Class - java.io.SequenceInputStream
Created by : Mr Dk.
2020 / 09 / 24 10:54
Nanjing, Jiangsu, China
Definition
实现多个输入流之间的逻辑拼接。当一个流读完后，接着去读下一个流。
/**
* A <code>SequenceInputStream</code> represents
* the logical concatenation of other input
* streams. It starts out with an ordered
* collection of input streams and reads from
* the first one until end of file is reached,
* whereupon it reads from the second one,
* and so on, until end of file is reached
* on the last of the contained input streams.
*
* @author Author van Hoff
* @since JDK1.0
*/
public
class SequenceInputStream extends InputStream {
}
Members
e用于维护所有的输入流集合 - 在一个输入流结束后，通过e可以拿到下一个输入流in用于维护当前正在使用的输入流 - 调用read()时，将作用在这个输入流上
Enumeration<? extends InputStream> e;
InputStream in;
Constructor
可以直接使用两个现有输入流直接构造对象。首先将输入流构造到一个 Vector 里，然后调用 Vector 的 .elements() 赋值成员变量 Enumeration<? extends InputStream> e。然后调用 nextStream() 获取第一个 InputStream。
/**
* Initializes a newly
* created <code>SequenceInputStream</code>
* by remembering the two arguments, which
* will be read in order, first <code>s1</code>
* and then <code>s2</code>, to provide the
* bytes to be read from this <code>SequenceInputStream</code>.
*
* @param s1 the first input stream to read.
* @param s2 the second input stream to read.
*/
public SequenceInputStream(InputStream s1, InputStream s2) {
Vector<InputStream> v = new Vector<>(2);
v.addElement(s1);
v.addElement(s2);
e = v.elements();
try {
nextStream();
} catch (IOException ex) {
// This should never happen
throw new Error("panic");
}
}
当然也可以自行构造 Enumeration<? extends InputStream> e 然后直接构造对象。那么这样就支持多个输入流之间的逻辑拼接了。具体的操作方式与上一个函数类似。
/**
* Initializes a newly created <code>SequenceInputStream</code>
* by remembering the argument, which must
* be an <code>Enumeration</code> that produces
* objects whose run-time type is <code>InputStream</code>.
* The input streams that are produced by
* the enumeration will be read, in order,
* to provide the bytes to be read from this
* <code>SequenceInputStream</code>. After
* each input stream from the enumeration
* is exhausted, it is closed by calling its
* <code>close</code> method.
*
* @param e an enumeration of input streams.
* @see java.util.Enumeration
*/
public SequenceInputStream(Enumeration<? extends InputStream> e) {
this.e = e;
try {
nextStream();
} catch (IOException ex) {
// This should never happen
throw new Error("panic");
}
}
NextStream
关闭正在使用的输入流，然后将下一个可用的输入流复制到成员变量 in 中，以便对其进行 read()。只在前一个输入流被读尽时，这个函数才会被调用：
/**
* Continues reading in the next stream if an EOF is reached.
*/
final void nextStream() throws IOException {
if (in != null) {
in.close();
}
if (e.hasMoreElements()) {
in = (InputStream) e.nextElement();
if (in == null)
throw new NullPointerException();
}
else in = null;
}
Available
估算目前的输入流中还有多少字节能被读取。
/**
* Returns an estimate of the number of bytes that can be read (or
* skipped over) from the current underlying input stream without
* blocking by the next invocation of a method for the current
* underlying 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.
* <p>
* This method simply calls {@code available} of the current underlying
* input stream and returns the result.
*
* @return an estimate of the number of bytes that can be read (or
* skipped over) from the current underlying input stream
* without blocking or {@code 0} if this input stream
* has been closed by invoking its {@link #close()} method
* @exception IOException if an I/O error occurs.
*
* @since JDK1.1
*/
public int available() throws IOException {
if (in == null) {
return 0; // no way to signal EOF from available()
}
return in.available();
}
Read
当目前的输入流被读尽时，调用 nextStream() 将下一个输入流复制到成员变量 in 中。
/**
* Reads the next byte of data from this input stream. The byte is
* returned as an <code>int</code> in the range <code>0</code> to
* <code>255</code>. If no byte is available because the end of the
* stream has been reached, the value <code>-1</code> is returned.
* This method blocks until input data is available, the end of the
* stream is detected, or an exception is thrown.
* <p>
* This method
* tries to read one character from the current substream. If it
* reaches the end of the stream, it calls the <code>close</code>
* method of the current substream and begins reading from the next
* substream.
*
* @return the next byte of data, or <code>-1</code> if the end of the
* stream is reached.
* @exception IOException if an I/O error occurs.
*/
public int read() throws IOException {
while (in != null) {
int c = in.read();
if (c != -1) {
return c;
}
nextStream();
}
return -1;
}
/**
* Reads up to <code>len</code> bytes of data from this input stream
* into an array of bytes. If <code>len</code> is not zero, the method
* blocks until at least 1 byte of input is available; otherwise, no
* bytes are read and <code>0</code> is returned.
* <p>
* The <code>read</code> method of <code>SequenceInputStream</code>
* tries to read the data from the current substream. If it fails to
* read any characters because the substream has reached the end of
* the stream, it calls the <code>close</code> method of the current
* substream and begins reading from the next substream.
*
* @param b the buffer into which the data is read.
* @param off the start offset in array <code>b</code>
* at which the data is written.
* @param len the maximum number of bytes read.
* @return int the number of bytes read.
* @exception NullPointerException If <code>b</code> is <code>null</code>.
* @exception IndexOutOfBoundsException If <code>off</code> is negative,
* <code>len</code> is negative, or <code>len</code> is greater than
* <code>b.length - off</code>
* @exception IOException if an I/O error occurs.
*/
public int read(byte b[], int off, int len) throws IOException {
if (in == null) {
return -1;
} else 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;
}
do {
int n = in.read(b, off, len);
if (n > 0) {
return n;
}
nextStream();
} while (in != null);
return -1;
}
Close
关闭当前输入流之后，关闭后续的所有输入流。
/**
* Closes this input stream and releases any system resources
* associated with the stream.
* A closed <code>SequenceInputStream</code>
* cannot perform input operations and cannot
* be reopened.
* <p>
* If this stream was created
* from an enumeration, all remaining elements
* are requested from the enumeration and closed
* before the <code>close</code> method returns.
*
* @exception IOException if an I/O error occurs.
*/
public void close() throws IOException {
do {
nextStream();
} while (in != null);
}