java - Jackson - 在序列化 HashMap 对象时跳过具有空值的键

Question

我正在尝试使用 Jackson 对象映射器将带有 hashmap 的 Java 对象序列化为 json 字符串。

下面是Ext的类定义 -

        import com.fasterxml.jackson.annotation.JsonAnyGetter;
        import com.fasterxml.jackson.annotation.JsonAnySetter;
        import com.fasterxml.jackson.annotation.JsonIgnore;
        import com.fasterxml.jackson.annotation.JsonInclude;
        import com.fasterxml.jackson.annotation.JsonPropertyOrder;
        import com.fasterxml.jackson.annotation.JsonInclude.Include;
        import java.io.Serializable;
        import java.util.HashMap;
        import java.util.Map;
        import java.util.Objects;

        @JsonInclude(Include.NON_NULL)
        @JsonPropertyOrder({})
        public class Ext implements Serializable {

            @JsonIgnore
            private Map<String, Object> additionalProperties = new HashMap();
            private static final long serialVersionUID = -4500317258794294335L;

            public Ext() {
            }

            @JsonAnyGetter
            public Map<String, Object> getAdditionalProperties() {
                return this.additionalProperties;
            }

            @JsonAnySetter
            public void setAdditionalProperty(String name, Object value) {
                this.additionalProperties.put(name, value);
            }

            // ignore toString, equals and hascode

            public static class ExtBuilder {
                protected Ext instance;

                public ExtBuilder() {
                    if (this.getClass().equals(Ext.ExtBuilder.class)) {
                        this.instance = new Ext();
                    }
                }

                public Ext build() {
                    Ext result = this.instance;
                    this.instance = null;
                    return result;
                }

                public Ext.ExtBuilder withAdditionalProperty(String name, Object value) {
                    this.instance.getAdditionalProperties().put(name, value);
                    return this;
                }
            }
        }

下面是示例测试用例 -

    @Test
    public void testNullObjectSerialization() throws Exception {

        ObjectMapper mapper = new ObjectMapper();
        mapper.setSerializationInclusion(JsonInclude.Include.NON_NULL);
        mapper.configure(SerializationFeature.WRITE_NULL_MAP_VALUES, false);
        mapper.setDefaultPropertyInclusion(
              JsonInclude.Value.construct(JsonInclude.Include.NON_NULL, JsonInclude.Include.NON_NULL));

        ByteArrayOutputStream out = new ByteArrayOutputStream(1000);
        mapper.writeValue(out, new Ext.ExtBuilder().withAdditionalProperty("unexpected", null).withAdditionalProperty("expected", true).build());
        String result = new String(out.toByteArray());
        Assert.assertEquals("{\"expected\":true}", result);
    }

我用过

mapper.setDefaultPropertyInclusion(
              JsonInclude.Value.construct(JsonInclude.Include.NON_NULL, JsonInclude.Include.NON_NULL));` 

通过使用堆栈溢出中提供的答案 question .

我期望结果为 {"expected":true} 但不知何故，具有 null 值的键包含在结果中。

如何解决这个问题？

注意:

1. 代码位于 github here .
2. 我使用的是 jacksersion 版本 2.9.8

Answer 1

当您将@JsonInclude放在字段上时，这意味着如果字段(不是字段的元素)为null，那么它将在转换中被忽略。

• 就您的情况而言，当您在 additionalProperties 上使用 @JsonInclude 时，这意味着，如果 additionalProperties(本身)为空，那么它将在转换中被忽略。

so @JsonInclude only checks additionalProperties itself for null value, not its elements.

<小时/>

示例:假设您有一个带有 additionalProperties=null 的 Ext 对象，当您想要序列化它时，additionalProperties 会被忽略，因为它是空。

• 但在您的场景中，additionalProperties 映射的元素包含null，并且 Jackson 不会忽略它们。

<小时/>

解决方案1

You can serialize additionalProperties map (itself) not the whole Ext object.

public static void main(String[] args) throws JsonProcessingException {

        //Create Ext  Object
        Ext ext = new Ext.ExtBuilder().withAdditionalProperty("unexpected", null).withAdditionalProperty("expected", true).build();

        //Config
        ObjectMapper mapper = new ObjectMapper();
        mapper.setSerializationInclusion(JsonInclude.Include.NON_NULL);

        //Conversion the additionalProperties map (ext.getAdditionalProperties())
        String filterMap = mapper.writeValueAsString(ext.getAdditionalProperties());

        //Result ({"expected":true})
        System.out.println(filterMap);


    }
}        

Convert JSON to Ext Object

您还可以从生成的字符串中获取Ext对象。在这种情况下，您的对象有一个过滤的additionalProperties(没有任何带有空键或空值的元素)

public static void main(String[] args) throws JsonProcessingException {

        //Create Ext  Object
        Ext ext = new Ext.ExtBuilder().withAdditionalProperty("unexpected", null).withAdditionalProperty("expected", true).build();

        //Config
        ObjectMapper mapper = new ObjectMapper();
        mapper.setSerializationInclusion(JsonInclude.Include.NON_NULL);

        //Conversion the additionalProperties map (ext.getAdditionalProperties())
        String filterMap = mapper.writeValueAsString(ext.getAdditionalProperties());

        //Result ({"expected":true})
        System.out.println(filterMap);

        //Convert back JSON string to Ext object
        Ext filterdExt = mapper.readValue(filterMap, Ext.class);

        //Print AdditionalProperties of filterdExt ({"expected":true})
        System.out.println(filterdExt.getAdditionalProperties());

    }

<小时/>

解决方案2

您可以编写自定义序列化程序。

<强> This link 可能对此目的有用