| 1 | // Copyright (c) 2012 The Chromium Authors. All rights reserved. |
| 2 | // Use of this source code is governed by a BSD-style license that can be |
| 3 | // found in the LICENSE file. |
| 4 | |
| 5 | package org.chromium.content.common; |
| 6 | |
| 7 | import android.os.Handler; |
| 8 | import android.os.Looper; |
| 9 | import android.os.Message; |
| 10 | import android.util.Log; |
| 11 | |
| 12 | import java.lang.ref.ReferenceQueue; |
| 13 | import java.lang.ref.WeakReference; |
| 14 | import java.util.HashSet; |
| 15 | import java.util.Set; |
| 16 | |
| 17 | /** |
| 18 | * Handles running cleanup tasks when an object becomes eligible for GC. Cleanup tasks |
| 19 | * are always executed on the main thread. In general, classes should not have |
| 20 | * finalizers and likewise should not use this class for the same reasons. The |
| 21 | * exception is where public APIs exist that require native side resources to be |
| 22 | * cleaned up in response to java side GC of API objects. (Private/internal |
| 23 | * interfaces should always favor explicit resource releases / destroy() |
| 24 | * protocol for this rather than depend on GC to trigger native cleanup). |
| 25 | * NOTE this uses WeakReference rather than PhantomReference, to avoid delaying the |
| 26 | * cleanup processing until after finalizers (if any) have run. In general usage of |
| 27 | * this class indicates the client does NOT use finalizers anyway (Good), so this should |
| 28 | * not be a visible difference in practice. |
| 29 | */ |
| 30 | public class CleanupReference extends WeakReference<Object> { |
| 31 | private static final String TAG = "CleanupReference"; |
| 32 | |
| 33 | private static final boolean DEBUG = false; // Always check in as false! |
| 34 | |
| 35 | // The VM will enqueue CleanupReference instance onto sGcQueue when it becomes eligible for |
| 36 | // garbage collection (i.e. when all references to the underlying object are nullified). |
| 37 | // |sReaperThread| processes this queue by forwarding the references on to the UI thread |
| 38 | // (via REMOVE_REF message) to perform cleanup. |
| 39 | private static ReferenceQueue<Object> sGcQueue = new ReferenceQueue<Object>(); |
| 40 | private static Object sCleanupMonitor = new Object(); |
| 41 | |
| 42 | static private final Thread sReaperThread = new Thread(TAG) { |
| 43 | public void run() { |
| 44 | while (true) { |
| 45 | try { |
| 46 | CleanupReference ref = (CleanupReference) sGcQueue.remove(); |
| 47 | if (DEBUG) Log.d(TAG, "removed one ref from GC queue"); |
| 48 | synchronized (sCleanupMonitor) { |
| 49 | Message.obtain(sHandler, REMOVE_REF, ref).sendToTarget(); |
| 50 | // Give the UI thread chance to run cleanup before looping around and |
| 51 | // taking the next item from the queue, to avoid Message bombing it. |
| 52 | sCleanupMonitor.wait(500); |
| 53 | } |
| 54 | } catch (Exception e) { |
| 55 | Log.e(TAG, "Queue remove exception:", e); |
| 56 | } |
| 57 | } |
| 58 | } |
| 59 | }; |
| 60 | |
| 61 | static { |
| 62 | sReaperThread.setDaemon(true); |
| 63 | sReaperThread.start(); |
| 64 | } |
| 65 | |
| 66 | // Message's sent in the |what| field to |sHandler|. |
| 67 | |
| 68 | // Add a new reference to sRefs. |msg.obj| is the CleanupReference to add. |
| 69 | private static final int ADD_REF = 1; |
| 70 | // Remove reference from sRefs. |msg.obj| is the CleanupReference to remove. |
| 71 | private static final int REMOVE_REF = 2; |
| 72 | |
| 73 | /** |
| 74 | * This {@link Handler} polls {@link #sRefs}, looking for cleanup tasks that |
| 75 | * are ready to run. |
| 76 | */ |
| 77 | private static Handler sHandler = new Handler(Looper.getMainLooper()) { |
| 78 | @Override |
| 79 | public void handleMessage(Message msg) { |
| 80 | TraceEvent.begin(); |
| 81 | CleanupReference ref = (CleanupReference) msg.obj; |
| 82 | switch (msg.what) { |
| 83 | case ADD_REF: |
| 84 | sRefs.add(ref); |
| 85 | break; |
| 86 | case REMOVE_REF: |
| 87 | ref.runCleanupTaskInternal(); |
| 88 | break; |
| 89 | default: |
| 90 | Log.e(TAG, "Bad message=" + msg.what); |
| 91 | break; |
| 92 | } |
| 93 | |
| 94 | if (DEBUG) Log.d(TAG, "will try and cleanup; max = " + sRefs.size()); |
| 95 | |
| 96 | synchronized (sCleanupMonitor) { |
| 97 | // Always run the cleanup loop here even when adding or removing refs, to avoid |
| 98 | // falling behind on rapid garbage allocation inner loops. |
| 99 | while ((ref = (CleanupReference) sGcQueue.poll()) != null) { |
| 100 | ref.runCleanupTaskInternal(); |
| 101 | } |
| 102 | sCleanupMonitor.notifyAll(); |
| 103 | } |
| 104 | TraceEvent.end(); |
| 105 | } |
| 106 | }; |
| 107 | |
| 108 | /** |
| 109 | * Keep a strong reference to {@link CleanupReference} so that it will |
| 110 | * actually get enqueued. |
| 111 | * Only accessed on the UI thread. |
| 112 | */ |
| 113 | private static Set<CleanupReference> sRefs = new HashSet<CleanupReference>(); |
| 114 | |
| 115 | private Runnable mCleanupTask; |
| 116 | |
| 117 | /** |
| 118 | * @param obj the object whose loss of reachability should trigger the |
| 119 | * cleanup task. |
| 120 | * @param cleanupTask the task to run once obj loses reachability. |
| 121 | */ |
| 122 | public CleanupReference(Object obj, Runnable cleanupTask) { |
| 123 | super(obj, sGcQueue); |
| 124 | if (DEBUG) Log.d(TAG, "+++ CREATED ONE REF"); |
| 125 | mCleanupTask = cleanupTask; |
| 126 | handleOnUiThread(ADD_REF); |
| 127 | } |
| 128 | |
| 129 | /** |
| 130 | * Clear the cleanup task {@link Runnable} so that nothing will be done |
| 131 | * after garbage collection. |
| 132 | */ |
| 133 | public void cleanupNow() { |
| 134 | handleOnUiThread(REMOVE_REF); |
| 135 | } |
| 136 | |
| 137 | private void handleOnUiThread(int what) { |
| 138 | Message msg = Message.obtain(sHandler, what, this); |
| 139 | if (Looper.myLooper() == sHandler.getLooper()) { |
| 140 | sHandler.handleMessage(msg); |
| 141 | msg.recycle(); |
| 142 | } else { |
| 143 | msg.sendToTarget(); |
| 144 | } |
| 145 | } |
| 146 | |
| 147 | private void runCleanupTaskInternal() { |
| 148 | if (DEBUG) Log.d(TAG, "runCleanupTaskInternal"); |
| 149 | sRefs.remove(this); |
| 150 | if (mCleanupTask != null) { |
| 151 | if (DEBUG) Log.i(TAG, "--- CLEANING ONE REF"); |
| 152 | mCleanupTask.run(); |
| 153 | mCleanupTask = null; |
| 154 | } |
| 155 | clear(); |
| 156 | } |
| 157 | } |