1 // Copyright 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.android_webview;
6 
7 import android.annotation.SuppressLint;
8 import android.os.Handler;
9 import android.os.Looper;
10 import android.os.Message;
11 
12 import org.chromium.base.Log;
13 import org.chromium.base.ThreadUtils;
14 import org.chromium.base.TraceEvent;
15 
16 import java.lang.ref.ReferenceQueue;
17 import java.lang.ref.WeakReference;
18 import java.util.HashSet;
19 import java.util.Set;
20 
21 /**
22  * Handles running cleanup tasks when an object becomes eligible for GC. Cleanup tasks
23  * are always executed on the main thread. In general, classes should not have
24  * finalizers and likewise should not use this class for the same reasons. The
25  * exception is where public APIs exist that require native side resources to be
26  * cleaned up in response to java side GC of API objects. (Private/internal
27  * interfaces should always favor explicit resource releases / destroy()
28  * protocol for this rather than depend on GC to trigger native cleanup).
29  * NOTE this uses WeakReference rather than PhantomReference, to avoid delaying the
30  * cleanup processing until after finalizers (if any) have run. In general usage of
31  * this class indicates the client does NOT use finalizers anyway (Good), so this should
32  * not be a visible difference in practice.
33  */
34 public class CleanupReference extends WeakReference<Object> {
35     private static final String TAG = "CleanupReference";
36 
37     private static final boolean DEBUG = false;  // Always check in as false!
38 
39     // The VM will enqueue CleanupReference instance onto sGcQueue when it becomes eligible for
40     // garbage collection (i.e. when all references to the underlying object are nullified).
41     // |sReaperThread| processes this queue by forwarding the references on to the UI thread
42     // (via REMOVE_REF message) to perform cleanup.
43     private static ReferenceQueue<Object> sGcQueue = new ReferenceQueue<Object>();
44     private static Object sCleanupMonitor = new Object();
45 
46     private static final Thread sReaperThread = new Thread(TAG) {
47         @Override
48         @SuppressWarnings("WaitNotInLoop")
49         public void run() {
50             while (true) {
51                 try {
52                     CleanupReference ref = (CleanupReference) sGcQueue.remove();
53                     if (DEBUG) Log.d(TAG, "removed one ref from GC queue");
54                     synchronized (sCleanupMonitor) {
55                         Message.obtain(LazyHolder.sHandler, REMOVE_REF, ref).sendToTarget();
56                         // Give the UI thread chance to run cleanup before looping around and
57                         // taking the next item from the queue, to avoid Message bombing it.
58                         sCleanupMonitor.wait(500);
59                     }
60                 } catch (Exception e) {
61                     Log.e(TAG, "Queue remove exception:", e);
62                 }
63             }
64         }
65     };
66 
67     static {
68         sReaperThread.setDaemon(true);
sReaperThread.start()69         sReaperThread.start();
70     }
71 
72     // Message's sent in the |what| field to |sHandler|.
73 
74     // Add a new reference to sRefs. |msg.obj| is the CleanupReference to add.
75     private static final int ADD_REF = 1;
76     // Remove reference from sRefs. |msg.obj| is the CleanupReference to remove.
77     private static final int REMOVE_REF = 2;
78 
79     /**
80      * This {@link Handler} polls {@link #sRefs}, looking for cleanup tasks that
81      * are ready to run.
82      * This is lazily initialized as ThreadUtils.getUiThreadLooper() may not be
83      * set yet early in startup.
84      */
85     @SuppressLint("HandlerLeak")
86     private static class LazyHolder {
87         static final Handler sHandler = new Handler(ThreadUtils.getUiThreadLooper()) {
88             @Override
89             public void handleMessage(Message msg) {
90                 try {
91                     TraceEvent.begin("CleanupReference.LazyHolder.handleMessage");
92                     CleanupReference ref = (CleanupReference) msg.obj;
93                     switch (msg.what) {
94                         case ADD_REF:
95                             sRefs.add(ref);
96                             break;
97                         case REMOVE_REF:
98                             ref.runCleanupTaskInternal();
99                             break;
100                         default:
101                             Log.e(TAG, "Bad message=%d", msg.what);
102                             break;
103                     }
104 
105                     if (DEBUG) Log.d(TAG, "will try and cleanup; max = %d", sRefs.size());
106 
107                     synchronized (sCleanupMonitor) {
108                         // Always run the cleanup loop here even when adding or removing refs, to
109                         // avoid falling behind on rapid garbage allocation inner loops.
110                         while ((ref = (CleanupReference) sGcQueue.poll()) != null) {
111                             ref.runCleanupTaskInternal();
112                         }
113                         sCleanupMonitor.notifyAll();
114                     }
115                 } finally {
116                     TraceEvent.end("CleanupReference.LazyHolder.handleMessage");
117                 }
118             }
119         };
120     }
121 
122     /**
123      * Keep a strong reference to {@link CleanupReference} so that it will
124      * actually get enqueued.
125      * Only accessed on the UI thread.
126      */
127     private static Set<CleanupReference> sRefs = new HashSet<CleanupReference>();
128 
129     private Runnable mCleanupTask;
130 
131     /**
132      * @param obj the object whose loss of reachability should trigger the
133      *            cleanup task.
134      * @param cleanupTask the task to run once obj loses reachability.
135      */
CleanupReference(Object obj, Runnable cleanupTask)136     public CleanupReference(Object obj, Runnable cleanupTask) {
137         super(obj, sGcQueue);
138         if (DEBUG) Log.d(TAG, "+++ CREATED ONE REF");
139         mCleanupTask = cleanupTask;
140         handleOnUiThread(ADD_REF);
141     }
142 
143     /**
144      * Clear the cleanup task {@link Runnable} so that nothing will be done
145      * after garbage collection.
146      */
cleanupNow()147     public void cleanupNow() {
148         handleOnUiThread(REMOVE_REF);
149     }
150 
hasCleanedUp()151     public boolean hasCleanedUp() {
152         return mCleanupTask == null;
153     }
154 
handleOnUiThread(int what)155     private void handleOnUiThread(int what) {
156         Message msg = Message.obtain(LazyHolder.sHandler, what, this);
157         if (Looper.myLooper() == msg.getTarget().getLooper()) {
158             msg.getTarget().handleMessage(msg);
159             msg.recycle();
160         } else {
161             msg.sendToTarget();
162         }
163     }
164 
runCleanupTaskInternal()165     private void runCleanupTaskInternal() {
166         if (DEBUG) Log.d(TAG, "runCleanupTaskInternal");
167         sRefs.remove(this);
168         Runnable cleanupTask = mCleanupTask;
169         mCleanupTask = null;
170         if (cleanupTask != null) {
171             if (DEBUG) Log.i(TAG, "--- CLEANING ONE REF");
172             cleanupTask.run();
173         }
174         clear();
175     }
176 }
177