Jackson is the JSON serializer/deserializer of choice. Here are some tiny examples, modified from our friend at mkyong. The following JARs were used to do this:

• jackson-core-asl-1.9.2.jar
• jackson-mapper-asl-1.9.2.jar

There are two ways of using Jackson to serialize...

User test

POJO has accessors (getters and setters) that are consumed by Jackson.

User.java:

package com.etretatlogiciels.examples;

import java.util.List;
import java.util.ArrayList;

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" );
        }
    };

    // in order for UserTest to work, these accessors must be present...
    public int getAge() { return age; }
    public void setAge( int age ) { this.age = age; }
    public String getName() { return name; }
    public void setName( String name ) { this.name = name; }
    public List< String > getMessages() { return messages; }
    public void setMessages( List< String > messages ) { this.messages = messages; }
}

UserTest.java:

package com.etretatlogiciels.examples;

import java.io.IOException;

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

public class UserTest
{
    public static void main( String[] args )
    {
        User         user   = new User();
        ObjectMapper mapper = new ObjectMapper();

        try
        {
            // convert user object to JSON string and display to console...
            System.out.println( mapper.defaultPrettyPrintingWriter().writeValueAsString( user ) );
        }
        catch( JsonGenerationException e )
        {
            e.printStackTrace();
        }
        catch( JsonMappingException e )
        {
            e.printStackTrace();
        }
        catch( IOException e )
        {
            e.printStackTrace();
        }
    }
}

Output:

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

Customer test

POJO has no accessors, but annotations instruct Jackson to use the field directly and not to use fields that are null.

Customer.java:

package com.etretatlogiciels.examples;

import org.codehaus.jackson.annotate.JsonAutoDetect;
import org.codehaus.jackson.annotate.JsonAutoDetect.Visibility;
import org.codehaus.jackson.map.annotate.JsonSerialize;

@JsonSerialize(  include            = JsonSerialize.Inclusion.NON_NULL )
@JsonAutoDetect( fieldVisibility    = Visibility.ANY,
             getterVisibility   = Visibility.NONE,
             isGetterVisibility = Visibility.NONE,
             setterVisibility   = Visibility.NONE )
public class Customer
{
    String  name;
    Integer id;
}

CustomerTest.java:

package com.etretatlogiciels.examples;

import java.io.IOException;

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

public class CustomerTest
{
    public static void main( String[] args )
    {
        Customer     customer = new Customer();
        ObjectMapper mapper   = new ObjectMapper();

        try
        {
            customer.name = "mkyong";
            customer.id   = 99;
            System.out.println( mapper.defaultPrettyPrintingWriter().writeValueAsString( customer ) );

            customer.name = "Jack-and-the-Beanstalk";
            customer.id   = null;
            System.out.println( mapper.defaultPrettyPrintingWriter().writeValueAsString( customer ) );
        }
        catch( JsonGenerationException e )
        {
            e.printStackTrace();
        }
        catch( JsonMappingException e )
        {
            e.printStackTrace();
        }
        catch( IOException e )
        {
            e.printStackTrace();
        }
    }
}

Output:

    {
      "name" : "mkyong",
      "id" : 99
    }
    {
      "name" : "Jack-and-the-Beanstalk"
    }

Annotations

@JsonSerialize

include = JsonSerialize.Inclusion.NON_NULL, don't serialize any fields in this POJO that are null

@JsonAutoDetect

fieldVisibility = Visibility.ANY, sees any field in POJO

getterVisibility = Visibility.ANY, don't use (serialize results from) this POJO's getters

siGetterVisibility = Visibility.ANY, don't use this POJO's "is" getters

setterVisibility = Visibility.ANY, don't use this POJO's setters

When you do something like this:

ObjectMapper mapper = new ObjectMapper();
Pojo         pojo   = mapper.readValue( jsonString, Pojo.class );

...and you get an except with the message (in this note's title), you can annotate with @JsonIgnoreProperties, but this is likely not the problem. It's that you probably don't have get-accessors ("getters") in your POJO. These are used by Jackson.