Check for required fields

[piegamesde]

This is a pretty common request for Gson, so I thought maybe we could implement this using a custom PostProcessor?

---

I came up with this little thing, tell me what you think of it:

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import java.lang.reflect.Field;
import java.util.ArrayList;
import java.util.List;

import com.google.gson.Gson;
import com.google.gson.JsonElement;
import com.google.gson.JsonParseException;
import com.google.gson.annotations.SerializedName;

import io.gsonfire.PostProcessor;

public class DeserializeNullChecker implements PostProcessor<Object> {

    @Documented
    @Retention(RetentionPolicy.RUNTIME)
    @Target({ ElementType.FIELD })
    public @interface DeserializeNonNull {
    }

    @Override
    public void postDeserialize(Object result, JsonElement src, Gson gson) {
        check(result, src);
    }

    @Override
    public void postSerialize(JsonElement result, Object src, Gson gson) {
    }

    private void check(Object result, JsonElement src) {
        try {
            List<String> nullFields = new ArrayList<>();
            for (Field f : result.getClass().getDeclaredFields()) {
                if (f.isAnnotationPresent(DeserializeNonNull.class)) {
                    String name = f.getName();
                    if (f.isAnnotationPresent(SerializedName.class))
                        name = f.getAnnotation(SerializedName.class).value();
                    if (f.get(result) == null)
                        nullFields.add('"' + name + '"');
                }
            }
            if (!nullFields.isEmpty()) {
                if (nullFields.size() == 1)
                    throw new JsonParseException("Field " + nullFields.get(0) + " in object '"
                            + ellipsisString(src.toString())
                            + "' must be specified");
                else
                    throw new JsonParseException("Fields " + nullFields + " in object '"
                            + ellipsisString(src.toString())
                            + "' must be specified");
            }
        } catch (IllegalArgumentException | IllegalAccessException e) {
            throw new JsonParseException("Could not check all null fields", e);
        }
    }

    private String ellipsisString(String text) {
        if (text.length() < 40)
            return text;
        else
            return text.substring(0, 30) + "[…]" + text.substring(text.length() - 10, text.length());
    }
}

[julman99]

A @Required annotation seems like a very useful feature. I will put it for 1.9.0, however I don't have release date yet for it.