使用Jackson进行Java Mongo DBObject的高效POJO映射

Question

虽然与Convert DBObject to a POJO using MongoDB Java Driver类似，但我的问题不同，因为我特别对使用Jackson进行映射感兴趣。

我有一个对象，我想转换为Mongo DBObject实例。我想使用Jackson JSON框架来完成这项工作。

这样做的一种方法是：

DBObject dbo = (DBObject)JSON.parse(m_objectMapper.writeValueAsString(entity));

然而，根据https://github.com/FasterXML/jackson-docs/wiki/Presentation:-Jackson-Performance，这是最糟糕的方式。所以，我正在寻找替代方案。理想情况下，我希望能够挂钩到JSON生成管道并动态填充DBObject实例。这是可能的，因为在我的情况下，目标是BasicDBObject实例，它实现了Map接口。因此，它应该很容易适应管道。

现在，我知道我可以使用ObjectMapper.convertValue函数将对象转换为Map，然后使用BasicDBObject类型的地图构造函数将地图递归转换为BasicDBObject实例。但是，我想知道我是否可以删除中间地图并直接创建BasicDBObject。

注意，因为BasicDBObject本质上是一个地图，所以相反的转换，即从标量DBObject到POJO是微不足道的，应该非常有效：

DBObject dbo = getDBO();
Class clazz = getObjectClass();
Object pojo = m_objectMapper.convertValue(dbo, clazz);

最后，我的POJO没有任何JSON注释，我希望它能保持这种状态。

Answer 1

您可以使用Mixin注释来注释您的POJO和BasicDBObject（或DBObject），因此注释不是问题。由于BasicDBOject是地图，因此您可以在put方法上使用@JsonAnySetter。

m_objectMapper.addMixInAnnotations(YourMixIn.class, BasicDBObject.class);

public interface YourMixIn.class {
    @JsonAnySetter
    void put(String key, Object value);
}

由于我对MongoDB Object没有任何经验，所以我能想到这一切。

更新： MixIn基本上是一种杰克逊机制，可以在不修改所述类的情况下向类中添加注释。如果您无法控制要编组的类（例如，当它来自外部jar）或者您不希望使用注释使类混乱时，这是完美的选择。

在这种情况下，您说BasicDBObject实现了Map接口，因此该类具有方法put，由地图接口定义。通过向该方法添加@JsonAnySetter，您可以告诉Jackson，每当他在类内省后发现他不知道的属性时，使用该方法将属性插入到对象中。关键是属性的名称，值是属性的值。

所有这些组合使得中间地图消失，因为Jackson将直接转换为BasicDBOject，因为它现在知道如何从Json反序列化该类。使用该配置，您可以执行以下操作：

DBObject dbo = m_objectMapper.convertValue(pojo, BasicDBObject.class);

请注意，我没有对此进行测试，因为我不使用MongoDB，因此可能会有一些松散的结果。但是，我对相似的用例使用了相同的机制，没有任何问题。 YMMV取决于班级。

Answer 2

以下是从POJO到BsonDocument 的简单序列化程序（用Scala编写）的示例，它可以与Mongo驱动程序的版本3一起使用。反序列化器写起来会有些困难。

创建一个BsonObjectGenerator对象，直接对Mongo Bson进行流式序列化：

val generator = new BsonObjectGenerator
mapper.writeValue(generator, POJO)
generator.result()

以下是序列化程序的代码：

class BsonObjectGenerator extends JsonGenerator {

  sealed trait MongoJsonStreamContext extends JsonStreamContext

  case class MongoRoot(root: BsonDocument = BsonDocument()) extends MongoJsonStreamContext {
    _type = JsonStreamContext.TYPE_ROOT

    override def getCurrentName: String = null

    override def getParent: MongoJsonStreamContext = null
  }

  case class MongoArray(parent: MongoJsonStreamContext, arr: BsonArray = BsonArray()) extends MongoJsonStreamContext {
    _type = JsonStreamContext.TYPE_ARRAY

    override def getCurrentName: String = null

    override def getParent: MongoJsonStreamContext = parent
  }

  case class MongoObject(name: String, parent: MongoJsonStreamContext, obj: BsonDocument = BsonDocument()) extends MongoJsonStreamContext {
    _type = JsonStreamContext.TYPE_OBJECT

    override def getCurrentName: String = name

    override def getParent: MongoJsonStreamContext = parent
  }

  private val root = MongoRoot()
  private var node: MongoJsonStreamContext = root

  private var fieldName: String = _

  def result(): BsonDocument = root.root

  private def unsupported(): Nothing = throw new UnsupportedOperationException

  override def disable(f: Feature): JsonGenerator = this

  override def writeStartArray(): Unit = {
    val array = new BsonArray
    node match {
      case MongoRoot(o) =>
        o.append(fieldName, array)
        fieldName = null
      case MongoArray(_, a) =>
        a.add(array)
      case MongoObject(_, _, o) =>
        o.append(fieldName, array)
        fieldName = null
    }
    node = MongoArray(node, array)
  }

