Jackson中DeserializationFeature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT和ACCEPT_EMPTY_STRING_AS_NULL_OBJECT

Jackson使用過程中的一些疑惑和跟蹤。

DeserializationFeature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT

首先先來看看這個配置項對應的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.
     *<p>
     * 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}.
     *<p>
     * Feature is disabled by default.
     */

大致意思如下：

它允許將JSON中的空字串（""）作為null值繫結到一個POJO或者Map或者Collection集合物件。如果禁用該配置項，那麼值為空字串的欄位在反序列化成一個POJO、Map、Collection時將會報錯。

⚠️注意！

不能理解成：他會把JSON字串中的“空字串”值在反序列化時轉為物件中對應欄位的null值。

具體來看下面例子：

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));

如果我們理解成他會把JSON字串中的“空字串”值在反序列化時轉為物件中對應欄位的null值，那麼我們期望反序列化之後address的值會從"“變為null，但是並沒有，在user物件中address的值依然是”"。

事實上作為Address物件的addressObj在反序列化之後從""變為了null。

因此我們應該正確理解

ACCEPT_EMPTY_STRING_AS_NULL_OBJECT

這個配置項的註釋真正要表達的意思，那就是：
空字串""必須是要繫結到一個POJO、Map、Cpllection、陣列這樣的物件上（而不是普通的基本型別和String型別）時，才會在反序列化時轉為null。

對於上面的例子，我們如果禁用

ACCEPT_EMPTY_STRING_AS_NULL_OBJECT

（將

objectMapper.configure(DeserializationFeature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT,true);

註釋掉）會發現報錯如下：

DeserializationFeature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT

還是先看對應的JavaDoc：

/**
     * Feature that can be enabled to allow empty JSON Array
     * value (that is, <code>[ ]</code>) 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.
     *<p>
     * Feature is disabled by default.
     * 
     * @since 2.5
     */

大致的意思是：

它允許將JSON中的空陣列（[]）作為null繫結到POJO等其他物件上。

事實真的是這樣嗎？

還是基於上面的例子稍微改動：

        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，我們期望JSON字串裡面的hobbies欄位在反序列化以後變為null。

但是結果並沒有。它依舊被反序列化為了一個length=0的陣列

GitHub上有人丟擲了同樣的問題：ACCEPT_EMPTY_ARRAY_AS_NULL_OBJECT does not convert empty JSON array to null value

作者是這麼回答的：

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：

需要說明的是：在Jackson檔案中，“ POJO”的含義與“ Bean”大致相同； 作為一組屬性/值對處理的Java型別。 這不包括Collections，Maps或陣列（或標量型別）。 因此，儘管對ACCEPT_EMPTY_ARRAY_AS_NULL_OBJECT的命名過於模糊（實際上應該說是POJO），但對於容器型別，當前不打算將[]強制為null。

但更一般而言，在可以正常解釋此類輸入的情況下，我不希望從JSON陣列新增強制。 也就是說，不打算將空的List / array更改為Java的null。
我可能需要更新Javadocs來闡明此行為。

簡單來說，這個配置項並不能把List/array更改為Java的null。

那問題來了，既然不能把list/array轉為null，那這個配置項有什麼用呢？

作者這麼回答的：

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：

我認為用例從文件和評論中都很清楚，但是我認為這是一個比我最初想象的更大的誤解。意圖和實現的是“強制”（隱式地從不相容型別轉換）從（空）JSON陣列作為null到非陣列/非集合型別，例如POJO或Map甚至是Scalar。
這是為了支援某些特定的平臺（我認為PHP）將null編碼為[]。
如果不進行設定，您將獲得基本的“無法從JsonToken.START_ARRAY反序列化MyValue”異常。
新的CoercionConfig（請參閱＃2113）將使這一點更加清晰和可配置。
至於將空的Collection / array轉換為null，我想這就是您想要的：目前只有相反的做法-將null轉換為“ empty”，跳過設定或引發Exception。這樣做的功能是增加的，因為避免空值一直是使用者希望用例提出的。
我認為有時也要求使用逆功能-將“空”值設為null-但到目前為止，尚未新增任何支援。
我總是對新功能和特性的可能性持開放態度，但是在這種情況下，擴充套件已存在的事物不是問題（上面提到的機制非常具體，部分原因是空值處理與針對歷史的一般JsonDeserializer反序列化流程完全不同原因…）。

簡而言之，這個選項是為了支援特定平臺（PHP等語言）會將null轉為[]的情況。如果不設定，在反序列化時會丟擲“無法從JsonToken.START_ARRAY反序列化MyValue”異常。

總結

1. DeserializationFeature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT是對POJO、Map、集合、陣列有效，而不是一個普通的string型別欄位。
2. DeserializationFeature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT在Java中不能達到預期的效果。