1. Overview

1.概述

This quick tutorial will show how to serialize a Java entity with Jackson 2 using a Custom Serializer.

这个快速教程将展示如何使用自定义序列化器用Jackson 2序列化一个Java实体。

If you want to dig deeper and learn other cool things you can do with the Jackson 2 – head on over to the main Jackson tutorial.

如果你想深入了解并学习你可以用杰克逊2做的其他很酷的事情–请到杰克逊主教程。

2. Standard Serialization of an Object Graph

2.对象图的标准序列化

Let’s define 2 simple entities and see how Jackson serializes these without any custom logic:

让我们定义2个简单的实体，看看Jackson是如何在没有任何自定义逻辑的情况下序列化这些实体的。

public class User {
    public int id;
    public String name;
}
public class Item {
    public int id;
    public String itemName;
    public User owner;
}

Now, let’s serialize an Item entity with a User entity:

现在，让我们将一个Item实体与一个User实体序列化。

Item myItem = new Item(1, "theItem", new User(2, "theUser"));
String serialized = new ObjectMapper().writeValueAsString(myItem);

This will result in a full JSON representation for both entities:

这将导致两个实体的完整JSON表示。

{
    "id": 1,
    "itemName": "theItem",
    "owner": {
        "id": 2,
        "name": "theUser"
    }
}

3. Custom Serializer on the ObjectMapper

**3.在ObjectMapper上的自定义串行器**

Now, let’s simplify the JSON output above by only serializing the id of the User, not the entire User object; we’d like to get the following, simpler JSON:

现在，让我们简化上面的JSON输出，只对User的id进行序列化，而不是整个User对象；我们想得到下面这个更简单的JSON。

{
    "id": 25,
    "itemName": "FEDUfRgS",
    "owner": 15
}

Simply put, we’ll have to define a custom Serializer for Item objects:

简单地说，我们必须为Item对象定义一个自定义Serializer。

public class ItemSerializer extends StdSerializer<Item> {
    
    public ItemSerializer() {
        this(null);
    }
  
    public ItemSerializer(Class<Item> t) {
        super(t);
    }

    @Override
    public void serialize(
      Item value, JsonGenerator jgen, SerializerProvider provider) 
      throws IOException, JsonProcessingException {
 
        jgen.writeStartObject();
        jgen.writeNumberField("id", value.id);
        jgen.writeStringField("itemName", value.itemName);
        jgen.writeNumberField("owner", value.owner.id);
        jgen.writeEndObject();
    }
}

Now, we need to register this custom serializer with the ObjectMapper for the Item class, and perform the serialization:

现在，我们需要为Item类的ObjectMapper注册这个自定义序列化器，并执行序列化。

Item myItem = new Item(1, "theItem", new User(2, "theUser"));
ObjectMapper mapper = new ObjectMapper();

SimpleModule module = new SimpleModule();
module.addSerializer(Item.class, new ItemSerializer());
mapper.registerModule(module);

String serialized = mapper.writeValueAsString(myItem);

That’s it – we now have a simpler, custom JSON serialization of the Item->User entities.

就是这样–我们现在有了一个更简单的、自定义的JSON序列化的Item->User实体。

4. Custom Serializer on the Class

4.类上的自定义串行器

We can also register the serializer directly on the class, instead of on the ObjectMapper:

我们还可以直接在类上注册序列化器，而不是在ObjectMapper上注册。

@JsonSerialize(using = ItemSerializer.class)
public class Item {
    ...
}

Now, when performing standard serialization:

现在，在执行标准序列化时。

Item myItem = new Item(1, "theItem", new User(2, "theUser"));
String serialized = new ObjectMapper().writeValueAsString(myItem);

We will get the custom JSON output, created by the serializer, specified via @JsonSerialize:

我们将得到自定义的JSON输出，由序列化器创建，通过@JsonSerialize指定。

{
    "id": 25,
    "itemName": "FEDUfRgS",
    "owner": 15
}

This is helpful when the ObjectMapper cannot be accessed and configured directly.

当ObjectMapper不能被直接访问和配置时，这很有帮助。

5. Conclusion

5.结论

This article illustrated how to get to a custom JSON output with Jackson 2, by using Serializers.

本文说明了如何通过使用序列化器，用Jackson 2获得自定义的JSON输出。

The implementation of all these examples and code snippets can be found on GitHub – this is a Maven-based project, so it should be easy to import and run as it is.

所有这些例子和代码片段的实现都可以在GitHub – 这是一个基于Maven的项目，所以应该很容易导入并按原样运行。