fix: Default to GsonFactory and lazy-load default JsonFactory

src/main/java/com/google/firebase/internal/ApiClientUtils.java

@@ -16,15 +16,13 @@
 
 package com.google.firebase.internal;
 
-import com.google.api.client.googleapis.util.Utils;
 import com.google.api.client.http.HttpRequestFactory;
 import com.google.api.client.http.HttpResponse;
 import com.google.api.client.http.HttpTransport;
 import com.google.api.client.json.JsonFactory;
-import com.google.api.client.json.jackson2.JacksonFactory;
+import com.google.api.client.json.gson.GsonFactory;
 import com.google.common.collect.ImmutableList;
 import com.google.firebase.FirebaseApp;
-
 import java.io.IOException;
 
 /**
@@ -80,17 +78,30 @@ public static void disconnectQuietly(HttpResponse response) {
     }
   }
 
+  /**
+   * Returns the default {@link JsonFactory} instance used across the SDK when one is not
+   * explicitly configured via {@link com.google.firebase.FirebaseOptions}.
+   *
+   * @return The default {@link JsonFactory} instance.
+   */
   public static JsonFactory getDefaultJsonFactory() {
-    // Force using the Jackson2 parser for this project for now. Eventually we should switch
-    // to Gson, but there are some issues that's preventing this migration at the moment.
-    // See https://github.com/googleapis/google-api-java-client/issues/1779 for details.
-    return JacksonFactory.getDefaultInstance();
+    return JsonFactoryInstanceHolder.INSTANCE;
   }
 
+  /**
+   * Returns the default {@link HttpTransport} instance used across the SDK when one is not
+   * explicitly configured via {@link com.google.firebase.FirebaseOptions}.
+   *
+   * @return The default {@link HttpTransport} instance.
+   */
   public static HttpTransport getDefaultTransport() {
     return TransportInstanceHolder.INSTANCE;
   }
 
+  private static class JsonFactoryInstanceHolder {
+    static final JsonFactory INSTANCE = GsonFactory.getDefaultInstance();
+  }
+
   private static class TransportInstanceHolder {
     static final HttpTransport INSTANCE = new ApacheHttp2Transport();
   }

src/test/java/com/google/firebase/FirebaseOptionsTest.java

@@ -116,6 +116,7 @@ public void createOptionsWithOnlyMandatoryValuesSet() throws IOException {
             .setCredentials(GoogleCredentials.fromStream(ServiceAccount.EDITOR.asStream()))
             .build();
     assertNotNull(firebaseOptions.getJsonFactory());
+    assertTrue(firebaseOptions.getJsonFactory() instanceof GsonFactory);
     assertNotNull(firebaseOptions.getHttpTransport());
     assertNotNull(firebaseOptions.getThreadManager());
     assertNull(firebaseOptions.getDatabaseUrl());
@@ -145,6 +146,7 @@ public AccessToken refreshAccessToken() {
             .build();
 
     assertNotNull(firebaseOptions.getJsonFactory());
+    assertTrue(firebaseOptions.getJsonFactory() instanceof GsonFactory);
     assertNotNull(firebaseOptions.getHttpTransport());
     assertNull(firebaseOptions.getDatabaseUrl());
     assertNull(firebaseOptions.getStorageBucket());

src/test/java/com/google/firebase/internal/ApiClientUtilsTest.java

@@ -18,7 +18,10 @@
 
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertSame;
+import static org.junit.Assert.assertThrows;
 import static org.junit.Assert.assertTrue;
 
 import com.google.api.client.http.GenericUrl;
@@ -27,15 +30,27 @@
 import com.google.api.client.http.HttpResponse;
 import com.google.api.client.http.HttpTransport;
 import com.google.api.client.http.HttpUnsuccessfulResponseHandler;
+import com.google.api.client.json.GenericJson;
+import com.google.api.client.json.JsonFactory;
+import com.google.api.client.json.gson.GsonFactory;
 import com.google.api.client.testing.http.MockHttpTransport;
 import com.google.api.client.testing.http.MockLowLevelHttpResponse;
 import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableSet;
+import com.google.common.io.ByteStreams;
 import com.google.firebase.FirebaseApp;
 import com.google.firebase.FirebaseOptions;
 import com.google.firebase.TestOnlyImplFirebaseTrampolines;
 import com.google.firebase.auth.MockGoogleCredentials;
 import com.google.firebase.internal.RetryInitializer.RetryHandlerDecorator;
+import com.google.firebase.messaging.FirebaseMessaging;
+import com.google.firebase.messaging.Message;
+import com.google.firebase.remoteconfig.Template;
 import java.io.IOException;
+import java.io.InputStream;
+import java.lang.reflect.Method;
+import java.util.HashMap;
+import java.util.Set;
 import org.junit.After;
 import org.junit.Test;
 
@@ -137,4 +152,154 @@ public void testVerifyDefaultTransportReused() {
     assertEquals(t1, t2);
   }
 
