Java 1.x: DynamoDBMapper

The Amazon SDK for Java provides a DynamoDBMapper class, allowing you to map your client-side classes to Amazon DynamoDB tables. To use DynamoDBMapper, you define the relationship between items in a DynamoDB table and their corresponding object instances in your code. The DynamoDBMapper class enables you to perform various create, read, update, and delete (CRUD) operations on items, and run queries and scans against tables.

The SDK for Java provides a set of annotation types so that you can map your classes to tables. For example, consider a ProductCatalog table that has Id as the partition key.

ProductCatalog(Id, ...)

You can map a class in your client application to the ProductCatalog table as shown in the following Java code. This code defines a plain old Java object (POJO) named CatalogItem, which uses annotations to map object fields to DynamoDB attribute names.

package com.amazonaws.codesamples;

import java.util.Set;

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

@DynamoDBTable(tableName="ProductCatalog")
public class CatalogItem {

    private Integer id;
    private String title;
    private String ISBN;
    private Set<String> bookAuthors;
    private String someProp;

    @DynamoDBHashKey(attributeName="Id")
    public Integer getId() { return id; }
    public void setId(Integer id) {this.id = id; }

    @DynamoDBAttribute(attributeName="Title")
    public String getTitle() {return title; }
    public void setTitle(String title) { this.title = title; }

    @DynamoDBAttribute(attributeName="ISBN")
    public String getISBN() { return ISBN; }
    public void setISBN(String ISBN) { this.ISBN = ISBN; }

    @DynamoDBAttribute(attributeName="Authors")
    public Set<String> getBookAuthors() { return bookAuthors; }
    public void setBookAuthors(Set<String> bookAuthors) { this.bookAuthors = bookAuthors; }

    @DynamoDBIgnore
    public String getSomeProp() { return someProp; }
    public void setSomeProp(String someProp) { this.someProp = someProp; }
}

In the preceding code, the @DynamoDBTable annotation maps the CatalogItem class to the ProductCatalog table. You can store individual class instances as items in the table. In the class definition, the @DynamoDBHashKey annotation maps the Id property to the primary key.

By default, the class properties map to the same name attributes in the table. The properties Title and ISBN map to the same name attributes in the table.

The @DynamoDBAttribute annotation is optional when the name of the DynamoDB attribute matches the name of the property declared in the class. When they differ, use this annotation with the attributeName parameter to specify which DynamoDB attribute this property corresponds to.

In the preceding example, the @DynamoDBAttribute annotation is added to each property to ensure that the property names match exactly with the tables created in a previous step, and to be consistent with the attribute names used in other code examples in this guide.

Your class definition can have properties that don't map to any attributes in the table. You identify these properties by adding the @DynamoDBIgnore annotation. In the preceding example, the SomeProp property is marked with the @DynamoDBIgnore annotation. When you upload a CatalogItem instance to the table, your DynamoDBMapper instance does not include the SomeProp property. In addition, the mapper does not return this attribute when you retrieve an item from the table.

After you define your mapping class, you can use DynamoDBMapper methods to write an instance of that class to a corresponding item in the Catalog table. The following code example demonstrates this technique.

AmazonDynamoDB client = AmazonDynamoDBClientBuilder.standard().build();

DynamoDBMapper mapper = new DynamoDBMapper(client);

CatalogItem item = new CatalogItem();
item.setId(102);
item.setTitle("Book 102 Title");
item.setISBN("222-2222222222");
item.setBookAuthors(new HashSet<String>(Arrays.asList("Author 1", "Author 2")));
item.setSomeProp("Test");

mapper.save(item);

The following code example shows how to retrieve the item and access some of its attributes.

CatalogItem partitionKey = new CatalogItem();

partitionKey.setId(102);
DynamoDBQueryExpression<CatalogItem> queryExpression = new DynamoDBQueryExpression<CatalogItem>()
    .withHashKeyValues(partitionKey);

List<CatalogItem> itemList = mapper.query(CatalogItem.class, queryExpression);

for (int i = 0; i < itemList.size(); i++) {
    System.out.println(itemList.get(i).getTitle());
    System.out.println(itemList.get(i).getBookAuthors());
}

DynamoDBMapper offers an intuitive, natural way of working with DynamoDB data within Java. It also provides several built-in features, such as optimistic locking, ACID transactions, autogenerated partition key and sort key values, and object versioning.