在java中将Arrow转换为Parquet，反之亦然

在Java中将Arrow转换为Parquet，反之亦然，可以通过使用Apache Arrow和Apache Parquet库来实现。

Apache Arrow是一个内存数据结构和计算平台，用于在不同系统之间高效地传输和处理大规模数据集。它提供了一种统一的数据模型，可以在不同的编程语言和计算框架之间进行数据交换和共享。Arrow的主要优势包括高性能、低内存占用和跨平台支持。

Apache Parquet是一种列式存储格式，用于高效地存储和处理大规模结构化数据。它支持压缩、谓词下推、列式存储和高效的读写操作，适用于大数据分析和数据仓库场景。Parquet的主要优势包括高性能、高压缩比和灵活的数据模型。

要在Java中将Arrow转换为Parquet，可以按照以下步骤进行：

导入所需的依赖库：

import org.apache.arrow.vector.VectorSchemaRoot;
import org.apache.arrow.vector.ipc.ArrowFileReader;
import org.apache.arrow.vector.ipc.ArrowFileWriter;
import org.apache.arrow.vector.ipc.SeekableReadChannel;
import org.apache.arrow.vector.ipc.WriteChannel;
import org.apache.parquet.column.ColumnDescriptor;
import org.apache.parquet.column.ParquetProperties;
import org.apache.parquet.column.page.PageReadStore;
import org.apache.parquet.example.data.Group;
import org.apache.parquet.example.data.simple.SimpleGroupFactory;
import org.apache.parquet.hadoop.ParquetFileWriter;
import org.apache.parquet.hadoop.ParquetReader;
import org.apache.parquet.hadoop.ParquetWriter;
import org.apache.parquet.hadoop.metadata.CompressionCodecName;
import org.apache.parquet.hadoop.metadata.ParquetMetadata;
import org.apache.parquet.hadoop.util.HadoopInputFile;
import org.apache.parquet.hadoop.util.HadoopOutputFile;
import org.apache.parquet.io.ColumnIOFactory;
import org.apache.parquet.io.MessageColumnIO;
import org.apache.parquet.io.ParquetDecodingException;
import org.apache.parquet.io.RecordReader;
import org.apache.parquet.io.api.Binary;
import org.apache.parquet.io.api.RecordMaterializer;
import org.apache.parquet.schema.MessageType;
import org.apache.parquet.schema.PrimitiveType;
import org.apache.parquet.schema.Type;
import org.apache.parquet.schema.Types;

将Arrow转换为Parquet：

// 读取Arrow文件
try (SeekableReadChannel arrowChannel = SeekableReadChannel.fromFile(arrowFilePath);
     ArrowFileReader arrowReader = new ArrowFileReader(arrowChannel, new RootAllocator(Long.MAX_VALUE))) {
    VectorSchemaRoot arrowRoot = arrowReader.getVectorSchemaRoot();
    MessageType parquetSchema = convertArrowSchemaToParquet(arrowRoot.getSchema());

    // 创建Parquet写入器
    try (HadoopOutputFile parquetFile = HadoopOutputFile.fromPath(parquetFilePath, new Configuration());
         ParquetWriter<Group> parquetWriter = createParquetWriter(parquetFile, parquetSchema)) {
        // 逐行读取Arrow数据并写入Parquet文件
        while (arrowReader.loadNextBatch()) {
            writeArrowBatchToParquet(arrowRoot, parquetWriter);
        }
    }
}

// 将Arrow的Schema转换为Parquet的Schema
private static MessageType convertArrowSchemaToParquet(org.apache.arrow.vector.types.pojo.Schema arrowSchema) {
    List<Type> parquetFields = new ArrayList<>();
    for (org.apache.arrow.vector.types.pojo.Field arrowField : arrowSchema.getFields()) {
        Type parquetField = convertArrowFieldToParquet(arrowField);
        parquetFields.add(parquetField);
    }
    return new MessageType("root", parquetFields);
}

// 将Arrow的Field转换为Parquet的Field
private static Type convertArrowFieldToParquet(org.apache.arrow.vector.types.pojo.Field arrowField) {
    PrimitiveType.PrimitiveTypeName parquetType = convertArrowTypeToParquet(arrowField.getFieldType().getTypeID());
    return Types.buildMessage().addField(new PrimitiveType(Type.Repetition.OPTIONAL, parquetType,
            arrowField.getName())).named(arrowField.getName());
}

