Add strategies for unknown and missing fields

Author: Marcono1234

gson/src/main/java/com/google/gson/Gson.java

@@ -151,6 +151,8 @@ public final class Gson {
   static final FieldNamingStrategy DEFAULT_FIELD_NAMING_STRATEGY = FieldNamingPolicy.IDENTITY;
   static final ToNumberStrategy DEFAULT_OBJECT_TO_NUMBER_STRATEGY = ToNumberPolicy.DOUBLE;
   static final ToNumberStrategy DEFAULT_NUMBER_TO_NUMBER_STRATEGY = ToNumberPolicy.LAZILY_PARSED_NUMBER;
+  static final MissingFieldValueStrategy DEFAULT_MISSING_FIELD_VALUE_STRATEGY = MissingFieldValueStrategy.DO_NOTHING;
+  static final UnknownFieldStrategy DEFAULT_UNKNOWN_FIELD_STRATEGY = UnknownFieldStrategy.IGNORE;
 
   private static final String JSON_NON_EXECUTABLE_PREFIX = ")]}'\n";
 
@@ -195,6 +197,8 @@ public final class Gson {
   final List<TypeAdapterFactory> builderHierarchyFactories;
   final ToNumberStrategy objectToNumberStrategy;
   final ToNumberStrategy numberToNumberStrategy;
+  final MissingFieldValueStrategy missingFieldValueStrategy;
+  final UnknownFieldStrategy unknownFieldStrategy;
   final List<ReflectionAccessFilter> reflectionFilters;
 
   /**
@@ -242,6 +246,7 @@ public Gson() {
         LongSerializationPolicy.DEFAULT, DEFAULT_DATE_PATTERN, DateFormat.DEFAULT, DateFormat.DEFAULT,
         Collections.<TypeAdapterFactory>emptyList(), Collections.<TypeAdapterFactory>emptyList(),
         Collections.<TypeAdapterFactory>emptyList(), DEFAULT_OBJECT_TO_NUMBER_STRATEGY, DEFAULT_NUMBER_TO_NUMBER_STRATEGY,
+        DEFAULT_MISSING_FIELD_VALUE_STRATEGY, DEFAULT_UNKNOWN_FIELD_STRATEGY,
         Collections.<ReflectionAccessFilter>emptyList());
   }
 
@@ -255,6 +260,7 @@ public Gson() {
       List<TypeAdapterFactory> builderHierarchyFactories,
       List<TypeAdapterFactory> factoriesToBeAdded,
       ToNumberStrategy objectToNumberStrategy, ToNumberStrategy numberToNumberStrategy,
+      MissingFieldValueStrategy missingFieldValueStrategy, UnknownFieldStrategy unknownFieldStrategy,
       List<ReflectionAccessFilter> reflectionFilters) {
     this.excluder = excluder;
     this.fieldNamingStrategy = fieldNamingStrategy;
@@ -276,6 +282,8 @@ public Gson() {
     this.builderHierarchyFactories = builderHierarchyFactories;
     this.objectToNumberStrategy = objectToNumberStrategy;
     this.numberToNumberStrategy = numberToNumberStrategy;
+    this.missingFieldValueStrategy = missingFieldValueStrategy;
+    this.unknownFieldStrategy = unknownFieldStrategy;
     this.reflectionFilters = reflectionFilters;
 
     List<TypeAdapterFactory> factories = new ArrayList<>();
@@ -341,7 +349,8 @@ public Gson() {
     factories.add(jsonAdapterFactory);
     factories.add(TypeAdapters.ENUM_FACTORY);
     factories.add(new ReflectiveTypeAdapterFactory(
-        constructorConstructor, fieldNamingStrategy, excluder, jsonAdapterFactory, reflectionFilters));
+        constructorConstructor, fieldNamingStrategy, excluder, jsonAdapterFactory,
+        missingFieldValueStrategy, unknownFieldStrategy, reflectionFilters));
 
     this.factories = Collections.unmodifiableList(factories);
   }

gson/src/main/java/com/google/gson/GsonBuilder.java

@@ -22,10 +22,12 @@
 import static com.google.gson.Gson.DEFAULT_FORMATTING_STYLE;
 import static com.google.gson.Gson.DEFAULT_JSON_NON_EXECUTABLE;
 import static com.google.gson.Gson.DEFAULT_LENIENT;
+import static com.google.gson.Gson.DEFAULT_MISSING_FIELD_VALUE_STRATEGY;
 import static com.google.gson.Gson.DEFAULT_NUMBER_TO_NUMBER_STRATEGY;
 import static com.google.gson.Gson.DEFAULT_OBJECT_TO_NUMBER_STRATEGY;
 import static com.google.gson.Gson.DEFAULT_SERIALIZE_NULLS;
 import static com.google.gson.Gson.DEFAULT_SPECIALIZE_FLOAT_VALUES;
+import static com.google.gson.Gson.DEFAULT_UNKNOWN_FIELD_STRATEGY;
 import static com.google.gson.Gson.DEFAULT_USE_JDK_UNSAFE;
 
