FieldMask: JSON serialisation

wire-runtime-swift/src/main/swift/FieldMask.swift

@@ -79,14 +79,17 @@ extension FieldMask: Codable {
     public init(from decoder: Decoder) throws {
         let container = try decoder.singleValueContainer()
         let value = try container.decode(String.self)
-        self.paths = value.isEmpty
-            ? []
-            : value.split(separator: ",").map { FieldMask.protoName(String($0)) }
+        self.paths = value.split(separator: ",").map { FieldMask.protoName(String($0)) }
     }
 
     public func encode(to encoder: Encoder) throws {
         var container = encoder.singleValueContainer()
-        try container.encode(paths.map(FieldMask.jsonName).joined(separator: ","))
+        try container.encode(
+            paths
+                .filter { !$0.isEmpty }
+                .map(FieldMask.jsonName)
+                .joined(separator: ",")
+        )
     }
 
     private static func jsonName(_ path: String) -> String {
@@ -113,7 +116,7 @@ extension FieldMask: Codable {
                 result.append(String(scalar).uppercased())
                 capitalizeNext = false
             } else {
-                result.append(String(scalar))
+                result.append(String(scalar).lowercased())
             }
         }
         return result
@@ -128,7 +131,7 @@ extension FieldMask: Codable {
                 }
                 result.append(String(scalar).lowercased())
             } else {
-                result.append(String(scalar))
+                result.append(String(scalar).lowercased())
             }
         }
         return result

wire-runtime-swift/src/test/swift/FieldMaskTests.swift

@@ -54,6 +54,19 @@ final class FieldMaskTests: XCTestCase {
         XCTAssertEqual(String(data: encoded, encoding: .utf8), #""user.displayName,photo""#)
     }
 
+    func testJSONSkipsEmptyPaths() throws {
+        let fieldMask = FieldMask(paths: ["", "photo", ""])
+
+        let encoded = try JSONEncoder().encode(fieldMask)
+        let decoded = try JSONDecoder().decode(
+            FieldMask.self,
+            from: Foundation.Data("\",photo,\"".utf8)
+        )
+
+        XCTAssertEqual(String(data: encoded, encoding: .utf8), #""photo""#)
+        XCTAssertEqual(decoded, FieldMask(paths: ["photo"]))
+    }
+
     func testGeneratedMessageWithFieldMask() throws {
         let message = MessageContainingFieldMask {
             $0.mask = FieldMask(paths: ["user.display_name", "photo"])

wire-runtime/src/jvmMain/kotlin/com/squareup/wire/internal/FieldMaskJsonFormatter.kt

@@ -0,0 +1,58 @@
+/*
+ * Copyright (C) 2026 Square, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.squareup.wire.internal
+
+import com.squareup.wire.FieldMask
+
+/** Encodes a field mask as a JSON string like "user.displayName,photo". */
+object FieldMaskJsonFormatter : JsonFormatter<FieldMask> {
+  override fun toStringOrNumber(value: FieldMask): String = value.paths
+    .filter { it.isNotEmpty() }
+    .joinToString(separator = ",") { it.lowerUnderscoreToLowerCamel() }
+
+  override fun fromString(value: String): FieldMask = FieldMask(
+    value.split(',')
+      .filter { it.isNotEmpty() }
+      .map { it.lowerCamelToLowerUnderscore() },
+  )
+
+  private fun String.lowerUnderscoreToLowerCamel(): String {
+    val result = StringBuilder()
+    var capitalizeNext = false
+    for (c in this) {
+      if (c == '_') {
+        capitalizeNext = true
+      } else if (capitalizeNext) {
+        result.append(c.uppercaseChar())
+        capitalizeNext = false
+      } else {
+        result.append(c.lowercaseChar())
+      }
+    }
+    return result.toString()
+  }
+
+  private fun String.lowerCamelToLowerUnderscore(): String {
+    val result = StringBuilder()
+    for (c in this) {
+      if (c in 'A'..'Z') {
+        result.append('_')
+      }
+      result.append(c.lowercaseChar())
+    }
+    return result.toString()
+  }
+}

wire-runtime/src/jvmMain/kotlin/com/squareup/wire/internal/JsonIntegration.kt

@@ -103,6 +103,7 @@ abstract class JsonIntegration<F, A> {
       ProtoAdapter.BYTES_VALUE,
       -> return ByteStringJsonFormatter
       ProtoAdapter.DURATION -> return DurationJsonFormatter
+      ProtoAdapter.FIELD_MASK -> return FieldMaskJsonFormatter
       ProtoAdapter.INSTANT -> return InstantJsonFormatter
       is EnumAdapter<*> -> return EnumJsonFormatter(protoAdapter)
     }

