java 用于条件保存的 DynamoDBMapper
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DynamoDBMapper for conditional saves
提问by n00b
I'm using DynamoDBMapper and would like to conditionally save if and only if the hashkey and range key combination does not exist. I know there are ways to use UUIDs to reduce the possibility of a collision but I would like to protect myself by using conditional saves.
我正在使用 DynamoDBMapper 并希望在且仅当 hashkey 和 range 键组合不存在时有条件地保存。我知道有一些方法可以使用 UUID 来减少碰撞的可能性，但我想通过使用条件保存来保护自己。
I came across this articlethat uses DynamoDBSaveExpressionhowever I'm not able to specify that the condition is "hashkey AND rangekey" cannot exist. The APIspecifies a withConditionalOperatormethod but I'm not able to see this in my class. I am using the latest aws java sdk also from here.
我遇到了这篇使用DynamoDBSaveExpression 的文章，但是我无法指定条件是“hashkey AND rangekey”不能存在。该API指定一个withConditionalOperator方法，但我不能看到这在我的课。我也从这里使用最新的 aws java sdk 。
Any suggestions on how to conditionally save? Or what I may be doing incorrectly?
关于如何有条件地保存的任何建议？或者我可能做错了什么？
回答by mkobit
DynamoDBSaveExpression saveExpression = new DynamoDBSaveExpression();
Map<String, ExpectedAttributeValue> expectedAttributes =
ImmutableMap.<String, ExpectedAttributeValue>builder()
.put("hashKey", new ExpectedAttributeValue(false))
.put("rangeKey", new ExpectedAttributeValue(false))
.build();
saveExpression.setExpected(expectedAttributes);
saveExpression.setConditionalOperator(ConditionalOperator.AND);
try {
dynamoDBMapper.save(objectToSave, saveExpression);
} catch (ConditionalCheckFailedException e) {
//Handle conditional check
}
This uses the public ExpectedAttributeValue(Boolean exists)constructor, which just internally calls setExists.
这使用public ExpectedAttributeValue(Boolean exists)构造函数，它只是在内部调用setExists.
