Question

当我尝试下面的代码时，“result”包含null。

    Collection<Data> result = ObjectifyService.ofy().load().type(Data.class).list();

result.size（）不为零。

在“结果”中包含null的原因是什么？

Answer 1

请检查以下代码，描述Objectify中的所有基本操作。

获取Objectify中所有对象的列表

List<T> list = ofy().load().type(clazz).list();

您还可以在下面的代码中使用GenericDAO及其实现在Objectify中找到一些其他查询。

GenericDAO接口

package com.myweb.webservices.common.dao;

import java.util.List;

import com.myweb.webservices.common.exception.DatabaseException;

public interface GenericDao {
public <T> void create(T t);
public <T> String createWithKey(T t);
public <T> Long createWithID(T t);
public <T> void update(Class<T> clazz, Long id, T t) throws DatabaseException;
public <T> void update(Class<T> clazz, String key, T t) throws DatabaseException;
public <T> T getById(Class<T> clazz, Long id) throws DatabaseException;
public <T> T getByKey(Class<T> clazz, String key) throws DatabaseException;
public <T> List<T> list(Class<T> clazz);
public <T> void delete(Class<T> clazz, Long id) throws DatabaseException;
public <T> void deleteByKey(Class<T> clazz, String key) throws DatabaseException;   
}

GenericDAO实施

package com.myweb.webservices.common.dao;

import static com.googlecode.objectify.ObjectifyService.ofy;

import java.util.List;

import com.googlecode.objectify.Key;
import com.googlecode.objectify.ObjectifyService;
import com.myweb.webservices.common.exception.DatabaseException;
import com.myweb.webservices.model.User;

public abstract class AbstractGenericDaoImpl implements GenericDao{

static {
    ObjectifyService.register(User.class);
}

@Override
public <T> void create(T t) {
    ofy().save().entity(t).now();
}

@Override
public <T> String createWithKey(T t) {
    Key<T> key =  ofy().save().entity(t).now();
    return key.getString();
}

@Override
public <T> Long createWithID(T t) {
    Key<T> key =  ofy().save().entity(t).now();
    return key.getId();
}

@Override
public <T> void update(Class<T> clazz, Long id, T t) throws DatabaseException {
    if (id == null) {
        throw new DatabaseException("ID cannot be null");
    }
    T tnew = ofy().load().type(clazz).id(id).get();
    ofy().save().entity(tnew).now();
}

@Override
public <T> void update(Class<T> clazz, String key, T t) throws DatabaseException {
    if (key == null) {
        throw new DatabaseException("ID cannot be null");
    }
    T tnew = ofy().load().type(clazz).id(key).get();
    ofy().save().entity(tnew).now();

}

@Override
public <T> T getById(Class<T> clazz, Long id) throws DatabaseException {
    if (id == null) {
        throw new DatabaseException("ID cannot be null");
    }
    return ofy().load().type(clazz).id(id).get();
}

@Override
public <T> T getByKey(Class<T> clazz, String key) throws DatabaseException {
    if (key == null) {
        throw new DatabaseException("ID cannot be null");
    }
    return ofy().load().type(clazz).id(key).get();
}

@Override
public <T> List<T> list(Class<T> clazz) {
    List<T> list = ofy().load().type(clazz).list();
    return list;
}

@Override
public <T> void delete(Class<T> clazz, Long id) throws DatabaseException {
    if (id == null) {
        throw new DatabaseException("ID cannot be null");
    }
    T t = ofy().load().type(clazz).id(id).get();
    if(t != null){
        ofy().delete().entity(t).now();
    }
}

@Override
public <T> void deleteByKey(Class<T> clazz, String key) throws DatabaseException {
    if (key == null) {
        throw new DatabaseException("ID cannot be null");
    }
    T t = ofy().load().type(clazz).id(key).get();
    if(t != null){
        ofy().delete().entity(t).now();
    }
}
}

Objectify - 查询结果中的列表包含null

1 个答案: