java 如果 jackson 在 json 中的值为 null，则为属性提供默认值
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give a default value for an attribute if the value is null in json by Hymanson
提问by Trying
Suppose i have class i.e.
假设我有课，即
private class Student {
private Integer x = 1000;
public Integer getX() {
return x;
}
public void setX(Integer x) {
this.x = x;
}
}
Now suppose json is "{x:12}"and doing deserialization then the xwill have the value is 12. But if the json is "{}"then the value of x = 1000(get is from the default value of the attribute declared in the class).
现在假设 json is"{x:12}"并进行反序列化，那么它x的值为12. 但是，如果JSON是 "{}"那么的值x = 1000（get是从类中声明的属性的默认值）。
Now if the json is "{x:null}"then value of x becomes nullbut here even in this case i want value of xto be 1000. How to do it via Hymanson. Thanks in advance.
现在，如果 json 是 "{x:null}"x 的值，null但即使在这种情况下，我也希望值x是1000。如何通过 Hymanson 做到这一点。提前致谢。
I am deserializing via below method, if it helps in anyway:
objectMapper.readValue(<json string goes here>, Student.class);
我正在通过以下方法反序列化，如果它有帮助的话：
objectMapper.readValue(<json string goes here>, Student.class);
采纳答案by Trying
public class Student {
private Integer x = Integer.valueOf(1000);
public Integer getX() {
return x;
}
public void setX(Integer x) {
if(x != null) {
this.x = x;
}
}
}
This works for me........
这对我有用......
Test code 1:
测试代码1：
public static void main(String[] args) throws IOException {
String s = "{\"x\":null}";
ObjectMapper mapper = new ObjectMapper();
Student ss = mapper.readValue(s, Student.class);
System.out.println(ss.getX());
}
output:
输出：
1000
1000
Test code 2:
测试代码2：
public static void main(String[] args) throws IOException {
String s = "{}";
ObjectMapper mapper = new ObjectMapper();
Student ss = mapper.readValue(s, Student.class);
System.out.println(ss.getX());
}
output:
输出：
1000
1000
回答by Steve K
You should be able to override the setter. Add the @JsonProperty(value="x")annotations to the getter and setter to let Hymanson know to use them:
您应该能够覆盖 setter。将@JsonProperty(value="x")注释添加到 getter 和 setter 以让 Hymanson 知道使用它们：
private class Student {
private static final Integer DEFAULT_X = 1000;
private Integer x = DEFAULT_X;
@JsonProperty(value="x")
public Integer getX() {
return x;
}
@JsonProperty(value="x")
public void setX(Integer x) {
this.x = x == null ? DEFAULT_X : x;
}
}
回答by Sergey Pauk
Consider extending JsonDeserializer
考虑扩展JsonDeserializer
custom deserializer:
自定义解串器：
public class StudentDeserializer extends JsonDeserializer<Student> {
@Override
public Student deserialize(JsonParser p, DeserializationContext ctxt)
throws IOException, JsonProcessingException {
JsonNode node = p.getCodec().readTree(p);
// if JSON is "{}" or "{"x":null}" then create Student with default X
if (node == null || node.get("x").isNull()) {
return new Student();
}
// otherwise create Student with a parsed X value
int x = (Integer) ((IntNode) node.get("x")).numberValue();
Student student = new Student();
student.setX(x);
return student;
}
}
and it's use:
它的用途是：
ObjectMapper mapper = new ObjectMapper();
SimpleModule module = new SimpleModule();
module.addDeserializer(Student.class, new StudentDeserializer());
mapper.registerModule(module);
Student readValue = mapper.readValue(<your json string goes here>", Student.class);
回答by Stefaan Neyts
From json to object, you could fix this in the setter and tell Hymanson to not use Field access but use the setter for unmarshalling.
从 json 到对象，您可以在 setter 中修复此问题，并告诉 Hymanson 不要使用 Field 访问，而是使用 setter 进行解组。
