Query from AWS DynamoDB for Android

Question

我正在尝试从我的AWS DynamoDB数据库中获取数据。我已经设置了以下代码，但是我遇到了错误。不清楚如何从这里开始查询基于主键的数据

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import com.amazonaws.auth.CognitoCachingCredentialsProvider;
import com.amazonaws.regions.Regions;
import com.amazonaws.services.dynamodbv2.*;
import com.amazonaws.mobileconnectors.dynamodbv2.dynamodbmapper.*;
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
}
CognitoCachingCredentialsProvider credentialsProvider = new CognitoCachingCredentialsProvider(
        getApplicationContext(),
        "**********************", // Identity Pool ID
        Regions.US_EAST_1 // Region
);
AmazonDynamoDBClient ddbClient = new AmazonDynamoDBClient(credentialsProvider);
DynamoDBMapper mapper = new DynamoDBMapper(ddbClient);

Answer 1

第一条评论，看起来你正在一个活动中尝试这样做——这肯定会在Android中引发Network on Main Thread异常。

其次，作为一般指南，您需要设置一个单独的类，其中DynamoDB字段已设置并链接到类中的变量。例如：

@DynamoDBTable(tableName = SA_AWSConstants.QUESTIONTABLENAME)
public class SA_Question implements Comparable<SA_Question> {
    // fields coming from AWS table
    private String questionguid_str; // : STRING, PRIMARY HASH KEY
    private String json_str;
    private long ordinalposition_int;
    ...
    @DynamoDBHashKey(attributeName = SA_AWSConstants.DBSCHEMA_QUESTIONGUID)
    public String getQuestionguid_str() {
        return questionguid_str;
    }
    public void setQuestionguid_str(String questionguid_str) {
        this.questionguid_str = questionguid_str;
    }
    @DynamoDBRangeKey(attributeName = SA_AWSConstants.DBSCHEMA_ORDINALPOS)
    public long getOrdinalposition_int() {
        return ordinalposition_int;
    }
    public void setOrdinalposition_int(long ordinalposition_int) {
        this.ordinalposition_int = ordinalposition_int;
    }
    @DynamoDBAttribute(attributeName = SA_AWSConstants.DBSCHEMA_QUESTION_JSONSTR)
    public String getJson_str() {
        return json_str;
    }
    public void setJson_str(String json_str) {
        this.json_str = json_str;
    }
}

然后你可以做这样的事情：

                DynamoDBMapper mapper = new DynamoDBMapper(ddb);
                DynamoDBScanExpression scanExpression = new DynamoDBScanExpression();
                PaginatedScanList<SA_Question> paginatedq = mapper.scan(SA_Question.class, scanExpression);
                ArrayList<SA_Question> awsqlist = new ArrayList<SA_Question>();
                for (SA_Question q : paginatedq) {
                    q.initiateFromJSON();
                    awsqlist.add(q);
                }

您必须注意AWS对单个查询返回数据的限制（因此是PaginatedScan——如果您希望超过限制，则需要检查是否存在更多要返回的数据）。