Jackson中DeserializationFeature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT和DeserializationFeature. ACCEPT_EMP

Jackson使用过程中的一些疑惑和跟踪。

DeserializationFeature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT

首先先来看看这个配置项对应的JavaDoc：

/**
     * Feature that can be enabled to allow JSON empty String
     * value ("") to be bound as `null` for POJOs and other structured
     * values ({@link java.util.Map}s, {@link java.util.Collection}s).
     * If disabled, standard POJOs can only be bound from JSON `null` or
     * JSON Object (standard meaning that no custom deserializers or
     * constructors are defined; both of which can add support for other
     * kinds of JSON values); if enabled, empty JSON String can be taken
     * to be equivalent of JSON null.
     *

     * NOTE: this does NOT apply to scalar values such as booleans and numbers;
     * whether they can be coerced depends on
     * {@link MapperFeature#ALLOW_COERCION_OF_SCALARS}.
     *

     * Feature is disabled by default.
     */

大致意思如下：

它允许将JSON中的空字符串（""）作为null值绑定到一个POJO或者Map或者Collection集合对象。如果禁用该配置项，那么值为空字符串的字段在反序列化成一个POJO、Map、Collection时将会报错。

⚠️注意！

不能理解成：他会把JSON字符串中的“空字符串”值在反序列化时转为对象中对应字段的null值。

具体来看下面例子：

public class Address {
    private String street;
    private String building;
    //省略getter/setter
}

public class User {
    private Integer id;
    private String name;
    private String address;
    private Address addressObj;
    private String[] hobbies;
    private Date birthDate;
    //省略getter/setter
}

        String userStr = "{\"id\":1,\"name\":\"徐健\",\"address\":\"\",\"addressObj\":\"\"}";
        ObjectMapper objectMapper = new ObjectMapper();
        objectMapper.configure(DeserializationFeature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT,true);
        User user = objectMapper.readValue(userStr,User.class);
        System.out.println(objectMapper.writeValueAsString(user));

如果我们理解成他会把JSON字符串中的“空字符串”值在反序列化时转为对象中对应字段的null值，那么我们期望反序列化之后address的值会从"“变为null，但是并没有，在user对象中address的值依然是”"。

事实上作为Address对象的addressObj在反序列化之后从""变为了null。

因此我们应该正确理解

ACCEPT_EMPTY_STRING_AS_NULL_OBJECT

这个配置项的注释真正要表达的意思，那就是：
空字符串""必须是要绑定到一个POJO、Map、Cpllection、数组这样的对象上（而不是普通的基本类型和String类型）时，才会在反序列化时转为null。

对于上面的例子，我们如果禁用

ACCEPT_EMPTY_STRING_AS_NULL_OBJECT

（将

objectMapper.configure(DeserializationFeature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT,true);

注释掉）会发现报错如下：

DeserializationFeature.ACCEPT_EMPTY_ARRAY_AS_NULL_OBJECT

还是先看对应的JavaDoc：

/**
     * Feature that can be enabled to allow empty JSON Array
     * value (that is, [ ]) to be bound to POJOs (and
     * with 2.9, other values too) as `null`.
     * If disabled, standard POJOs can only be bound from JSON `null` or
     * JSON Object (standard meaning that no custom deserializers or
     * constructors are defined; both of which can add support for other
     * kinds of JSON values); if enabled, empty JSON Array will be taken
     * to be equivalent of JSON null.
     *

     * Feature is disabled by default.
     * 
     * @since 2.5
     */

大致的意思是：

它允许将JSON中的空数组（[]）作为null绑定到POJO等其他对象上。

事实真的是这样吗？

还是基于上面的例子稍微改动：

        String userStr = "{\"id\":1,\"name\":\"徐健\",\"address\":\"\",\"addressObj\":\"\",\"hobbies\":[]}";
        ObjectMapper objectMapper = new ObjectMapper();
        objectMapper.configure(DeserializationFeature.ACCEPT_EMPTY_ARRAY_AS_NULL_OBJECT,true);
        User user = objectMapper.readValue(userStr,User.class);
        System.out.println(objectMapper.writeValueAsString(user));

按照上面的JavaDoc，我们期望JSON字符串里面的hobbies字段在反序列化以后变为null。

但是结果并没有。它依旧被反序列化为了一个length=0的数组

GitHub上有人抛出了同样的问题：ACCEPT_EMPTY_ARRAY_AS_NULL_OBJECT does not convert empty JSON array to null value

作者是这么回答的：

Just to make clear: in Jackson documentation, "POJO" means roughly same as "Bean"; Java type that is handled as a set of property/value pairs. This does not include Collections, Maps or arrays (or scalar types). So while naming of ACCEPT_EMPTY_ARRAY_AS_NULL_OBJECT is too vague (it really should say POJO), it is not currently intended to coerce [ ] into null for container types.

