Querying Tables Using the AWS SDK for Java Low-Level API

The query method supports several optional parameters. For example, you can optionally filter the query result in the preceding query to return replies in the past two weeks by specifying a condition. The condition is called range condition because Amazon DynamoDB evaluates the query condition that you specify against the range attribute of the primary key. You can specify other optional parameters to retrieve only a specific list of attributes from items in the query result. For more information about the parameters and the API, see Query.

The following Java code snippet retrieves forum thread replies posted in the past 15 days. The snippet specifies optional parameters using:

long twoWeeksAgoMilli = (new Date()).getTime() - (15L*24L*60L*60L*1000L);
Date twoWeeksAgo = new Date();
twoWeeksAgo.setTime(twoWeeksAgoMilli);
SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSS'Z'");
String twoWeeksAgoStr = df.format(twoWeeksAgo);

Map<String, Condition> keyConditions = new HashMap<String, Condition>();

Condition hashKeyCondition = new Condition()
    .withComparisonOperator(ComparisonOperator.EQ.toString())
    .withAttributeValueList(new AttributeValue().withS("Amazon DynamoDB#DynamoDB Thread 1"));
keyConditions.put("Id", hashKeyCondition);

Condition rangeKeyCondition = new Condition()
    .withComparisonOperator(ComparisonOperator.GT.toString())
    .withAttributeValueList(new AttributeValue().withS(twoWeeksAgoStr));
keyConditions.put("ReplyDateTime", rangeKeyCondition);

QueryRequest queryRequest = new QueryRequest()
    .withTableName("Reply")
    .withKeyConditions(keyConditions)
    .withAttributesToGet(Arrays.asList("Subject", "ReplyDateTime"))
    .withRangeKeyCondition(rangeKeyCondition)
    .withConsistentRead(true);

QueryResult result = client.query(queryRequest);
for (Map<String, AttributeValue> item : result.getItems()){
    printItem(item);
}

You can also optionally limit the page size, or the number of items per page, by using the withLimit method of the request. Each time you execute the query method, you get one page of results that has the specified number of items. To fetch the next page, you execute the query method again by providing the primary key value of the last item in the previous page so that the method can return the next set of items. You provide this information in the request by using the withExclusiveStartKey method. Initially, the parameter for this method can be null. To retrieve subsequent pages, you must update this property value to the primary key of the last item in the preceding page.

The following Java code snippet queries the Reply table. In the request, the withLimit and withExclusiveStartKey methods are used. The do/while loop continues to scan one page at time until the getLastEvaluatedKey method of the result returns a null value.

Condition hashKeyCondition = new Condition()
    .withComparisonOperator(ComparisonOperator.EQ.toString())
    .withAttributeValueList(new AttributeValue().withS("Amazon DynamoDB#DynamoDB Thread 1"));

Map<String, Condition> keyConditions = new HashMap<String, Condition>();
keyConditions.put("Id", hashKeyCondition);

Map<String, AttributeValue> lastEvaluatedKey = null;
do {
    QueryRequest queryRequest = new QueryRequest()
            .withTableName("Reply")
            .withKeyConditions(keyConditions)
            .withLimit(10)
            .withExclusiveStartKey(lastEvaluatedKey);

    QueryResult result = client.query(queryRequest);
    for (Map<String, AttributeValue> item : result.getItems()) {
        printItem(item);
    }
    lastEvaluatedKey = result.getLastEvaluatedKey();
} while (lastEvaluatedKey != null);

The examples in this section use the AWS SDK for Java low-level API. Additionally, the SDK provides Object Persistence Model API that enable you to map your client-side classes to your Amazon DynamoDB tables and perform query operations on them. For more information, see Using the AWS SDK for Java.

The following tables store information about a collection of forums. For more information about table schemas, see Example Tables and Data in Amazon DynamoDB.

Forum ( Name, ... ) 
Thread ( ForumName, Subject, Message, LastPostedBy, LastPostDateTime, ...) 
Reply ( Id, ReplyDateTime, Message, PostedBy, ...)

In this Java code example, you execute variations of finding replies for a thread 'DynamoDB Thread 1' in forum 'Amazon DynamoDB'.

import java.io.IOException;
import java.text.SimpleDateFormat;
import java.util.Arrays;
import java.util.Date;
import java.util.HashMap;
import java.util.Map;

import com.amazonaws.auth.AWSCredentials;
import com.amazonaws.auth.PropertiesCredentials;
import com.amazonaws.services.dynamodbv2.AmazonDynamoDBClient;
import com.amazonaws.services.dynamodbv2.model.AttributeValue;
import com.amazonaws.services.dynamodbv2.model.ComparisonOperator;
import com.amazonaws.services.dynamodbv2.model.Condition;
import com.amazonaws.services.dynamodbv2.model.QueryRequest;
import com.amazonaws.services.dynamodbv2.model.QueryResult;

public class LowLevelQuery {

    static AmazonDynamoDBClient client;
    static String tableName = "Reply";

    public static void main(String[] args) throws Exception {
        
        String forumName = "Amazon DynamoDB";
        String threadSubject = "DynamoDB Thread 1";

        createClient();

        findRepliesForAThread(forumName, threadSubject);
        findRepliesForAThreadSpecifyOptionalLimit(forumName, threadSubject);
        findRepliesInLast15DaysWithConfig(forumName, threadSubject);
        findRepliesPostedWithinTimePeriod(forumName, threadSubject);

