如何让Jackson库在反序列化空值时使用我的默认初始化值？

Question

我有以下JSON输入：

{
    "teachers": null,
    "students": [],
    "janitors": ["J1", "J2"]
}

它将映射到这个学校对象。

public class School {
    
    // Child JSON arrays reflecting JSON input
    private List<String> teachers = new ArrayList<>();
    private List<String> students = new ArrayList<>();
    private List<String> janitors = new ArrayList<>();
    
    // Getters and Setters
    public List<String> getTeachers() {
        return teachers;
    }
    public void setTeachers(List<String> teachers) {
        this.teachers = teachers;
    }

    public List<String> getStudents() {
        return students;
    }
    public void setStudents(List<String> students) {
        this.students = students;
    }

    public List<String> getJanitors() {
        return janitors;
    }
    public void setJanitors(List<String> janitors) {
        this.janitors = janitors;
    }
}

这是我到目前为止的映射器配置：

ObjectMapper mapper = new ObjectMapper();
mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);

Jackson库反序列化JSON输入后，尽管将其初始化为数组，但我仍然得到了school.教师=null。我初始化这些数组的原因是为了避免不必要的空检查。

如何让Jackson反序列化器忽略空值或忽略它无法映射到的空节点？

Answer 1

可以在类中使用@jsoninclude(jsoninclude.include.non_null)。

import com.fasterxml.jackson.annotation.JsonInclude;
@JsonInclude(JsonInclude.Include.NON_NULL)
public class School {
    
    // Child JSON arrays reflecting JSON input
    private List<String> teachers = new ArrayList<>();
    private List<String> students = new ArrayList<>();
    private List<String> janitors = new ArrayList<>();
}

也可以在字段级别使用；以防您希望某些字段为空。

import com.fasterxml.jackson.annotation.JsonInclude;

public class School {

    // Child JSON arrays reflecting JSON input
    @JsonInclude(JsonInclude.Include.NON_NULL)
    private List<String> teachers = new ArrayList<>();

    @JsonInclude(JsonInclude.Include.NON_NULL)
    private List<String> students = new ArrayList<>();
    
    private List<String> janitors = new ArrayList<>();
}

因此，在上面的场景中，如果janitors为null，它将显示为null。