java.lang.ClassCastException: 无法转换为java.lang.Object

Question

我想按照自己的需求编写一个自定义serde，但卡在了一个点上，出现了类转换异常。

输入数据为：

john,miller

我希望将这些数据作为 fname string,lname string 插入到Hive中，所以我编写了一个定制的SerDe。

我只实现了SerDe接口的反序列化方法，如下所示:

    package com.datametica.serde;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Properties;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.hive.serde.Constants;
import org.apache.hadoop.hive.serde2.SerDe;
import org.apache.hadoop.hive.serde2.SerDeException;
import org.apache.hadoop.hive.serde2.SerDeStats;
import org.apache.hadoop.hive.serde2.objectinspector.ObjectInspector;
import org.apache.hadoop.hive.serde2.objectinspector.ObjectInspectorFactory;
import org.apache.hadoop.hive.serde2.objectinspector.StructObjectInspector;
import org.apache.hadoop.hive.serde2.typeinfo.TypeInfo;
import org.apache.hadoop.hive.serde2.typeinfo.TypeInfoUtils;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.io.Writable;

public class CustomSerde implements SerDe {

    int numColumns;
    StructObjectInspector rowOI;
    List<String> columnNames;
    List<Object> rows;
    List<TypeInfo> columnTypes;

    @Override
    public void initialize(Configuration conf, Properties tblProps)
            throws SerDeException {
        String columnNameProperty = tblProps
                .getProperty(Constants.LIST_COLUMNS);
        columnNames = Arrays.asList(columnNameProperty.split(","));

        String columnTypeProperty = tblProps
                .getProperty(Constants.LIST_COLUMN_TYPES);
        columnTypes = TypeInfoUtils
                .getTypeInfosFromTypeString(columnTypeProperty);
        numColumns = columnNames.size();

        List<ObjectInspector> columnOIs = new ArrayList<ObjectInspector>(
                columnNames.size());
        ObjectInspector oi;

        for (int c = 0; c < numColumns; c++) {
            oi = TypeInfoUtils
                    .getStandardJavaObjectInspectorFromTypeInfo(columnTypes
                            .get(c));
            columnOIs.add(oi);
        }

        /*
         * for (int c = 0; c < numColumns; c++) { rows.add(); }
         */

        rowOI = ObjectInspectorFactory.getStandardStructObjectInspector(
                columnNames, columnOIs);

    }

    @Override
    public CustomDataFormat deserialize(Writable record) throws SerDeException {
        Text text = (Text) record;
        String[] valArray = text.toString().split(",");
        System.out.println("----------------------------\n");
        System.out.println("yo yo yo "+text.toString() + "\n");
        System.out.println("----------------------------\n");
        CustomDataFormat dataObject = new CustomDataFormat();
        dataObject.setFname(valArray[0]);
        dataObject.setLname(valArray[1]);
        return dataObject;
    }

    @Override
    public ObjectInspector getObjectInspector() throws SerDeException {
        return rowOI;
    }

    @Override
    public SerDeStats getSerDeStats() {
        return null;
    }

    @Override
    public Class<? extends Writable> getSerializedClass() {
        return null;
    }

    @Override
    public Writable serialize(Object arg0, ObjectInspector arg1)
            throws SerDeException {
        return null;
    }

}

将保存数据的类

package com.datametica.serde;

import java.util.ArrayList;
import java.util.List;

public class CustomDataFormat {
    String fname;

    String lname;

    /*List<LevelOneStruct> arrayOfLevelTwoStruct = new ArrayList<LevelOneStruct>();

    public List<LevelOneStruct> getArrayOfLevelTwoStruct() {
        return arrayOfLevelTwoStruct;
    }

    public void setArrayOfLevelTwoStruct(
            List<LevelOneStruct> arrayOfLevelTwoStruct) {
        this.arrayOfLevelTwoStruct = arrayOfLevelTwoStruct;
    }*/

    public String getFname() {
        return fname;
    }

    public void setFname(String fname) {
        this.fname = fname;
    }

    public String getLname() {
        return lname;
    }

    public void setLname(String lname) {
        this.lname = lname;
    }

}

CustomDataFormat类的ObjectInspector

package com.datametica.serde;

import java.util.ArrayList;
import java.util.List;

import org.apache.hadoop.hive.serde2.objectinspector.StandardStructObjectInspector;
import org.apache.hadoop.hive.serde2.objectinspector.StructField;

public class CustomStructObjectInspector extends StandardStructObjectInspector {

    @Override
    public Object getStructFieldData(Object data, StructField fieldRef) {
        Object dataToReturn = new Object();
        CustomDataFormat customSerde = (CustomDataFormat) data;
        switch (fieldRef.getFieldName()) {
        case "fname":
            dataToReturn = customSerde.getFname();
            break;
        /*
         * case "arrayOfLevelTwoStruct": dataToReturn =
         * customSerde.getArrayOfLevelTwoStruct(); break;
         */
        case "lname":
            dataToReturn = customSerde.getLname();
            break;
        default:
            dataToReturn = null;
        }
        return dataToReturn;
    }

    @Override
    public List<Object> getStructFieldsDataAsList(Object data) {
        List<Object> listOfData = new ArrayList<Object>();
        CustomDataFormat customSerde = (CustomDataFormat) data;
        listOfData.add(customSerde.getFname());
        listOfData.add(customSerde.getLname());
        return listOfData;
    }

}

创建jar文件后，我正在创建Hive表，如下所示：

create table customserde (fname string,lname string) row format serde 'com.datametica.serde.CustomSerde';

将数据加载到表中，如下所示：

load data inpath '/user/dm3/tables_data/customserde' into table customserde;

到目前为止一切都很好，但是当我对表执行选择操作时

select * from customserde;

获取异常

Caused by: java.lang.ClassCastException: com.datametica.serde.CustomDataFormat cannot be cast to [Ljava.lang.Object;

非常感谢你的帮助，我在这一点上完全陷入了困境。

提前致谢。

- sachingupta

Answer 1

我发现我的错误是deserialize()方法并不返回customdataformat.class对象，而是返回行的对象，也就是ArrayList，如下所示：

public Object deserialize(Writable record) throws SerDeException {
        Text text = (Text) record;
        String[] valArray = text.toString().split(",");
        CustomDataFormat dataObject = new CustomDataFormat();
        dataObject.setFname(valArray[0]);
        dataObject.setLname(valArray[1]);

        rows.set(0, dataObject.getFname());
        rows.set(1, dataObject.getLname());
        return rows;
    }

- sachingupta