How can use reflection to get the property names and values from a POJO?

Question

So I'm writing an "POJO to JSON" converter. I want to be able to pass in a List<T> object and convert to JSON.

Hopefully this will make sense

/**
 *
     * NOT COMPLETE!!!  OBVIOUSLY!!!
 */
public abstract class Jsonator<T> implements Serializable {

    private Class<T> entityClass;
    private JSONObject json;
    private JSONArray jsonArray;

    public Jsonator(Class<T> entityClass) {
        this.entityClass = entityClass;
    }

    public void convert(List<T> paObjectList) throws IllegalArgumentException, IllegalAccessException {
        json = new JSONObject();
        jsonArray = new JSONArray();

        try {

            for (Object obj : paObjectList) {
                JSONObject objJson = new JSONObject();

                Class<?> kls = obj.getClass();

                Field[] fields = kls.getFields();
                for (Field field : fields) {
                    objJson.put(field.getName(), (T) field.get(obj));
                }

                jsonArray.add(objJson);
            }

            json.put("results", jsonArray);

        }
        catch (Exception ex) {
        }
    }

    public String error() {
        return "ERROR";
    }

    public String results() {
        if (json != null) {
            return json.toJSONString();
        }

        return "[]";
    }
}

When I get to the Object obj section, my obj is correct. I can debug it and see the name and value of the class.

Let's say that class is this:

public class User {
    private firstName;
    private lastName;

    ... getters....setters....etc...

}

So that now, obj is a Site. OK, I then try to get the field names (firstName, lastName) but the fields object is empty.

What am I doing wrong?

Thanks

EDIT

I got it to work! This is not finished code, but it's doing exactly what I want for now. I've read that Google and Jackson will do this too. If someone can provide a good link on how to selectively pick properties from a POJO, then I'm all ears.

Or better yet, I'd like to know WHY I shouldn't be doing this, this way?

Thanks!

Jsonator (NOT FINISHED)

import java.io.Serializable;
import java.lang.reflect.Field;
import java.util.List;
import org.json.simple.JSONArray;
import org.json.simple.JSONObject;

/**
 *
 * @author Cecil.Meeks
 */
public abstract class Jsonator<T> implements Serializable {

    private Class<T> entityClass;
    private JSONObject json;
    private JSONArray jsonArray;

    public Jsonator(Class<T> entityClass) {
        this.entityClass = entityClass;
    }

    public void convert(List<T> paObjectList) throws IllegalArgumentException, IllegalAccessException {
        json = new JSONObject();
        jsonArray = new JSONArray();

        try {

            for (Object obj : paObjectList) {
                JSONObject objJson = new JSONObject();

                Class<?> kls = obj.getClass();

                Field[] fields = kls.getDeclaredFields();
                for (Field field : fields) {
                    field.setAccessible(true);
                    objJson.put(field.getName(), field.get(obj));
                }

                jsonArray.add(objJson);
            }

            json.put("results", jsonArray);

        }
        catch (SecurityException ex) {
            ex.printStackTrace();
        }
        catch (Exception ex) {
            ex.printStackTrace();
        }
    }

    public String error() {
        return "ERROR";
    }

    public String results() {
        if (json != null) {
            return json.toJSONString();
        }

        return "[]";
    }
}

Site Class

import java.io.Serializable;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Table;
import org.hibernate.annotations.GenericGenerator;

@Entity
@Table(name = "Sites")
public class Site implements Serializable {

    private String siteKey;
    private String site;
    private String siteType;
    private String address1;
    private String address2;
    private String city;
    private String zipCode;
    private String createdBy;
    private String glCode;

    public Site() {
    }

    @Id
    @GenericGenerator(name = "generator", strategy = "guid", parameters = {})
    @GeneratedValue(generator = "generator")
    public String getSiteKey() {
        return siteKey;
    }

    public void setSiteKey(String siteKey) {
        this.siteKey = siteKey;
    }

    @Column(name = "Site", unique = true, length = 125, nullable = false)
    public String getSite() {
        return site;
    }

    public void setSite(String site) {
        this.site = site;
    }

    @Column(name = "SiteType", unique = false, length = 8, nullable = true)
    public String getSiteType() {
        return siteType;
    }

    public void setSiteType(String siteType) {
        this.siteType = siteType;
    }

    @Column(name = "Address1", unique = false, length = 125, nullable = true)
    public String getAddress1() {
        return address1;
    }

    public void setAddress1(String address1) {
        this.address1 = address1;
    }

    @Column(name = "Address2", unique = false, length = 125, nullable = true)
    public String getAddress2() {
        return address2;
    }

    public void setAddress2(String address2) {
        this.address2 = address2;
    }

    @Column(name = "City", unique = false, length = 125, nullable = true)
    public String getCity() {
        return city;
    }

    public void setCity(String city) {
        this.city = city;
    }

    @Column(name = "ZipCode", unique = false, length = 50, nullable = true)
    public String getZipCode() {
        return zipCode;
    }

    public void setZipCode(String zipCode) {
        this.zipCode = zipCode;
    }

    @Column(name = "CreatedBy", unique = false, length = 125, nullable = true)
    public String getCreatedBy() {
        return createdBy;
    }

    public void setCreatedBy(String createdBy) {
        this.createdBy = createdBy;
    }

    @Column(name = "GLCode", unique = false, length = 11, nullable = true)
    public String getGlCode() {
        return glCode;
    }

    public void setGlCode(String glCode) {
        this.glCode = glCode;
    }


}

EXAMPLE

public class SiteJsonator extends Jsonator<Site> {

    public SiteJsonator() {
        super(Site.class);
    }

}

@Controller
@RequestMapping(value = "/sites")
public class SitesController {

    @Autowired
    private SiteService siteService;

    @RequestMapping(value = "/", method = RequestMethod.GET, headers = "Accept=application/json")
    @ResponseBody
    public String index(ModelMap map) {

        SiteJsonator list  = new SiteJsonator();;
        try {
            list.convert(siteService.getAll());
            return list.results();
        }
        catch (Exception ex) {
            return list.error();
        } 
    }
}

UPDATE 2

Here is the better Jsonator for those interested:

https://gist.github.com/3893242

You can pass in an "exclude" String[] and it will not include those. Plus, it has a standard "results, message, etc" that we like to pass back in our AJAX requests. Good for ExtJS.

Answer 1

You need to use #getDeclaredFields() to include private fields, #getFields() only lists the public ones.

With private fields you will also run into access restriction problems, so you probably want to look into the Field#setAccessible() method as well.