// 将Arrow的Type转换为Parquet的Type
private static PrimitiveType.PrimitiveTypeName convertArrowTypeToParquet(org.apache.arrow.vector.types.Types.MinorType arrowType) {
    switch (arrowType) {
        case INT:
            return PrimitiveType.PrimitiveTypeName.INT32;
        case BIGINT:
            return PrimitiveType.PrimitiveTypeName.INT64;
        case FLOAT4:
            return PrimitiveType.PrimitiveTypeName.FLOAT;
        case FLOAT8:
            return PrimitiveType.PrimitiveTypeName.DOUBLE;
        case BINARY:
            return PrimitiveType.PrimitiveTypeName.BINARY;
        case STRING:
            return PrimitiveType.PrimitiveTypeName.BINARY;
        // 其他类型根据需要进行转换
        default:
            throw new UnsupportedOperationException("Unsupported Arrow type: " + arrowType);
    }
}

// 创建Parquet写入器
private static ParquetWriter<Group> createParquetWriter(HadoopOutputFile parquetFile, MessageType parquetSchema) throws IOException {
    return ParquetWriter.builder(new GroupWriteSupport(), parquetFile)
            .withType(parquetSchema)
            .withCompressionCodec(CompressionCodecName.SNAPPY)
            .withWriteMode(ParquetFileWriter.Mode.OVERWRITE)
            .build();
}

// 将Arrow的Batch数据写入Parquet文件
private static void writeArrowBatchToParquet(VectorSchemaRoot arrowRoot, ParquetWriter<Group> parquetWriter) throws IOException {
    int numRows = arrowRoot.getRowCount();
    for (int row = 0; row < numRows; row++) {
        Group parquetGroup = convertArrowRowToParquetGroup(arrowRoot, row);
        parquetWriter.write(parquetGroup);
    }
}

// 将Arrow的Row数据转换为Parquet的Group
private static Group convertArrowRowToParquetGroup(VectorSchemaRoot arrowRoot, int row) {
    SimpleGroupFactory groupFactory = new SimpleGroupFactory(arrowRoot.getSchema());
    Group parquetGroup = groupFactory.newGroup();
    for (int col = 0; col < arrowRoot.getFieldVectors().size(); col++) {
        String fieldName = arrowRoot.getFieldVectors().get(col).getField().getName();
        Object fieldValue = convertArrowValueToParquetValue(arrowRoot.getFieldVectors().get(col), row);
        parquetGroup.append(fieldName, fieldValue);
    }
    return parquetGroup;
}

// 将Arrow的Value转换为Parquet的Value
private static Object convertArrowValueToParquetValue(ValueVector arrowVector, int index) {
    Object value = arrowVector.getObject(index);
    if (arrowVector.isNull(index)) {
        return null;
    } else if (arrowVector instanceof VarCharVector) {
        return Binary.fromConstantByteArray(((VarCharVector) arrowVector).get(index));
    } else {
        return value;
    }
}

要在Java中将Parquet转换为Arrow，可以按照以下步骤进行：

导入所需的依赖库：

import org.apache.arrow.memory.RootAllocator;
import org.apache.arrow.vector.VectorSchemaRoot;
import org.apache.arrow.vector.ipc.ArrowFileWriter;
import org.apache.arrow.vector.ipc.ArrowStreamWriter;
import org.apache.arrow.vector.ipc.SeekableWriteChannel;
import org.apache.arrow.vector.ipc.message.ArrowRecordBatch;
import org.apache.parquet.column.ColumnDescriptor;
import org.apache.parquet.column.page.PageReadStore;
import org.apache.parquet.hadoop.ParquetFileReader;
import org.apache.parquet.hadoop.metadata.ParquetMetadata;
import org.apache.parquet.io.ColumnIOFactory;
import org.apache.parquet.io.MessageColumnIO;
import org.apache.parquet.io.RecordReader;
import org.apache.parquet.io.api.RecordMaterializer;
import org.apache.parquet.schema.MessageType;
import org.apache.parquet.schema.Type;
import org.apache.parquet.schema.Types;

将Parquet转换为Arrow：

// 读取Parquet文件
try (SeekableReadChannel parquetChannel = SeekableReadChannel.fromFile(parquetFilePath);
     ParquetFileReader parquetReader = ParquetFileReader.open(parquetChannel)) {
    ParquetMetadata parquetMetadata = parquetReader.getFooter();
    MessageType parquetSchema = parquetMetadata.getFileMetaData().getSchema();

    // 创建Arrow写入器
    try (SeekableWriteChannel arrowChannel = SeekableWriteChannel.fromFile(arrowFilePath);
         ArrowFileWriter arrowWriter = new ArrowFileWriter(new RootAllocator(Long.MAX_VALUE), null,
                 new ArrowStreamWriter(arrowChannel, new RootAllocator(Long.MAX_VALUE)))) {
        // 写入Arrow文件头
        arrowWriter.start(parquetSchema, null);

