Add a Java API for UMA histograms.

base/android/record_histogram.cc

Index: base/android/record_histogram.cc
diff --git a/base/android/record_histogram.cc b/base/android/record_histogram.cc
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+++ b/base/android/record_histogram.cc
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+// Copyright 2014 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_android.h"
+#include "base/android/jni_string.h"
+#include "base/android/record_histogram.h"
+#include "base/metrics/histogram.h"
+#include "base/metrics/statistics_recorder.h"
+#include "jni/RecordHistogram_jni.h"
+
+namespace base {
+namespace android {
+
+void RecordBooleanHistogram(JNIEnv* env,
+                            jclass clazz,
+                            jstring j_histogram_name,
+                            jboolean j_sample) {
+  std::string histogram_name = ConvertJavaStringToUTF8(env, j_histogram_name);

[jdduke (slow), 2015/01/23 17:33:41]

Ouch, JNI + string conversion isn't exactly the cheapest operation. Do we
typically pass a Java string to native when recording UMA? Or is the string
already embedded in the native code? We already have to modify the manifest with
the string, would it be absurd to have a shared enum type that's used to convey
the metric?

[Alexei Svitkine (slow), 2015/01/23 17:52:07]

Having a shared enum adds more boiler plate that this change is trying to avoid.
i.e. you'd need to update two extra files in addition to calling the API.

Here's another idea:

Have a map keyed by the Java string pointer that maps to the Histogram object.
As a first try, look up the entry in this map. If found, done - and no string
conversion was done in the process. If not found, use this code path (i.e.
looking in the internal FactoryGet map).

This way, in the common case, where a string literal from the Java side is
passed (which is interned), it can just do a map look up based on the pointer
without any conversion.

You only need to do the conversion the first time that histogram is hit (where
it's a cache miss) and in cases where the histogram name is not a string literal
(i.e. if people concatenate histogram names - which shouldn't be a common use
case).

[jdduke (slow), 2015/01/23 18:19:31]

That sounds pretty reasonable, any objections?

+  bool sample = static_cast<bool>(j_sample);
+
+  BooleanHistogram::FactoryGet(histogram_name,
+                               HistogramBase::kUmaTargetedHistogramFlag)
+      ->AddBoolean(sample);
+}
+
+void RecordEnumeratedHistogram(JNIEnv* env,
+                               jclass clazz,
+                               jstring j_histogram_name,
+                               jint j_sample,
+                               jint j_boundary) {
+  std::string histogram_name = ConvertJavaStringToUTF8(env, j_histogram_name);
+  int sample = static_cast<int>(j_sample);
+  int boundary = static_cast<int>(j_boundary);
+
+  LinearHistogram::FactoryGet(histogram_name, 1, boundary, boundary + 1,
+                              HistogramBase::kUmaTargetedHistogramFlag)
+      ->Add(sample);
+}
+
+void Initialize(JNIEnv* env, jclass) {
+  StatisticsRecorder::Initialize();
+}
+
+// This backs a Java test util for testing histograms -
+// MetricsUtils.HistogramDelta. It should live in a test-specific file, but we
+// currently can't have test-specific native code packaged in test-specific Java
+// targets - see http://crbug.com/415945.
+jint GetHistogramValueCountForTesting(JNIEnv* env,
+                                      jclass clazz,
+                                      jstring histogram_name,
+                                      jint sample) {
+  HistogramBase* histogram = StatisticsRecorder::FindHistogram(
+      android::ConvertJavaStringToUTF8(env, histogram_name));
+  if (histogram == nullptr) {
+    // No samples have been recorded for this histogram (yet?).
+    return 0;
+  }
+
+  scoped_ptr<HistogramSamples> samples = histogram->SnapshotSamples();
+  return samples->GetCount(static_cast<int>(sample));
+}
+
+bool RegisterRecordHistogram(JNIEnv* env) {
+  return RegisterNativesImpl(env);
+}
+
+}  // namespace android
+}  // namespace base