我有一个用户

public class User {
    private String name;
    private String email;

    public User () {  }

    public User(String name) {
        this.name = name;
    }

    public User(String name, String email) {
        this(name);
        this.email = email;
    }
    // getters and setters
}

我还有简单的 POJO Comment

public class Comment {
    private String comment;
    private Date date;
    private String author;

    public Comment() { }

    public Comment(String comment, Date date, String author) {
        this.comment = comment;
        this.date = date;
        this.author = author;
    }
    // getters and setters
}

我想如何将新用户插入到集合中，并像这样对他发表一些评论:

public static void main(String[] args) {
    MongoClient client = new MongoClient();
    MongoDatabase db = client.getDatabase("example");
    MongoCollection<Document> collection = db.getCollection("object_arrays");

    collection.drop();

    List<Comment> reviews = new ArrayList<Comment>(){{
        add(new Comment("cool guy", new Date(), "John Doe"));
        add(new Comment("best joker", new Date(), "Vas Negas"));
        add(new Comment("very stupid but very funny man", new Date(), "Bill Murphy"));
    }};
    Document user = new Document();
    user.append("user", new User("0xFF", "email@email.com"))
            .append("reviews", reviews)
            .append("createDate", new Date());
    collection.insertOne(user);
}

不幸的是，我遇到了异常:

    Exception in thread "main" org.bson.codecs.configuration.CodecConfigurationException: Can't find a codec for class com.mongodb.course.com.mongodb.course.model.User.
    at org.bson.codecs.configuration.CodecCache.getOrThrow(CodecCache.java:46)
    at org.bson.codecs.configuration.ProvidersCodecRegistry.get(ProvidersCodecRegistry.java:63)
    at org.bson.codecs.configuration.ChildCodecRegistry.get(ChildCodecRegistry.java:51)
    at org.bson.codecs.DocumentCodec.writeValue(DocumentCodec.java:174)
    at org.bson.codecs.DocumentCodec.writeMap(DocumentCodec.java:189)
    at org.bson.codecs.DocumentCodec.encode(DocumentCodec.java:131)
    at org.bson.codecs.DocumentCodec.encode(DocumentCodec.java:45)
    at org.bson.codecs.BsonDocumentWrapperCodec.encode(BsonDocumentWrapperCodec.java:63)
    at org.bson.codecs.BsonDocumentWrapperCodec.encode(BsonDocumentWrapperCodec.java:29)
    at com.mongodb.connection.InsertCommandMessage.writeTheWrites(InsertCommandMessage.java:101)
    at com.mongodb.connection.InsertCommandMessage.writeTheWrites(InsertCommandMessage.java:43)
    at com.mongodb.connection.BaseWriteCommandMessage.encodeMessageBodyWithMetadata(BaseWriteCommandMessage.java:129)
    at com.mongodb.connection.RequestMessage.encodeWithMetadata(RequestMessage.java:160)
    at com.mongodb.connection.WriteCommandProtocol.sendMessage(WriteCommandProtocol.java:212)
    at com.mongodb.connection.WriteCommandProtocol.execute(WriteCommandProtocol.java:101)
    at com.mongodb.connection.InsertCommandProtocol.execute(InsertCommandProtocol.java:67)
    at com.mongodb.connection.InsertCommandProtocol.execute(InsertCommandProtocol.java:37)
    at com.mongodb.connection.DefaultServer$DefaultServerProtocolExecutor.execute(DefaultServer.java:159)
    at com.mongodb.connection.DefaultServerConnection.executeProtocol(DefaultServerConnection.java:286)
    at com.mongodb.connection.DefaultServerConnection.insertCommand(DefaultServerConnection.java:115)
    at com.mongodb.operation.MixedBulkWriteOperation$Run$2.executeWriteCommandProtocol(MixedBulkWriteOperation.java:455)
    at com.mongodb.operation.MixedBulkWriteOperation$Run$RunExecutor.execute(MixedBulkWriteOperation.java:646)
    at com.mongodb.operation.MixedBulkWriteOperation$Run.execute(MixedBulkWriteOperation.java:401)
    at com.mongodb.operation.MixedBulkWriteOperation$1.call(MixedBulkWriteOperation.java:179)
    at com.mongodb.operation.MixedBulkWriteOperation$1.call(MixedBulkWriteOperation.java:168)
    at com.mongodb.operation.OperationHelper.withConnectionSource(OperationHelper.java:230)
    at com.mongodb.operation.OperationHelper.withConnection(OperationHelper.java:221)
    at com.mongodb.operation.MixedBulkWriteOperation.execute(MixedBulkWriteOperation.java:168)
    at com.mongodb.operation.MixedBulkWriteOperation.execute(MixedBulkWriteOperation.java:74)
    at com.mongodb.Mongo.execute(Mongo.java:781)
    at com.mongodb.Mongo$2.execute(Mongo.java:764)
    at com.mongodb.MongoCollectionImpl.executeSingleWriteRequest(MongoCollectionImpl.java:515)
    at com.mongodb.MongoCollectionImpl.insertOne(MongoCollectionImpl.java:306)
    at com.mongodb.MongoCollectionImpl.insertOne(MongoCollectionImpl.java:297)
    at com.mongodb.course.week3.ArrayListWithObject.main(ArrayListWithObject.java:34)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)