  private def writeBsonValue(value: BsonValue): Unit = node match {
    case MongoRoot(o) =>
      o.append(fieldName, value)
      fieldName = null
    case MongoArray(_, a) =>
      a.add(value)
    case MongoObject(_, _, o) =>
      o.append(fieldName, value)
      fieldName = null
  }

  private def writeBsonString(text: String): Unit = {
    writeBsonValue(BsonString(text))
  }

  override def writeString(text: String): Unit = writeBsonString(text)

  override def writeString(text: Array[Char], offset: Int, len: Int): Unit = writeBsonString(new String(text, offset, len))

  override def writeString(text: SerializableString): Unit = writeBsonString(text.getValue)

  private def writeBsonFieldName(name: String): Unit = {
    fieldName = name
  }

  override def writeFieldName(name: String): Unit = writeBsonFieldName(name)

  override def writeFieldName(name: SerializableString): Unit = writeBsonFieldName(name.getValue)

  override def setCodec(oc: ObjectCodec): JsonGenerator = this

  override def useDefaultPrettyPrinter(): JsonGenerator = this

  override def getFeatureMask: Int = 0

  private def writeBsonBinary(data: Array[Byte]): Unit = {
    writeBsonValue(BsonBinary(data))
  }

  override def writeBinary(bv: Base64Variant, data: Array[Byte], offset: Int, len: Int): Unit = {
    val res = if (offset != 0 || len != data.length) {
      val subset = new Array[Byte](len)
      System.arraycopy(data, offset, subset, 0, len)
      subset
    } else {
      data
    }
    writeBsonBinary(res)
  }

  override def writeBinary(bv: Base64Variant, data: InputStream, dataLength: Int): Int = unsupported()

  override def isEnabled(f: Feature): Boolean = false

  override def writeRawUTF8String(text: Array[Byte], offset: Int, length: Int): Unit = writeBsonString(new String(text, offset, length, "UTF-8"))

  override def writeRaw(text: String): Unit = unsupported()

  override def writeRaw(text: String, offset: Int, len: Int): Unit = unsupported()

  override def writeRaw(text: Array[Char], offset: Int, len: Int): Unit = unsupported()

  override def writeRaw(c: Char): Unit = unsupported()

  override def flush(): Unit = ()

  override def writeRawValue(text: String): Unit = writeBsonString(text)

  override def writeRawValue(text: String, offset: Int, len: Int): Unit = writeBsonString(text.substring(offset, offset + len))

  override def writeRawValue(text: Array[Char], offset: Int, len: Int): Unit = writeBsonString(new String(text, offset, len))

  override def writeBoolean(state: Boolean): Unit = {
    writeBsonValue(BsonBoolean(state))
  }

  override def writeStartObject(): Unit = {
    node = node match {
      case p@MongoRoot(o) =>
        MongoObject(null, p, o)
      case p@MongoArray(_, a) =>
        val doc = new BsonDocument
        a.add(doc)
        MongoObject(null, p, doc)
      case p@MongoObject(_, _, o) =>
        val doc = new BsonDocument
        val f = fieldName
        o.append(f, doc)
        fieldName = null
        MongoObject(f, p, doc)
    }
  }

  override def writeObject(pojo: scala.Any): Unit = unsupported()

  override def enable(f: Feature): JsonGenerator = this

  override def writeEndArray(): Unit = {
    node = node match {
      case MongoRoot(_) => unsupported()
      case MongoArray(p, a) => p
      case MongoObject(_, _, _) => unsupported()
    }
  }

  override def writeUTF8String(text: Array[Byte], offset: Int, length: Int): Unit = writeBsonString(new String(text, offset, length, "UTF-8"))

  override def close(): Unit = ()

  override def writeTree(rootNode: TreeNode): Unit = unsupported()

  override def setFeatureMask(values: Int): JsonGenerator = this

  override def isClosed: Boolean = unsupported()

  override def writeNull(): Unit = {
    writeBsonValue(BsonNull())
  }

  override def writeNumber(v: Int): Unit = {
    writeBsonValue(BsonInt32(v))
  }

  override def writeNumber(v: Long): Unit = {
    writeBsonValue(BsonInt64(v))
  }

  override def writeNumber(v: BigInteger): Unit = unsupported()

  override def writeNumber(v: Double): Unit = {
    writeBsonValue(BsonDouble(v))
  }

  override def writeNumber(v: Float): Unit = {
    writeBsonValue(BsonDouble(v))
  }

  override def writeNumber(v: BigDecimal): Unit = unsupported()

  override def writeNumber(encodedValue: String): Unit = unsupported()

  override def version(): Version = unsupported()

  override def getCodec: ObjectCodec = unsupported()

  override def getOutputContext: JsonStreamContext = node

  override def writeEndObject(): Unit = {
    node = node match {
      case p@MongoRoot(_) => p
      case MongoArray(p, a) => unsupported()
      case MongoObject(_, p, _) => p
    }
  }
}

