Gson: Deserialization with Gson.fromJson() should *ignore* missing fields

[GoogleCodeExporter]

What steps will reproduce the problem?

1. Create a simple *non-static* class which initializes default values for some 
fields:

    public class Test {

        int x = -1;
        String s = "Hello";
    }

2. Create new Gson and call `fromJson()` with missing fields:

    Test t = new Gson().fromJson("{}", Test.class);
    System.out.println(String.format("x=[%s] s=[%s]", t.x, t.s));

>> Results: x=[0] s=[null]

    Test t = new Gson().fromJson("{\"x\": 1}", Test.class);
    System.out.println(String.format("x=[%s] s=[%s]", t.x, t.s));

>> Results: x=[1] s=[null]

What is the expected output? What do you see instead?

As the test cases, I would like to expect default value for each field to be 
the one that the class initializes. For instance with the first test case, `x` 
should be `-1` and `s` should be "Hello".

What version of the product are you using? On what operating system?

 - Gson 2.2.3
 - OSes:
   + Fedora 18, 64 bit with OpenJDK 1.7.0_19;
   + Android API 4 and API 17;

Thanks,
Hai

Original issue reported on code.google.com by [email protected] on 5 May 2013 at 8:23

[GoogleCodeExporter]

Is this still an issue.  Did u try it in 2.2.4?

Its working for me on Windows with GSON 2.2.4

Original comment by [email protected] on 3 Aug 2013 at 1:22

[GoogleCodeExporter]

Hari,

Yes it is. I've just tested with Gson 2.2.4 on Fedora 19, 64 bit with OpenJDK 
64-Bit Server VM (build 23.7-b01, mixed mode).

Original comment by [email protected] on 4 Aug 2013 at 11:35

[GoogleCodeExporter]

Apparently, if you have a default constructor for the class, it will work as 
you like.

public Test() { }

and that should be enough for the instance variables to be initialized as 
declared.

Original comment by [email protected] on 22 Nov 2013 at 1:12

[GoogleCodeExporter]

@krt...

Thanks for your help. Unfortunately it's not working. I still got "x=[0] 
s=[null]". Gson v2.2.4.

Original comment by [email protected] on 22 Nov 2013 at 7:28

[GoogleCodeExporter]

Confirm, this is still not working.
No way to assign default values for fields.
If field is missing in JSON it is deserialized as null.
Gson v2.2.4

Original comment by [email protected] on 31 Jan 2014 at 7:16

[GoogleCodeExporter]

This is most probably related to issue 495, see the links in the posting from 
Jun 23, 2013.

Original comment by [email protected] on 2 Feb 2014 at 12:32

[tommiller1]

This is still a problem, is anyone looking into this? How do we prevent null fields from being deserialized?

[vyshnavkr]

Ya man...i'm also stuck with this

[stayclean]

Is it solved? 2016 Nov, missing field should have method to get ignored.

[vyshnavkr]

@stayclean
I don't remember the solution but I have done it all well.
Will this help?
stackoverflow.com/questions/33435683/deserialize-with-gson-and-null-values

[vyshnavkr]

www.stackoverflow.com/questions/33435683/deserialize-with-gson-and-null-values

[anubiann00b]

I'm having the same problem right now. That link does not help, it's about something else @VyshnavKR

[vyshnavkr]

Have you posted in StackOverflow? If not kindly post it there and share the link here. Let's discuss it there.

[MiralDesai]

How has this not been fixed it? Is it still being looked at? It's a big problem.

Fields that have nothing to do with the JSON return should not be modified at all.

[transilvlad]

Any update on this?

[Begro]

I tried in gson 2.8.1,it is already fixed.
public class User { String name = "username"; int x = -1; }
public static void main(String[] args) { User u = new Gson().fromJson("{}", User.class); System.out.println(String.format("x=[%s] s=[%s]", u.name,u.x)); }
result:x=[username] s=[-1]

[pysaumont]

Not working for me with 2.8.1.

  public class User {
    String name = "username";
    int x = -1;
  }

  public static void main(String[] args) {
    User u = new Gson().fromJson("{}", User.class);
    System.out.println(String.format("x=[%s] s=[%s]", u.name,u.x));
  }

result:

x=[null] s=[0]

This seems to correspond to the documentation:

While deserializing, a missing entry in JSON results in setting the corresponding field in the object to null

[Begro]

I am sorry for my poor Engilsh.
I found somecode in gson.
` TypeAdapter typeAdapter = getAdapter(typeToken);

T object = typeAdapter.read(reader); And I go into the class ReflectiveTypeAdapterFactory and the method,I found T instance = constructor.construct();
try {
in.beginObject();
while (in.hasNext()) {
String name = in.nextName();
BoundField field = boundFields.get(name);
if (field == null || !field.deserialized) {
in.skipValue();
} else {
field.read(in, instance);
}
}`

First,the code user the java reflect Constructor.newInstance(Object... args),
although args ==null ,the instance initialize its parameter.
In my case,the name is initialize to username.
And in.hasNext() == false , return instance.
The result is username not null.

