PushbackInputStream adds
* functionality to another input stream, namely
* the ability to "push back" or "unread" bytes,
* by storing pushed-back bytes in an internal buffer.
* This is useful in situations where
* it is convenient for a fragment of code
* to read an indefinite number of data bytes
* that are delimited by a particular byte
* value; after reading the terminating byte,
* the code fragment can "unread" it, so that
* the next read operation on the input stream
* will reread the byte that was pushed back.
* For example, bytes representing the characters
* constituting an identifier might be terminated
* by a byte representing an operator character;
* a method whose job is to read just an identifier
* can read until it sees the operator and
* then push the operator back to be re-read.
*
* @author David Connelly
* @author Jonathan Payne
* @since 1.0
*/
public
class PushbackInputStream extends FilterInputStream {
/**
* The pushback buffer.
* @since 1.1
*/
protected byte[] buf;
/**
* The position within the pushback buffer from which the next byte will
* be read. When the buffer is empty, pos is equal to
* buf.length; when the buffer is full, pos is
* equal to zero.
*
* @since 1.1
*/
protected int pos;
/**
* Check to make sure that this stream has not been closed
*/
private void ensureOpen() throws IOException {
if (in == null)
throw new IOException("Stream closed");
}
/**
* Creates a PushbackInputStream
* with a pushback buffer of the specified size,
* and saves its argument, the input stream
* in, for later use. Initially,
* the pushback buffer is empty.
*
* @param in the input stream from which bytes will be read.
* @param size the size of the pushback buffer.
* @exception IllegalArgumentException if {@code size <= 0}
* @since 1.1
*/
public PushbackInputStream(InputStream in, int size) {
super(in);
if (in == null) {
throw new NullPointerException();
}
if (size <= 0) {
throw new IllegalArgumentException("size <= 0");
}
this.buf = new byte[size];
this.pos = size;
}
/**
* Creates a PushbackInputStream
* with a 1-byte pushback buffer, and saves its argument, the input stream
* in, for later use. Initially,
* the pushback buffer is empty.
*
* @param in the input stream from which bytes will be read.
*/
public PushbackInputStream(InputStream in) {
this(in, 1);
}
/**
* Reads the next byte of data from this input stream. The value
* byte is returned as an int in the range
* 0 to 255. If no byte is available
* because the end of the stream has been reached, the value
* -1 is returned. This method blocks until input data
* is available, the end of the stream is detected, or an exception
* is thrown.
*
* This method returns the most recently pushed-back byte, if there is
* one, and otherwise calls the read method of its underlying
* input stream and returns whatever value that method returns.
*
* @return the next byte of data, or -1 if the end of the
* stream has been reached.
* @exception IOException if this input stream has been closed by
* invoking its {@link #close()} method,
* or an I/O error occurs.
* @see java.io.InputStream#read()
*/
public int read() throws IOException {
ensureOpen();
if (pos < buf.length) {
return buf[pos++] & 0xff;
}
return super.read();
}
/**
* Reads up to len bytes of data from this input stream into
* an array of bytes. This method first reads any pushed-back bytes; after
* that, if fewer than len bytes have been read then it
* reads from the underlying input stream. If len is not zero, the method
* blocks until at least 1 byte of input is available; otherwise, no
* bytes are read and 0 is returned.
*
* @param b the buffer into which the data is read.
* @param off the start offset in the destination array b
* @param len the maximum number of bytes read.
* @return the total number of bytes read into the buffer, or
* -1 if there is no more data because the end of
* the stream has been reached.
* @exception NullPointerException If b is null.
* @exception IndexOutOfBoundsException If off is negative,
* len is negative, or len is greater than
* b.length - off
* @exception IOException if this input stream has been closed by
* invoking its {@link #close()} method,
* or an I/O error occurs.
* @see java.io.InputStream#read(byte[], int, int)
*/
public int read(byte[] b, int off, int len) throws IOException {
ensureOpen();
if (b == null) {
throw new NullPointerException();
} else if (off < 0 || len < 0 || len > b.length - off) {
throw new IndexOutOfBoundsException();
} else if (len == 0) {
return 0;
}
int avail = buf.length - pos;
if (avail > 0) {
if (len < avail) {
avail = len;
}
System.arraycopy(buf, pos, b, off, avail);
pos += avail;
off += avail;
len -= avail;
}
if (len > 0) {
len = super.read(b, off, len);
if (len == -1) {
return avail == 0 ? -1 : avail;
}
return avail + len;
}
return avail;
}
/**
* Pushes back a byte by copying it to the front of the pushback buffer.
* After this method returns, the next byte to be read will have the value
* (byte)b.
*
* @param b the int value whose low-order
* byte is to be pushed back.
* @exception IOException If there is not enough room in the pushback
* buffer for the byte, or this input stream has been closed by
* invoking its {@link #close()} method.
