Java 如何将DynamoDB扫描结果添加到对象列表?
我正在对DynamoDB表执行扫描,然后需要将返回项中的各个属性添加到类型为Java 如何将DynamoDB扫描结果添加到对象列表?,java,iterator,amazon-dynamodb,Java,Iterator,Amazon Dynamodb,我正在对DynamoDB表执行扫描,然后需要将返回项中的各个属性添加到类型为User的列表中(User具有单个构造函数User(String uuid))。代码当前成功扫描数据库并返回扫描结果的列表。然而,由于某种原因,我的迭代似乎返回null AmazonDynamoDBClient client = dynamoClient.getDynamoClient(); DynamoDBMapper mapper = new DynamoDBMapper(client);
User
的列表中(User
具有单个构造函数User(String uuid)
)。代码当前成功扫描数据库并返回扫描结果的列表。然而,由于某种原因,我的迭代似乎返回null
AmazonDynamoDBClient client = dynamoClient.getDynamoClient();
DynamoDBMapper mapper = new DynamoDBMapper(client);
try {
DynamoDBScanExpression scanExpression = new DynamoDBScanExpression();
Map<String, Condition> scanFilter = new HashMap<String, Condition>();
Condition scanCondition =
new Condition().withComparisonOperator(ComparisonOperator.NOT_NULL);
scanFilter.put("uuid", scanCondition);
scanExpression.setScanFilter(scanFilter);
List scanResults = mapper.scan(UserAccounts.class, scanExpression);
for (Iterator it = scanResults.iterator(); it.hasNext();) {
//User user = (User) it.next();
allUserSummary.add(new User(scanResults.get(1).toString()));
}
} catch (Exception e) {
// TODO
}
AmazonDynamoDBClient=dynamoClient.getDynamoClient();
DynamoDBMapper mapper=新的DynamoDBMapper(客户机);
试一试{
DynamoDBScanExpression scanExpression=新的DynamoDBScanExpression();
Map scanFilter=new HashMap();
条件扫描条件=
新条件()。带有ComparisonOperator(ComparisonOperator.NOT_NULL);
scanFilter.put(“uuid”,scanCondition);
scanExpression.setScanFilter(scanFilter);
List scanResults=mapper.scan(UserAccounts.class,scanExpression);
for(Iterator it=scanResults.Iterator();it.hasNext();){
//User=(User)it.next();
添加新用户(scanResults.get(1.toString());
}
}捕获(例外e){
//待办事项
}
我建议您通过以下方式开始使用现代紧凑列表迭代,这有助于避免在使用旧迭代样式时出现许多常见错误:
[……]
迭代器只是杂乱无章。此外,这是一个发展的机会
错误。迭代器变量在每个循环中出现三次:即
两次出错的机会。for-each构造摆脱了
混乱和出错的机会。下面是示例的外观
对于每个构造,使用以下选项:
void cancelAll(Collection<TimerTask> c) {
for (TimerTask t : c)
t.cancel();
}
对于您的示例,这显然会产生以下结果:
List<UserAccounts> scanResults = mapper.scan(UserAccounts.class, scanExpression);
for (UserAccounts userAccounts : scanResults) {
allUserSummary.add(new User(userAccounts.getKey()));
}
List scanResults=mapper.scan(UserAccounts.class,scanExpression);
用于(用户帐户用户帐户:扫描结果){
添加(新用户(userAccounts.getKey());
}
将打印输出放在for循环的一侧,查看它是否打印您的用户。。。同时,也要确保它是在事后完成的,而不是在你们的getter中完成的allUserSummary@Daniel我可以确认问题来自这一行:allUserSummary.add(新用户(scanResults.get(1.toString()))代码>如果我在迭代器中放入一个print语句,我会得到正确的返回用户数,但显然用户是空的!
@DynamoDBTable(tableName = "UserAccounts")
public class UserAccounts{
private String key; // or uuid right away
@DynamoDBHashKey
public String getKey() {
return key;
}
public void setKey(String key) {
this.key = key;
}
// ...
}
List<UserAccounts> scanResults = mapper.scan(UserAccounts.class, scanExpression);
for (UserAccounts userAccounts : scanResults) {
allUserSummary.add(new User(userAccounts.getKey()));
}