我知道 MongoDB 的 Java 驱动程序无法将我的对象转换为 Document，它需要某种转换器。我还知道 Codec、CodecRegistry 和 CodecProvider 接口(interface)。顺便说一句，有没有更简单的方法将对象转换为mongo文档？你能告诉我我该怎么做吗？ 谢谢。

最佳答案

您发布的代码的问题是默认情况下它不知道如何将您的 pojo 对象序列化为 Json 以将它们保存到数据库中。您可以使用 MongoDB Java 驱动程序执行此操作，但您需要做一些工作来序列化 Comment ArrayList 和 User pojo。如果您添加一些 Jackson 映射代码，您可以按如下方式执行此操作:

import java.util.ArrayList;
import java.util.Date;
import java.util.List;
import java.util.function.Function;
import java.util.stream.Collectors;

import org.bson.Document;

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.mongodb.MongoClient;
import com.mongodb.client.MongoCollection;
import com.mongodb.client.MongoDatabase;

public class Problem {
    public static void main(String[] args) {
        try (final MongoClient client = new MongoClient()) {
            final MongoDatabase db = client.getDatabase("example");
            final MongoCollection<Document> collection = db.getCollection("object_arrays");

            collection.drop();

            final List<Comment> reviews = new ArrayList<Comment>() {
                {
                    add(new Comment("cool guy", new Date(), "John Doe"));
                    add(new Comment("best joker", new Date(), "Vas Negas"));
                    add(new Comment("very stupid but very funny man", new Date(), "Bill Murphy"));
                }
            };

            final ObjectMapper mapper = new ObjectMapper();
            final User user = new User("0xFF", "email@email.com");
            try {
                //Create a Document representation of the User object
                final String userJson = mapper.writeValueAsString(user);
                final Document userDoc = Document.parse(userJson);

                //Convert the review ArrayList into a Mongo Document.  Need to amend this if not using Java8
                final List<Document> reviewDocs = reviews.stream().map(convertToJson())
                        .map(reviewJson -> Document.parse(reviewJson)).collect(Collectors.toList());

                //Wrap it all up to it can be saved to the database
                final Document wrapperDoc = new Document();
                wrapperDoc.append("user", userDoc).append("reviews", reviewDocs).append("createDate", new Date());
                collection.insertOne(wrapperDoc);
            } catch (final JsonProcessingException e) {
                e.printStackTrace();
            }
        }
    }

    private static Function<Comment, String> convertToJson() {
        final ObjectMapper mapper = new ObjectMapper();
        return review -> {
            try {
                return mapper.writeValueAsString(review);
            } catch (final JsonProcessingException e) {
                e.printStackTrace();
            }
            return "";
        };
    }
}

*这使用了一些 Java8 代码，您可能需要根据您使用的 Java 版本更改这些代码

正如这个问题的另一个答案所说，有一些框架可以将对象的序列化和与 MongoDB 的交互结合起来，这样您就不需要手动编写序列化代码。例如 Spring 有一个 Mongo 驱动程序，我使用了另一个名为 Jongo 的驱动程序，我发现它非常好。

关于java - 如何在 MongoDB 3.2 文档中插入对象？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/36402690/