[pysaumont]

Do you mean you find some code and used it in you own application? Perhaps you have registered a type adapter somewhere before. If so, can you post a working example? (To preserve formatting, insert you code examples between two lines containing only three backticks (`), such as:

public class User {
    String name = "username";
    int x = -1;
}

[Begro]

Thank you.
I didn't describe clearly.
I debug the Gson source code.And I found why my result is username not null.
You can follow the call chain,you will found the method in Gson.java

public <T> T fromJson(JsonReader reader, Type typeOfT) throws JsonIOException, JsonSyntaxException {
    boolean isEmpty = true;
   boolean oldLenient = reader.isLenient();
   reader.setLenient(true);
   try {
     reader.peek();
     isEmpty = false;
     TypeToken<T> typeToken = (TypeToken<T>) TypeToken.get(typeOfT);
     TypeAdapter<T> typeAdapter = getAdapter(typeToken);
     T object = typeAdapter.read(reader);
     return object;
   } 
  ```
and the typeAdapter is instanceof  Adapter in ReflectiveTypeAdapterFactory.
and the method read() is

T instance = constructor.construct();

  try {
    in.beginObject();
    while (in.hasNext()) {
      String name = in.nextName();
      BoundField field = boundFields.get(name);
      if (field == null || !field.deserialized) {
        in.skipValue();
      } else {
        field.read(in, instance);
      }
    }
  }
 return instance;

'T instance = constructor.construct();'  After this,instance is initialize its parameter.
In my code, the parameter 'name' is initialize to 'username'.
And in.hasNext()==false, so the method just return instance.
For more detail, the code ' constructor.construct()' user the java reflect 

Constructor.newInstance(Object... args)
```
although 'args ==null' ,the instance initialize its parameter with default value.
I did in jdk1.7 and jdk1.8 .
Did I describe clearly ? I tried my best.
Sorry , I will improve my English.

[ahoyland]

This is still a problem. I'm going to have to use a different library because of this. Very disappointing...

[bulldozer75]

Works in 2.8.2. Declare the User class static or define it outside of another class.
Update: actually, it works in 2.2.4 too.

[mario222k]

For me it works with 2.8.5 only if all fields have defaults

    private data class Foo(
        @SerializedName("claus") val santa: Boolean = false
        @SerializedName("norris") val chuck: Boolean = true
    )

   val foo = Gson().fromJson<Foo>("{}", Foo::class.java) // --> Foo(santa=false, chuck=true)

but when one has no default

    private data class Foo(
        @SerializedName("claus") val santa: Boolean
        @SerializedName("norris") val chuck: Boolean = true
    )

   val foo = Gson().fromJson<Foo>("{}", Foo::class.java) // --> Foo(santa=false, chuck=false)

[Sdghasemi]

However I'm disappointed, I'm currently using this approach:

public class Test {
    @SerializedName("f")
    private int foo;
    @SerializedName("b")
    private int bar;

    public Test() {
        bar = -1;    // Uses -1 as default value for bar variable when it's not present in the JSON
    }
    ...
}

BTW I'm currently using v2.8.5.

[transilvlad]

I changed the logic of my code to have a defaults JSON file that then gets mapped over and adds values where missing.

[lll-01]

modify ReflectiveTypeAdapterFactory

 @Override
 void read(JsonReader reader, Object value)
         throws IOException, IllegalAccessException {
     Object fieldValue = typeAdapter.read(reader);
     if (fieldValue == null && !context.deserializeNulls()) return;// Do not write null value
     if (fieldValue != null || !isPrimitive) {
         field.set(value, fieldValue);
     }
 }

add deserializeNulls to GsonBuilder and Gson

/**
 * Configure Gson to deserialize null fields. By default, Gson omits all fields that are null
 * during deserialization.
 *
 * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
 * @since 1.2
 */
public GsonBuilder deserializeNulls() {
    this.deserializeNulls = true;
    return this;
}

[moodi1999]

@ahoyland

This is still a problem. I'm going to have to use a different library because of this. Very disappointing...

did you find solution in other lib?
if you did please mention it.🙏🏻

[mochadwi]

any actual updates for this?

[mochadwi]

modify ReflectiveTypeAdapterFactory

 @Override
 void read(JsonReader reader, Object value)
         throws IOException, IllegalAccessException {
     Object fieldValue = typeAdapter.read(reader);
     if (fieldValue == null && !context.deserializeNulls()) return;// Do not write null value
     if (fieldValue != null || !isPrimitive) {
         field.set(value, fieldValue);
     }
 }

add deserializeNulls to GsonBuilder and Gson

/**
 * Configure Gson to deserialize null fields. By default, Gson omits all fields that are null
 * during deserialization.
 *
 * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
 * @since 1.2
 */
public GsonBuilder deserializeNulls() {
    this.deserializeNulls = true;
    return this;
}

the ReflectiveTypeAdapterFactory is final, we cannot inherit it. any other approach? @lll-01

[Marcono1234]

1. Create a simple non-static class which initializes default values for some
   fields:

This part of the reproduction steps hints where the issue is. Since the class is an non-static (i.e. inner) nested class the compiler adds a synthetic constructor parameter for passing the enclosing instance. Therefore Gson is not able to find a no-parameter constructor and falls back to using the JDK internal method sun.misc.Unsafe.allocateInstance whose documentation says:

Allocates an instance but does not run any constructor.

(which apparently includes the initializers assigning the field values as well)

There are at least two solutions for this:

• make the inner class static; often this is possible without any additional code changes and likely will also improve the performance of your code
• use an InstanceCreator; this can be more tricky since you somehow have to access an instance of the enclosing class to create an instance of the inner class. Implementing this correctly and in a sane way depends on your use case.

The Kotlin case described in https://github.com/google/gson/issues/513#issuecomment-447299253 seems to be similar. It appears when all fields have a default value the Kotlin compiler creates a no-parameter constructor. However, as soon as one field has no default value there won't be a no-parameter constructor so Gson falls back to using Unsafe as described above.
The solution here could be to add a default value for all fields (even if you expect some of them to always be overwritten) or to write a custom TypeAdapter.

[a1573595]

Still a problem.