本文属于机器翻译版本。若本译文内容与英语原文存在差异，则一律以英文原文为准。 # 处理中的项目 DynamoDB 在中 DynamoDB，项目是*属性的集合，每个属性*都有一个*名称*和一个*值*。属性值可以为标量、集或文档类型。有关更多信息，请参阅《 Amazon DynamoDB 开发人员指南》中的[命名规则和数据类型](https://docs.aws.amazon.com//amazondynamodb/latest/developerguide/HowItWorks.NamingRulesDataTypes.html)。 ## 检索（获取）表中的项目 调用 DynamoDbClient's `getItem` 方法并向其传递一个包含所需项目的表名和主键值的[GetItemRequest](https://sdk.amazonaws.com/java/api/latest/software/amazon/awssdk/services/dynamodb/model/GetItemRequest.html)对象。它返回一个包含该项目所有属性的[GetItemResponse](https://sdk.amazonaws.com/java/api/latest/software/amazon/awssdk/services/dynamodb/model/GetItemResponse.html)对象。您可以在 [ 中指定一个或多个](https://docs.aws.amazon.com//amazondynamodb/latest/developerguide/Expressions.ProjectionExpressions.html)投影表达式`GetItemRequest`以检索特定属性。 您可以使用返回`GetItemResponse`对象的`item()`方法来检索与该项目关联的键（字符串 [AttributeValue](https://sdk.amazonaws.com/java/api/latest/software/amazon/awssdk/services/dynamodb/model/AttributeValue.html)）和值（）对的[映射](https://docs.oracle.com/javase/8/docs/api/index.html?java/util/Map.html)。 **导入** ``` import software.amazon.awssdk.regions.Region; import software.amazon.awssdk.services.dynamodb.model.DynamoDbException; import software.amazon.awssdk.services.dynamodb.DynamoDbClient; import software.amazon.awssdk.services.dynamodb.model.AttributeValue; import software.amazon.awssdk.services.dynamodb.model.GetItemRequest; import java.util.HashMap; import java.util.Map; import java.util.Set; ``` **代码** ``` public static void getDynamoDBItem(DynamoDbClient ddb,String tableName,String key,String keyVal ) { HashMap keyToGet = new HashMap(); keyToGet.put(key, AttributeValue.builder() .s(keyVal).build()); GetItemRequest request = GetItemRequest.builder() .key(keyToGet) .tableName(tableName) .build(); try { Map returnedItem = ddb.getItem(request).item(); if (returnedItem != null) { Set keys = returnedItem.keySet(); System.out.println("Amazon DynamoDB table attributes: \n"); for (String key1 : keys) { System.out.format("%s: %s\n", key1, returnedItem.get(key1).toString()); } } else { System.out.format("No item found with the key %s!\n", key); } } catch (DynamoDbException e) { System.err.println(e.getMessage()); System.exit(1); } } ``` 请参阅上的[完整示例](https://github.com/awsdocs/aws-doc-sdk-examples/blob/bc964a243276990f05c180618ea8b34777c68f0e/javav2/example_code/dynamodb/src/main/java/com/example/dynamodb/GetItem.java) GitHub。 ## 使用异步客户端从表中检索（获取）项目 调用`getItem`的方法， DynamoDbAsyncClient 然后向其传递一个包含所需项目的表名和主键值的[GetItemRequest](https://sdk.amazonaws.com/java/api/latest/software/amazon/awssdk/services/dynamodb/model/GetItemRequest.html)对象。 您可以返回包含该项目的所有属性的 [Collection](https://docs.oracle.com/javase/8/docs/api/index.html?java/util/Collection.html) 实例（请参阅以下示例）。 **导入** ``` import software.amazon.awssdk.regions.Region; import software.amazon.awssdk.services.dynamodb.model.GetItemRequest; import software.amazon.awssdk.services.dynamodb.model.AttributeValue; import software.amazon.awssdk.services.dynamodb.DynamoDbAsyncClient; import java.util.HashMap; import java.util.Map; import java.util.Set; import java.util.stream.Collectors; import software.amazon.awssdk.services.dynamodb.model.DynamoDbException; ``` **代码** ``` public static void getItem(DynamoDbAsyncClient client, String