wire-tests/fixtures/shared/proto/proto3/contains_field_mask.proto

@@ -0,0 +1,25 @@
+/*
+ * Copyright 2026 Square Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+syntax = "proto3";
+
+package squareup.proto3;
+
+import "google/protobuf/field_mask.proto";
+
+message ContainsFieldMask {
+  google.protobuf.FieldMask mask = 1;
+  repeated google.protobuf.FieldMask masks = 2;
+}

wire-tests/jvm-json-java/build.gradle.kts

@@ -37,6 +37,7 @@ wire {
       "all_structs.proto",
       "all_wrappers.proto",
       "camel_case.proto",
+      "contains_field_mask.proto",
       "map_types.proto",
       "pizza.proto",
     )

wire-tests/jvm-json-kotlin/build.gradle.kts

@@ -37,6 +37,7 @@ wire {
       "all_structs.proto",
       "all_wrappers.proto",
       "camel_case.proto",
+      "contains_field_mask.proto",
       "map_types.proto",
       "pizza.proto",
     )

wire-tests/wire-json-shared-kotlin-tests/com/squareup/wire/WireJsonTest.kt

@@ -45,6 +45,7 @@ import squareup.proto3.AllWrappers
 import squareup.proto3.BuyOneGetOnePromotion
 import squareup.proto3.CamelCase
 import squareup.proto3.CamelCase.NestedCamelCase
+import squareup.proto3.ContainsFieldMask
 import squareup.proto3.FreeDrinkPromotion
 import squareup.proto3.FreeGarlicBreadPromotion
 import squareup.proto3.MapTypes
@@ -267,6 +268,33 @@ class WireJsonTest {
     )
   }
 
+  @Test fun fieldMask() {
+    val value = ContainsFieldMask.Builder()
+      .mask(FieldMask(listOf("user.display_name", "photo", "foo_bar.baz_qux")))
+      .masks(
+        listOf(
+          FieldMask(listOf("display_name")),
+          FieldMask(listOf("updated_at.seconds")),
+        ),
+      )
+      .build()
+    val json = """
+      |{
+      |  "mask": "user.displayName,photo,fooBar.bazQux",
+      |  "masks": ["displayName", "updatedAt.seconds"]
+      |}
+    """.trimMargin()
+
+    assertJsonEquals(json, jsonLibrary.toJson(value, ContainsFieldMask::class.java))
+
+    val parsed = jsonLibrary.fromJson(json, ContainsFieldMask::class.java)
+    assertThat(parsed).isEqualTo(value)
+    assertJsonEquals(
+      jsonLibrary.toJson(parsed, ContainsFieldMask::class.java),
+      jsonLibrary.toJson(value, ContainsFieldMask::class.java),
+    )
+  }
+
   @Test fun anyMessageWithUnregisteredTypeOnReading() {
     try {
       jsonLibrary.fromJson(PIZZA_DELIVERY_UNKNOWN_TYPE_JSON, PizzaDelivery::class.java)

wire-tests/wire-json-shared-kotlin-tests/com/squareup/wire/internal/FieldMaskJsonFormatterTest.kt

@@ -0,0 +1,45 @@
+/*
+ * Copyright (C) 2026 Square, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.squareup.wire.internal
+
+import assertk.assertThat
+import assertk.assertions.isEqualTo
+import com.squareup.wire.FieldMask
+import com.squareup.wire.internal.FieldMaskJsonFormatter.fromString
+import com.squareup.wire.internal.FieldMaskJsonFormatter.toStringOrNumber
+import org.junit.Test
+
+class FieldMaskJsonFormatterTest {
+  @Test fun `field mask to string`() {
+    assertThat(
+      toStringOrNumber(FieldMask(listOf("user.display_name", "photo", "foo_bar.baz_qux"))),
+    ).isEqualTo("user.displayName,photo,fooBar.bazQux")
+  }
+
+  @Test fun `string to field mask`() {
+    assertThat(fromString("user.displayName,photo,fooBar.bazQux"))
+      .isEqualTo(FieldMask(listOf("user.display_name", "photo", "foo_bar.baz_qux")))
+  }
+
+  @Test fun `empty paths are skipped`() {
+    assertThat(toStringOrNumber(FieldMask(listOf("", "photo", ""))))
+      .isEqualTo("photo")
+    assertThat(fromString(",photo,"))
+      .isEqualTo(FieldMask(listOf("photo")))
+    assertThat(fromString(""))
+      .isEqualTo(FieldMask())
+  }
+}