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# 範例：使用 適用於 Java 的 AWS SDK 文件 API 處理二進位類型屬性
<a name="JavaDocumentAPIBinaryTypeExample"></a>

下列 Java 程式碼範例示範二進位類型屬性的處理。範例會將項目新增至 `Reply` 表。該項目包含存放壓縮資料的二進位類型屬性 (`ExtendedMessage`)。範例接著會擷取項目，並列印所有屬性值。為了進行示範，該範例使用 `GZIPOutputStream` 類別來壓縮範例串流，並將其指派給 `ExtendedMessage` 屬性。擷取二進制屬性時，其便會使用 `GZIPInputStream` 類別進行解壓縮。

**注意**  
適用於 Java 的開發套件也提供物件持久性模型，讓您將用戶端類別映射至 DynamoDB 資料表。此方法可以減少您必須撰寫的程式碼數量。如需詳細資訊，請參閱 [Java 1.x：DynamoDBMapper](DynamoDBMapper.md)。

如已完成[在 DynamoDB 中建立資料表，以及載入程式碼範例的資料](SampleData.md)一節，應已建立 `Reply` 表。您也可以透過編寫程式的方式建立此資料表。如需詳細資訊，請參閱 [使用 建立範例資料表和上傳資料 適用於 Java 的 AWS SDK](AppendixSampleDataCodeJava.md)。

如需測試下列範例的逐步說明，請參閱 [Java 程式碼範例](CodeSamples.Java.md)。

**Example**  

```
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;
    }
}
```