如何使用反射从POJO获取属性名称和值？

Question

所以我正在写一个“ POJO to JSON”转换器。我希望能够传递一个List<T>对象并转换为JSON。

希望这会有意义

/**
 *
     * 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 "[]";
    }
}

当我进入本Object obj节时，我obj是正确的。我可以调试它，并查看类的名称和值。

假设该类是这样的：

public class User {
    private firstName;
    private lastName;

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

}

所以现在obj是一个站点。OK，然后我尝试获取字段名称（firstName，lastName），但是fields对象为空。

我究竟做错了什么？

谢谢

编辑

我知道了！这不是完成的代码，但它确实正在执行我现在想要的操作。我读过Google和Jackson也会这样做。如果有人可以提供一个很好的链接，说明如何有选择地从POJO中选择属性，那么我无所不能。

还是更好，我想知道 为什么 我不应该这样吗？

谢谢！

Jsonator（未完成）

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 "[]";
    }
}

网站类别

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;
    }


}

例

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 html" target="_blank">list.results();
        }
        catch (Exception ex) {
            return list.error();
        } 
    }
}

更新2

Jsonator对于那些有兴趣的人来说，这是更好的选择：

https://gist.github.com/3893242

您可以传递“ exclude” String
[]，并且其中不包括那些。另外，它具有标准的“结果，消息等”，我们希望将其传递回我们的AJAX请求中。对ExtJS有好处。

Answer 1

您需要使用#getDeclaredFields()包括私有字段，#getFields()仅列出公共字段。

使用private字段，您还将遇到访问限制问题，因此您可能也想研究该Field#setAccessible()方法。