使用Google App Engine，我为Client课程创建了一个端点。当我使用简单的GET语句访问它时，它会出错。即使我只插入了简单数据，也会发生这种情况。
我在最后放了一些我的课程的骨架代码。我猜我需要对private List<Assessment> assessment类中的行Client做一些奇特的注释，但我不知道该怎么做。
有没有人知道如何解决这个问题，如果可能的话，我想知道为什么这个错误与设置有关，因为评估字段中没有数据。
插入数据
{
"firstName" : "Jon",
"lastName" : "Doe"
}
插入后，执行此简单语句以选择所有 firstName 和 lastName 字段。
GET http://localhost:8888/_ah/api/clientendpoint/v1/client?fields=items(firstName%2Cid%2ClastName)
这是我收到错误的地方
错误
com.google.appengine.repackaged.org.codehaus.jackson.map.JsonMappingException：您刚刚尝试访问字段“评估”，但是当您分离对象时，此字段未分离。要么不访问此字段，要么在分离对象时将其分离。 （通过参考链：com.google.api.server.spi.response.CollectionResponse [\“items \”] - ＆gt; com.google.appengine.datanucleus.query.StreamingQueryResult [0] - ＆gt; com.my.app .client.Client [\ “评估\”]）
更新：我通过Eclipse Google API插件生成了端点。我把它放在最后，这对应于GET语句。
我进一步追踪，可能只是我对如何创建返回的对象缺乏了解。
错误似乎在
com.my.app.client.Client.jdoGetassessment（Client.java） com.my.app.client.Client.getAssessment（Client.java:247）
但我的代码中不存在jdoGetAssessment()，所以我猜这是在Google App Engine中生成的。 getAssessment()由。{/ p>定义
public List<Assessment> getAssessment() {
return assessment;
}
这似乎触发了错误消息，但我仍然不明白为什么在没有数据的情况下触发它。
客户端
@Entity
public class Client {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Key id;
private String firstName;
private String lastName;
@ElementCollection
private List<Assessment> assessment;
public Key getId() {
return id;
}
public void setId(Key id) {
this.id = id;
}
public String getFirstName() {
return firstName;
}
public void setFirstName(String firstName) {
this.firstName = firstName;
}
public String getLastName() {
return lastName;
}
public void setLastName(String lastName) {
this.lastName = lastName;
}
public List<Assessment> getAssessment() {
return assessment;
}
public void setAssessment(List<Assessment> assessment) {
this.assessment = assessment;
}
}
评估
@Embeddable
public class Assessment {
private Date date;
public Date getDate() {
return date;
}
public void setDate(Date date) {
this.date = date;
}
}
生成ClientEndpoint.java
@Api(name = "clientendpoint", namespace = @ApiNamespace(ownerDomain = "myapp.com", ownerName = "myapp.com", packagePath = "mypackage.client"))
public class ClientEndpoint {
/**
* This method lists all the entities inserted in datastore.
* It uses HTTP GET method and paging support.
*`
* @return A CollectionResponse class containing the list of all entities
* persisted and a cursor to the next page.
*/
@SuppressWarnings({ "unchecked", "unused" })
@ApiMethod(name = "listClient")
public CollectionResponse<Client> listClient(
@Nullable @Named("cursor") String cursorString,
@Nullable @Named("limit") Integer limit) {
EntityManager mgr = null;
Cursor cursor = null;
List<Client> execute = null;
try {
mgr = getEntityManager();
Query query = mgr.createQuery("select from Client as Client");
if (cursorString != null && cursorString != "") {
cursor = Cursor.fromWebSafeString(cursorString);
query.setHint(JPACursorHelper.CURSOR_HINT, cursor);
}
if (limit != null) {
query.setFirstResult(0);
query.setMaxResults(limit);
}
execute = (List<Client>) query.getResultList();
cursor = JPACursorHelper.getCursor(execute);
if (cursor != null)
cursorString = cursor.toWebSafeString();
// Tight loop for fetching all entities from datastore and accomodate
// for lazy fetch.
for (Client obj : execute)
;
} finally {
mgr.close();
}
return CollectionResponse.<Client> builder().setItems(execute)
.setNextPageToken(cursorString).build();
}
/**
* This method gets the entity having primary key id. It uses HTTP GET method.
*
* @param id the primary key of the java bean.
* @return The entity with primary key id.
*/
@ApiMethod(name = "getClient")
public Client getClient(@Named("id") Long id) {
EntityManager mgr = getEntityManager();
Client client = null;
try {
client = mgr.find(Client.class, id);
} finally {
mgr.close();
}
return client;
}
/**
* This inserts a new entity into App Engine datastore. If the entity already
* exists in the datastore, an exception is thrown.
* It uses HTTP POST method.
*
* @param client the entity to be inserted.
* @return The inserted entity.
*/
@ApiMethod(name = "insertClient")
public Client insertClient(Client client) {
EntityManager mgr = getEntityManager();
try {
if (containsClient(client)) {
throw new EntityExistsException("Object already exists");
}
mgr.persist(client);
} finally {
mgr.close();
}
return client;
}
/**
* This method is used for updating an existing entity. If the entity does not
* exist in the datastore, an exception is thrown.
* It uses HTTP PUT method.
*
* @param client the entity to be updated.
* @return The updated entity.
*/
@ApiMethod(name = "updateClient")
public Client updateClient(Client client) {
EntityManager mgr = getEntityManager();
try {
if (!containsClient(client)) {
throw new EntityNotFoundException("Object does not exist");
}
mgr.persist(client);
} finally {
mgr.close();
}
return client;
}
/**
* This method removes the entity with primary key id.
* It uses HTTP DELETE method.
*
* @param id the primary key of the entity to be deleted.
* @return The deleted entity.
*/
@ApiMethod(name = "removeClient")
public Client removeClient(@Named("id") Long id) {
EntityManager mgr = getEntityManager();
Client client = null;
try {
client = mgr.find(Client.class, id);
mgr.remove(client);
} finally {
mgr.close();
}
return client;
}
private boolean containsClient(Client client) {
if (client.getId() == null)
return false;
EntityManager mgr = getEntityManager();
boolean contains = true;
try {
Client item = mgr.find(Client.class, client.getId());
if (item == null) {
contains = false;
}
} finally {
mgr.close();
}
return contains;
}
private static EntityManager getEntityManager() {
return EMF.get().createEntityManager();
}
}
答案 0 :(得分：3)
问题可能是JDO延迟加载 - 您正在请求对象，而JDO在您尝试使用它们之前不会获取它们的属性，此时与数据存储区的连接已经消失。您可以在自动生成的示例中看到一个代码示例来阻止延迟加载：
// Tight loop for fetching all entities from datastore and accomodate
// for lazy fetch.
for (Client obj : execute)
;
这会在返回之前加载列表中的所有Client个对象。但是，List上的Client属性也会延迟加载，因此您需要为每个Client对象的assessment属性重复此过程。
编辑：删除了使用FetchType.EAGER的建议，因为它在App Engine上不起作用。