How to convert input string to json string or json object using jackson in java

Question

How to convert input string to json string or json object using jackson in java.

Thanks in advance

Answer 1

This is documented on mkyong and quoted here:

Jackson is a High-performance JSON processor Java library. In this tutorial, we show you how to use Jackson’s data binding to convert Java object to / from JSON.

For object/json conversion, you need to know following two methods :

//1. Convert Java object to JSON format
ObjectMapper mapper = new ObjectMapper();
mapper.writeValue(new File("c:\\user.json"), user);
//2. Convert JSON to Java object
ObjectMapper mapper = new ObjectMapper();
User user = mapper.readValue(new File("c:\\user.json"), User.class);

Note: Both writeValue() and readValue() has many overloaded methods to support different type of inputs and outputs. Make sure check it out.

1. Jackson Dependency Jackson contains 6 separate jars for different purpose, check here. In this case, you only need “jackson-mapper-asl” to handle the conversion, just declares following dependency in your pom.xml

   <repositories>
       <repository>
           <id>codehaus</id>
           <url>http://repository.codehaus.org/org/codehaus</url>
       </repository>
   </repositories>
   
   <dependencies>
       <dependency>
           <groupId>org.codehaus.jackson</groupId>
           <artifactId>jackson-mapper-asl</artifactId>
           <version>1.8.5</version>
       </dependency>
   </dependencies>
   

   For non-maven user, just get the Jackson library here.

2. POJO

   An user object, initialized with some values. Later use Jackson to convert this object to / from JSON.

   package com.mkyong.core;
   
   import java.util.ArrayList;
   import java.util.List;
   
   public class User {
   
       private int age = 29;
       private String name = "mkyong";
       private List<String> messages = new ArrayList<String>() {
           {
               add("msg 1");
               add("msg 2");
               add("msg 3");
           }
       };
   
       //getter and setter methods
   
       @Override
       public String toString() {
           return "User [age=" + age + ", name=" + name + ", " +
                   "messages=" + messages + "]";
       }
   }
   

3. Java Object to JSON Convert an “user” object into JSON formatted string, and save it into a file “user.json“.

   package com.mkyong.core;
   
   import java.io.File;
   import java.io.IOException;
   import org.codehaus.jackson.JsonGenerationException;
   import org.codehaus.jackson.map.JsonMappingException;
   import org.codehaus.jackson.map.ObjectMapper;
   
   public class JacksonExample {
       public static void main(String[] args) {
   
       User user = new User();
       ObjectMapper mapper = new ObjectMapper();
   
       try {
   
           // convert user object to json string, and save to a file
           mapper.writeValue(new File("c:\\user.json"), user);
   
           // display to console
           System.out.println(mapper.writeValueAsString(user));
   
       } catch (JsonGenerationException e) {
   
           e.printStackTrace();
   
       } catch (JsonMappingException e) {
   
           e.printStackTrace();
   
       } catch (IOException e) {
   
           e.printStackTrace();
   
       }
   
     }
   
   }
   

   Output

   {"age":29,"messages":["msg 1","msg 2","msg 3"],"name":"mkyong"}
   

   Note Above JSON output is hard to read. You can enhance it by enable the pretty print feature.

4. JSON to Java Object

   Read JSON string from file “user.json“, and convert it back to Java object.

   package com.mkyong.core;
   
   import java.io.File;
   import java.io.IOException;
   import org.codehaus.jackson.JsonGenerationException;
   import org.codehaus.jackson.map.JsonMappingException;
   import org.codehaus.jackson.map.ObjectMapper;
   
   public class JacksonExample {
       public static void main(String[] args) {
   
       ObjectMapper mapper = new ObjectMapper();
   
       try {
   
           // read from file, convert it to user class
           User user = mapper.readValue(new File("c:\\user.json"), User.class);
   
           // display to console
           System.out.println(user);
   
       } catch (JsonGenerationException e) {
   
           e.printStackTrace();
   
       } catch (JsonMappingException e) {
   
           e.printStackTrace();
   
       } catch (IOException e) {
   
           e.printStackTrace();
   
       }
   
     }
   
   }
   

Output

    User [age=29, name=mkyong, messages=[msg 1, msg 2, msg 3]]

Answer 2

In java we can convert String to json by lot of methods.You can use collection for this purpose HashMap which gives you value in {key:value} pair which could be useful for you and if you are using Spring this can be helpful for you.

   //****************** update The user Info**********************//
        @RequestMapping(value = "/update/{id}",  method = RequestMethod.POST, consumes = MediaType.APPLICATION_JSON_VALUE)
        public @ResponseBody
        Status updateMsBuddy(@RequestBody MsBuddyInfo msBuddyInfo){

            try{
                if(msBuddyService.updateMsBuddy(msBuddyInfo))
                return new Status(1, "MsBuddyInfo updated Successfully",+msBuddyInfo.getId());
                else
                    return new Status(1, "unable to updated Profile",+msBuddyInfo.getId());
            }
            catch(Exception e){

                return new Status(0,e.toString());
            }
        }

Answer 3

Very nice tutorial from journaldev Jackson-JSON processing

you can use com.fasterxml.jackson.databind.ObjectMapper class of jackson API to read or wirte a string formatted json object.