Jackson无法反序列化通过REST传递的MongoDB对象 [英] Jackson can't deserialize MongoDB object passed through REST

问题描述

尝试设置应用程序以在Web应用程序中显示MongoDB嵌套数据（嵌套和嵌入的两个级别）（用于业务分析）。

Trying to set up an application to display MongoDB nested data (two levels of nesting and embedding) in a web application (for business analysis).

我是使用Dropwizard，所以我复制了 dropwizard-mongo 示例，将其减少到必要的部分（没有删除，没有插入，没有指标或任何东西）。

I'm using Dropwizard, so I copied the dropwizard-mongo example, reduced it to the necessary parts (no delete, no insert, no metrics or anything).

App.java

package test;


import io.dropwizard.Application;
import io.dropwizard.setup.Bootstrap;
import io.dropwizard.setup.Environment;

import com.meltmedia.dropwizard.mongo.MongoBundle;
import test.Res;

public class App extends Application<Config> {
  public static void main(String[] args) throws Exception {
    new App().run(args);
  }

  MongoBundle<Config> mongoBundle;

  @Override
  public void initialize(Bootstrap<Config> bootstrap) {
    bootstrap.addBundle(mongoBundle =
        MongoBundle.<Config> builder()
            .withConfiguration(Config::getMongo).build());
  }

  @Override
  public void run(Config config, Environment env) throws Exception {
    env.jersey().register(new Res(mongoBundle.getDB()));
  }


}

Res.java （资源类）

Res.java (Resource class)

package test;

import java.util.List;
import java.util.Set;


import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.PathParam;
import javax.ws.rs.Produces;
import javax.ws.rs.WebApplicationException;
import javax.ws.rs.core.Response;
import javax.ws.rs.core.Response.Status;

import org.jongo.Jongo;
import org.jongo.MongoCollection;

import com.fasterxml.jackson.databind.node.ObjectNode;
import com.mongodb.DB;

@Path("/")
public class Res {

  DB database;
  Jongo jongo;

  public Res(DB database) {
    this.database = database;
    this.jongo = new Jongo(database);
  }

  @GET
  @Produces("application/json")
  public Set<String> collectionNames() {
    return database.getCollectionNames();
  }

  @Path("{collectionName}")
  public CollectionResource collection(@PathParam("collectionName") String name) {
    return new CollectionResource(jongo.getCollection(name));
  }

  public class CollectionResource {

    MongoCollection collection;

    public CollectionResource(MongoCollection collection) {
      this.collection = collection;
    }

    @GET
    @Produces("application/json")
    public List<String> list() {
      return collection.distinct("_id").as(String.class);
    }

    @GET
    @Path("{id}")
    @Produces("application/json")
    public ObjectNode getDocument(@PathParam("id") String id) {
      ObjectNode node = collection.findOne("{_id: #}", id).as(ObjectNode.class);
      if (node == null) {
        throw new WebApplicationException(Response.status(Status.NOT_FOUND).build());
      }
      return node;
    }

  }

}

我可以卷曲到/并将返回集合，所以我会说与数据库的连接有效。但是，如果我卷曲任何集合，无论是实际数据还是只有普通KVP字母和数字的虚拟对象，甚至只是一个空对象（仅限_id），我总是得到

I can curl to / and will get the collections returned, so I'll say the connection to the DB works. However, if I curl any of the collections, be it the actual data or a dummy with just plain KVP of letters and numbers or even just an empty object (_id only), I always get

com.fasterxml.jackson.databind.JsonMappingException：org.bson.types.ObjectId无法强制转换为java.lang.String

com.fasterxml.jackson.databind.JsonMappingException: org.bson.types.ObjectId cannot be cast to java.lang.String

鉴于我对这个例子几乎没有做任何事情，我有点想知道如何处理这个问题。

Given I hardly did anything to the example, I'm somewhat at loss for ideas what to do about this.

推荐答案

你有一个 POST 方法创建数据还是你正在使用现有的集合？

Do you have a POST method creating data or are you using an existing collection?

你可以看到这里在他们的例子中，他们是明确地处理 _id 作为字符串而不是 ObjectId 。我的猜测就是绕过你在这里看到的问题。

You can see here that in their example, they are are explicitly treating _id as a String and NOT an ObjectId. My guess is that was to get around the very issue you are seeing here.

Jongo有一个映射 ObjectId 的许多方法，但它们都需要使用自定义POJO并注释 id 字段，因此它们不会像显示的示例那样灵活。

Jongo has many ways of mapping an ObjectId, but they all requiring using a custom POJO and annotating the id field so they wont be as flexible as the example shown.

如果可能，转换所有 _id 数据库中的字段是纯字符串，否则你需要找到一些处理 ObjectId 类型（输入到POJO或修改Jongo Mapper 或许）。

If possible, convert all your _id fields in the database to be plain strings, otherwise you will need to find someway to deal with the ObjectId type (either typing to to a POJO or modifying the Jongo Mapper perhaps).

祝你好运：）

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