我已经创建了一个类 User 来扩展 Document。 User 只有一些简单的构造函数和围绕一些字符串和整数的 getter/setter。但是，当我尝试将 User 类插入 Mongo 时，出现以下错误:

Exception in thread "main" org.bson.codecs.configuration.CodecConfigurationException: Can't find a codec for class com.foo.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.ProvidersCodecRegistry.get(ProvidersCodecRegistry.java:37)
    at org.bson.BsonDocumentWrapper.asBsonDocument(BsonDocumentWrapper.java:62)
    at com.mongodb.MongoCollectionImpl.documentToBsonDocument(MongoCollectionImpl.java:507)
    at com.mongodb.MongoCollectionImpl.insertMany(MongoCollectionImpl.java:292)
    at com.mongodb.MongoCollectionImpl.insertMany(MongoCollectionImpl.java:282)
    at com.foo.bar.main(bar.java:27)

听起来我需要使用一些 Mongo 编解码器的东西，但我不熟悉它，一些快速谷歌搜索返回一些看起来非常高级的结果。

如何正确编写用于 Mongo 的 User 类？这是我的类(class)供引用:

public class User extends Document {
    User(String user, List<Document > history, boolean isActive, String location){
        this.append("_id", user)
                .append("history", history)
                .append("isActive", isActive)
                .append("location", location);
    }

    public List<Document > getHistory(){
        return this.get("history", ArrayList.class);
    }

    public void addToHistory(Document event){
       List<Document> history = this.getHistory();
        history.add(event);
        this.append("history", history);
    }

    public boolean hasMet(User otherUser){
        List<String> usersIveMet = this.getUsersMet(),
                     usersTheyMet = otherUser.getUsersMet();
        return !Collections.disjoint(usersIveMet, usersTheyMet);
    }

    public List<String> getUsersMet() {
        List<Document> usersHistory = this.getHistory();
        List<String> usersMet = usersHistory.stream()
                .map(doc -> Arrays.asList(doc.getString("user1"), doc.getString("user1")))
                .filter(u -> !u.equals(this.getUser()))
                .flatMap(u -> u.stream())
                .collect(Collectors.toList());
        return usersMet;
    }

    public String getUser(){
        return this.getString("_id");
    }
}

最佳答案

由于您正在尝试创建新对象(即使您从 Document 扩展)，Mongo 无法识别它，因此您需要提供编码/解码以便让 Mongo 了解您的对象(至少我看不到其他方式..)。

我试了一下你的 User 类，让它工作了。 所以，这是我定义用户类的方式:

public class User {

    private List<Document> history;
    private String id;
    private Boolean isActive;
    private String location;

    // Getters and setters. Omitted for brevity..
}

然后你需要为你的用户类提供编码/解码逻辑:

public class UserCodec implements Codec<User> {

    private CodecRegistry codecRegistry;

    public UserCodec(CodecRegistry codecRegistry) {
        this.codecRegistry = codecRegistry;
    }

    @Override
    public User decode(BsonReader reader, DecoderContext decoderContext) {
        reader.readStartDocument();
        String id = reader.readString("id");
        Boolean isActive = reader.readBoolean("isActive");
        String location = reader.readString("location");

        Codec<Document> historyCodec = codecRegistry.get(Document.class);
        List<Document> history = new ArrayList<>();
        reader.readStartArray();
        while (reader.readBsonType() != BsonType.END_OF_DOCUMENT) {
            history.add(historyCodec.decode(reader, decoderContext));
        }
        reader.readEndArray();
        reader.readEndDocument();

        User user = new User();
        user.setId(id);
        user.setIsActive(isActive);
        user.setLocation(location);
        user.setHistory(history);
        return user;
    }

    @Override
    public void encode(BsonWriter writer, User user, EncoderContext encoderContext) {
        writer.writeStartDocument();
        writer.writeName("id");
        writer.writeString(user.getId());
        writer.writeName("isActive");
        writer.writeBoolean(user.getIsActive());
        writer.writeName("location");
        writer.writeString(user.getLocation());

        writer.writeStartArray("history");
        for (Document document : user.getHistory()) {
            Codec<Document> documentCodec = codecRegistry.get(Document.class);
            encoderContext.encodeWithChildContext(documentCodec, writer, document);
        }
        writer.writeEndArray();
        writer.writeEndDocument();
    }

    @Override
    public Class<User> getEncoderClass() {
        return User.class;
    }
}

那么在开始序列化/反序列化之前，你需要一个编解码器来进行类型检查。

public class UserCodecProvider implements CodecProvider {

    @Override
    @SuppressWarnings("unchecked")
    public <T> Codec<T> get(Class<T> clazz, CodecRegistry registry) {
        if (clazz == User.class) {
            return (Codec<T>) new UserCodec(registry);
        }
        return null;
    }
}

最后，您需要将您的提供商注册到您的 MongoClient，仅此而已。

public class MongoDb {

    private MongoDatabase db;

    public MongoDb() {
        CodecRegistry codecRegistry = CodecRegistries.fromRegistries(
                CodecRegistries.fromProviders(new UserCodecProvider()),
                MongoClient.getDefaultCodecRegistry());
        MongoClientOptions options = MongoClientOptions.builder()
                .codecRegistry(codecRegistry).build();
        MongoClient mongoClient = new MongoClient(new ServerAddress(), options);
        db = mongoClient.getDatabase("test");
    }

    public void addUser(User user) {
        MongoCollection<User> collection = db.getCollection("user").withDocumentClass(User.class);
        collection.insertOne(user);
    }

    public static void main(String[] args) {
        MongoDb mongoDb = new MongoDb();

        Document history1 = new Document();
        history1.append("field1", "value1");
        history1.append("field2", "value2");
        history1.append("field3", "value3");

        List<Document> history = new ArrayList<>();
        history.add(history1);

        User user = new User();
        user.setId("someId1");
        user.setIsActive(true);
        user.setLocation("someLocation");
        user.setHistory(history);
        mongoDb.addUser(user);
    }
}

关于java - 如何正确扩展 Mongo 的文档类？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/33326572/