| /* |
| * Copyright (C) 2021 The Android Open Source Project |
| * |
| * Licensed under the Apache License, Version 2.0 (the "License"); |
| * you may not use this file except in compliance with the License. |
| * You may obtain a copy of the License at |
| * |
| * http://www.apache.org/licenses/LICENSE-2.0 |
| * |
| * Unless required by applicable law or agreed to in writing, software |
| * distributed under the License is distributed on an "AS IS" BASIS, |
| * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| * See the License for the specific language governing permissions and |
| * limitations under the License. |
| */ |
| |
| package com.android.server.utils; |
| |
| import android.annotation.NonNull; |
| import android.annotation.Nullable; |
| |
| import java.util.WeakHashMap; |
| import java.util.concurrent.atomic.AtomicInteger; |
| |
| /** |
| * A class that caches snapshots. Instances are instantiated on a {@link Watchable}; when the |
| * {@link Watchable} reports a change, the cache is cleared. The snapshot() method fetches the |
| * cache if it is valid, or rebuilds the cache if it has been cleared. |
| * |
| * The class is abstract; clients must implement the createSnapshot() method. |
| * |
| * @param <T> The type returned by the snapshot() method. |
| */ |
| public abstract class SnapshotCache<T> extends Watcher{ |
| |
| /** |
| * Global snapshot cache enable flag. Set to false for testing or debugging. |
| */ |
| private static final boolean ENABLED = true; |
| |
| /** |
| * The statistics for a single cache. The object records the number of times a |
| * snapshot was reused and the number of times a snapshot was rebuilt. |
| */ |
| private static class Statistics { |
| final String mName; |
| private final AtomicInteger mReused = new AtomicInteger(0); |
| private final AtomicInteger mRebuilt = new AtomicInteger(0); |
| Statistics(@NonNull String n) { |
| mName = n; |
| } |
| } |
| |
| // The source object from which snapshots are created. This may be null if createSnapshot() |
| // does not require it. |
| protected final T mSource; |
| |
| // The cached snapshot |
| private volatile T mSnapshot = null; |
| |
| // True if the snapshot is sealed and may not be modified. |
| private volatile boolean mSealed = false; |
| |
| // The statistics for this cache. This may be null. |
| private final Statistics mStatistics; |
| |
| /** |
| * The global list of caches. |
| */ |
| private static final WeakHashMap<SnapshotCache, Void> sCaches = new WeakHashMap<>(); |
| |
| /** |
| * Create a cache with a source object for rebuilding snapshots and a |
| * {@link Watchable} that notifies when the cache is invalid. If the name is null |
| * then statistics are not collected for this cache. |
| * @param source Source data for rebuilding snapshots. |
| * @param watchable The object that notifies when the cache is invalid. |
| * @param name The name of the cache, for statistics reporting. |
| */ |
| public SnapshotCache(@Nullable T source, @NonNull Watchable watchable, @Nullable String name) { |
| mSource = source; |
| watchable.registerObserver(this); |
| if (name != null) { |
| mStatistics = new Statistics(name); |
| sCaches.put(this, null); |
| } else { |
| mStatistics = null; |
| } |
| } |
| |
| /** |
| * Create a cache with a source object for rebuilding snapshots and a |
| * {@link Watchable} that notifies when the cache is invalid. The name is null in |
| * this API. |
| * @param source Source data for rebuilding snapshots. |
| * @param watchable The object that notifies when the cache is invalid. |
| */ |
| public SnapshotCache(@Nullable T source, @NonNull Watchable watchable) { |
| this(source, watchable, null); |
| } |
| |
| /** |
| * A private constructor that sets fields to null and mSealed to true. This supports |
| * the Sealed subclass. |
| */ |
| public SnapshotCache() { |
| mSource = null; |
| mSealed = true; |
| mStatistics = null; |
| } |
| |
| /** |
| * Notify the object that the source object has changed. If the local object is sealed then |
| * IllegalStateException is thrown. Otherwise, the cache is cleared. |
| */ |
| public final void onChange(@Nullable Watchable what) { |
| if (mSealed) { |
| throw new IllegalStateException("attempt to change a sealed object"); |
| } |
| mSnapshot = null; |
| } |
| |
| /** |
| * Seal the cache. Attempts to modify the cache will generate an exception. |
| */ |
| public final void seal() { |
| mSealed = true; |
| } |
| |
| /** |
| * Return a snapshot. This uses the cache if it is non-null. Otherwise it creates a |
| * new snapshot and saves it in the cache. |
| * @return A snapshot as returned by createSnapshot() and possibly cached. |
| */ |
| public final T snapshot() { |
| T s = mSnapshot; |
| if (s == null || !ENABLED) { |
| s = createSnapshot(); |
| mSnapshot = s; |
| if (mStatistics != null) mStatistics.mRebuilt.incrementAndGet(); |
| } else { |
| if (mStatistics != null) mStatistics.mReused.incrementAndGet(); |
| } |
| return s; |
| } |
| |
| /** |
| * Create a single, uncached snapshot. Clients must implement this per local rules. |
| * @return A snapshot |
| */ |
| public abstract T createSnapshot(); |
| |
| /** |
| * A snapshot cache suitable for sealed snapshots. Attempting to retrieve the |
| * snapshot will throw an UnsupportedOperationException. |
| * @param <T> the type of object being cached. This is needed for compilation only. It |
| * has no effect on execution. |
| */ |
| public static class Sealed<T> extends SnapshotCache<T> { |
| /** |
| * Create a sealed SnapshotCache that cannot be used to create new snapshots. |
| */ |
| public Sealed() { |
| } |
| /** |
| * Provide a concrete implementation of createSnapshot() that throws |
| * UnsupportedOperationException. |
| */ |
| public T createSnapshot() { |
| throw new UnsupportedOperationException("cannot snapshot a sealed snaphot"); |
| } |
| } |
| |
| /** |
| * A snapshot cache suitable for Snappable types. The key is that Snappable types |
| * have a known implementation of createSnapshot() so that this class is concrete. |
| * @param <T> The class whose snapshot is being cached. |
| */ |
| public static class Auto<T extends Snappable<T>> extends SnapshotCache<T> { |
| public Auto(@NonNull T source, @NonNull Watchable watchable, @Nullable String name) { |
| super(source, watchable, name); |
| } |
| public Auto(@NonNull T source, @NonNull Watchable watchable) { |
| this(source, watchable, null); |
| } |
| /** |
| * Concrete createSnapshot() using the snapshot() method of <T>. |
| */ |
| public T createSnapshot() { |
| return mSource.snapshot(); |
| } |
| } |
| |
| } |