如何在Jackson序列化时忽略空Optional字段

问题原因 #

你当前配置的JsonInclude.Include.NON_NULL仅会忽略字段本身为null的情况，但secondName对应的Optional对象本身并非null，只是它包裹的值为null，因此Jackson会把这个空Optional序列化成null，无法达到你想要的忽略效果。

解决方案 #

方案一：改用JsonInclude.Include.NON_ABSENT #

将序列化包含规则替换为NON_ABSENT，它会同时忽略空Optional对象和null值：

ObjectMapper objectMapper = new ObjectMapper();
objectMapper.registerModule(new Jdk8Module());
objectMapper.setSerializationInclusion(JsonInclude.Include.NON_ABSENT);
System.out.println(objectMapper.writeValueAsString(employee));

方案二：配置Jdk8Module忽略空Optional #

注册Jdk8Module时，关闭configureAbsentsAsNulls配置，让空Optional直接被忽略而非序列化成null：

ObjectMapper objectMapper = new ObjectMapper();
Jdk8Module jdk8Module = new Jdk8Module();
jdk8Module.configureAbsentsAsNulls(false);
objectMapper.registerModule(jdk8Module);
objectMapper.setSerializationInclusion(JsonInclude.Include.NON_NULL);
System.out.println(objectMapper.writeValueAsString(employee));

方案三：针对特定字段添加注解 #

如果只需对部分Optional字段生效，可以在对应的get方法上添加@JsonInclude(JsonInclude.Include.NON_ABSENT)注解：

class Employee {
    private String firstName;
    private String secondName;

    @JsonInclude(JsonInclude.Include.NON_ABSENT)
    public Optional<String> getFirstName() {
        return Optional.ofNullable(firstName);
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    @JsonInclude(JsonInclude.Include.NON_ABSENT)
    public Optional<String> getSecondName() {
        return Optional.ofNullable(secondName);
    }

    public void setSecondName(String secondName) {
        this.secondName = secondName;
    }
}

应用上述任意方案后，都会得到你期望的序列化结果：

{"firstName":"foo"}

内容的提问来源于stack exchange，提问作者Zeeshan