jackson-序列化HashMap对象时跳过具有空值的键

Question

我正在尝试使用杰克逊对象映射器将带有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question中提供的答案。

我期望结果为{"expected":true}，但结果中包含具有null值的键。

如何解决此问题？

注意：Cod eis位于github here。

Answer 1

WriteValue不会以任何方式考虑包含：它仅适用于JSON的编写。转换是先写后读的顺序；但是在完成读取时，输出不再包含排除的值。

替换此代码：

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);

作者

ByteArrayOutputStream out = new ByteArrayOutputStream(1000);
            mapper.writeValue(out, mapper.convertValue(new Ext.ExtBuilder().withAdditionalProperty("unexpected", null).withAdditionalProperty("expected", true).build(), Ext.class));

            String result = new String(out.toByteArray());
            Assert.assertEquals("{\"expected\":true}", result);

Answer 2

[当您将@JsonInclude放在字段上或在映射器中使用（setSerializationInclusion）时，这意味着如果该字段（不是字段的元素）为null，则它将被忽略转换。

• 在您的情况下，当您在@JsonInclude上使用additionalProperties时，这意味着，如果additionalProperties（itself）为null，则将从转换中将其忽略。

所以@JsonInclude仅检查additionalProperties本身而不检查其元素。

---

示例：假设您有一个带有Ext的additionalProperties=null对象，当您要对其进行序列化时，additionalProperties被忽略，因为它是null。

但是在您的情况下，additionalProperties映射的elements包含null，Jackson不会忽略它们。

您可以序列化其他属性映射（本身），而不是整个Ext对象。

    //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);

而且您还可以从生成的字符串中获取Ext对象。在这种情况下，您的对象具有已过滤的AdditionalProperties（不包含具有null键或null值的任何元素）

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

//Print the map of filterdExt 
System.out.println(filterdExt.getAdditionalProperties());