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("[email protected]")) {
				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();
			else
				return 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 == "[email protected]">: 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)