 import com.google.gson.annotations.Since;
@@ -56,7 +58,7 @@
  * use {@code new Gson()}. {@code GsonBuilder} is best used by creating it, and then invoking its
  * various configuration methods, and finally calling create.</p>
  *
- * <p>The following is an example shows how to use the {@code GsonBuilder} to construct a Gson
+ * <p>The following example shows how to use the {@code GsonBuilder} to construct a Gson
  * instance:
  *
  * <pre>
@@ -73,8 +75,8 @@
  *
  * <p>NOTES:
  * <ul>
- * <li> the order of invocation of configuration methods does not matter.</li>
- * <li> The default serialization of {@link Date} and its subclasses in Gson does
+ * <li>the order of invocation of configuration methods does not matter.</li>
+ * <li>the default serialization of {@link Date} and its subclasses in Gson does
  *  not contain time-zone information. So, if you are using date/time instances,
  *  use {@code GsonBuilder} and its {@code setDateFormat} methods.</li>
  * </ul>
@@ -104,6 +106,8 @@ public final class GsonBuilder {
   private boolean useJdkUnsafe = DEFAULT_USE_JDK_UNSAFE;
   private ToNumberStrategy objectToNumberStrategy = DEFAULT_OBJECT_TO_NUMBER_STRATEGY;
   private ToNumberStrategy numberToNumberStrategy = DEFAULT_NUMBER_TO_NUMBER_STRATEGY;
+  private MissingFieldValueStrategy missingFieldValueStrategy = DEFAULT_MISSING_FIELD_VALUE_STRATEGY;
+  private UnknownFieldStrategy unknownFieldStrategy = DEFAULT_UNKNOWN_FIELD_STRATEGY;
   private final ArrayDeque<ReflectionAccessFilter> reflectionFilters = new ArrayDeque<>();
 
   /**
@@ -141,6 +145,8 @@ public GsonBuilder() {
     this.useJdkUnsafe = gson.useJdkUnsafe;
     this.objectToNumberStrategy = gson.objectToNumberStrategy;
     this.numberToNumberStrategy = gson.numberToNumberStrategy;
+    this.missingFieldValueStrategy = gson.missingFieldValueStrategy;
+    this.unknownFieldStrategy = gson.unknownFieldStrategy;
     this.reflectionFilters.addAll(gson.reflectionFilters);
   }
 
@@ -388,6 +394,36 @@ public GsonBuilder setObjectToNumberStrategy(ToNumberStrategy objectToNumberStra
     return this;
   }
 
+  /**
+   * Configures Gson to apply a specific missing field value strategy during deserialization.
+   * The strategy is used during reflection-based deserialization when the JSON data does
+   * not contain a value for a field. A field with explicit JSON null is not considered missing.
+   *
+   * @param missingFieldValueStrategy strategy handling missing field values
+   * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
+   * @see MissingFieldValueStrategy#DO_NOTHING The default missing field value strategy
+   * @since $next-version$
+   */
+  public GsonBuilder setMissingFieldValueStrategy(MissingFieldValueStrategy missingFieldValueStrategy) {
+    this.missingFieldValueStrategy = Objects.requireNonNull(missingFieldValueStrategy);
+    return this;
+  }
+
+  /**
+   * Configures Gson to apply a specific unknown field strategy during deserialization.
+   * The strategy is used during reflection-based deserialization when an unknown field
+   * is encountered in the JSON data.
+   *
+   * @param unknownFieldStrategy strategy handling unknown fields
+   * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
+   * @see UnknownFieldStrategy#IGNORE The default unknown field strategy
+   * @since $next-version$
+   */
+  public GsonBuilder setUnknownFieldStrategy(UnknownFieldStrategy unknownFieldStrategy) {
+    this.unknownFieldStrategy = Objects.requireNonNull(unknownFieldStrategy);
+    return this;
+  }
+
   /**
    * Configures Gson to apply a specific number strategy during deserialization of {@link Number}.
    *
@@ -782,7 +818,8 @@ public Gson create() {
         serializeSpecialFloatingPointValues, useJdkUnsafe, longSerializationPolicy,
         datePattern, dateStyle, timeStyle, new ArrayList<>(this.factories),
         new ArrayList<>(this.hierarchyFactories), factories,
-        objectToNumberStrategy, numberToNumberStrategy, new ArrayList<>(reflectionFilters));
+        objectToNumberStrategy, numberToNumberStrategy,
+        missingFieldValueStrategy, unknownFieldStrategy, new ArrayList<>(reflectionFilters));
   }
 
