojo.setStringVal("test string");
     pojo.setList(List.of("item1", "item2"));
     List<MyObject> list = List.of(pojo);
     
     ObjectMapper om = new ObjectMapper();
     String s = om.writeValueAsString(list);
     System.out.println(s);
  

  [{"intVal":3,"list":["item1","item2"],"stringVal":"test string"}]
  

  To Pojo:

     String s = "[{\"intVal\":3,\"list\":[\"item1\",\"item2\"],\"stringVal\":\"test string\"}]";
     ObjectMapper om = new ObjectMapper();
     List<MyObject> list = om.readValue(s, List.class);
     System.out.println(list.get(0));
     System.out.println(list.get(0).getClass());
  

  {intVal=3, list=[item1, item2], stringVal=test string}
  java.lang.ClassCastException: java.base/java.util.LinkedHashMap cannot be cast to com.logicbig.example.MyObject
        at com.logicbig.example.PojoTypeReference.toPojo(PojoTypeReference.java:34)
  

  As seen in above output, Jackson cannot infer the actual type (because of Java type erasure problem) and returns 'list of Map' (LinkedHashMap) instead of 'list of MyObject'. To fix that, we need to indicate actual type by providing Jackson's TypeReference anonymous instance:

     String s = "[{\"intVal\":3,\"list\":[\"item1\",\"item2\"],\"stringVal\":\"test string\"}]";
     ObjectMapper om = new ObjectMapper();
     List<MyObject> list = om.readValue(s, new TypeReference<List<MyObject>>() { });
     System.out.println(list.get(0));
     System.out.println(list.get(0).getClass());
  

  MyObject{intVal=3, StringVal='test string', list=[item1, item2]}
  class com.logicbig.example.MyObject
  

  
  
  

• Reading Writing files

     MyObject myObject = new MyObject();
     myObject.setIntVal(3);
     myObject.setStringVal("test string");
     myObject.setList(List.of("item1", "item2"));
     
     //create temp file
     File tempFile = File.createTempFile("jackson-", ".txt");
     System.out.println("-- saving to file --");
     System.out.println(tempFile);
     
     //write myObject as JSON to file
     ObjectMapper om = new ObjectMapper();
     om.writeValue(tempFile, myObject);
     
     //reading
     System.out.println("-- reading as text from file --");
     String s = new String(Files.readAllBytes(tempFile.toPath()));
     System.out.println(s);
     
     System.out.println("-- reading as Object from file --");
     MyObject myObject2 = om.readValue(tempFile, MyObject.class);
     System.out.println(myObject2);
  

  -- saving to file --
  C:\Users\Joe\AppData\Local\Temp\jackson-3890415751750072504.txt
  -- reading as text from file --
  {"intVal":3,"list":["item1","item2"],"stringVal":"test string"}
  -- reading as Object from file --
  MyObject{intVal=3, StringVal='test string', list=[item1, item2]}
  

  
  
  

• Reading from URL

     URL url = new URL("https://jsonplaceholder.typicode.com/posts/1");
     ObjectMapper om = new ObjectMapper();
     Object o = om.readValue(url, Object.class);
     System.out.println(o);
     System.out.println("Read as: "+o.getClass().getName());
  

  {userId=1, id=1, title=sunt aut facere repellat provident occaecati excepturi optio reprehenderit, body=quia et suscipit
  suscipit recusandae consequuntur expedita et cum
  reprehenderit molestiae ut ut quas totam
  nostrum rerum est autem sunt rem eveniet architecto}
  Read as: java.util.LinkedHashMap
  

  
  
  

Example Project

Dependencies and Technologies Used:

• jackson-databind 2.9.2: General data-binding functionality for Jackson: works on core streaming API.
• JDK 9
• Maven 3.3.9