+  @Test
+  public void testDefaultJsonFactoryUsesGsonAndReused() throws IOException {
+    JsonFactory f1 = ApiClientUtils.getDefaultJsonFactory();
+    JsonFactory f2 = ApiClientUtils.getDefaultJsonFactory();
+    assertTrue(f1 instanceof GsonFactory);
+    assertSame(f1, f2);
+    assertEquals("0.2", f1.toString(0.2f));
+    assertThrows(
+        IOException.class,
+        () -> f1.createJsonParser("not json").parseAndClose(new HashMap<String, Object>()));
+    assertThrows(
+        IOException.class,
+        () -> f1.createJsonParser("not json").parseAndClose(GenericJson.class));
+  }
+
+  @Test
+  public void testMessagingWorksWithoutJackson2OnClasspath() throws Exception {
+    ClassLoader isolatedLoader = new PackageBlockingClassLoader(
+        ApiClientUtilsTest.class.getClassLoader(),
+        ImmutableSet.of(
+            "com.google.api.client.json.jackson2.",
+            "com.fasterxml.jackson."));
+    assertThrows(
+        ClassNotFoundException.class,
+        () -> isolatedLoader.loadClass("com.google.api.client.json.jackson2.JacksonFactory"));
+
+    Class<?> probe = isolatedLoader.loadClass(JacksonExcludedProbe.class.getName());
+    Method method = probe.getMethod("sendWithDefaultJsonFactory");
+    String messageId = (String) method.invoke(null);
+    assertEquals("projects/test-project/messages/msg-123", messageId);
+  }
+
+  @Test
+  public void testCustomJsonFactoryDoesNotLoadDefaultJsonFactoryImpl() throws Exception {
+    ClassLoader isolatedLoader = new PackageBlockingClassLoader(
+        ApiClientUtilsTest.class.getClassLoader(),
+        ImmutableSet.of(
+            "com.google.api.client.json.jackson2.",
+            "com.fasterxml.jackson.",
+            "com.google.api.client.json.gson."));
+    assertThrows(
+        ClassNotFoundException.class,
+        () -> isolatedLoader.loadClass("com.google.api.client.json.gson.GsonFactory"));
+
+    Class<?> probe = isolatedLoader.loadClass(JacksonExcludedProbe.class.getName());
+    Method method = probe.getMethod("sendWithCustomJsonFactory", JsonFactory.class);
+    String messageId = (String) method.invoke(null, GsonFactory.getDefaultInstance());
+    assertEquals("projects/test-project/messages/msg-456", messageId);
+  }
+
+  /**
+   * Loaded inside {@link PackageBlockingClassLoader} to verify SDK initialization and FCM requests
+   * when optional JSON parser artifacts are absent from the classpath.
+   */
+  public static final class JacksonExcludedProbe {
+
+    private JacksonExcludedProbe() { }
+
+    /**
+     * Exercises default JsonFactory paths (FCM send and Remote Config Template) without Jackson2.
+     */
+    public static String sendWithDefaultJsonFactory() throws Exception {
+      Template template = Template.fromJSON("{\"etag\": \"etag-123\"}");
+      assertEquals("etag-123", template.getETag());
+      assertNotNull(template.toJSON());
+
+      MockLowLevelHttpResponse response = new MockLowLevelHttpResponse()
+          .setContent("{\"name\": \"projects/test-project/messages/msg-123\"}");
+      HttpTransport transport = new MockHttpTransport.Builder()
+          .setLowLevelHttpResponse(response)
+          .build();
+      FirebaseOptions options = FirebaseOptions.builder()
+          .setCredentials(new MockGoogleCredentials("test-token"))
+          .setProjectId("test-project")
+          .setHttpTransport(transport)
+          .build();
+      FirebaseApp app = FirebaseApp.initializeApp(options);
+      try {
+        return FirebaseMessaging.getInstance(app)
+            .send(Message.builder().setTopic("test-topic").build());
+      } finally {
+        TestOnlyImplFirebaseTrampolines.clearInstancesForTest();
+      }
+    }
+
+    /** Sends an FCM message using an explicit JsonFactory when default impls are absent. */
+    public static String sendWithCustomJsonFactory(JsonFactory jsonFactory) throws Exception {
+      MockLowLevelHttpResponse response = new MockLowLevelHttpResponse()
+          .setContent("{\"name\": \"projects/test-project/messages/msg-456\"}");
+      HttpTransport transport = new MockHttpTransport.Builder()
+          .setLowLevelHttpResponse(response)
+          .build();
+      FirebaseOptions options = FirebaseOptions.builder()
+          .setCredentials(new MockGoogleCredentials("test-token"))
+          .setProjectId("test-project")
+          .setHttpTransport(transport)
+          .setJsonFactory(jsonFactory)
+          .build();
+      FirebaseApp app = FirebaseApp.initializeApp(options);
+      try {
+        return FirebaseMessaging.getInstance(app)
+            .send(Message.builder().setTopic("test-topic").build());
+      } finally {
+        TestOnlyImplFirebaseTrampolines.clearInstancesForTest();
+      }
+    }
+  }
+
+  private static final class PackageBlockingClassLoader extends ClassLoader {
+
+    private final Set<String> blockedPrefixes;
+
+    PackageBlockingClassLoader(ClassLoader parent, Set<String> blockedPrefixes) {
+      super(parent);
+      this.blockedPrefixes = blockedPrefixes;
+    }
+
+    @Override
+    protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
+      synchronized (getClassLoadingLock(name)) {
+        Class<?> loaded = findLoadedClass(name);
+        if (loaded != null) {
+          return loaded;
+        }
+        for (String prefix : blockedPrefixes) {
+          if (name.startsWith(prefix)) {
+            throw new ClassNotFoundException(name);
+          }
+        }
+        if (name.startsWith("com.google.firebase.")) {
+          String resourcePath = name.replace('.', '/') + ".class";
+          try (InputStream in = getParent().getResourceAsStream(resourcePath)) {
+            if (in == null) {
+              throw new ClassNotFoundException(name);
+            }
+            byte[] bytes = ByteStreams.toByteArray(in);
+            Class<?> defined = defineClass(name, bytes, 0, bytes.length);
+            if (resolve) {
+              resolveClass(defined);
+            }
+            return defined;
+          } catch (IOException e) {
+            throw new ClassNotFoundException(name, e);
+          }
+        }
+        return super.loadClass(name, resolve);
+      }
+    }
+  }
+
 }