*/
public void unread(int b) throws IOException {
ensureOpen();
if (pos == 0) {
throw new IOException("Push back buffer is full");
}
buf[--pos] = (byte)b;
}
/**
* Pushes back a portion of an array of bytes by copying it to the front
* of the pushback buffer. After this method returns, the next byte to be
* read will have the value b[off], the byte after that will
* have the value b[off+1], and so forth.
*
* @param b the byte array to push back.
* @param off the start offset of the data.
* @param len the number of bytes to push back.
* @exception NullPointerException If b is null.
* @exception IOException If there is not enough room in the pushback
* buffer for the specified number of bytes,
* or this input stream has been closed by
* invoking its {@link #close()} method.
* @since 1.1
*/
public void unread(byte[] b, int off, int len) throws IOException {
ensureOpen();
if (len > pos) {
throw new IOException("Push back buffer is full");
}
pos -= len;
System.arraycopy(b, off, buf, pos, len);
}
/**
* Pushes back an array of bytes by copying it to the front of the
* pushback buffer. After this method returns, the next byte to be read
* will have the value b[0], the byte after that will have the
* value b[1], and so forth.
*
* @param b the byte array to push back
* @exception NullPointerException If b is null.
* @exception IOException If there is not enough room in the pushback
* buffer for the specified number of bytes,
* or this input stream has been closed by
* invoking its {@link #close()} method.
* @since 1.1
*/
public void unread(byte[] b) throws IOException {
unread(b, 0, b.length);
}
/**
* Returns an estimate of the number of bytes that can be read (or
* skipped over) from this input stream without blocking by the next
* invocation of a method for this input stream. The next invocation might be
* the same thread or another thread. A single read or skip of this
* many bytes will not block, but may read or skip fewer bytes.
*
*
The method returns the sum of the number of bytes that have been
* pushed back and the value returned by {@link
* java.io.FilterInputStream#available available}.
*
* @return the number of bytes that can be read (or skipped over) from
* the input stream without blocking.
* @exception IOException if this input stream has been closed by
* invoking its {@link #close()} method,
* or an I/O error occurs.
* @see java.io.FilterInputStream#in
* @see java.io.InputStream#available()
*/
public int available() throws IOException {
ensureOpen();
int n = buf.length - pos;
int avail = super.available();
return n > (Integer.MAX_VALUE - avail)
? Integer.MAX_VALUE
: n + avail;
}
/**
* Skips over and discards n bytes of data from this
* input stream. The skip method may, for a variety of
* reasons, end up skipping over some smaller number of bytes,
* possibly zero. If n is negative, no bytes are skipped.
*
*
The skip method of PushbackInputStream
* first skips over the bytes in the pushback buffer, if any. It then
* calls the skip method of the underlying input stream if
* more bytes need to be skipped. The actual number of bytes skipped
* is returned.
*
* @param n {@inheritDoc}
* @return {@inheritDoc}
* @throws IOException if the stream has been closed by
* invoking its {@link #close()} method,
* {@code in.skip(n)} throws an IOException,
* or an I/O error occurs.
* @see java.io.FilterInputStream#in
* @see java.io.InputStream#skip(long n)
* @since 1.2
*/
public long skip(long n) throws IOException {
ensureOpen();
if (n <= 0) {
return 0;
}
long pskip = buf.length - pos;
if (pskip > 0) {
if (n < pskip) {
pskip = n;
}
pos += pskip;
n -= pskip;
}
if (n > 0) {
pskip += super.skip(n);
}
return pskip;
}
/**
* Tests if this input stream supports the mark and
* reset methods, which it does not.
*
* @return false, since this class does not support the
* mark and reset methods.
* @see java.io.InputStream#mark(int)
* @see java.io.InputStream#reset()
*/
public boolean markSupported() {
return false;
}
/**
* Marks the current position in this input stream.
*
*
The mark method of PushbackInputStream
* does nothing.
*
* @param readlimit the maximum limit of bytes that can be read before
* the mark position becomes invalid.
* @see java.io.InputStream#reset()
*/
public synchronized void mark(int readlimit) {
}
/**
* Repositions this stream to the position at the time the
* mark method was last called on this input stream.
*
*
The method reset for class
* PushbackInputStream does nothing except throw an
* IOException.
*
* @exception IOException if this method is invoked.
* @see java.io.InputStream#mark(int)
* @see java.io.IOException
*/
public synchronized void reset() throws IOException {
throw new IOException("mark/reset not supported");
}
/**
* Closes this input stream and releases any system resources
* associated with the stream.
* Once the stream has been closed, further read(), unread(),
* available(), reset(), or skip() invocations will throw an IOException.
* Closing a previously closed stream has no effect.
*
* @exception IOException if an I/O error occurs.
*/
public synchronized void close() throws IOException {
if (in == null)
return;
in.close();
in = null;
buf = null;
}
}