   private void addTypeAdaptersForDate(String datePattern, int dateStyle, int timeStyle,

gson/src/main/java/com/google/gson/MissingFieldValueStrategy.java

@@ -0,0 +1,77 @@
+package com.google.gson;
+
+import com.google.gson.internal.reflect.ReflectionHelper;
+import com.google.gson.reflect.TypeToken;
+import java.lang.reflect.Field;
+
+/**
+ * A strategy defining how to handle missing field values during reflection-based deserialization.
+ *
+ * @see GsonBuilder#setMissingFieldValueStrategy(MissingFieldValueStrategy)
+ * @since $next-version$
+ */
+public interface MissingFieldValueStrategy {
+  /**
+   * This strategy does nothing when a missing field is detected, it preserves the initial field
+   * value, if any.
+   *
+   * <p>This is the default missing field value strategy.
+   */
+  MissingFieldValueStrategy DO_NOTHING = new MissingFieldValueStrategy() {
+    @Override
+    public Object handleMissingField(TypeToken<?> declaringType, Object instance, Field field, TypeToken<?> resolvedFieldType) {
+      // Preserve initial field value
+      return null;
+    }
+
+    @Override
+    public String toString() {
+      return "MissingFieldValueStrategy.DO_NOTHING";
+    }
+  };
+
+  /**
+   * This strategy throws an exception when a missing field is detected.
+   */
+  MissingFieldValueStrategy THROW_EXCEPTION = new MissingFieldValueStrategy() {
+    @Override
+    public Object handleMissingField(TypeToken<?> declaringType, Object instance, Field field, TypeToken<?> resolvedFieldType) {
+      // TODO: Proper exception
+      throw new RuntimeException("Missing value for field '" + ReflectionHelper.fieldToString(field) + "'");
+    }
+
+    @Override
+    public String toString() {
+      return "MissingFieldValueStrategy.THROW_EXCEPTION";
+    }
+  };
+
+  /**
+   * Called when a missing field value is detected. Implementations can either throw an exception or
+   * return a default value.
+   *
+   * <p>Returning {@code null} will keep the initial field value, if any. For example when returning
+   * {@code null} for the field {@code String f = "default"}, the field will still have the value
+   * {@code "default"} afterwards (assuming the constructor of the class was called, see also
+   * {@link GsonBuilder#disableJdkUnsafe()}). The type of the returned value has to match the
+   * type of the field, no narrowing or widening numeric conversion is performed.
+   *
+   * <p>The {@code instance} represents an instance of the declaring type with the so far already
+   * deserialized fields. It is intended to be used for looking up existing field values to derive
+   * the missing field value from them. Manipulating {@code instance} in any way is not recommended.<br>
+   * For Record classes (Java 16 feature) the {@code instance} is {@code null}.
+   *
+   * <p>{@code resolvedFieldType} is the type of the field with type variables being resolved, if
+   * possible. For example if {@code class MyClass<T>} has a field {@code T myField} and
+   * {@code MyClass<String>} is deserialized, then {@code resolvedFieldType} will be {@code String}.
+   *
+   * @param declaringType type declaring the field
+   * @param instance instance of the declaring type, {@code null} for Record classes
+   * @param field field whose value is missing
+   * @param resolvedFieldType resolved type of the field
+   * @return the field value, or {@code null}
+   */
+  // TODO: Should this really expose `instance`? Only use case would be to derive value from other fields
+  //   but besides that user should not directly manipulate `instance` but return new value instead
+  Object handleMissingField(TypeToken<?> declaringType, Object instance, Field field, TypeToken<?> resolvedFieldType);
+}

gson/src/main/java/com/google/gson/ReflectionAccessFilter.java

@@ -124,6 +124,10 @@ enum FilterResult {
         ? FilterResult.BLOCK_INACCESSIBLE
         : FilterResult.INDECISIVE;
     }
+
+    @Override public String toString() {
+      return "ReflectionAccessFilter.BLOCK_INACCESSIBLE_JAVA";
+    }
   };
 
