DynamoDBMapper仅在对象不存在时保存 [英] DynamoDBMapper save only if object doesn't exist

问题描述

使用Java DynamoDBMapper，如何仅在对象不存在时（基于主键）保存对象。如果确实存在，我希望引发异常或失败，而不是更新现有条目。

Using the Java DynamoDBMapper, how can I save an object only if it doesn't already exist (based on primary key). If it does exist, I want an exception or failure to be thrown, rather than having the existing entry updated.

推荐答案

我相信您应该可以使用

I believe you should be able to do this with a DynamoDbSaveExpression object that can apply to the mapper.

AWS网站上有一个教程此处，代码如下：

There's a tutorial on the AWS site here, code shown below:

try {
    DynamoDBSaveExpression saveExpression = new       DynamoDBSaveExpression();
    Map expected = new HashMap();
    expected.put("status", new ExpectedAttributeValue().withExists(false));

    saveExpression.setExpected(expected);

    mapper.save(obj, saveExpression);
} catch (ConditionalCheckFailedException e) {
    // This means our save wasn't recorded, since our constraint wasn't met
    // If this happens, the worker can simply look for a new task to work on
}

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