CleanupReference.java revision 1e9bf3e0803691d0a228da41fc608347b6db4340
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 5package org.chromium.content.common; 6 7import android.os.Handler; 8import android.os.Looper; 9import android.os.Message; 10import android.util.Log; 11 12import org.chromium.base.ThreadUtils; 13 14import java.lang.ref.ReferenceQueue; 15import java.lang.ref.WeakReference; 16import java.util.HashSet; 17import java.util.Set; 18 19/** 20 * Handles running cleanup tasks when an object becomes eligible for GC. Cleanup tasks 21 * are always executed on the main thread. In general, classes should not have 22 * finalizers and likewise should not use this class for the same reasons. The 23 * exception is where public APIs exist that require native side resources to be 24 * cleaned up in response to java side GC of API objects. (Private/internal 25 * interfaces should always favor explicit resource releases / destroy() 26 * protocol for this rather than depend on GC to trigger native cleanup). 27 * NOTE this uses WeakReference rather than PhantomReference, to avoid delaying the 28 * cleanup processing until after finalizers (if any) have run. In general usage of 29 * this class indicates the client does NOT use finalizers anyway (Good), so this should 30 * not be a visible difference in practice. 31 */ 32public class CleanupReference extends WeakReference<Object> { 33 private static final String TAG = "CleanupReference"; 34 35 private static final boolean DEBUG = false; // Always check in as false! 36 37 // The VM will enqueue CleanupReference instance onto sGcQueue when it becomes eligible for 38 // garbage collection (i.e. when all references to the underlying object are nullified). 39 // |sReaperThread| processes this queue by forwarding the references on to the UI thread 40 // (via REMOVE_REF message) to perform cleanup. 41 private static ReferenceQueue<Object> sGcQueue = new ReferenceQueue<Object>(); 42 private static Object sCleanupMonitor = new Object(); 43 44 static private final Thread sReaperThread = new Thread(TAG) { 45 public void run() { 46 while (true) { 47 try { 48 CleanupReference ref = (CleanupReference) sGcQueue.remove(); 49 if (DEBUG) Log.d(TAG, "removed one ref from GC queue"); 50 synchronized (sCleanupMonitor) { 51 Message.obtain(LazyHolder.sHandler, REMOVE_REF, ref).sendToTarget(); 52 // Give the UI thread chance to run cleanup before looping around and 53 // taking the next item from the queue, to avoid Message bombing it. 54 sCleanupMonitor.wait(500); 55 } 56 } catch (Exception e) { 57 Log.e(TAG, "Queue remove exception:", e); 58 } 59 } 60 } 61 }; 62 63 static { 64 sReaperThread.setDaemon(true); 65 sReaperThread.start(); 66 } 67 68 // Message's sent in the |what| field to |sHandler|. 69 70 // Add a new reference to sRefs. |msg.obj| is the CleanupReference to add. 71 private static final int ADD_REF = 1; 72 // Remove reference from sRefs. |msg.obj| is the CleanupReference to remove. 73 private static final int REMOVE_REF = 2; 74 75 /** 76 * This {@link Handler} polls {@link #sRefs}, looking for cleanup tasks that 77 * are ready to run. 78 * This is lazily initialized as ThreadUtils.getUiThreadLooper() may not be 79 * set yet early in startup. 80 */ 81 private static class LazyHolder { 82 static final Handler sHandler = new Handler(ThreadUtils.getUiThreadLooper()) { 83 @Override 84 public void handleMessage(Message msg) { 85 TraceEvent.begin(); 86 CleanupReference ref = (CleanupReference) msg.obj; 87 switch (msg.what) { 88 case ADD_REF: 89 sRefs.add(ref); 90 break; 91 case REMOVE_REF: 92 ref.runCleanupTaskInternal(); 93 break; 94 default: 95 Log.e(TAG, "Bad message=" + msg.what); 96 break; 97 } 98 99 if (DEBUG) Log.d(TAG, "will try and cleanup; max = " + sRefs.size()); 100 101 synchronized (sCleanupMonitor) { 102 // Always run the cleanup loop here even when adding or removing refs, to avoid 103 // falling behind on rapid garbage allocation inner loops. 104 while ((ref = (CleanupReference) sGcQueue.poll()) != null) { 105 ref.runCleanupTaskInternal(); 106 } 107 sCleanupMonitor.notifyAll(); 108 } 109 TraceEvent.end(); 110 } 111 }; 112 } 113 114 /** 115 * Keep a strong reference to {@link CleanupReference} so that it will 116 * actually get enqueued. 117 * Only accessed on the UI thread. 118 */ 119 private static Set<CleanupReference> sRefs = new HashSet<CleanupReference>(); 120 121 private Runnable mCleanupTask; 122 123 /** 124 * @param obj the object whose loss of reachability should trigger the 125 * cleanup task. 126 * @param cleanupTask the task to run once obj loses reachability. 127 */ 128 public CleanupReference(Object obj, Runnable cleanupTask) { 129 super(obj, sGcQueue); 130 if (DEBUG) Log.d(TAG, "+++ CREATED ONE REF"); 131 mCleanupTask = cleanupTask; 132 handleOnUiThread(ADD_REF); 133 } 134 135 /** 136 * Clear the cleanup task {@link Runnable} so that nothing will be done 137 * after garbage collection. 138 */ 139 public void cleanupNow() { 140 handleOnUiThread(REMOVE_REF); 141 } 142 143 private void handleOnUiThread(int what) { 144 Message msg = Message.obtain(LazyHolder.sHandler, what, this); 145 if (Looper.myLooper() == msg.getTarget().getLooper()) { 146 msg.getTarget().handleMessage(msg); 147 msg.recycle(); 148 } else { 149 msg.sendToTarget(); 150 } 151 } 152 153 private void runCleanupTaskInternal() { 154 if (DEBUG) Log.d(TAG, "runCleanupTaskInternal"); 155 sRefs.remove(this); 156 if (mCleanupTask != null) { 157 if (DEBUG) Log.i(TAG, "--- CLEANING ONE REF"); 158 mCleanupTask.run(); 159 mCleanupTask = null; 160 } 161 clear(); 162 } 163} 164