    }

    private static void createClient() throws IOException {
        
        AWSCredentials credentials = new PropertiesCredentials(
                LowLevelQuery.class.getResourceAsStream("AwsCredentials.properties"));

        client = new AmazonDynamoDBClient(credentials);

    }
    
    private static void findRepliesForAThread(String forumName, String threadSubject) {

        String replyId = forumName + "#" + threadSubject;
        
        Condition hashKeyCondition = new Condition()
            .withComparisonOperator(ComparisonOperator.EQ.toString())
            .withAttributeValueList(new AttributeValue().withS(replyId));
        
        Map<String, Condition> keyConditions = new HashMap<String, Condition>();
        keyConditions.put("Id", hashKeyCondition);
        
        QueryRequest queryRequest = new QueryRequest()
            .withTableName(tableName)
            .withKeyConditions(keyConditions);

        QueryResult result = client.query(queryRequest);
        for (Map<String, AttributeValue> item : result.getItems()) {
            printItem(item);
        }
    }

    private static void findRepliesForAThreadSpecifyOptionalLimit(String forumName, String threadSubject) {
        
        Map<String, AttributeValue> lastEvaluatedKey = null;
        do {
            QueryRequest queryRequest = new QueryRequest()
                    .withTableName(tableName)
                    .withKeyConditions(makeReplyKeyConditions(forumName, threadSubject))
                    .withLimit(1)
                    .withExclusiveStartKey(lastEvaluatedKey);

            QueryResult result = client.query(queryRequest);
            for (Map<String, AttributeValue> item : result.getItems()) {
                printItem(item);
            }
            lastEvaluatedKey = result.getLastEvaluatedKey();
        } while (lastEvaluatedKey != null);        
    }

    private static void findRepliesInLast15DaysWithConfig(String forumName, String threadSubject) {

        long twoWeeksAgoMilli = (new Date()).getTime() - (15L*24L*60L*60L*1000L);
        Date twoWeeksAgo = new Date();
        twoWeeksAgo.setTime(twoWeeksAgoMilli);
        SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSS'Z'");
        String twoWeeksAgoStr = df.format(twoWeeksAgo);

        Condition rangeKeyCondition = new Condition()
            .withComparisonOperator(ComparisonOperator.GT.toString())
            .withAttributeValueList(new AttributeValue().withS(twoWeeksAgoStr));
        
        Map<String, Condition> keyConditions = makeReplyKeyConditions(forumName, threadSubject);
        keyConditions.put("ReplyDateTime", rangeKeyCondition);
        
        QueryRequest queryRequest = new QueryRequest().withTableName(tableName)
            .withKeyConditions(keyConditions)
            .withAttributesToGet(Arrays.asList("Message", "ReplyDateTime", "PostedBy"));

        QueryResult result = client.query(queryRequest);
        for (Map<String, AttributeValue> item : result.getItems()) {
            printItem(item);
        }

    }

    private static void findRepliesPostedWithinTimePeriod(String forumName, String threadSubject) {
        
        long startDateMilli = (new Date()).getTime() - (15L*24L*60L*60L*1000L); 
        long endDateMilli = (new Date()).getTime() - (5L*24L*60L*60L*1000L);    
        java.text.SimpleDateFormat df = new java.text.SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSS'Z'");
        String startDate = df.format(startDateMilli);
        String endDate = df.format(endDateMilli);

        Condition rangeKeyCondition = new Condition()
            .withComparisonOperator(ComparisonOperator.BETWEEN.toString())
            .withAttributeValueList(
                new AttributeValue().withS(startDate), 
                new AttributeValue().withS(endDate));
        
        Map<String, Condition> keyConditions = makeReplyKeyConditions(forumName, threadSubject);
        keyConditions.put("ReplyDateTime", rangeKeyCondition);
        
        QueryRequest queryRequest = new QueryRequest()
            .withTableName(tableName)
            .withKeyConditions(keyConditions)
            .withAttributesToGet(Arrays.asList("Message", "ReplyDateTime", "PostedBy"));

        QueryResult result = client.query(queryRequest);
        for (Map<String, AttributeValue> item : result.getItems()) {
            printItem(item);
        }        
    }

    private static void printItem(Map<String, AttributeValue> attributeList) {
        for (Map.Entry<String, AttributeValue> item : attributeList.entrySet()) {
            String attributeName = item.getKey();
            AttributeValue value = item.getValue();
            System.out.println(attributeName + " "
                    + (value.getS() == null ? "" : "S=[" + value.getS() + "]")
                    + (value.getN() == null ? "" : "N=[" + value.getN() + "]")
                    + (value.getB() == null ? "" : "B=[" + value.getB() + "]")
                    + (value.getSS() == null ? "" : "SS=[" + value.getSS() + "]")
                    + (value.getNS() == null ? "" : "NS=[" + value.getNS() + "]")
                    + (value.getBS() == null ? "" : "BS=[" + value.getBS() + "] \n"));
        }
    }
    
    /**
     * A simple helper function that returns a KeyCondition map filled in with the hash key equality condition
     * for a the Reply example table, given a forumName and threadSubject.
     */
    private static Map<String, Condition> makeReplyKeyConditions(String forumName, String threadSubject) {
        String replyId = forumName + "#" + threadSubject;
        
        Condition hashKeyCondition = new Condition()
            .withComparisonOperator(ComparisonOperator.EQ.toString())
            .withAttributeValueList(new AttributeValue().withS(replyId));
        
        Map<String, Condition> keyConditions = new HashMap<String, Condition>();
        keyConditions.put("Id", hashKeyCondition);
        
        return keyConditions;
    }
}