1/*
2 * Copyright (C) 2014 The Android Open Source Project
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 *      http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17package com.android.printspooler.model;
18
19import android.util.Log;
20
21import java.io.File;
22import java.io.IOException;
23
24/**
25 * This class provides a shared file to several threads. Only one thread
26 * at a time can use the file. To acquire the file a thread has to
27 * request it in a blocking call to {@link #acquireFile(OnReleaseRequestCallback)}.
28 * The provided callback is optional and is used to notify the owning thread
29 * when another one wants to acquire the file. In case a release is requested
30 * the thread owning the file must release it as soon as possible. If no
31 * callback is provided a thread that acquires the file must release it
32 * as soon as possible, i.e. even if callback was provided the thread cannot
33 * have the file for less time.
34 */
35public final class MutexFileProvider {
36    private static final String LOG_TAG = "MutexFileProvider";
37
38    private static final boolean DEBUG = true;
39
40    private final Object mLock = new Object();
41
42    private final File mFile;
43
44    private Thread mOwnerThread;
45
46    private OnReleaseRequestCallback mOnReleaseRequestCallback;
47
48    public interface OnReleaseRequestCallback {
49        public void onReleaseRequested(File file);
50    }
51
52    public MutexFileProvider(File file) throws IOException {
53        mFile = file;
54        if (file.exists()) {
55            file.delete();
56        }
57        file.createNewFile();
58    }
59
60    public File acquireFile(OnReleaseRequestCallback callback) {
61        synchronized (mLock) {
62            // If this thread has the file, nothing to do.
63            if (mOwnerThread == Thread.currentThread()) {
64                return mFile;
65            }
66
67            // Another thread wants file ask for a release.
68            if (mOwnerThread != null && mOnReleaseRequestCallback != null) {
69                mOnReleaseRequestCallback.onReleaseRequested(mFile);
70            }
71
72            // Wait until the file is released.
73            while (mOwnerThread != null) {
74                try {
75                    mLock.wait();
76                } catch (InterruptedException ie) {
77                    /* ignore */
78                }
79            }
80
81            // Update the owner and the callback.
82            mOwnerThread = Thread.currentThread();
83            mOnReleaseRequestCallback = callback;
84
85            if (DEBUG) {
86                Log.i(LOG_TAG, "Acquired file: " + mFile + " by thread: " + mOwnerThread);
87            }
88
89            return mFile;
90        }
91    }
92
93    public void releaseFile() {
94        synchronized (mLock) {
95            if (mOwnerThread != Thread.currentThread()) {
96                return;
97            }
98
99            if (DEBUG) {
100                Log.i(LOG_TAG, "Released file: " + mFile + " from thread: " + mOwnerThread);
101            }
102
103            // Update the owner and the callback.
104            mOwnerThread = null;
105            mOnReleaseRequestCallback = null;
106
107            mLock.notifyAll();
108        }
109    }
110}
111