好久没有在这里写点东西了，要养成书写记录的习惯。
这里简单的讨论一下java设计网络程序中如何控制上传和下载速度，我们常见的FTP，HTTP，BT等协议都是TCP的，但是现在流行的utorrent却基于UDP实现了自己UTP协议(UDP+拥塞控制)，不管使用什么协议，站在I/O的角度来说，限速的控制思路都是一样的。
思路很简单，如下：
1.假设下载或者上传速度上限是m (KB/s),那么发送一个固定的字节数据(假设是n字节)的时间花费是：n/m；
2.假设现在要发送n字节的数据，那么理论所需的时间应该是n/m，而在实际情况下，发送n字节的数据只花费了t秒，那么发送该发送线程就应该睡眠n/m-t秒，这样就基本实现了速度的控制。
代码以TCP为例
速度控制
1 package com.actiontec.net.bandwidth;
2
3 /**
4 *
5 * @author Le
6 *
7 */
8 public class BandwidthLimiter {
9
10 /* KB */
11 private static Long KB = 1024l;
12
13 /* The smallest count chunk length in bytes */
14 private static Long CHUNK_LENGTH = 1024l;
15
16 /* How many bytes will be sent or receive */
17 private int bytesWillBeSentOrReceive = 0;
18
19 /* When the last piece was sent or receive */
20 private long lastPieceSentOrReceiveTick = System.nanoTime();
21
22 /* Default rate is 1024KB/s */
23 private int maxRate = 1024;
24
25 /* Time cost for sending CHUNK_LENGTH bytes in nanoseconds */
26 private long timeCostPerChunk = (1000000000l * CHUNK_LENGTH)
27 / (this.maxRate * KB);
28
29 /**
30 * Initialize a BandwidthLimiter object with a certain rate.
31 *
32 * @param maxRate
33 * the download or upload speed in KBytes
34 */
35 public BandwidthLimiter(int maxRate) {
36 this.setMaxRate(maxRate);
37 }
38
39 /**
40 * Set the max upload or download rate in KB/s. maxRate must be grater than
41 * 0. If maxRate is zero, it means there is no bandwidth limit.
42 *
43 * @param maxRate
44 * If maxRate is zero, it means there is no bandwidth limit.
45 * @throws IllegalArgumentException
46 */
47 public synchronized void setMaxRate(int maxRate)
48 throws IllegalArgumentException {
49 if (maxRate < 0) {
50 throw new IllegalArgumentException("maxRate can not less than 0");
51 }
52 this.maxRate = maxRate < 0 ? 0 : maxRate;
53 if (maxRate == 0)
54 this.timeCostPerChunk = 0;
55 else
56 this.timeCostPerChunk = (1000000000l * CHUNK_LENGTH)
57 / (this.maxRate * KB);
58 }
59
60 /**
61 * Next 1 byte should do bandwidth limit.
62 */
63 public synchronized void limitNextBytes() {
64 this.limitNextBytes(1);
65 }
66
67 /**
68 * Next len bytes should do bandwidth limit
69 *
70 * @param len
71 */
72 public synchronized void limitNextBytes(int len) {
73 this.bytesWillBeSentOrReceive += len;
74
75 /* We have sent CHUNK_LENGTH bytes */
76 while (this.bytesWillBeSentOrReceive > CHUNK_LENGTH) {
77 long nowTick = System.nanoTime();
78 long missedTime = this.timeCostPerChunk
79 - (nowTick - this.lastPieceSentOrReceiveTick);
80 if (missedTime > 0) {
81 try {
82 Thread.sleep(missedTime / 1000000,
83 (int) (missedTime % 1000000));
84 } catch (InterruptedException e) {
85 e.printStackTrace();
86 }
87 }
88 this.bytesWillBeSentOrReceive -= CHUNK_LENGTH;
89 this.lastPieceSentOrReceiveTick = nowTick
90 + (missedTime > 0 ? missedTime : 0);
91 }
92 }
93 }
94
下载控制
1 package com.actiontec.net.bandwidth;
2
3 import java.io.IOException;
4 import java.io.InputStream;
5
6 /**
7 * @author Le
8 *
9 */
10 public class DownloadLimiter extends InputStream {
11 private InputStream is = null;
12 private BandwidthLimiter bandwidthLimiter = null;
13
14 public DownloadLimiter(InputStream is, BandwidthLimiter bandwidthLimiter)
15 {
16 this.is = is;
17 this.bandwidthLimiter = bandwidthLimiter;
18 }
19 @Override
20 public int read() throws IOException {
21 if(this.bandwidthLimiter != null)
22 this.bandwidthLimiter.limitNextBytes();
23 return this.is.read();
24 }
25
26 public int read(byte b[], int off, int len) throws IOException
27 {
28 if (bandwidthLimiter != null)
29 bandwidthLimiter.limitNextBytes(len);
30 return this.is.read(b, off, len);
31 }
32 }
同样，上传控制
1 package com.actiontec.net.bandwidth;
2
3 import java.io.IOException;
4 import java.io.OutputStream;
5
6 /**
7 * @author Le
8 *
9 */
10 public class UploadLimiter extends OutputStream {
11 private OutputStream os = null;
12 private BandwidthLimiter bandwidthLimiter = null;
13
14 public UploadLimiter(OutputStream os, BandwidthLimiter bandwidthLimiter)
15 {
16 this.os = os;
17 this.bandwidthLimiter = bandwidthLimiter;
18 }
19
20 @Override
21 public void write(int b) throws IOException {
22 if (bandwidthLimiter != null)
23 bandwidthLimiter.limitNextBytes();
24 this.os.write(b);
25 }
26
27 public void write(byte[] b, int off, int len) throws IOException {
28 if (bandwidthLimiter != null)
29 bandwidthLimiter.limitNextBytes(len);
30 this.os.write(b, off, len);
31 }
32
33 }
对于一个TCP socket
1 ServerSocket socket = new ServerSocket();
2 //其它初始化略
1 //从socket中以一定的速率读数据
2 //```java
3 DownloadLimiter dl = new DownloadLimiter(socket.getInputStream(), new BandwidthLimiter(6250));
4 is = new DataInputStream(dl);
5
6 //读数据
7 int len = is.readInt();
8 ByteBuffer buffer = ByteBuffer.allocate(4 + len);
9 buffer.putInt(len);
10 is.readFully(buffer.array(), 4, buffer.remaining());
11 //```
12
13 //以一定的速率写数据到socket
14 //```java
15 UploadLimiter ul = new UploadLimiter(socket.getOutputStream(), new BandwidthLimiter(6250));
16 ul.write(
);
17 //``` 在多线程环境下也可以使用上述的方法。最后附图是任务管理器的网络利用率图6250KB/s（也就是50000kb/s,附图中网络利用率也在5%左右，所以应该这个做法还算准确）