   /**
@@ -149,6 +153,10 @@ enum FilterResult {
         ? FilterResult.BLOCK_ALL
         : FilterResult.INDECISIVE;
     }
+
+    @Override public String toString() {
+      return "ReflectionAccessFilter.BLOCK_ALL_JAVA";
+    }
   };
 
   /**
@@ -173,6 +181,10 @@ enum FilterResult {
         ? FilterResult.BLOCK_ALL
         : FilterResult.INDECISIVE;
     }
+
+    @Override public String toString() {
+      return "ReflectionAccessFilter.BLOCK_ALL_ANDROID";
+    }
   };
 
   /**
@@ -198,6 +210,10 @@ enum FilterResult {
         ? FilterResult.BLOCK_ALL
         : FilterResult.INDECISIVE;
     }
+
+    @Override public String toString() {
+      return "ReflectionAccessFilter.BLOCK_ALL_PLATFORM";
+    }
   };
 
   /**

gson/src/main/java/com/google/gson/UnknownFieldStrategy.java

@@ -0,0 +1,79 @@
+package com.google.gson;
+
+import com.google.gson.reflect.TypeToken;
+import com.google.gson.stream.JsonReader;
+import java.io.IOException;
+
+/**
+ * A strategy defining how to handle unknown fields during reflection-based deserialization.
+ *
+ * @see GsonBuilder#setUnknownFieldStrategy(UnknownFieldStrategy)
+ * @since $next-version$
+ */
+public interface UnknownFieldStrategy {
+  /**
+   * This strategy ignores the unknown field.
+   *
+   * <p>This is the default unknown field strategy.
+   */
+  UnknownFieldStrategy IGNORE = new UnknownFieldStrategy() {
+    @Override
+    public void handleUnknownField(TypeToken<?> declaringType, Object instance, String fieldName,
+        JsonReader jsonReader, Gson gson) throws IOException {
+      jsonReader.skipValue();
+    }
+
+    @Override
+    public String toString() {
+      return "UnknownFieldStrategy.IGNORE";
+    }
+  };
+
+  /**
+   * This strategy throws an exception when an unknown field is encountered.
+   *
+   * <p><b>Note:</b> Be careful when using this strategy; while it might sound tempting
+   * to strictly validate that the JSON data matches the expected format, this strategy
+   * makes it difficult to add new fields to the JSON structure in a backward compatible way.
+   * Usually it suffices to use only {@link MissingFieldValueStrategy#THROW_EXCEPTION} for
+   * validation and to ignore unknown fields.
+   */
+  UnknownFieldStrategy THROW_EXCEPTION = new UnknownFieldStrategy() {
+    @Override
+    public void handleUnknownField(TypeToken<?> declaringType, Object instance, String fieldName,
+        JsonReader jsonReader, Gson gson) throws IOException {
+      // TODO: Proper exception
+      throw new RuntimeException("Unknown field '" + fieldName + "' for " + declaringType.getRawType() + " at path " + jsonReader.getPath());
+    }
+
+    @Override
+    public String toString() {
+      return "UnknownFieldStrategy.THROW_EXCEPTION";
+    }
+  };
+
+  /**
+   * Called when an unknown field is encountered. Implementations can throw an exception,
+   * store the field value in {@code instance} or ignore the unknown field.
+   *
+   * <p>The {@code jsonReader} is positioned to read the value of the unknown field. If an
+   * implementation of this method does not throw an exception it must consume the value, either
+   * by reading it with methods like {@link JsonReader#nextString()} (possibly after peeking
+   * at the value type first), or by skipping it with {@link JsonReader#skipValue()}.<br>
+   * The {@code gson} object can be used to read from the {@code jsonReader}. It is the same
+   * instance which was originally used to perform the deserialization.
+   *
+   * <p>The {@code instance} represents an instance of the declaring type with the so far already
+   * deserialized fields. It can be used to store the value of the unknown field, for example
+   * if it declares a {@code transient Map<String, Object>} field for all unknown values.<br>
+   * For Record classes (Java 16 feature) the {@code instance} is {@code null}.
+   *
+   * @param declaringType type declaring the field
+   * @param instance instance of the declaring type, {@code null} for Record classes
+   * @param fieldName name of the unknown field
+   * @param jsonReader reader to be used to read or skip the field value
+   * @param gson {@code Gson} instance which can be used to read the field value from {@code jsonReader}
+   * @throws IOException if reading or skipping the field value fails
+   */
+  void handleUnknownField(TypeToken<?> declaringType, Object instance, String fieldName, JsonReader jsonReader, Gson gson) throws IOException;
+}