Objectify - 查詢結果中的列表包含空值

Question

當我嘗試下面的代碼時，「result」包含null。

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

result.size（）不爲零。

在「result」中包含null的原因是什麼？

Answer 1

請檢查以下代碼描述Objectify中的所有基本操作。

讓所有對象列表中物化

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

，你也可以找到在下面的代碼使用GenericDAO及其FPGA實現在物化一些額外的查詢。

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實施

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(); 
    } 
} 
}