在GAE中,我有一个充满“一次性”的表-诸如“最后使用的序列号”之类的东西并没有真正落入其他表中。这是带有字符串值对的简单字符串键。
我有一些代码来获取一个命名的整数并将其递增,如下所示:
@PersistenceCapable(detachable="true")
public class OneOff
{
@PrimaryKey
@Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
private Key key;
@Persistent
private String dataKey;
@Persistent
private String value;
public OneOff(String kk, String vv)
{
this.dataKey = kk;
this.value = vv;
}
public static OneOff persistOneOff(String kk, String vv)
{
OneOff oneoff= new OneOff(kk, vv);
PersistenceManager pm = PMF.get().getPersistenceManager();
try
{
pm.makePersistent(oneoff);
}
finally
{
pm.close();
}
return oneoff;
}
// snip...
@SuppressWarnings("unchecked")
synchronized
public static int getIntValueForKeyAndIncrement(String kk, int deFltValue)
{
int result = 0;
OneOff oneOff = null;
PersistenceManager pm = PMF.get().getPersistenceManager();
Query query = pm.newQuery(OneOff.class);
query.setFilter("dataKey == kkParam");
query.declareParameters("String kkParam");
List<OneOff> oneOffs = (List<OneOff>) query.execute(kk);
int count = oneOffs.size();
if (count == 1)
{
oneOff = oneOffs.get(0);
result = Integer.parseInt(oneOff.value);
}
else if (count == 0)
{
oneOff = new OneOff(kk, "default");
result = deFltValue;
}
else
{
// Log WTF error.
}
// update object in DB.
oneOff.value = "" + (result+1);
try
{
pm.makePersistent(oneOff);
}
finally
{
pm.close();
}
return result;
}
// etc...
但是,当我拨打这些电话时:
int val1 = OneOff.getIntValueForKeyAndIncrement("someKey", 100);
int val2 = OneOff.getIntValueForKeyAndIncrement("someKey", 100);
int val3 = OneOff.getIntValueForKeyAndIncrement("someKey", 100);
有时我得到所需的增量,有时我得到相同的值。当我想为此特定事务锁定数据库时,我的数据库访问似乎正在异步运行。
我认为
synchronized
public static
本来应该为我这样做的,但显然不是(可能由于多个实例正在运行!)
无论如何-我该怎么做我想要的事情? (我想在获取并更新此值时锁定我的数据库,以使整个事情并发安全。)
谢谢!
==编辑==
我已经接受了罗伯特的正确答案,因为交易确实是我想要的。但是,为了完整起见,我在下面添加了更新的代码。我认为这是正确的,尽管我不确定
if(oneOff==null)
子句(try-catch位)。public static int getIntValueForKeyAndIncrement(String kk, int defltValue)
{
int result = 0;
Entity oneOff = null;
int retries = 3;
// Using Datastore Transactions
DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
while (true)
{
com.google.appengine.api.datastore.Transaction txn = datastore.beginTransaction();
try
{
Key oneOffKey = KeyFactory.createKey("OneOff", kk);
oneOff = datastore.get (oneOffKey);
result = Integer.parseInt((String) oneOff.getProperty("value"));
oneOff.setProperty("value", "" + (result+1));
datastore.put(oneOff);
txn.commit();
break;
}
catch (EntityNotFoundException ex)
{
result = defltValue;
}
catch (ConcurrentModificationException ex)
{
if (--retries < 0)
{
throw ex;
}
}
if (oneOff == null)
{
try
{
Key oneOffKey = KeyFactory.createKey("OneOff", kk);
oneOff = new Entity(oneOffKey);
oneOff.setProperty("value", "" + (defltValue+1));
datastore.put(txn, oneOff);
datastore.put(oneOff);
txn.commit();
break;
}
finally
{
if (txn.isActive())
{
txn.rollback();
}
}
}
else
{
if (txn.isActive())
{
txn.rollback();
}
}
}
return result;
}
最佳答案
您应该在transaction中更新值。只要您的读取和写入都在一个事务中,App Engine的事务就可以防止两个更新相互覆盖。请务必注意有关实体组的讨论。
关于google-app-engine - 如何在App Engine(GAE)中锁定数据库?,我们在Stack Overflow上找到一个类似的问题:https://stackoverflow.com/questions/6963510/