perf(android): Parse app start profiling config without JsonSerializer

CHANGELOG.md

@@ -22,6 +22,7 @@
 
 ### Performance
 
+- Parse the app start profiling config with only the deserializer it needs instead of building a full `JsonSerializer` and `SentryOptions`, cutting 188 of 221 allocations on the main thread before `Application.onCreate` ([#5867](https://github.com/getsentry/sentry-java/pull/5867))
 - Create the outbox and cache directories lazily in their consumers instead of during SDK init, moving the `mkdirs()` calls off the init (main) thread ([#5792](https://github.com/getsentry/sentry-java/pull/5792))
 - Reduce the number of SDK threads: `LifecycleWatcher` now schedules the session-end task on the shared timer executor instead of creating a dedicated `java.util.Timer` thread ([#5819](https://github.com/getsentry/sentry-java/pull/5819))
 - Reduce the number of SDK threads: `RateLimiter` now schedules its rate-limit-lifted notifications on the shared timer executor instead of creating a dedicated `java.util.Timer` thread ([#5814](https://github.com/getsentry/sentry-java/pull/5814))

sentry-android-core/src/main/java/io/sentry/android/core/SentryPerformanceProvider.java

@@ -13,7 +13,7 @@
 import io.sentry.ILogger;
 import io.sentry.ISentryLifecycleToken;
 import io.sentry.ITransactionProfiler;
-import io.sentry.JsonSerializer;
+import io.sentry.JsonObjectReader;
 import io.sentry.SentryAppStartProfilingOptions;
 import io.sentry.SentryExecutorService;
 import io.sentry.SentryLevel;
@@ -119,8 +119,7 @@ private void launchAppStartProfiler(final @NotNull AppStartMetrics appStartMetri
     try (final @NotNull Reader reader =
             new BufferedReader(new InputStreamReader(new FileInputStream(configFile)))) {
       final @Nullable SentryAppStartProfilingOptions profilingOptions =
-          new JsonSerializer(SentryOptions.empty())
-              .deserialize(reader, SentryAppStartProfilingOptions.class);
+          deserializeProfilingConfig(reader);
 
       if (profilingOptions == null) {
         logger.log(
@@ -151,6 +150,29 @@ private void launchAppStartProfiler(final @NotNull AppStartMetrics appStartMetri
     }
   }
 
+  /**
+   * Parses the app start profiling config with only the deserializer it needs. Going through {@link
+   * io.sentry.JsonSerializer} would allocate a full {@link SentryOptions} plus all 56 registered
+   * deserializers on the main thread before {@code Application.onCreate}, to use exactly one of
+   * them.
+   *
+   * <p>Returns null on malformed input, matching what {@code JsonSerializer.deserialize} did, so
+   * callers keep reporting it as a deserialization failure rather than a read error. The vendored
+   * JSON reader signals bad input with {@link java.io.IOException} (including {@code
+   * MalformedJsonException} and {@code EOFException}), {@link IllegalStateException} on a token
+   * type mismatch, and {@link NumberFormatException} on an unparseable number — all {@link
+   * Exception} subclasses, so {@code Error} propagates instead of being swallowed here.
+   */
+  private @Nullable SentryAppStartProfilingOptions deserializeProfilingConfig(
+      final @NotNull Reader reader) {
+    try (final @NotNull JsonObjectReader jsonReader = new JsonObjectReader(reader)) {
+      return new SentryAppStartProfilingOptions.Deserializer().deserialize(jsonReader, logger);
+    } catch (Exception e) {
+      logger.log(SentryLevel.ERROR, "Error when deserializing", e);
+      return null;
+    }
+  }
+
   private void createAndStartContinuousProfiler(
       final @NotNull Context context,
       final @NotNull SentryAppStartProfilingOptions profilingOptions,

sentry-android-core/src/test/java/io/sentry/android/core/SentryPerformanceProviderTest.kt

@@ -153,6 +153,21 @@ class SentryPerformanceProviderTest {
       )
   }
 
+  @Test
+  fun `when config file is malformed, profiler is not started`() {
+    fixture.getSut { config -> config.writeText("{\"profile_sampled\": tru") }
+    assertNull(AppStartMetrics.getInstance().appStartProfiler)
+    assertNull(AppStartMetrics.getInstance().appStartContinuousProfiler)
+    verify(fixture.logger).log(eq(SentryLevel.ERROR), eq("Error when deserializing"), any())
+    verify(fixture.logger)
+      .log(
+        eq(SentryLevel.WARNING),
+        eq(
+          "Unable to deserialize the SentryAppStartProfilingOptions. App start profiling will not start."
+        ),
+      )
+  }
+
   @Test
   fun `when profiling is disabled, profiler is not started`() {
     fixture.getSut { config ->