Add MemoryMonitorAndroid

content/browser/memory/memory_monitor_android.cc

+// Copyright (c) 2016 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 "content/browser/memory/memory_monitor_android.h"
+
+#include "base/android/context_utils.h"
+#include "base/android/jni_android.h"
+#include "base/memory/ptr_util.h"
+#include "jni/MemoryMonitorAndroid_jni.h"
+
+namespace content {
+
+namespace {
+const size_t kMBShift = 20;
+}
+
+// static
+std::unique_ptr<MemoryMonitorAndroid> MemoryMonitorAndroid::Create() {
+  return base::WrapUnique(new MemoryMonitorAndroid);
+}
+
+MemoryMonitorAndroid::MemoryMonitorAndroid() {
+  InitializeFieldIDs();
+}
+
+MemoryMonitorAndroid::~MemoryMonitorAndroid() {}
+
+int MemoryMonitorAndroid::GetFreeMemoryUntilCriticalMB() {
+  DCHECK(CanGetMemoryInfo());
+  MemoryInfo info;
+  GetMemoryInfo(&info);
+  return (info.avail_mem - info.threshold) >> kMBShift;
+}
+
+bool MemoryMonitorAndroid::CanGetMemoryInfo() {
+  return (avail_mem_id_ != 0 && low_memory_id_ != 0 && threshold_id_ != 0 &&
+          total_mem_id_ != 0);
+}
+
+void MemoryMonitorAndroid::GetMemoryInfo(MemoryInfo* out) {
+  DCHECK(out);
+  DCHECK(CanGetMemoryInfo());
+  JNIEnv* env = base::android::AttachCurrentThread();
+  base::android::ScopedJavaLocalRef<jobject> info =
+      Java_MemoryMonitorAndroid_getMemoryInfo(
+          env, base::android::GetApplicationContext());
+  out->avail_mem = env->GetLongField(info.obj(), avail_mem_id_);
+  out->low_memory = env->GetBooleanField(info.obj(), low_memory_id_);
+  out->threshold = env->GetLongField(info.obj(), threshold_id_);
+  out->total_mem = env->GetLongField(info.obj(), total_mem_id_);
+}
+
+void MemoryMonitorAndroid::InitializeFieldIDs() {
+  JNIEnv* env = base::android::AttachCurrentThread();
+  base::android::ScopedJavaLocalRef<jclass> clazz =
+      base::android::GetClass(env, "android/app/ActivityManager$MemoryInfo");
+  avail_mem_id_ = env->GetFieldID(clazz.obj(), "availMem", "J");

[boliu, 2016/09/27 00:19:11]

we go to great lengths to not hand write jni code. I'm not entirely sure if jni
generation works for sdk classes, and if there's an existing example already

how about returning long[], and convert with jni_array.h?

or if this is called frequently and causing gc is not idea, could just have java
callback into native with the 4 parameters

I know neither is ideal, but still better than turning sdk mis-use into runtime
errors imo

[agrieve, 2016/09/27 00:35:05]

You can generate wrappers for sdk classes, but I don't think the generator has a
good answer for field access. This seems fine to me, as it even has tests.

[boliu, 2016/09/27 03:00:23]

Yeah, ok. There really is nothing great here :/

Could you save these fields, so they are not looked up every call?

[bashi, 2016/09/27 03:51:36]

Thank you for suggestions. PS#4 uses native callback instead of GetFieldID. I
slightly prefer this approach but I'm happy to go back to use GetFieldID. What
do you think?
The previous CL stores these fields at constructor so this lookups happens only
once.

+  low_memory_id_ = env->GetFieldID(clazz.obj(), "lowMemory", "Z");
+  threshold_id_ = env->GetFieldID(clazz.obj(), "threshold", "J");
+  total_mem_id_ = env->GetFieldID(clazz.obj(), "totalMem", "J");
+}
+
+// Implementation of a factory function defined in memory_monitor.h.
+std::unique_ptr<MemoryMonitor> CreateMemoryMonitor() {
+  return MemoryMonitorAndroid::Create();
+}
+
+}  // namespace content