Answer 3

您可能会检查jongo如何做到这一点。它是开源的，代码可以在github找到。或者您也可以简单地使用他们的库。当我需要更多的灵活性时，我会使用jongo和plain DBObject的混合。

他们声称他们（几乎）与直接使用Java驱动程序一样快，所以我认为他们的方法是有效的。

我使用下面的小助手实用程序类，它受到代码库的启发，并使用Jongo（MongoBsonFactory）和Jackson的混合来在DBObjects和POJO之间进行转换。请注意，getDbObject方法执行DBObject的深层副本以使其可编辑 - 如果您不需要自定义任何内容，则可以删除该部分并提高性能。

import com.fasterxml.jackson.annotation.JsonAutoDetect;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.ObjectReader;
import com.fasterxml.jackson.databind.ObjectWriter;
import com.fasterxml.jackson.databind.introspect.VisibilityChecker;
import com.mongodb.BasicDBObject;
import com.mongodb.DBEncoder;
import com.mongodb.DBObject;
import com.mongodb.DefaultDBEncoder;
import com.mongodb.LazyWriteableDBObject;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import org.bson.LazyBSONCallback;
import org.bson.io.BasicOutputBuffer;
import org.bson.io.OutputBuffer;
import org.jongo.marshall.jackson.bson4jackson.MongoBsonFactory;

public class JongoUtils {

    private final static ObjectMapper mapper = new ObjectMapper(MongoBsonFactory.createFactory());

    static {
        mapper.setVisibilityChecker(VisibilityChecker.Std.defaultInstance().withFieldVisibility(
                JsonAutoDetect.Visibility.ANY));
    }

    public static DBObject getDbObject(Object o) throws IOException {
        ObjectWriter writer = mapper.writer();
        ByteArrayOutputStream baos = new ByteArrayOutputStream();

        writer.writeValue(baos, o);
        DBObject dbo = new LazyWriteableDBObject(baos.toByteArray(), new LazyBSONCallback());
        //turn it into a proper DBObject otherwise it can't be edited.
        DBObject result = new BasicDBObject();
        result.putAll(dbo);
        return result;
    }

    public static <T> T getPojo(DBObject o, Class<T> clazz) throws IOException {
        ObjectReader reader = mapper.reader(clazz);
        DBEncoder dbEncoder = DefaultDBEncoder.FACTORY.create();
        OutputBuffer buffer = new BasicOutputBuffer();
        dbEncoder.writeObject(buffer, o);

        T pojo = reader.readValue(buffer.toByteArray());

        return pojo;
    }
}

样本用法：

Pojo pojo = new Pojo(...);
DBObject o = JongoUtils.getDbObject(pojo);
//you can customise it if you want:
o.put("_id", pojo.getId());

Answer 4

我知道这是一个非常古老的问题，但如果今天被问到，我会建议在官方的Mongo Java驱动程序上使用built-in POJO support。

Answer 5

以下是对assylias'答案的更新，该答案不需要Jongo并且与Mongo 3.x驱动程序兼容。它还处理嵌套的对象图，我无法使用已经在mongo 3.x驱动程序中删除的LazyWritableDBObject。

这个想法是告诉Jackson如何将对象序列化为BSON字节数组，然后将BSON字节数组反序列化为BasicDBObject。如果你想将BSON字节直接发送到数据库，我相信你可以在mongo-java-drivers中找到一些低级API。当您致电ObjectMapper时，您需要依赖bson4jackson才能让writeValues(ByteArrayOutputStream, Object)序列化BSON：

import com.fasterxml.jackson.databind.ObjectMapper;
import com.mongodb.BasicDBObject;
import com.mongodb.DBObject;
import de.undercouch.bson4jackson.BsonFactory;
import de.undercouch.bson4jackson.BsonParser;
import org.bson.BSON;
import org.bson.BSONObject;

import java.io.ByteArrayOutputStream;
import java.io.IOException;

public class MongoUtils {

    private static ObjectMapper mapper;

    static {
        BsonFactory bsonFactory = new BsonFactory();
        bsonFactory.enable(BsonParser.Feature.HONOR_DOCUMENT_LENGTH);
        mapper = new ObjectMapper(bsonFactory);
    }

    public static DBObject getDbObject(Object o) {
        try {
            ByteArrayOutputStream baos = new ByteArrayOutputStream();
            mapper.writeValue(baos, o);

            BSONObject decode = BSON.decode(baos.toByteArray());
            return new BasicDBObject(decode.toMap());
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }
}