But more generally I don't think I want to add coercions from JSON Array in cases where such input can be interpreted normally; that is, it is not intended to change empty List/array into Java null value.
I may need to update Javadocs to clarify this behavior.

大致意思如下：

需要说明的是：在Jackson文件中，“ POJO”的含义与“ Bean”大致相同； 作为一组属性/值对处理的Java类型。 这不包括Collections，Maps或数组（或标量类型）。 因此，尽管对ACCEPT_EMPTY_ARRAY_AS_NULL_OBJECT的命名过于模糊（实际上应该说是POJO），但对于容器类型，当前不打算将[]强制为null。

但更一般而言，在可以正常解释此类输入的情况下，我不希望从JSON数组添加强制。 也就是说，不打算将空的List / array更改为Java的null。
我可能需要更新Javadocs来阐明此行为。

简单来说，这个配置项并不能把List/array更改为Java的null。

那问题来了，既然不能把list/array转为null，那这个配置项有什么用呢？

作者这么回答的：

I thought use case was clear from document and my comment, but I think this is a bigger misunderstanding than I originally thought. What was intended and implemented is for "coercion" -- implicit conversion from incompatible type -- from (empty) JSON Array into non-array/non-collection type such as POJO or Map or even Scalar, as null.
This was to support some specific platform (PHP I think) that encodes nulls as [ ].
Without setting you would get basic "can not deserialize MyValue from JsonToken.START_ARRAY" exception.
New CoercionConfig (see #2113) will make this bit more clear as well as configurable.

As to converting empty Collection/array into null, which I think is what you want: only the reverse -- converting nulls into "empty", or skipping setting, or throwing Exception -- is possible currently. Functionality for doing that was added because avoiding nulls has been consistently brought up as a use case users want.
Inverse functionality -- making null out of "empty" value -- has been sometimes requested too, I think, but so far no support has been added.
I am always open to possibility of new functionality, features, but in this case it is not a matter of extending something that already is (mechanisms mentioned above are quite specific, partly since null value handling is quite separate from general JsonDeserializer deserialization flow for historical reasons...).

大致意思如下：

我认为用例从文档和评论中都很清楚，但是我认为这是一个比我最初想象的更大的误解。意图和实现的是“强制”（隐式地从不兼容类型转换）从（空）JSON数组作为null到非数组/非集合类型，例如POJO或Map甚至是Scalar。
这是为了支持某些特定的平台（我认为PHP）将null编码为[]。
如果不进行设置，您将获得基本的“无法从JsonToken.START_ARRAY反序列化MyValue”异常。
新的CoercionConfig（请参阅＃2113）将使这一点更加清晰和可配置。
至于将空的Collection / array转换为null，我想这就是您想要的：目前只有相反的做法-将null转换为“ empty”，跳过设置或引发Exception。这样做的功能是增加的，因为避免空值一直是用户希望用例提出的。
我认为有时也要求使用逆功能-将“空”值设为null-但到目前为止，尚未添加任何支持。
我总是对新功能和特性的可能性持开放态度，但是在这种情况下，扩展已存在的事物不是问题（上面提到的机制非常具体，部分原因是空值处理与针对历史的一般JsonDeserializer反序列化流程完全不同原因…）。

简而言之，这个选项是为了支持特定平台（PHP等语言）会将null转为[]的情况。如果不设置，在反序列化时会抛出“无法从JsonToken.START_ARRAY反序列化MyValue”异常。

总结

1. DeserializationFeature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT是对POJO、Map、集合、数组有效，而不是一个普通的string类型字段。
2. DeserializationFeature.ACCEPT_EMPTY_ARRA_AS_NULL_OBJECT在Java中不能达到预期的效果。