如何防止Map中的空值和bean内的空字段通过Jackson序列化 [英] How to prevent null values inside a Map and null fields inside a bean from getting serialized through Jackson

问题描述

我有一个 Map< String，Foo> foosMap 我想通过Jackson序列化。现在我想在序列化过程中遵循两个设置：

I have a a Map<String,Foo> foosMap that I want to serialize through Jackson . Now I want following two settings on the serialization process:

1. Map可以有足够的空值和空键，我不知道希望空值被序列化。


2. 对于所有那些被序列化的Foo，我不想序列化Foo中引用的空对象。

实现这一目标的最佳方法是什么？我在我的项目中使用jackson-core1.9和jackson-mapper1.9罐子。

What is the best way to achieve this ? I am using jackson-core1.9 and jackson-mapper1.9 jars in my project.

推荐答案

如果将原来的 Map 数据结构改为是合理的序列化以更好地代表想要序列化的实际值，这可能是一个不错的方法，这可能会减少必要的Jackson配置量。例如，在调用Jackson之前，如果可能，只删除 null 键条目。那说...

If it's reasonable to alter the original Map data structure to be serialized to better represent the actual value wanted to be serialized, that's probably a decent approach, which would possibly reduce the amount of Jackson configuration necessary. For example, just remove the null key entries, if possible, before calling Jackson. That said...

要禁止序列化 Map 条目空值：

To suppress serializing Map entries with null values:

在杰克逊2.9之前

你仍然可以使用 WRITE_NULL_MAP_VALUES ，但要注意，它移动到的 SerializationFeature ：

you can still make use of WRITE_NULL_MAP_VALUES, but note that it's moved to SerializationFeature:

mapper.configure(SerializationFeature.WRITE_NULL_MAP_VALUES, false);

自Jackson 2.9

不推荐使用 WRITE_NULL_MAP_VALUES ，您可以使用以下等价物：

The WRITE_NULL_MAP_VALUES is deprecated, you can use the below equivalent:

mapper.setDefaultPropertyInclusion(
   JsonInclude.Value.construct(Include.ALWAYS, Include.NON_NULL))

---

要禁止序列化具有空值的属性，可以直接配置 ObjectMapper ，或者使用 @JsonInclude 注释：

---

To suppress serializing properties with null values, you can configure the ObjectMapper directly, or make use of the @JsonInclude annotation:

mapper.setSerializationInclusion(Include.NON_NULL);

或：

@JsonInclude(Include.NON_NULL)
class Foo
{
  public String bar;

  Foo(String bar)
  {
    this.bar = bar;
  }
}

---

要处理null Map 键，我需要进行一些自定义序列化。

---

To handle null Map keys, some custom serialization is necessary, as best I understand.

将 null 键序列化为空字符串的简单方法（包括前面提到的两个配置的完整示例）：

A simple approach to serialize null keys as empty strings (including complete examples of the two previously mentioned configurations):

import java.io.IOException;
import java.util.HashMap;
import java.util.Map;

import com.fasterxml.jackson.annotation.JsonInclude.Include;
import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationFeature;
import com.fasterxml.jackson.databind.SerializerProvider;

public class JacksonFoo
{
  public static void main(String[] args) throws Exception
  {
    Map<String, Foo> foos = new HashMap<String, Foo>();
    foos.put("foo1", new Foo("foo1"));
    foos.put("foo2", new Foo(null));
    foos.put("foo3", null);
    foos.put(null, new Foo("foo4"));

    // System.out.println(new ObjectMapper().writeValueAsString(foos));
    // Exception: Null key for a Map not allowed in JSON (use a converting NullKeySerializer?)

    ObjectMapper mapper = new ObjectMapper();
    mapper.configure(SerializationFeature.WRITE_NULL_MAP_VALUES, false);
    mapper.setSerializationInclusion(Include.NON_NULL);
    mapper.getSerializerProvider().setNullKeySerializer(new MyNullKeySerializer());
    System.out.println(mapper.writeValueAsString(foos));
    // output: 
    // {"":{"bar":"foo4"},"foo2":{},"foo1":{"bar":"foo1"}}
  }
}

class MyNullKeySerializer extends JsonSerializer<Object>
{
  @Override
  public void serialize(Object nullKey, JsonGenerator jsonGenerator, SerializerProvider unused) 
      throws IOException, JsonProcessingException
  {
    jsonGenerator.writeFieldName("");
  }
}

class Foo
{
  public String bar;

  Foo(String bar)
  {
    this.bar = bar;
  }
}

禁止序列化 Map 带有 null 键的条目，需要进一步的自定义序列化处理。

To suppress serializing Map entries with null keys, further custom serialization processing would be necessary.

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