PipedInputStream类

  1 package java.io;
  2 
  3 public class PipedInputStream extends InputStream {
  4     // “管道输出流”是否关闭的标记
  5     boolean closedByWriter = false;
  6     // “管道输入流”是否关闭的标记
  7     volatile boolean closedByReader = false;
  8     // “管道输入流”与“管道输出流”是否连接的标记
  9     // 它在PipedOutputStream的connect()连接函数中被设置为true
 10     boolean connected = false;
 11 
 12     Thread readSide;    // 读取“管道”数据的线程
 13     Thread writeSide;    // 向“管道”写入数据的线程
 14 
 15     // “管道”的默认大小
 16     private static final int DEFAULT_PIPE_SIZE = 1024;
 17 
 18     protected static final int PIPE_SIZE = DEFAULT_PIPE_SIZE;
 19 
 20     // 缓冲区
 21     protected byte buffer[];
 22 
 23     //MG注 in为写指针 下一个写入字节的位置。in==out代表满，说明“写入的数据”全部被读取了或者是代表in多走了length。
 24     protected int in = -1;
 25     //MG注 out为读指针 下一个读取字节的位置。in==out代表满，说明“写入的数据”全部被读取了或者是代表in多走了length。
 26     protected int out = 0;
 27 
 28     // 构造函数：指定与“管道输入流”关联的“管道输出流”
 29     public PipedInputStream(PipedOutputStream src) throws IOException {
 30         this(src, DEFAULT_PIPE_SIZE);
 31     }
 32 
 33     // 构造函数：指定与“管道输入流”关联的“管道输出流”，以及“缓冲区大小”
 34     public PipedInputStream(PipedOutputStream src, int pipeSize)
 35             throws IOException {
 36          initPipe(pipeSize);
 37          connect(src);
 38     }
 39 
 40     // 构造函数：默认缓冲区大小是1024字节
 41     public PipedInputStream() {
 42         initPipe(DEFAULT_PIPE_SIZE);
 43     }
 44 
 45     // 构造函数：指定缓冲区大小是pipeSize
 46     public PipedInputStream(int pipeSize) {
 47         initPipe(pipeSize);
 48     }
 49 
 50     // 初始化“管道”：新建缓冲区大小
 51     private void initPipe(int pipeSize) {
 52          if (pipeSize <= 0) {
 53             throw new IllegalArgumentException("Pipe Size <= 0");
 54          }
 55          buffer = new byte[pipeSize];
 56     }
 57 
 58     // 将“管道输入流”和“管道输出流”绑定。
 59     // 实际上，这里调用的是PipedOutputStream的connect()函数
 60     public void connect(PipedOutputStream src) throws IOException {
 61         src.connect(this);
 62     }
 63 
 64     // 接收int类型的数据b。
 65     // 它只会在PipedOutputStream的write(int b)中会被调用
 66     protected synchronized void receive(int b) throws IOException {
 67         // 检查管道状态
 68         checkStateForReceive();
 69         // 获取“写入管道”的线程
 70         writeSide = Thread.currentThread();
 71         //MG注 此处只有in比out多走了length的长度才会进入
 72         if (in == out)
 73             awaitSpace();
 74         if (in < 0) {
 75             in = 0;
 76             out = 0;
 77         }
 78         // 将b保存到缓冲区
 79         buffer[in++] = (byte)(b & 0xFF);
 80         if (in >= buffer.length) {
 81             in = 0;
 82         }
 83     }
 84 
 85     // 接收字节数组b。
 86     synchronized void receive(byte b[], int off, int len)  throws IOException {
 87         // 检查管道状态
 88         checkStateForReceive();
 89         // 获取“写入管道”的线程
 90         writeSide = Thread.currentThread();
 91         int bytesToTransfer = len;
 92         while (bytesToTransfer > 0) {
 93             // 若“写入管道”的数据正好全部被读取完，则等待。
 94             if (in == out)
 95                 awaitSpace();
 96             int nextTransferAmount = 0;
 97             // 如果“管道中被读取的数据，少于写入管道的数据”；
 98             // 则设置nextTransferAmount=“buffer.length - in”
 99             if (out < in) {
100                 nextTransferAmount = buffer.length - in;
101             } else if (in < out) { // 如果“管道中被读取的数据，大于/等于写入管道的数据”，则执行后面的操作
102                 // 若in==-1(即管道的写入数据等于被读取数据)，此时nextTransferAmount = buffer.length - in;
103                 // 否则，nextTransferAmount = out - in;
104                 if (in == -1) {
105                     in = out = 0;
106                     nextTransferAmount = buffer.length - in;
107                 } else {
108                     nextTransferAmount = out - in;
109                 }
110             }
111             if (nextTransferAmount > bytesToTransfer)
112                 nextTransferAmount = bytesToTransfer;
113             // assert断言的作用是，若nextTransferAmount <= 0，则终止程序。
114             assert(nextTransferAmount > 0);
115             // 将数据写入到缓冲中
116             System.arraycopy(b, off, buffer, in, nextTransferAmount);
117             bytesToTransfer -= nextTransferAmount;
118             off += nextTransferAmount;
119             in += nextTransferAmount;
120             if (in >= buffer.length) {
121                 in = 0;
122             }
123         }
124     }
125 
126     // 检查管道状态
127     private void checkStateForReceive() throws IOException {
128         if (!connected) {
129             throw new IOException("Pipe not connected");
130         } else if (closedByWriter || closedByReader) {
