Add "Ignore" annotation to ignore some fields during serialization or d…

[raomuyang]

Ignore is an annotation that indicates this member should be ignored for JSON
serialization or deserialization.

This annotation has no effect unless you build com.google.gson.Gson
with a com.google.gson.GsonBuilder and invoke GsonBuilder.excludeFieldsWithIgnoreAnnotation() method.

• An example:

public class Account {
  private String accountName;
  @Ignore(deserialize = false) private String info;
  @Ignore private String password;
}

If you create Gson via new Gson(), the toJson() and fromJson()
methods will use all fields for serialization and deserialization. However, you can create
Gson via new GsonBuilder().excludeFieldsWithIgnoreAnnotation().create(), then the
toJson() will exclude the info field and thepassword field,
because those fields were marked with the@Ignore annotation and set
serialize = true. Similarly, the fromJson() will exclude password since deserialize is set to false.

---

What's the difference fromExpose annotation?
TheExpose annotation indicates this member should be exposed for JSON serialization
or deserialization, it means all the fields that you wanna expose, you should be marked with Expose:

public class Student {
  @Expose private Account account;
  @Expose private String class;
}

You can Create Gson with Gson gson = new GsonBuilder().excludeFieldsWithoutExposeAnnotation().create(),
but the fields of Account class will be excluded because they are not marked by Expose.

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[NightlyNexus]

If you need this functionality, you can mark the fields transient.

[raomuyang]

@NightlyNexus Thanks for you review. But transient can not replace @Ignore(deserialize = false)

[kyle-cackett]

I home-brewed the functionality in this pull request because transient was not sufficient for my use case. The problem with transient is that it has implications outside of GSON. In particular, I needed a particular field to be serialized by Java when writing objects to streams but not by GSON -- this is not possible with the transient field.

[Johnny850807]

Agreed, @expose makes class so ugly.
And there is a case that I want my field to be non-transient, but be ignored from Gson.