如何在Java或Scala中读取和写入来自/到镶木地板文件的Map <String,Object>？

Question

寻找一个关于如何Map<String, Object>在Java或Scala中读取/ 写入镶木地板文件的简明示例？

这是预期的结构,com.fasterxml.jackson.databind.ObjectMapper 在Java中用作序列化器(即使用镶木地板寻找等效物):

public static Map<String, Object> read(InputStream inputStream) throws IOException {
    ObjectMapper objectMapper = new ObjectMapper();

    return objectMapper.readValue(inputStream, new TypeReference<Map<String, Object>>() {

    });
}

public static void write(OutputStream outputStream, Map<String, Object> map) throws IOException {
    ObjectMapper objectMapper = new ObjectMapper();

    objectMapper.writeValue(outputStream, map);        
}

Answer 1

我不太擅长镶木地板，但是，从这里开始：

Schema schema = new Schema.Parser().parse(Resources.getResource("map.avsc").openStream());

    File tmp = File.createTempFile(getClass().getSimpleName(), ".tmp");
    tmp.deleteOnExit();
    tmp.delete();
    Path file = new Path(tmp.getPath());

    AvroParquetWriter<GenericRecord> writer = 
        new AvroParquetWriter<GenericRecord>(file, schema);

    // Write a record with an empty map.
    ImmutableMap emptyMap = new ImmutableMap.Builder<String, Integer>().build();
    GenericData.Record record = new GenericRecordBuilder(schema)
        .set("mymap", emptyMap).build();
    writer.write(record);
    writer.close();

    AvroParquetReader<GenericRecord> reader = new AvroParquetReader<GenericRecord>(file);
    GenericRecord nextRecord = reader.read();

    assertNotNull(nextRecord);
    assertEquals(emptyMap, nextRecord.get("mymap"));

根据您的情况，ImmutableMap使用默认地图更改（Google Collections），如下所示：

Schema schema = new Schema.Parser().parse( Resources.getResource( "map.avsc" ).openStream() );

        File tmp = File.createTempFile( getClass().getSimpleName(), ".tmp" );
        tmp.deleteOnExit();
        tmp.delete();
        Path file = new Path( tmp.getPath() );

        AvroParquetWriter<GenericRecord> writer = new AvroParquetWriter<GenericRecord>( file, schema );

        // Write a record with an empty map.
        Map<String,Object> emptyMap = new HashMap<String, Object>();

        // not empty any more
        emptyMap.put( "SOMETHING", new SOMETHING() );
        GenericData.Record record = new GenericRecordBuilder( schema ).set( "mymap", emptyMap ).build();
        writer.write( record );
        writer.close();

        AvroParquetReader<GenericRecord> reader = new AvroParquetReader<GenericRecord>( file );
        GenericRecord nextRecord = reader.read();

        assertNotNull( nextRecord );
        assertEquals( emptyMap, nextRecord.get( "mymap" ) );

我没有测试代码，但尝试一下..