131             throw new IOException("Pipe closed");
132         } else if (readSide != null && !readSide.isAlive()) {
133             throw new IOException("Read end dead");
134         }
135     }
136 
137     // 等待。
138     // 若“写入管道”的数据正好全部被读取完(例如，管道缓冲满)，则执行awaitSpace()操作；
139     // 它的目的是让“读取管道的线程”管道产生读取数据请求，从而才能继续的向“管道”中写入数据。
140     private void awaitSpace() throws IOException {
141         
142         // 如果“管道中被读取的数据，等于写入管道的数据”时，
143         // 则每隔1000ms检查“管道状态”，并唤醒管道操作：若有“读取管道数据线程被阻塞”，则唤醒该线程。
144         while (in == out) {
145             checkStateForReceive();
146 
147             /* full: kick any waiting readers */
148             notifyAll();
149             try {
150                 wait(1000);
151             } catch (InterruptedException ex) {
152                 throw new java.io.InterruptedIOException();
153             }
154         }
155     }
156 
157     // 当PipedOutputStream被关闭时，被调用
158     synchronized void receivedLast() {
159         closedByWriter = true;
160         notifyAll();
161     }
162 
163     // 从管道(的缓冲)中读取一个字节，并将其转换成int类型
164     public synchronized int read()  throws IOException {
165         if (!connected) {
166             throw new IOException("Pipe not connected");
167         } else if (closedByReader) {
168             throw new IOException("Pipe closed");
169         } else if (writeSide != null && !writeSide.isAlive()
170                    && !closedByWriter && (in < 0)) {
171             throw new IOException("Write end dead");
172         }
173 
174         readSide = Thread.currentThread();
175         int trials = 2;
176         while (in < 0) {
177             if (closedByWriter) {
178                 /* closed by writer, return EOF */
179                 return -1;
180             }
181             if ((writeSide != null) && (!writeSide.isAlive()) && (--trials < 0)) {
182                 throw new IOException("Pipe broken");
183             }
184             /* might be a writer waiting */
185             notifyAll();
186             try {
187                 wait(1000);
188             } catch (InterruptedException ex) {
189                 throw new java.io.InterruptedIOException();
190             }
191         }
192         int ret = buffer[out++] & 0xFF;
193         if (out >= buffer.length) {
194             out = 0;
195         }
196         if (in == out) {
197             /* now empty */
198             in = -1;
199         }
200 
201         return ret;
202     }
203 
204     // 从管道(的缓冲)中读取数据，并将其存入到数组b中
205     public synchronized int read(byte b[], int off, int len)  throws IOException {
206         if (b == null) {
207             throw new NullPointerException();
208         } else if (off < 0 || len < 0 || len > b.length - off) {
209             throw new IndexOutOfBoundsException();
210         } else if (len == 0) {
211             return 0;
212         }
213 
214         /* possibly wait on the first character */
215         int c = read();
216         if (c < 0) {
217             return -1;
218         }
219         b[off] = (byte) c;
220         int rlen = 1;
221         while ((in >= 0) && (len > 1)) {
222 
223             int available;
224 
225             if (in > out) {
226                 available = Math.min((buffer.length - out), (in - out));
227             } else {
228                 available = buffer.length - out;
229             }
230 
231             // A byte is read beforehand outside the loop
232             if (available > (len - 1)) {
233                 available = len - 1;
234             }
235             System.arraycopy(buffer, out, b, off + rlen, available);
236             out += available;
237             rlen += available;
238             len -= available;
239 
240             if (out >= buffer.length) {
241                 out = 0;
242             }
243             if (in == out) {
244                 /* now empty */
245                 in = -1;
246             }
247         }
248         return rlen;
249     }
250 
251     // 返回不受阻塞地从此输入流中读取的字节数。
252     public synchronized int available() throws IOException {
253         if(in < 0)
254             return 0;
255         else if(in == out)
256             return buffer.length;
257         else if (in > out)
258             return in - out;
259         else
260             return in + buffer.length - out;
261     }
262 
263     // 关闭管道输入流
264     public void close()  throws IOException {
265         closedByReader = true;
266         synchronized (this) {
267             in = -1;
268         }
269     }
270 }