java - MongoDB java : Perist POJO class with generic field

Question

我有一个 POJO 类，看起来像这样-

public class CacheEntity<V> {
  private String cacheId;
  private V value;

  public static final String ID_KEY = "cacheId";

  public CacheEntity() {
  }

  public CacheEntity(String cache_id, V value) {
    this.cacheId = cache_id;
    this.value = value;
  }

  public V getValue() {
    return value;
  }

  public void setValue(V value) {
    this.value = value;
  }

  public String getCacheId() {
    return cacheId;
  }

  public void setCacheId(String cacheId) {
    this.cacheId = cacheId;
  }
}

我使用的是官方驱动 mongodb-driver-sync version 3.8.1

如文档中所述 here ，我正在使用带有自动 PojoCodecProvider 的 CodecRegistry。当我尝试保留此实体时出现以下错误。

Exception in thread "main" org.bson.codecs.configuration.CodecConfigurationException: CacheEntity contains generic types that have not been specialised.
Top level classes with generic types are not supported by the PojoCodec.
    at org.bson.codecs.pojo.PojoCodecImpl.encode(PojoCodecImpl.java:93)
    at org.bson.codecs.BsonDocumentWrapperCodec.encode(BsonDocumentWrapperCodec.java:63)
    at org.bson.codecs.BsonDocumentWrapperCodec.encode(BsonDocumentWrapperCodec.java:29)
    at com.mongodb.operation.BulkWriteBatch$WriteRequestEncoder.encode(BulkWriteBatch.java:398)
    at com.mongodb.operation.BulkWriteBatch$WriteRequestEncoder.encode(BulkWriteBatch.java:377)
    at org.bson.codecs.BsonDocumentWrapperCodec.encode(BsonDocumentWrapperCodec.java:63)
    at org.bson.codecs.BsonDocumentWrapperCodec.encode(BsonDocumentWrapperCodec.java:29)
    at com.mongodb.internal.connection.BsonWriterHelper.writeDocument(BsonWriterHelper.java:75)
    at com.mongodb.internal.connection.BsonWriterHelper.writePayload(BsonWriterHelper.java:59)
    at com.mongodb.internal.connection.CommandMessage.encodeMessageBodyWithMetadata(CommandMessage.java:143)
    at com.mongodb.internal.connection.RequestMessage.encode(RequestMessage.java:138)
    at com.mongodb.internal.connection.CommandMessage.encode(CommandMessage.java:57)
    at com.mongodb.internal.connection.InternalStreamConnection.sendAndReceive(InternalStreamConnection.java:244)
    at com.mongodb.internal.connection.UsageTrackingInternalConnection.sendAndReceive(UsageTrackingInternalConnection.java:99)
    at com.mongodb.internal.connection.DefaultConnectionPool$PooledConnection.sendAndReceive(DefaultConnectionPool.java:444)
    at com.mongodb.internal.connection.CommandProtocolImpl.execute(CommandProtocolImpl.java:72)
    at com.mongodb.internal.connection.DefaultServer$DefaultServerProtocolExecutor.execute(DefaultServer.java:200)
    at com.mongodb.internal.connection.DefaultServerConnection.executeProtocol(DefaultServerConnection.java:269)
    at com.mongodb.internal.connection.DefaultServerConnection.command(DefaultServerConnection.java:131)
    at com.mongodb.operation.MixedBulkWriteOperation.executeCommand(MixedBulkWriteOperation.java:418)
    at com.mongodb.operation.MixedBulkWriteOperation.executeBulkWriteBatch(MixedBulkWriteOperation.java:256)
    at com.mongodb.operation.MixedBulkWriteOperation.access$700(MixedBulkWriteOperation.java:67)
    at com.mongodb.operation.MixedBulkWriteOperation$1.call(MixedBulkWriteOperation.java:200)
    at com.mongodb.operation.MixedBulkWriteOperation$1.call(MixedBulkWriteOperation.java:191)
    at com.mongodb.operation.OperationHelper.withReleasableConnection(OperationHelper.java:419)
    at com.mongodb.operation.MixedBulkWriteOperation.execute(MixedBulkWriteOperation.java:191)
    at com.mongodb.operation.MixedBulkWriteOperation.execute(MixedBulkWriteOperation.java:67)
    at com.mongodb.client.internal.MongoClientDelegate$DelegateOperationExecutor.execute(MongoClientDelegate.java:193)
    at com.mongodb.client.internal.MongoCollectionImpl.executeSingleWriteRequest(MongoCollectionImpl.java:960)
    at com.mongodb.client.internal.MongoCollectionImpl.executeReplaceOne(MongoCollectionImpl.java:602)
    at com.mongodb.client.internal.MongoCollectionImpl.replaceOne(MongoCollectionImpl.java:578)
    at com.mongodb.client.internal.MongoCollectionImpl.replaceOne(MongoCollectionImpl.java:567)

我该怎么做才能使这项工作正常进行？谢谢。

Answer 1

有以下选项可以使其正常工作。

通过子类化

如果你每次都使用具体的泛型类型进行子类化，它会起作用:

public class MyClassCacheEntity extends CacheEntity<MyClass> {}

通过使用 BsonDocument + Jackson

为避免子类化，您可以尝试将任意对象存储为 BsonDocument-s 并使用 Jackson 对它们进行序列化/反序列化:

public class CacheEntity<V> {
  private String cacheId;
  private BsonDocument rawValue;

  // [...]

  // sets the value
  public void withValue(T o) {
    this.rawValue = BsonDocument.parse(new ObjectMapper().writeValueAsString(o))
  }

  // recovers the value
  public T value(Class<T> clazz) {
    return new ObjectMapper().readValue(this.rawValue.toJson(), clazz);
  }
}

除非您避免 withValue() 和 value() 方法的 getters/setters 约定，否则 Mongo 会开始提示相同的泛型问题。

>

与纯bson相同

我认为，您也可以尝试使用 org.bson.codecs.pojo.PojoCodec 执行与上述相同的操作，您可以创建或从当前的 mongo 编解码器注册表中提取它。如果你提前知道并给它真正的类，它不应该提示泛型。