Android: lazy initialization for method id.

base/android/jni_method_id_unittest.cc

+// Copyright (c) 2012 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "base/android/jni_method_id.h"
+
+#include "base/android/jni_android.h"
+#include "base/logging.h"
+#include "base/time.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace base {
+namespace android {
+
+namespace {
+
+base::subtle::AtomicWord g_atomic_id = 0;
+int LazyMethodIDCall(JNIEnv* env, jclass clazz, int p) {
+  jmethodID id = base::android::MethodID::LazyGet<
+      base::android::MethodID::METHODTYPE_STATIC,
+      base::android::MethodID::EXCEPTIONCHECK_NO>(
+      env, clazz,
+      "abs",
+      "(I)I",
+      &g_atomic_id);
+
+  return env->CallStaticIntMethod(clazz, id, p);
+}
+
+int MethodIDCall(JNIEnv* env, jclass clazz, jmethodID id, int p) {
+  return env->CallStaticIntMethod(clazz, id, p);
+}
+
+}  // namespace
+
+TEST(JNIAndroidMicrobenchmark, MethodId) {
+  JNIEnv* env = AttachCurrentThread();
+  ScopedJavaLocalRef<jclass> clazz(GetClass(env, "java/lang/Math"));
+  base::Time start_lazy = base::Time::Now();
+  int o = 0;
+  for (int i = 0; i < 1024; ++i)
+    o += LazyMethodIDCall(env, clazz.obj(), i);

[joth, 2012/10/03 17:31:52]

depending on your goals, this test is skewed against LazyMethodIDCall as that
has to do the initial JNIEnv::GetMethodID() call, where as non-lazy version
below gets this called "for free" on startup.

Put another-way -- this cost on Lazy version is amortized as the size of the
loop is increased (e.g. 1024 -> 1M), whereas the one below should be "constant"
regardless of the iterations.

+  base::Time end_lazy = base::Time::Now();
+
+  jmethodID id = reinterpret_cast<jmethodID>(g_atomic_id);
+  base::Time start = base::Time::Now();
+  for (int i = 0; i < 1024; ++i)
+    o += MethodIDCall(env, clazz.obj(), id, i);
+  base::Time end = base::Time::Now();
+
+  // On a Galaxy Nexus, results were in the range of:
+  // JNI LazyMethodIDCall (us) 1984
+  // JNI MethodIDCall (us) 1861
+  LOG(ERROR) << "JNI LazyMethodIDCall (us) " <<
+      base::TimeDelta(end_lazy - start_lazy).InMicroseconds();
+  LOG(ERROR) << "JNI MethodIDCall (us) " <<
+      base::TimeDelta(end - start).InMicroseconds();
+  LOG(ERROR) << "JNI " << o;
+}
+
+}  // namespace android
+}  // namespace base