tableName, String key, String keyVal) { HashMap keyToGet = new HashMap(); keyToGet.put(key, AttributeValue.builder() .s(keyVal).build()); try { // Create a GetItemRequest instance GetItemRequest request = GetItemRequest.builder() .key(keyToGet) .tableName(tableName) .build(); // Invoke the DynamoDbAsyncClient object's getItem java.util.Collection returnedItem = client.getItem(request).join().item().values(); // Convert Set to Map Map map = returnedItem.stream().collect(Collectors.toMap(AttributeValue::s, s->s)); Set keys = map.keySet(); for (String sinKey : keys) { System.out.format("%s: %s\n", sinKey, map.get(sinKey).toString()); } } catch (DynamoDbException e) { System.err.println(e.getMessage()); System.exit(1); } ``` 请参阅上的[完整示例](https://github.com/awsdocs/aws-doc-sdk-examples/blob/bc964a243276990f05c180618ea8b34777c68f0e/javav2/example_code/dynamodbasync/src/main/java/com/example/dynamodbasync/DynamoDBAsyncGetItem.java) GitHub。 ## 向表添加新项目 创建表示项目属性的键值对的[映射](https://docs.oracle.com/javase/8/docs/api/index.html?java/util/Map.html)。其中必须包括表的主键字段的值。如果主键标识的项目已存在，那么其字段将通过该请求*更新*。 **注意** 如果您的账户和地区的命名表不存在，[ResourceNotFoundException](https://sdk.amazonaws.com/java/api/latest/software/amazon/awssdk/services/dynamodb/model/ResourceNotFoundException.html)则会抛出 a。 **导入** ``` import software.amazon.awssdk.regions.Region; import software.amazon.awssdk.services.dynamodb.model.DynamoDbException; import software.amazon.awssdk.services.dynamodb.DynamoDbClient; import software.amazon.awssdk.services.dynamodb.model.AttributeValue; import software.amazon.awssdk.services.dynamodb.model.PutItemRequest; import software.amazon.awssdk.services.dynamodb.model.ResourceNotFoundException; import java.util.HashMap; ``` **代码** ``` public static void putItemInTable(DynamoDbClient ddb, String tableName, String key, String keyVal, String albumTitle, String albumTitleValue, String awards, String awardVal, String songTitle, String songTitleVal){ HashMap itemValues = new HashMap(); // Add all content to the table itemValues.put(key, AttributeValue.builder().s(keyVal).build()); itemValues.put(songTitle, AttributeValue.builder().s(songTitleVal).build()); itemValues.put(albumTitle, AttributeValue.builder().s(albumTitleValue).build()); itemValues.put(awards, AttributeValue.builder().s(awardVal).build()); PutItemRequest request = PutItemRequest.builder() .tableName(tableName) .item(itemValues) .build(); try { ddb.putItem(request); System.out.println(tableName +" was successfully updated"); } catch (ResourceNotFoundException e) { System.err.format("Error: The Amazon DynamoDB table \"%s\" can't be found.\n", tableName); System.err.println("Be sure that it exists and that you've typed its name correctly!"); System.exit(1); } catch (DynamoDbException e) { System.err.println(e.getMessage()); System.exit(1); } } ``` 请参阅上的[完整示例](https://github.com/awsdocs/aws-doc-sdk-examples/blob/f4eaf2b2971805cfb2b87a8e5ab408f83169432e/javav2/example_code/dynamodb/src/main/java/com/example/dynamodb/PutItem.java) GitHub。 ## 更新表中现有项目 可以使用 DynamoDbClient 的 `updateItem` 方法，通过提供要更新的表名称、主键值和字段映射，更新表中已有项目的属性。 **注意** 如果您的账户和地区的命名表不存在，或者您传入的主键标识的项目不存在，则会抛出 a [ResourceNotFoundException](https://sdk.amazonaws.com/java/api/latest/software/amazon/awssdk/services/dynamodb/model/ResourceNotFoundException.html)。 **导入** ``` import software.amazon.awssdk.regions.Region; import software.amazon.awssdk.services.dynamodb.model.DynamoDbException; import software.amazon.awssdk.services.dynamodb.model.AttributeAction; import software.amazon.awssdk.services.dynamodb.model.AttributeValue; import software.amazon.awssdk.services.dynamodb.model.AttributeValueUpdate; import software.amazon.awssdk.services.dynamodb.model.ResourceNotFoundException; import software.amazon.awssdk.services.dynamodb.model.UpdateItemRequest; import software.amazon.awssdk.services.dynamodb.DynamoDbClient; import java.util.HashMap; ``` **代码** ``` public static void updateTableItem(DynamoDbClient ddb, String tableName, String key, String keyVal, String name, String updateVal){ HashMap itemKey = new HashMap(); itemKey.put(key, AttributeValue.builder().s(keyVal).build()); HashMap updatedValues = new HashMap(); // Update the column specified by name with updatedVal updatedValues.put(name, AttributeValueUpdate.builder() .value(AttributeValue.builder().s(updateVal).build()) .action(AttributeAction.PUT) .build()); UpdateItemRequest request = UpdateItemRequest.builder() .tableName(tableName) .key(itemKey) .attributeUpdates(updatedValues) .build(); try { ddb.updateItem(request); } catch (ResourceNotFoundException e) { System.err.println(e.getMessage()); System.exit(1); } catch (DynamoDbException e) { System.err.println(e.getMessage()); System.exit(1); } System.out.println("Done!"); } ``` 请参阅上的[完整示例](https://github.com/awsdocs/aws-doc-sdk-examples/blob/f4eaf2b2971805cfb2b87a8e5ab408f83169432e/javav2/example_code/dynamodb/src/main/java/com/example/dynamodb/UpdateItem.java) GitHub。 ## 删除表中现有项目 您可以通过使用 DynamoDbClient's `deleteItem` 方法并提供表名和主键值来删除表中存在的项目。 **注意** 如果您的账户和地区的命名表不存在，或者您传入的主键标识的项目不存在，则会抛出 a [ResourceNotFoundException](https://sdk.amazonaws.com/java/api/latest/software/amazon/awssdk/services/dynamodb/model/ResourceNotFoundException.html)。 **导入** ``` import software.amazon.awssdk.regions.Region; import software.amazon.awssdk.services.dynamodb.DynamoDbClient; import software.amazon.awssdk.services.dynamodb.model.AttributeValue; import software.amazon.awssdk.services.dynamodb.model.DeleteItemRequest; import software.amazon.awssdk.services.dynamodb.model.DynamoDbException; import java.util.HashMap; ``` **代码** ``` public static void deleteDynamoDBItem(DynamoDbClient ddb, String tableName, String key, String keyVal) { HashMap keyToGet = new HashMap(); keyToGet.put(key, AttributeValue.builder() .s(keyVal) .build()); DeleteItemRequest deleteReq = DeleteItemRequest.builder() .tableName(tableName) .key(keyToGet) .build(); try { ddb.deleteItem(deleteReq); } catch (DynamoDbException e) { System.err.println(e.getMessage()); System.exit(1); } } ``` 请参阅上的[完整示例](https://github.com/awsdocs/aws-doc-sdk-examples/blob/f4eaf2b2971805cfb2b87a8e5ab408f83169432e/javav2/example_code/dynamodb/src/main/java/com/example/dynamodb/DeleteItem.java) GitHub。 ## 更多信息 + 《 Amazon DynamoDB 开发人员指南》中的[项目处理准则](https://docs.aws.amazon.com//amazondynamodb/latest/developerguide/best-practices.html) + [使用《 Amazon DynamoDB 开发者指南》 DynamoDB中的项目](https://docs.aws.amazon.com//amazondynamodb/latest/developerguide/WorkingWithItems.html)