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]]

A slight variation on Richards answer but readTree can take a string so you can simplify it to:

ObjectMapper mapper = new ObjectMapper();
JsonNode actualObj = mapper.readTree("{\"k1\":\"v1\"}");

Try this,

You can't create a new string like your doing.

    String string = "{\"id\":100,\"firstName\":\"Adam\"}";
    Student student = mapper.readValue(string, Student.class);

To convert your object in JSON with Jackson:

import com.fasterxml.jackson.databind.ObjectMapper; 
import com.fasterxml.jackson.databind.ObjectWriter; 

ObjectWriter ow = new ObjectMapper().writer().withDefaultPrettyPrinter();
String json = ow.writeValueAsString(object);

Try,

JSONObject object = ...;
ObjectMapper mapper = new ObjectMapper();
String jsonString = mapper.writeValueAsString(object);

The code should pass an instance of Type A to the writeValueAsString method not the actual class.

The ObjectMapper will be able to determine the instance's type via reflection, however if you don't pass an instance it cannot determine the field value's to place in the generated JSON.

Also be sure to catch or throw the appropriate exceptions.

public void save() {

try {
        A a = new A();
        ObjectMapper mapper = new ObjectMapper();
        String temp = mapper.writeValueAsString(a);

    } catch (JsonGenerationException e) {
       e.printStackTrace();
    } catch (JsonMappingException e) {
       e.printStackTrace();
    } catch (IOException e) {
       e.printStackTrace();
    }

}

You should also have acessors for the fields on class A and class B. NOTICE I included B in the previous sentence, since Jackson must be able to map all fields on the instance we provide to it.

public A   {
    private List<B> b;
    private int c;

    private B getB(){ return b;}
    private void setB(B b){this.b = b;}
    private int getC(){return c;}
    private void setC(int c){this.c = c;}
}

Here is a complete working example that I have tested.

import java.io.IOException;
import java.util.List;

import org.codehaus.jackson.JsonGenerationException;
import org.codehaus.jackson.map.JsonMappingException;
import org.codehaus.jackson.map.ObjectMapper;

public class A {

    private List<B> b;
    private int c;

    public List<B> getB() {
        return b;
    }

    public void setB(List<B> b) {
        this.b = b;
    }

    public int getC() {
        return c;
    }

    public void setC(int c) {
        this.c = c;
    }

    public static void main(String[] args) throws JsonGenerationException, JsonMappingException, IOException {
        A a = new A();
        ObjectMapper mapper = new ObjectMapper();
        String temp = mapper.writeValueAsString(a);
        System.out.println(temp);   
    }
}

class B{


}

If this simple example does not work the jackson-mapper-asl.jar file is most likely not on the build path. Download the file from here and place it somewhere on your local machine.

Next via Eclipse, Right Click on your Project and select properties.

Then select Build Path > Libraries > Add External Jars. Locate the jar and then hit OK.