1 /*
2  * Copyright (C) 2017 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 
17 package android.util.apk;
18 
19 import java.io.IOException;
20 import java.nio.ByteBuffer;
21 import java.security.DigestException;
22 
23 /**
24  * {@link DataSource} which provides data from a {@link ByteBuffer}.
25  */
26 class ByteBufferDataSource implements DataSource {
27     /**
28      * Underlying buffer. The data is stored between position 0 and the buffer's capacity.
29      * The buffer's position is 0 and limit is equal to capacity.
30      */
31     private final ByteBuffer mBuf;
32 
ByteBufferDataSource(ByteBuffer buf)33     ByteBufferDataSource(ByteBuffer buf) {
34         // Defensive copy, to avoid changes to mBuf being visible in buf, and to ensure position is
35         // 0 and limit == capacity.
36         mBuf = buf.slice();
37     }
38 
39     @Override
size()40     public long size() {
41         return mBuf.capacity();
42     }
43 
44     @Override
feedIntoDataDigester(DataDigester md, long offset, int size)45     public void feedIntoDataDigester(DataDigester md, long offset, int size)
46             throws IOException, DigestException {
47         // There's no way to tell MessageDigest to read data from ByteBuffer from a position
48         // other than the buffer's current position. We thus need to change the buffer's
49         // position to match the requested offset.
50         //
51         // In the future, it may be necessary to compute digests of multiple regions in
52         // parallel. Given that digest computation is a slow operation, we enable multiple
53         // such requests to be fulfilled by this instance. This is achieved by serially
54         // creating a new ByteBuffer corresponding to the requested data range and then,
55         // potentially concurrently, feeding these buffers into MessageDigest instances.
56         ByteBuffer region;
57         synchronized (mBuf) {
58             mBuf.position(0);
59             mBuf.limit((int) offset + size);
60             mBuf.position((int) offset);
61             region = mBuf.slice();
62         }
63 
64         md.consume(region);
65     }
66 }
67