        // 逐行读取Parquet数据并写入Arrow文件
        PageReadStore pageReadStore;
        while ((pageReadStore = parquetReader.readNextRowGroup()) != null) {
            RecordReader<Group> recordReader = createParquetRecordReader(parquetSchema, pageReadStore);
            Group parquetGroup;
            while ((parquetGroup = recordReader.read()) != null) {
                ArrowRecordBatch arrowBatch = convertParquetGroupToArrowBatch(parquetGroup, parquetSchema);
                arrowWriter.writeBatch(arrowBatch);
            }
        }

        // 写入Arrow文件尾
        arrowWriter.end();
    }
}

// 创建Parquet记录读取器
private static RecordReader<Group> createParquetRecordReader(MessageType parquetSchema, PageReadStore pageReadStore) {
    MessageColumnIO columnIO = new ColumnIOFactory().getColumnIO(parquetSchema);
    RecordMaterializer<Group> recordMaterializer = columnIO.getRecordMaterializer();
    return columnIO.getRecordReader(pageReadStore, recordMaterializer);
}

// 将Parquet的Group转换为Arrow的RecordBatch
private static ArrowRecordBatch convertParquetGroupToArrowBatch(Group parquetGroup, MessageType parquetSchema) {
    VectorSchemaRoot arrowRoot = VectorSchemaRoot.create(convertParquetSchemaToArrowSchema(parquetSchema),
            new RootAllocator(Long.MAX_VALUE));
    for (int col = 0; col < parquetGroup.getFieldRepetitionCount(); col++) {
        String fieldName = parquetGroup.getType().getFieldName(col);
        Object fieldValue = convertParquetValueToArrowValue(parquetGroup.getValueToString(col, 0));
        arrowRoot.getFieldVectors().get(col).setSafe(0, fieldValue);
    }
    arrowRoot.setRowCount(1);
    return new ArrowRecordBatch(1, arrowRoot);
}

// 将Parquet的Schema转换为Arrow的Schema
private static org.apache.arrow.vector.types.pojo.Schema convertParquetSchemaToArrowSchema(MessageType parquetSchema) {
    org.apache.arrow.vector.types.pojo.Schema.Builder arrowSchemaBuilder = org.apache.arrow.vector.types.pojo.Schema.builder();
    for (Type parquetField : parquetSchema.getFields()) {
        org.apache.arrow.vector.types.pojo.Field arrowField = convertParquetFieldToArrowField(parquetField);
        arrowSchemaBuilder.addField(arrowField);
    }
    return arrowSchemaBuilder.build();
}

// 将Parquet的Field转换为Arrow的Field
private static org.apache.arrow.vector.types.pojo.Field convertParquetFieldToArrowField(Type parquetField) {
    org.apache.arrow.vector.types.pojo.Field.Builder arrowFieldBuilder = org.apache.arrow.vector.types.pojo.Field.newBuilder();
    arrowFieldBuilder.setName(parquetField.getName());
    arrowFieldBuilder.setNullable(true); // Parquet中的字段都是可空的
    arrowFieldBuilder.setType(convertParquetTypeToArrowType(parquetField.asPrimitiveType()));
    return arrowFieldBuilder.build();
}

// 将Parquet的Type转换为Arrow的Type
private static org.apache.arrow.vector.types.pojo.ArrowType convertParquetTypeToArrowType(PrimitiveType parquetType) {
    switch (parquetType.getPrimitiveTypeName()) {
        case INT32:
            return org.apache.arrow.vector.types.pojo.ArrowType.Int32.INSTANCE;
        case INT64:
            return org.apache.arrow.vector.types.pojo.ArrowType.Int64.INSTANCE;
        case FLOAT:
            return org.apache.arrow.vector.types.pojo.ArrowType.Float.INSTANCE;
        case DOUBLE:
            return org.apache.arrow.vector.types.pojo.ArrowType.Double.INSTANCE;
        case BINARY:
            return org.apache.arrow.vector.types.pojo.ArrowType.Binary.INSTANCE;
        // 其他类型根据需要进行转换
        default:
            throw new UnsupportedOperationException("Unsupported Parquet type: " + parquetType.getPrimitiveTypeName());
    }
}

// 将Parquet的Value转换为Arrow的Value
private static Object convertParquetValueToArrowValue(String parquetValue) {
    // 根据Parquet的数据类型进行转换
    return parquetValue;
}

以上是在Java中将Arrow转换为Parquet，反之亦然的完整步骤和代码示例。这些步骤涵盖了读取Arrow和Parquet文件、转换Schema、逐行读取数据、转换数据类型等关键操作。通过使用Apache Arrow和Apache Parquet库，可以实现高效、灵活的数据转换和处理。