我无法弄清楚Google App Engine JDO的实现有问题。该文档(http://code.google.com/intl/sv-SE/appengine/docs/java/datastore/jdo/creatinggettinganddeletingdata.html)指出“对makePersistent()的调用是同步的,直到保存对象并更新索引时才会返回。”但我的经历不同。Google App Engine JDO makePersistent延迟
我想将(makePersistent)对象保存到数据存储中。保存完成后,我希望能够立即从数据存储中读取(查询执行)。我知道我不必获取它(因为我已经在内存中拥有该对象),但重点是我希望下一个请求能够从数据存储中检索数据。如果第二个请求足够快,那么这与当前实现无关。
我注意到的一件奇怪的事情是,如果我试图在数据存储中从数据存储中取回对象几次(下面的代码),对象返回非常快(通常为< 10ms)。但是如果我跳过循环,而是在makePersistent和查询执行之间运行Thread.sleep(..)5000毫秒,则不确定是否找到该对象。这些解决方案都不是我想要的。我希望能够在没有睡眠或循环之间立即获取数据。
下面的代码和访问DataStoreTestServlet的结果如您所见,包括“等待”数据的循环。再次,我不想要循环。
有谁知道我错过了什么?我想它必须是某种东西。这个实现不适合我:)。
我使用的是appengine-java-sdk-1.6.0。这是本地(开发服务器)和部署在Google服务器上的问题。
下面是访问该servlet的结果。
Created users:
User [password=password, userName=user1321190966416] took 18ms, 2 loop(s)
User [password=password, userName=user1321190966438] took 15ms, 6 loop(s)
User [password=password, userName=user1321190966456] took 2ms, 1 loop(s)
User [password=password, userName=user1321190966460] took 10ms, 5 loop(s)
User [password=password, userName=user1321190966472] took 0ms, 1 loop(s)
User [password=password, userName=user1321190966472] took 0ms, 1 loop(s)
User [password=password, userName=user1321190966472] took 16ms, 1 loop(s)
User [password=password, userName=user1321190966488] took 0ms, 2 loop(s)
User [password=password, userName=user1321190966488] took 0ms, 1 loop(s)
User [password=password, userName=user1321190966488] took 16ms, 1 loop(s)
代码和配置。
jdoconfig.xml
<?xml version="1.0" encoding="utf-8"?>
<jdoconfig xmlns="http://java.sun.com/xml/ns/jdo/jdoconfig" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="http://java.sun.com/xml/ns/jdo/jdoconfig">
<persistence-manager-factory name="transactions-optional">
<property name="javax.jdo.PersistenceManagerFactoryClass" value="org.datanucleus.store.appengine.jdo.DatastoreJDOPersistenceManagerFactory"/>
<property name="javax.jdo.option.ConnectionURL" value="appengine"/>
<property name="javax.jdo.option.NontransactionalRead" value="true"/>
<property name="javax.jdo.option.NontransactionalWrite" value="true"/>
<property name="javax.jdo.option.RetainValues" value="true"/>
<property name="datanucleus.appengine.autoCreateDatastoreTxns" value="true"/>
<property name="datanucleus.appengine.datastoreReadConsistency" value="STRONG" />
</persistence-manager-factory>
</jdoconfig>
PMF.java
import javax.jdo.JDOHelper;
import javax.jdo.PersistenceManagerFactory;
public final class PMF {
private static final PersistenceManagerFactory pmfInstance = JDOHelper.getPersistenceManagerFactory("transactions-optional");
private PMF() {
}
public static PersistenceManagerFactory get() {
return pmfInstance;
}
}
User.java
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 User {
@PrimaryKey
@Persistent
private String userName;
@Persistent
private String password;
public User(String userName, String password) {
super();
this.setUserName(userName);
this.setPassword(password);
}
public String getUserName() {
return userName;
}
public String getPassword() {
return password;
}
public void setUserName(String userName) {
this.userName = userName;
}
public void setPassword(String password) {
this.password = password;
}
public String toString() {
return "User [password=" + password + ", userName=" + userName + "]";
}
}
DataStoreTestServlet.java
import java.io.IOException;
import java.util.List;
import javax.jdo.PersistenceManager;
import javax.jdo.Query;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
@SuppressWarnings("serial")
public class DataStoreTestServlet extends HttpServlet {
public void doGet(HttpServletRequest req, HttpServletResponse resp) throws IOException {
StringBuffer sb = new StringBuffer();
sb.append("Created users:\n");
for (int i = 0; i < 10; i++) {
String uniqueName = "user" + System.currentTimeMillis();
User user = new User(uniqueName, "password");
save(user);
User userFromDS = null;
long startTime = System.currentTimeMillis();
long loop = 0;
while (userFromDS == null) {
userFromDS = get(uniqueName);
loop++;
if (userFromDS != null) {
long endTime = System.currentTimeMillis();
sb.append(userFromDS.toString() + " took " + (endTime - startTime) + "ms, " + loop + " loop(s)\n");
}
}
}
resp.setContentType("text/plain");
resp.getWriter().println(sb.toString());
}
public Object save(Object obj) {
PersistenceManager pm = PMF.get().getPersistenceManager();
Object savedObject = null;
try {
savedObject = pm.makePersistent(obj);
} finally {
pm.close();
}
return savedObject;
}
public User get(String userName) {
User user = null;
List<User> users = null;
PersistenceManager pm = PMF.get().getPersistenceManager();
Query query = pm.newQuery(User.class);
query.setFilter("userName == nameParam");
query.declareParameters("String nameParam");
try {
users = (List<User>) query.execute(userName);
if (users != null && users.size() > 0) {
user = users.get(0);
}
} finally {
query.closeAll();
pm.close();
}
return user;
}
}
感谢您的回复。我添加了该属性,但没有改变。我仍然需要循环几次,试图获取数据,然后它成功:(非常奇怪。 –
当我第一次发布这个答案时,我不小心发布了' '但它应该是' '(注意'STRONG'与'EVENTUAL'相对)这是我的错;但是你确定你尝试过'STRONG'设置吗? –
Dave
嘿嘿,是的,我试着用STRONG设置。我也在我的查询中尝试了以下内容:query.addExtension(“datanucleus.appengine.datastoreReadConsistency”,“STRONG”); 同样的问题 –