Add @Json.ignore (#1417)

* Default Json.name to an unset value

* Promote shared transient tests to DualKotlinTest

* Add new ignore property to Json

* Support it in ClassJsonAdapter

* Mention no enum/record support

* Support in KotlinJsonAdapter

* Rework code gen API to know of "ignored"

* Support in apt code gen

* Support in KSP

* Update old non-working transient example test

* Synthetic holders

* Use field on both

moshi/src/main/java/com/squareup/moshi/ClassJsonAdapter.java

@@ -134,6 +134,8 @@ final class ClassJsonAdapter<T> extends JsonAdapter<T> {
           boolean platformType = Util.isPlatformType(rawType);
           for (Field field : rawType.getDeclaredFields()) {
             if (!includeField(platformType, field.getModifiers())) continue;
+            Json jsonAnnotation = field.getAnnotation(Json.class);
+            if (jsonAnnotation != null && jsonAnnotation.ignore()) continue;
 
             // Look up a type adapter for this type.
             Type fieldType = resolve(type, rawType, field.getGenericType());
@@ -145,8 +147,10 @@ final class ClassJsonAdapter<T> extends JsonAdapter<T> {
             field.setAccessible(true);
 
             // Store it using the field's name. If there was already a field with this name, fail!
-            Json jsonAnnotation = field.getAnnotation(Json.class);
-            String name = jsonAnnotation != null ? jsonAnnotation.name() : fieldName;
+            String name =
+                jsonAnnotation != null && !Json.UNSET_NAME.equals(jsonAnnotation.name())
+                    ? jsonAnnotation.name()
+                    : fieldName;
             FieldBinding<Object> fieldBinding = new FieldBinding<>(name, field, adapter);
             FieldBinding<?> replaced = fieldBindings.put(name, fieldBinding);
             if (replaced != null) {

moshi/src/main/java/com/squareup/moshi/Json.java

@@ -29,8 +29,9 @@ import java.lang.annotation.Target;
  *
  * <ul>
  *   <li><strong>Java class fields</strong>
- *   <li><strong>Kotlin properties</strong> for use with {@code moshi-kotlin}. This includes both
- *       properties declared in the constructor and properties declared as members.
+ *   <li><strong>Kotlin properties</strong> for use with {@code moshi-kotlin} or {@code
+ *       moshi-kotlin-codegen}. This includes both properties declared in the constructor and
+ *       properties declared as members.
  * </ul>
  *
  * <p>Users of the <a href="https://github.com/rharter/auto-value-moshi">AutoValue: Moshi
@@ -39,5 +40,17 @@ import java.lang.annotation.Target;
 @Retention(RUNTIME)
 @Documented
 public @interface Json {
-  String name();
+  /** The default value of {@link #name()}. Should only be used to check if it's been set. */
+  String UNSET_NAME = "\u0000";
+
+  /** The name of the field when encoded as JSON. */
+  String name() default UNSET_NAME;
+
+  /**
+   * If true, this field/property will be ignored. This is semantically similar to use of {@code
+   * transient} on the JVM.
+   *
+   * <p><strong>Note:</strong> this has no effect in enums or record classes.
+   */
+  boolean ignore() default false;
 }

moshi/src/main/java/com/squareup/moshi/StandardJsonAdapters.java

@@ -275,7 +275,10 @@ final class StandardJsonAdapters {
         for (int i = 0; i < constants.length; i++) {
           T constant = constants[i];
           Json annotation = enumType.getField(constant.name()).getAnnotation(Json.class);
-          String name = annotation != null ? annotation.name() : constant.name();
+          String name =
+              annotation != null && !Json.UNSET_NAME.equals(annotation.name())
+                  ? annotation.name()
+                  : constant.name();
           nameStrings[i] = name;
         }
         options = JsonReader.Options.of(nameStrings);

moshi/src/main/java16/com/squareup/moshi/RecordJsonAdapter.java

@@ -91,7 +91,10 @@ final class RecordJsonAdapter<T> extends JsonAdapter<T> {
           Set<Annotation> qualifiers = null;
           for (var annotation : component.getDeclaredAnnotations()) {
             if (annotation instanceof Json jsonAnnotation) {
-              jsonName = jsonAnnotation.name();
+              var annotationName = jsonAnnotation.name();
+              if (!Json.UNSET_NAME.equals(annotationName)) {
+                jsonName = jsonAnnotation.name();
+              }
             } else {
               if (annotation.annotationType().isAnnotationPresent(JsonQualifier.class)) {
                 if (qualifiers == null) {

moshi/src/test/java/com/squareup/moshi/ClassJsonAdapterTest.java

@@ -153,6 +153,26 @@ public final class ClassJsonAdapterTest {
     assertThat(fromJson.b).isEqualTo(12);
   }
 
+  static class IgnoredFields {
+    @Json(ignore = true)
+    int a;
+
+    int b;
+  }
+
+  @Test
+  public void ignoredFieldsOmitted() throws Exception {
+    IgnoredFields value = new IgnoredFields();
+    value.a = 11;
+    value.b = 12;
+    String toJson = toJson(IgnoredFields.class, value);
+    assertThat(toJson).isEqualTo("{\"b\":12}");
+
+    IgnoredFields fromJson = fromJson(IgnoredFields.class, "{\"a\":13,\"b\":12}");
+    assertThat(fromJson.a).isEqualTo(0); // Not assigned.
+    assertThat(fromJson.b).isEqualTo(12);
+  }
+
   static class BaseA {
     int a;
   }