MongoDB在不特殊指认的情况下，默认的集合主键是“_id”，类型是ObjectId。ObjectId是一个12字节的BSON类型字符串，包含了UNIX时间戳，机器识别码，进程号，计数值信息。机器码用来防止分布式系统生成id时冲突的问题，保证每台机器生成的识别码不同，进程号保证多线程情况下生成的id不同。
ObjectId在java程序中是对象类型，JavaBean中常这样使用：

@Document(collection = "c_userinfo")
public class UserInfo{
    @Id
    private ObjectId id;

    private String name;

    // getter setter略
}

此时，如果直接实体类序列化为json，id将被作为对象处理，前段无法将此对象转为字符串，也无法将此id作为唯一标识调用其他数据。

{
id: {
        "time": 1494233455000,
        "timestamp": 1494233455,
        "date": 1494233455000,
        "new": false,
        "timeSecond": 1494233455,
        "inc": -125534200,
        "machine": -1248386109
      }
}

故有时需要将ObjectId的序列化做处理，将ObjectId直接序列化为字符串。
以springmvc为例，在接口返回数据前统一处理序列化问题。自定义类实现ResponseBodyAdvice接口，重写beforeBodyWrite方法，自定义序列化方法。

import com.alibaba.fastjson.JSON;
import com.alibaba.fastjson.JSONObject;
import com.alibaba.fastjson.serializer.SerializeConfig;
import org.bson.types.ObjectId;
import org.springframework.core.MethodParameter;
import org.springframework.http.MediaType;
import org.springframework.http.converter.HttpMessageConverter;
import org.springframework.http.server.ServerHttpRequest;
import org.springframework.http.server.ServerHttpResponse;
import org.springframework.web.bind.annotation.ControllerAdvice;
import org.springframework.web.servlet.mvc.method.annotation.ResponseBodyAdvice;

@ControllerAdvice
public class MyResponseBodyAdvice implements ResponseBodyAdvice<Object> {

    @Override
    public boolean supports(MethodParameter returnType, Class<? extends HttpMessageConverter<?>> converterType) {
        return true;
    }

    @Override
    public Object beforeBodyWrite(Object body, MethodParameter returnType, MediaType selectedContentType,
            Class<? extends HttpMessageConverter<?>> selectedConverterType, ServerHttpRequest request,
            ServerHttpResponse response) {

        SerializeConfig config = new SerializeConfig();
        config.put(ObjectId.class, new ObjectIdJsonSerializer());
        return JSONObject.parse(JSON.toJSONString(body, config));
    }
}

然后实现一个ObjectIdJsonSerializer

import com.alibaba.fastjson.serializer.JSONSerializer;
import com.alibaba.fastjson.serializer.ObjectSerializer;
import com.alibaba.fastjson.serializer.SerializeWriter;
import org.bson.types.ObjectId;

import java.io.IOException;
import java.lang.reflect.Type;

public class ObjectIdJsonSerializer implements ObjectSerializer {
    @Override
    public void write(JSONSerializer serializer, Object object, Object fieldName, Type fieldType,int features) throws IOException {
        SerializeWriter out = serializer.getWriter();
        if (object == null) {
            serializer.getWriter().writeNull();
            return;
        }
        out.write("\"" + ((ObjectId) object).toString() + "\"");
    }
}

这样处理后接口返回的id将变成字符串类型。

{
"id":"5ae078ada0f091000be702b8"
}

同理spring mvc对于Date类型的输出默认为时间戳格式，我们可以自定义输出为格式化字符串，这样可以免去在前端格式化处理。需要在MyResponseBodyAdvice 中注册DateJsonSerializer 。

import com.alibaba.fastjson.serializer.JSONSerializer;
import com.alibaba.fastjson.serializer.ObjectSerializer;
import com.alibaba.fastjson.serializer.SerializeWriter;
import org.bson.types.ObjectId;

import java.io.IOException;
import java.lang.reflect.Type;
import java.util.Date;

public class DateJsonSerializer implements ObjectSerializer {
    @Override
    public void write(JSONSerializer serializer, Object object, Object fieldName, Type fieldType,int features) throws IOException {
        SerializeWriter out = serializer.getWriter();
        if (object == null) {
            serializer.getWriter().writeNull();
            return;
        }
        out.write("\"" + DateUtil.format(((Date)object),"yyyy-MM-dd: HH:mm:ss") + "\"");
    }
}