如何使用 Java 确定 Google AppEngine 数据存储区中给定键的对象是否存在？

发布于 2024-07-22 07:16:54 字数 883 浏览 5 评论 0原文

我正在尝试将分片计数器示例 (code.google.com/appengine/articles/sharding_counters.html) 移植到 Java。唯一的问题是 Java API 没有类似于 Python 的“get_by_key_name”的调用。这是基本思想：

            Transaction tx = pm.currentTransaction();
            Key key = KeyFactory.createKey(CounterShard.class.getSimpleName(), counter + randomIndex);
            CounterShard shard = pm.getObjectById(CounterShard.class, key);
            if (shard == null) {  // API does not work this way...
                shard = new CounterShard(key, counter);
            }
            shard.increment();
            pm.makePersistent(shard);
            tx.commit();

不幸的是，当我第一次运行它时，它会抛出一个 JDOObjectNotFoundException 。我可以运行一个查询来确定给定名称的计数器是否存在，但这不是事务性的。另一个线程可以做同样的事情，最终两个线程都会创建一个具有相同键的对象。

据我了解，事务（对于 Java API）中支持的唯一操作是 get 和 put。那么我怎样才能通过键锁定一个尚不存在的对象（即没有“get”）并确保我是第一个也是唯一一个创建它的人？

原文

I'm trying to port the Sharding Counters example (code.google.com/appengine/articles/sharding_counters.html) to Java. The only problem is that the Java API does not have a call similar to Python's 'get_by_key_name'. This is the basic idea:

            Transaction tx = pm.currentTransaction();
            Key key = KeyFactory.createKey(CounterShard.class.getSimpleName(), counter + randomIndex);
            CounterShard shard = pm.getObjectById(CounterShard.class, key);
            if (shard == null) {  // API does not work this way...
                shard = new CounterShard(key, counter);
            }
            shard.increment();
            pm.makePersistent(shard);
            tx.commit();

Unfortunately, this throws a JDOObjectNotFoundException the first time I run it. I could run a query to determine if a counter for a given name exists, but that is not transactional. Another thread could do the same and in the end both would create an object with the same key.

From what I understand, the only operations supported within a transaction (for the Java API) are get and put. So how can I lock an object by key that does not exist yet (i.e. no 'get') and make sure that I am the first and only one creating it?

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