示例:使用 AWS SDK for Java 文档 API 处理二进制类型属性
以下 Java 代码示例介绍如何处理二进制类型属性。示例介绍将项目添加到 Reply
表。项目包含存储压缩数据的二进制类型属性 (ExtendedMessage
)。然后,示例检索该项目,并打印所有属性值。为方便说明,该示例使用 GZIPOutputStream
类压缩示例数据流,并将其分配至 ExtendedMessage
属性。检索到二进制属性后,使用 GZIPInputStream
类对其解压。
注意
SDK for Java 还提供一个对象持久化模型,可用来将客户端类映射到 DynamoDB 表。该方法可以减少需要编写的代码数量。有关更多信息,请参阅 Java 1.x:DynamoDBMapper。
如果您已按照为 DynamoDB 中的代码示例创建表和加载数据部分进行操作,您应当已经创建了 Reply
表。您还能够以编程方式创建此表。有关更多信息,请参阅 使用 AWS SDK for Java 创建表示例并上传数据。
有关测试以下示例的分步说明,请参阅 Java 代码示例。
例
package com.amazonaws.codesamples.document; import java.io.ByteArrayInputStream; import java.io.ByteArrayOutputStream; import java.io.IOException; import java.nio.ByteBuffer; import java.text.SimpleDateFormat; import java.util.Date; import java.util.TimeZone; import java.util.zip.GZIPInputStream; import java.util.zip.GZIPOutputStream; import com.amazonaws.services.dynamodbv2.AmazonDynamoDB; import com.amazonaws.services.dynamodbv2.AmazonDynamoDBClientBuilder; import com.amazonaws.services.dynamodbv2.document.DynamoDB; import com.amazonaws.services.dynamodbv2.document.Item; import com.amazonaws.services.dynamodbv2.document.Table; import com.amazonaws.services.dynamodbv2.document.spec.GetItemSpec; public class DocumentAPIItemBinaryExample { static AmazonDynamoDB client = AmazonDynamoDBClientBuilder.standard().build(); static DynamoDB dynamoDB = new DynamoDB(client); static String tableName = "Reply"; static SimpleDateFormat dateFormatter = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSS'Z'"); public static void main(String[] args) throws IOException { try { // Format the primary key values String threadId = "Amazon DynamoDB#DynamoDB Thread 2"; dateFormatter.setTimeZone(TimeZone.getTimeZone("UTC")); String replyDateTime = dateFormatter.format(new Date()); // Add a new reply with a binary attribute type createItem(threadId, replyDateTime); // Retrieve the reply with a binary attribute type retrieveItem(threadId, replyDateTime); // clean up by deleting the item deleteItem(threadId, replyDateTime); } catch (Exception e) { System.err.println("Error running the binary attribute type example: " + e); e.printStackTrace(System.err); } } public static void createItem(String threadId, String replyDateTime) throws IOException { Table table = dynamoDB.getTable(tableName); // Craft a long message String messageInput = "Long message to be compressed in a lengthy forum reply"; // Compress the long message ByteBuffer compressedMessage = compressString(messageInput.toString()); table.putItem(new Item().withPrimaryKey("Id", threadId).withString("ReplyDateTime", replyDateTime) .withString("Message", "Long message follows").withBinary("ExtendedMessage", compressedMessage) .withString("PostedBy", "User A")); } public static void retrieveItem(String threadId, String replyDateTime) throws IOException { Table table = dynamoDB.getTable(tableName); GetItemSpec spec = new GetItemSpec().withPrimaryKey("Id", threadId, "ReplyDateTime", replyDateTime) .withConsistentRead(true); Item item = table.getItem(spec); // Uncompress the reply message and print String uncompressed = uncompressString(ByteBuffer.wrap(item.getBinary("ExtendedMessage"))); System.out.println("Reply message:\n" + " Id: " + item.getString("Id") + "\n" + " ReplyDateTime: " + item.getString("ReplyDateTime") + "\n" + " PostedBy: " + item.getString("PostedBy") + "\n" + " Message: " + item.getString("Message") + "\n" + " ExtendedMessage (uncompressed): " + uncompressed + "\n"); } public static void deleteItem(String threadId, String replyDateTime) { Table table = dynamoDB.getTable(tableName); table.deleteItem("Id", threadId, "ReplyDateTime", replyDateTime); } private static ByteBuffer compressString(String input) throws IOException { // Compress the UTF-8 encoded String into a byte[] ByteArrayOutputStream baos = new ByteArrayOutputStream(); GZIPOutputStream os = new GZIPOutputStream(baos); os.write(input.getBytes("UTF-8")); os.close(); baos.close(); byte[] compressedBytes = baos.toByteArray(); // The following code writes the compressed bytes to a ByteBuffer. // A simpler way to do this is by simply calling // ByteBuffer.wrap(compressedBytes); // However, the longer form below shows the importance of resetting the // position of the buffer // back to the beginning of the buffer if you are writing bytes directly // to it, since the SDK // will consider only the bytes after the current position when sending // data to DynamoDB. // Using the "wrap" method automatically resets the position to zero. ByteBuffer buffer = ByteBuffer.allocate(compressedBytes.length); buffer.put(compressedBytes, 0, compressedBytes.length); buffer.position(0); // Important: reset the position of the ByteBuffer // to the beginning return buffer; } private static String uncompressString(ByteBuffer input) throws IOException { byte[] bytes = input.array(); ByteArrayInputStream bais = new ByteArrayInputStream(bytes); ByteArrayOutputStream baos = new ByteArrayOutputStream(); GZIPInputStream is = new GZIPInputStream(bais); int chunkSize = 1024; byte[] buffer = new byte[chunkSize]; int length = 0; while ((length = is.read(buffer, 0, chunkSize)) != -1) { baos.write(buffer, 0, length); } String result = new String(baos.toByteArray(), "UTF-8"); is.close(); baos.close(); bais.close(); return result; } }