Why when a constructor is annotated with JsonCreator its arguments must be annotated with JsonProperty

Question

In Jackson  when you annotate a constructor with  JsonCreator  you must annotate its arguments with  JsonProperty  So this constructor  public Point double x  double y        this x   x      this y   y      becomes this    JsonCreator public Point  JsonProperty  x   double x   JsonProperty  y   double y        this x   x      this y   y      I don t understand why it s necessary  Can you please explain

User · Answer

It is possible to avoid constructor annotations with jdk8 where optionally the compiler will introduce metadata with the names of the constructor parameters  Then with jackson-module-parameter-names module Jackson can use this constructor   You can see an example at post Jackson without annotations   The Java    Tutorials - Obtaining Names of Method Parameters

User · Answer

As precised in the annotation documentation  the annotation indicates that the argument name is used as the property name without any modifications  but it can be specified to non-empty value to specify different name

User · Answer

One can simply use java bean ConstructorProperties annotation - it s much less verbose and Jackson also accepts it  For example      import java beans ConstructorProperties      ConstructorProperties   answer   closed   language   interface   operation      public DialogueOutput String answer  boolean closed  String language  String anInterface  String operation        this answer   answer      this closed   closed      this language   language      this anInterface   anInterface      this operation   operation

User · Answer

Because Java bytecode does not retain the names of method or constructor arguments

User · Answer

Jackson has to know in what order to pass fields from a JSON object to the constructor  It is not possible to access parameter names in Java using reflection - that s why you have to repeat this information in annotations

User · Answer

Just come across it and got an answer somewhere  you can use below annotation since 2 7 0    JsonAutoDetect fieldVisibility   JsonAutoDetect Visibility ANY  public class Point       final private double x      final private double y        ConstructorProperties   x    y        public Point double x  double y            this x   x          this y   y

User · Answer

Parameter names are normally not accessible by the Java code at runtime  because it s drop by the compiler   so if you want that functionality you need to either use Java 8 s built-in functionality or use a library such as ParaNamer in order to gain access to it   So in order to not having to utilize annotations for the constructor arguments when using Jackson  you can make use of either of these 2 Jackson modules   jackson-module-parameter-names  This module allows you to get annotation-free constructor arguments when using Java 8  In order to use it you first need to register the module   ObjectMapper mapper   new ObjectMapper    mapper registerModule new ParameterNamesModule       Then compile your code using the -parameters flag   javac -parameters       Link  https   github com FasterXML jackson-modules-java8 tree master parameter-names  jackson-module-paranamer  This other one simply requires you to register the module or configure an annotation introspection  but not both as pointed out by the comments   It allows you to use annotation-free constructor arguments on versions of Java prior to 1 8   ObjectMapper mapper   new ObjectMapper       either via module mapper registerModule new ParanamerModule        or by directly assigning annotation introspector  but not both   mapper setAnnotationIntrospector new ParanamerOnJacksonAnnotationIntrospector       Link  https   github com FasterXML jackson-modules-base tree master paranamer

User · Answer

When I understand this correctly  you replace the default constructor with a parameterized one and therefore have to describe the JSON keys which are used to call the constructor with

[java] Why when a constructor is annotated with @JsonCreator, its arguments must be annotated with @JsonProperty?

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