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25 
26 package java.io;
27 
28 import java.io.InputStream;
29 import java.util.Enumeration;
30 import java.util.Vector;
31 
32 /**
33  * A <code>SequenceInputStream</code> represents
34  * the logical concatenation of other input
35  * streams. It starts out with an ordered
36  * collection of input streams and reads from
37  * the first one until end of file is reached,
38  * whereupon it reads from the second one,
39  * and so on, until end of file is reached
40  * on the last of the contained input streams.
41  *
42  * @author  Author van Hoff
43  * @since   1.0
44  */
45 public
46 class SequenceInputStream extends InputStream {
47     Enumeration<? extends InputStream> e;
48     InputStream in;
49 
50     /**
51      * Initializes a newly created <code>SequenceInputStream</code>
52      * by remembering the argument, which must
53      * be an <code>Enumeration</code>  that produces
54      * objects whose run-time type is <code>InputStream</code>.
55      * The input streams that are  produced by
56      * the enumeration will be read, in order,
57      * to provide the bytes to be read  from this
58      * <code>SequenceInputStream</code>. After
59      * each input stream from the enumeration
60      * is exhausted, it is closed by calling its
61      * <code>close</code> method.
62      *
63      * @param   e   an enumeration of input streams.
64      * @see     java.util.Enumeration
65      */
SequenceInputStream(Enumeration<? extends InputStream> e)66     public SequenceInputStream(Enumeration<? extends InputStream> e) {
67         this.e = e;
68         peekNextStream();
69     }
70 
71     /**
72      * Initializes a newly
73      * created <code>SequenceInputStream</code>
74      * by remembering the two arguments, which
75      * will be read in order, first <code>s1</code>
76      * and then <code>s2</code>, to provide the
77      * bytes to be read from this <code>SequenceInputStream</code>.
78      *
79      * @param   s1   the first input stream to read.
80      * @param   s2   the second input stream to read.
81      */
SequenceInputStream(InputStream s1, InputStream s2)82     public SequenceInputStream(InputStream s1, InputStream s2) {
83         Vector<InputStream> v = new Vector<>(2);
84         v.addElement(s1);
85         v.addElement(s2);
86         e = v.elements();
87         peekNextStream();
88     }
89 
90     /**
91      *  Continues reading in the next stream if an EOF is reached.
92      */
nextStream()93     final void nextStream() throws IOException {
94         if (in != null) {
95             in.close();
96         }
97         peekNextStream();
98     }
99 
peekNextStream()100     private void peekNextStream() {
101         if (e.hasMoreElements()) {
102             in = (InputStream) e.nextElement();
103             if (in == null)
104                 throw new NullPointerException();
105         } else {
106             in = null;
107         }
108     }
109 
110     /**
111      * Returns an estimate of the number of bytes that can be read (or
112      * skipped over) from the current underlying input stream without
113      * blocking by the next invocation of a method for the current
114      * underlying input stream. The next invocation might be
115      * the same thread or another thread.  A single read or skip of this
116      * many bytes will not block, but may read or skip fewer bytes.
117      * <p>
118      * This method simply calls {@code available} of the current underlying
119      * input stream and returns the result.
120      *
121      * @return an estimate of the number of bytes that can be read (or
122      *         skipped over) from the current underlying input stream
123      *         without blocking or {@code 0} if this input stream
124      *         has been closed by invoking its {@link #close()} method
125      * @exception  IOException  if an I/O error occurs.
126      *
127      * @since   1.1
128      */
available()129     public int available() throws IOException {
130         if (in == null) {
131             return 0; // no way to signal EOF from available()
132         }
133         return in.available();
134     }
135 
136     /**
137      * Reads the next byte of data from this input stream. The byte is
138      * returned as an <code>int</code> in the range <code>0</code> to
139      * <code>255</code>. If no byte is available because the end of the
140      * stream has been reached, the value <code>-1</code> is returned.
141      * This method blocks until input data is available, the end of the
142      * stream is detected, or an exception is thrown.
143      * <p>
144      * This method
145      * tries to read one character from the current substream. If it
146      * reaches the end of the stream, it calls the <code>close</code>
147      * method of the current substream and begins reading from the next
148      * substream.
149      *
150      * @return     the next byte of data, or <code>-1</code> if the end of the
151      *             stream is reached.
152      * @exception  IOException  if an I/O error occurs.
153      */
read()154     public int read() throws IOException {
155         while (in != null) {
156             int c = in.read();
157             if (c != -1) {
158                 return c;
159             }
160             nextStream();
161         }
162         return -1;
163     }
164 
165     /**
166      * Reads up to <code>len</code> bytes of data from this input stream
167      * into an array of bytes.  If <code>len</code> is not zero, the method
168      * blocks until at least 1 byte of input is available; otherwise, no
169      * bytes are read and <code>0</code> is returned.
170      * <p>
171      * The <code>read</code> method of <code>SequenceInputStream</code>
172      * tries to read the data from the current substream. If it fails to
173      * read any characters because the substream has reached the end of
174      * the stream, it calls the <code>close</code> method of the current
175      * substream and begins reading from the next substream.
176      *
177      * @param      b     the buffer into which the data is read.
178      * @param      off   the start offset in array <code>b</code>
179      *                   at which the data is written.
180      * @param      len   the maximum number of bytes read.
181      * @return     int   the number of bytes read.
182      * @exception  NullPointerException If <code>b</code> is <code>null</code>.
183      * @exception  IndexOutOfBoundsException If <code>off</code> is negative,
184      * <code>len</code> is negative, or <code>len</code> is greater than
185      * <code>b.length - off</code>
186      * @exception  IOException  if an I/O error occurs.
187      */
read(byte b[], int off, int len)188     public int read(byte b[], int off, int len) throws IOException {
189         if (in == null) {
190             return -1;
191         } else if (b == null) {
192             throw new NullPointerException();
193         } else if (off < 0 || len < 0 || len > b.length - off) {
194             throw new IndexOutOfBoundsException();
195         } else if (len == 0) {
196             return 0;
197         }
198         do {
199             int n = in.read(b, off, len);
200             if (n > 0) {
201                 return n;
202             }
203             nextStream();
204         } while (in != null);
205         return -1;
206     }
207 
208     /**
209      * Closes this input stream and releases any system resources
210      * associated with the stream.
211      * A closed <code>SequenceInputStream</code>
212      * cannot  perform input operations and cannot
213      * be reopened.
214      * <p>
215      * If this stream was created
216      * from an enumeration, all remaining elements
217      * are requested from the enumeration and closed
218      * before the <code>close</code> method returns.
219      *
220      * @exception  IOException  if an I/O error occurs.
221      */
close()222     public void close() throws IOException {
223         do {
224             nextStream();
225         } while (in != null);
226     }
227 }
228