Google App Engine(GAE) FOR JAVA中JDO的简单使用说明
大家都知道，GAE中是使用JDO来做持久层的。当初我在做自己的清风记账本的时候，去网上找了一些资料，但是国内用的人很少，找到的资料基本上少的可怜，先在将清风记账本这个应用中的JDO一些基本语法公布出来。
每个使用数据存储区的请求都新建一个 PersistenceManager 类的实例。使用 PersistenceManagerFactory 类的实例完成此操作。
PersistenceManagerFactory 实例的初始化需要时间。好在您对于应用程序只需一个实例，而该实例可存储在将由多个请求和多个类使用的静态变量中。简单的做法是为静态实例创建一个单独的包装器类。
一：增加：

/** * 保存 *  * @param accBean 要保存的实体对象 */public void saveAccount(Pay accBean) {PersistenceManager pm = PMF.get().getPersistenceManager();try {pm.makePersistent(accBean);} finally {pm.close();}}

二：查询

public List<Pay> qryAccount(int startNum, int endNum, User user) {if (startNum > endNum) {return null;}List<Pay> accountList2 = new ArrayList<Pay>();List<Pay> accountList3 = new ArrayList<Pay>();PersistenceManager pm = PMF.get().getPersistenceManager();Query query = null;try {List<Pay> accountList = null;pm.setDetachAllOnCommit(true);//类似于参数            String queryTemplate = "",filter="";              String username = user.getEmail();//替换的值            queryTemplate = "email == \"%s\" ";              filter = String.format(queryTemplate, username);  query = pm.newQuery(Pay.class,filter);  //排序query.setOrdering("useDate desc");accountList = (List<Pay>) query.execute();// 管理员if (!user.getEmail().equals("scholers@gmail.com")) {for (Pay tempBean : accountList) {if (tempBean.getUser().getEmail().equals(user.getEmail())) {accountList2.add(tempBean);}}} else { // 普通用户accountList2.addAll(accountList);}//if (accountList2 == null) {return new ArrayList<Pay>();}if (endNum > accountList2.size()) {endNum = accountList2.size();}accountList3.addAll(accountList2.subList(startNum, endNum));if (accountList3 != null) {return accountList3;} else {return new ArrayList<Pay>();} } catch (RuntimeException e) {              e.printStackTrace();  } finally {query.closeAll();pm.close();}return accountList3;}

三：统计：

public int qryAccCount(String email, String times) {//List<Pay> accountList2 = new ArrayList<Pay>();PersistenceManager pm = PMF.get().getPersistenceManager();Query query = null;try {List<Pay> accountList = null;pm.setDetachAllOnCommit(true);String queryTemplate = "", filter = "";//日期参数的传递//日期参数支持>=,<=的方式if(times != null && !times.equals("")) {queryTemplate = "email == \"%s\" && useDate == today";} else {queryTemplate = "email == \"%s\" ";}filter = String.format(queryTemplate, email);query = pm.newQuery(Pay.class, filter);//查询结果排序方式query.setOrdering("useDate desc");if(times != null && !times.equals("")) {//这里类似于声明，声明这个参数类型query.declareImports("import java.util.Date"); query.declareParameters("Date today"); //格式化参数DateFormat format1 = new SimpleDateFormat("yyyy-MM-dd"); Date today = null; try {   today = format1.parse(times);  } catch (ParseException e) {       e.printStackTrace();   }   //执行查询accountList = (List<Pay>) pm.newQuery(query).execute(today);} else {accountList = (List<Pay>) pm.newQuery(query).execute();}if (accountList != null)return accountList.size();elsereturn 0;} finally {pm.close();}}

多个参数的传递：

query.declareImports("import java.util.Date"); //开始日期--这里事先要声明query.declareParameters("Date today, Date today2"); DateFormat format1 = new SimpleDateFormat("yyyy-MM-dd"); Date today = null; try {   today = format1.parse(times);  } catch (ParseException e) {       e.printStackTrace();   } //结束日期//query.declareParameters("Date today2"); Date today2 = null; try {   today2 = format1.parse(times2);  } catch (ParseException e) {       e.printStackTrace();   } //查询结果排序方式query.setOrdering("useDate desc,id desc");accountList = (List<Pay>) query.executeWithArray(today, today2); //这里可以将两个参数的值传入

四：删除

public void delAccount(long accountId) {if (accountId <= 0) {return;}PersistenceManager pm = PMF.get().getPersistenceManager();try {Pay accBean = pm.getObjectById(Pay.class, accountId);pm.deletePersistent(accBean);} finally {pm.close();}}

注意：GAE不支持SUM的统计方法我用如下代码：

query.setResult("sum(price)");List results = (List)query.execute();

就会直接报出错误信息，而根据JDO的描述，JDO是支持的！

010-11-26 13:06:40 com.google.appengine.tools.development.DevAppServerImpl start
信息: The server is running at http://localhost:8888/
org.datanucleus.store.appengine.query.DatastoreQuery$UnsupportedDatastoreOperatorException: Problem with query <SELECT sum(ID) FROM com.scholers.account.bean.Pay WHERE email == "scholers@gmail.com">: App Engine datastore does not support operator sum.
at org.datanucleus.store.appengine.query.DatastoreQuery.validateResultExpression(DatastoreQuery.java:679)
at org.datanucleus.store.appengine.query.DatastoreQuery.validate(DatastoreQuery.java:618)
at org.datanucleus.store.appengine.query.DatastoreQuery.performExecute(DatastoreQuery.java:215)
at org.datanucleus.store.appengine.query.JDOQLQuery.performExecute(JDOQLQuery.java:89)
at org.datanucleus.store.query.Query.executeQuery(Query.java:1489)
at org.datanucleus.store.query.Query.executeWithArray(Query.java:1371)
at org.datanucleus.store.query.Query.execute(Query.java:1344)
at org.datanucleus.jdo.JDOQuery.execute(JDOQuery.java:221)

1 楼 alang 2010-11-08 总结的不错。 2 楼 alang 2010-11-08 明年2季度，GAE将推出纯sql版的支持，不过是收费服务。现在还都是Datastore方式。 3 楼 scholers 2010-11-08 alang 写道总结的不错。


谢谢！ 4 楼 scholers 2010-11-08 alang 写道明年2季度，GAE将推出纯sql版的支持，不过是收费服务。现在还都是Datastore方式。


这个也开始收钱了啊，，