How to exclude null-value fields when using Flexjson?

Question

I am serializing a class like this into JSON using Flexjson:

public class Item {
    private Long id;
    private String name;
    private String description;
    ...

    // Getters and setters
    ...
}

Many of the Item fields can be null (e.g., description). Consequently, when such an Item object is serialized using Flexjson, I get the following JSON:

 {"id":62,"name":"Item A","description":null,...}

Since, as I already mentioned, an Item object may contain many null-value fields, the outcoming JSON is longer than effectively needed. This is in so far a problem, because I would like to send the generated JSON from a web server to a mobile client over a wireless connection via WiFi, 3G, EDGE or GPRS (i.e., more bandwidth is required, which results in less speed).

Therefore, I wanted to ask how it is possible to (efficiently) exclude null-value attributes using Flexjson?

Thanks!

Answer 1

You can use the following transformer :

import flexjson.transformer.AbstractTransformer;

public class ExcludeTransformer extends AbstractTransformer {

  @Override
  public Boolean isInline() {
      return true;
  }

  @Override
  public void transform(Object object) {
      // Do nothing, null objects are not serialized.
      return;
  }
}

with the following usage :

new JSONSerializer().transform(new ExcludeTransformer(), void.class).serialize(yourObject)

Note that all null fields will be excluded.

Adding the Transformer by Path (vs by Class) is not supported as FlexJSON forces TypeTransformer for null values :

JSONContext.java : line 95 :

private Transformer getPathTransformer(Object object) {
    if (null == object) return getTypeTransformer(object);
    return pathTransformerMap.get(path);
}

Answer 2

I am a newbie,i had same problem and could not find any solution on source forge so i used regular expression to remove all the nulls from JSON String

/**
 * This Function removes all the key:value pairs from the Json String for which the value equals null
 * @param jsonStringWithNullKeys
 * @return jsonStringWithoutNullKeys
 */
public static String getJsonStringWithoutNullKeys(String jsonStringWithNullKeys)
{
    Pattern p = Pattern.compile( "([,]?\"[^\"]*\":null[,]?)+" );
    Matcher m = p.matcher( jsonStringWithNullKeys );
    StringBuffer newString = new StringBuffer( jsonStringWithNullKeys.length() );

    while( m.find() )
    {
        if( m.group().startsWith( "," ) & m.group().endsWith( "," ) ) m.appendReplacement( newString, "," );
        else
            m.appendReplacement( newString, "" );
    }
    m.appendTail( newString );
    return newString.toString();
}

Answer 3

I haven't tried out your situation exactly, but I believe the following should solve your problem:

Item item;
// Assign item here
JSONSerializer json = new JSONSerializer();
if ( item.description != null ) {
  json.exclude( field );
}
return json.serialize(item);

Clearly, you'd probably access the description field using a getter. Additionally, you might want to iterate your instance fields using reflection to exclude the null-fields.