问题
Here's the code
import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.databind.ObjectMapper;
import lombok.Data;
import lombok.ToString;
public class Main {
public static void main(String[] args) throws Exception {
Fields f1 = new Fields(1);
System.out.println(f1);
ObjectMapper mapper = new ObjectMapper();
String str = mapper.writeValueAsString(f1);
System.out.println(str);
Fields f2 = mapper.readValue(str, Fields.class);
System.out.println(f2);
}
@Data
@ToString
public static class Fields {
private final long value1;
private final long value2;
public Fields(@JsonProperty("blah") long value) {
this.value1 = value++;
this.value2 = value++;
System.out.println(this);
}
}
}
Output
Main.Fields(value1=1, value2=2)
Main.Fields(value1=1, value2=2)
{"value1":1,"value2":2}
Main.Fields(value1=0, value2=1)
Main.Fields(value1=1, value2=2)
My questions are:
- Why did jackson modify private final fields that do not have setters after finish constructing it? If this is intended, how do I turn it off?
- If jackson is able to set fields directly, why is it required that I annotate the constructor with @JsonProperty? (Removing @JsonProperty from Fields results in error; and I didn't even need to annotate with correct properties)
Thank you
回答1:
Why is it required that I annotate the constructor with @JsonProperty?
It's not. What is required is an accessible constructor. You can either have a parameterless constructor
public Fields() {
this.value1 = 0;
this.value2 = 0;
System.out.println("cons: " + this);
}
(that necessarily initializes the fields since they are final
) or you can have a constructor that Jackson will try to resolve based on the declared @JsonProperty
name. Note that JsonProperty#required
is false
by default.
Why did jackson modify private final fields that do not have setters after finish constructing it? If this is intended, how do I turn it off?
Because it can. It thus allows you to use immutable types with deserialization. There is no built-in way that I know of through which you can disable this feature.
回答2:
Why did jackson modify private final fields that do not have setters after finish constructing it? If this is intended, how do I turn it off?
You can set MapperFeature.ALLOW_FINAL_FIELDS_AS_MUTATORS
property to false
(it is true
by default) when configuring your mapper.
Example:
import com.fasterxml.jackson.annotation.JsonCreator;
import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.MapperFeature;
import com.fasterxml.jackson.databind.ObjectMapper;
public class Temp {
private static final String RAW = "{\"value1\": \"aabbcc\",\"value2\":\"zzzzz\"}";
public static void main(String[] args) throws Exception {
System.out.println(new ObjectMapper().readValue(RAW, TestClass.class));
ObjectMapper mapper = new ObjectMapper();
mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false); // you will receive UnrecognizedPropertyException without this line
mapper.configure(MapperFeature.ALLOW_FINAL_FIELDS_AS_MUTATORS, false);
System.out.println(mapper.readValue(RAW, TestClass.class));
}
public static class TestClass {
private final String value1;
private final String value2;
@JsonCreator
public static TestClass createTestClass(
@JsonProperty("value1") String value1,
@JsonProperty("blah") String value2) {
return new TestClass(value1, value2);
}
private TestClass(String value1, String value2) {
this.value1 = value1;
this.value2 = value2;
}
public String getValue1() {
return value1;
}
public String getValue2() {
return value2;
}
@Override
public String toString() {
return "TestClass{" + "value1=" + value1 + ", value2=" + value2 + '}';
}
}
}
Output:
TestClass{value1=aabbcc, value2=zzzzz}
TestClass{value1=aabbcc, value2=null}
来源:https://stackoverflow.com/questions/30340946/jackson-deserialization-circumventing-final-fields