Question

我使用dynamodbmapper在dynamodb表中实现了扫描操作，但我没有得到所有结果。每当我运行程序时，扫描都会返回不同数量的项目。

代码段：

DyanmoDBScanExpression scanExpression = new DynamoDBScanExpression();
List<Books> scanResult = mapper.scan(Books.class, scanExpression);

我调查了一下，发现了物品扫描返回的限制。但是我找不到使用mapper从表中获取所有项目的方法！有没有办法让我可以循环遍历表格中的所有项目。我在JVM中设置了足够的堆内存，因此不存在内存问题。

Answer 1

您需要迭代，直到不再返回LastEvaluatedKey。检查SDK中的一个官方示例中的完成方式：

https://github.com/awslabs/aws-dynamodb-examples/blob/23837f36944f4166c56988452475edee99868166/src/main/java/com/amazonaws/codesamples/lowlevel/LowLevelQuery.java#L70

Answer 2

在Java中使用不带任何过滤器的DynamoDBScanExpression，

// Change to your Table_Name (you can load dynamically from lambda env as well)
DynamoDBMapperConfig mapperConfig = new DynamoDBMapperConfig.Builder().withTableNameOverride(DynamoDBMapperConfig.TableNameOverride.withTableNameReplacement("Table_Name")).build();

DynamoDBMapper mapper = new DynamoDBMapper(client, mapperConfig);

DynamoDBScanExpression scanExpression = new DynamoDBScanExpression();

// Change to your model class   
List < ParticipantReport > scanResult = mapper.scan(ParticipantReport.class, scanExpression);



// Check the count and iterate the list and perform as desired.
scanResult.size();

Answer 3

扫描应该返回所有项目问题是返回的集合延迟加载。你需要遍历列表，当它消耗所有被提取的项目时，将在幕后进行额外的调用以引入更多项目（直到所有内容都被引入）。

http://docs.aws.amazon.com/amazondynamodb/latest/developerguide/JavaQueryScanORMModelExample.html

在那个例子中它是：

// Work with external files.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"
#include "std_lib_facilities.h"
#include "wfile.h"


int main()
{
    Point p;
    vector<Point> original_points;
    vector<Point> processed_points;

    cout << "Please enter 7 pairs of numbers: " << endl;

    for (int i = 0; i <= 6; ++i)
    {
        cout << "#" << i + 1 << " pair: ";
        cin >> p;

        original_points.push_back(p);
    }

    p.print_all(original_points);

    cout << original_points << endl;

    p.import_all(processed_points);

    cout << processed_points << endl;

    keep_window_open();

    return 0;
}

Answer 4

有点晚了，但是

import java.util.HashMap;
import java.util.Map;
import java.util.List;

import com.amazonaws.services.dynamodbv2.datamodeling.DynamoDBMapper;
import com.amazonaws.services.dynamodbv2.datamodeling.DynamoDBQueryExpression;
import com.amazonaws.services.dynamodbv2.AmazonDynamoDBClient;
import com.amazonaws.services.dynamodbv2.model.AttributeValue;

public final class LogFetcher {

    static AmazonDynamoDBClient client = new AmazonDynamoDBClient();
    static String tableName = "SystemLog";

    public static List<SystemLog> findLogsForDeviceWithMacID(String macID) {

        client.setRegion(Region.getRegion(Regions.EU_WEST_1));

        DynamoDBMapper mapper = new DynamoDBMapper(client);
        Map<String, AttributeValue> eav = new HashMap<String, AttributeValue>();
        eav.put(":val1", new AttributeValue().withS(macID));

        DynamoDBQueryExpression<SystemLog> queryExpression = new DynamoDBQueryExpression<SystemLog>()
                .withKeyConditionExpression("parentKey = :val1")
                .withExpressionAttributeValues(eav);

         List<SystemLog> requestedLogs = mapper.query(SystemLog.class, queryExpression);

        return requestedLogs;
    }
}

样本类

import com.amazonaws.services.dynamodbv2.datamodeling.DynamoDBAttribute;
import com.amazonaws.services.dynamodbv2.datamodeling.DynamoDBHashKey;
import com.amazonaws.services.dynamodbv2.datamodeling.DynamoDBRangeKey;
import com.amazonaws.services.dynamodbv2.datamodeling.DynamoDBTable;

@DynamoDBTable(tableName="SystemLog")
public final class SystemLog {
    public Integer pidValue;
    public String uniqueId;
    public String parentKey;

    //DynamoDB

    //Partition (hash) key
    @DynamoDBHashKey(attributeName="parentKey")
    public String getParentKey() { return parentKey; }
    public void setParentKey(String parentKey) { this.parentKey = parentKey; }

    //Range key
    @DynamoDBRangeKey(attributeName="uniqueId")
    public String getUniqueId() { return uniqueId; }
    public void setUniqueId(String uniqueId) { this.uniqueId = uniqueId;} 

    @DynamoDBAttribute(attributeName="pidValue")
    public Integer getPidValue() { return pidValue; }    
    public void setPidValue(Integer pidValue) { this.pidValue = pidValue; }
}

Answer 5

默认情况下， DynamoDBMapper#scan 方法返回“延迟加载” 集合。它最初只返回一页结果，然后提供服务如有需要，请致电下一页。要获取所有匹配项，遍历分页的结果集合。

但是，PaginatedScanList自带了 PaginatedScanList#loadAllResults 方法，该方法有助于为该列表快速加载所有结果。

注意：： ITERATION_ONLY 模式下不支持 loadAllResults 方法。

List<Books> scanResult = mapper.scan(Books.class, new DynamoDBScanExpression());
scanResult.loadAllResults();//Eagerly loads all results for this list.
//Total results loaded into the list
System.out.println(scanResult.size());

使用Java High Level API从DynamoDB表中获取所有表项

5 个答案: