Question

我正在使用Google App Engine和Java中的JDO为移动应用开发服务器端，我在同时访问数据库时遇到了一些问题。

我正在做的应用是聊天，当两个人同时访问数据库时，其中一条消息会丢失。我认为这是因为两个访问中的一个在另一个访问之前完成，这是在它保存在数据库之前，这就是为什么其中一个消息丢失了。

你知道一些解决方案吗？像访问数据库中的相同注册表并将它们放入队列或堆栈以便最近访问的东西。

提前致谢！

这是我的代码：

Conversation.java：

import java.util.LinkedList;
import javax.jdo.annotations.IdGeneratorStrategy;
import javax.jdo.annotations.IdentityType;
import javax.jdo.annotations.PersistenceCapable;
import javax.jdo.annotations.Persistent;
import javax.jdo.annotations.PrimaryKey;

@PersistenceCapable(identityType = IdentityType.APPLICATION)  
public class Conversation 
{
    @PrimaryKey  
    @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)  
    private Long id;

    @Persistent
    private Long idUser;

    @Persistent(mappedBy = "conversation")
    private LinkedList<Message> messages;

    public Conversation(Long idUser) 
    {
        this.idUser = idUser;

        messages = new LinkedList<Message>();       
    }

    public Long getId() {
        return id;
    }

    public Long getIdUser() {
        return idUser;
    }

    public LinkedList<Message> getMessages() {
        return messages;
    }
}

Message.java：

import javax.jdo.annotations.IdGeneratorStrategy;
import javax.jdo.annotations.IdentityType;
import javax.jdo.annotations.PersistenceCapable;
import javax.jdo.annotations.Persistent;
import javax.jdo.annotations.PrimaryKey;
import com.google.appengine.api.datastore.Key;

@PersistenceCapable(identityType = IdentityType.APPLICATION)  
public class Message 
{
    @PrimaryKey  
    @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)  
    private Key id;

    @Persistent
    private Conversation conversation;

    @Persistent
    private String text;

    @Persistent
    private String hour;

    public Message(String text, String hour)
    {
        this.text = text;
        this.hour = hour;
    }

    public String getText() 
    {
        return text;
    }

    public String getHour() 
    {
        return hour;
    }

    public Key getId() 
    {
        return id;
    }

    public Conversation getConversation() {
        return conversation;
    }
}

保存消息：

final PersistenceManager persistenceManager = PMF.get().getPersistenceManager();      

String strQuery = "select from " + Conversation.class.getName() + " where id == " + id;
persistenceManager.getFetchPlan().setGroup(FetchGroup.ALL);
final Query query = persistenceManager.newQuery(strQuery);

try 
{
    List<Conversation> conversations = (List<Conversation>) query.execute;

    Conversation detached = null;
    detached = persistenceManager.detachCopy(conversations.get(0));

    detached.getMessages().add(message);
    persistenceManager.makePersistent(detached);
} finally {
    persistenceManager.close();
}

同时访问数据库

0 个答案: