Java字节流：FilterInputStream FilterOutputStream

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　　FilterInputStream、FilterOutputStream 过滤器字节输入流、输出流，这里用到了装饰器模式，它的主要用途在于给一个对象动态的添加功能。
　　当我们在创建FilterInputStream、FilterOutputStream这两个类的实例时需要传入一个InputStream、OutPutStream的子类，比如：当构造FilterOutputStream时传递进去的是FileOutputStream，而FileOutputStream和FilterOutputStream实现的是同一个抽象类OutputStream,那么FilterOutputStrean对FileOutputStream的装饰对于客户端来说就是透明的,可以在FileOutputStream的方法执行之前或之后加上一些额外的操作来达到装饰的效果。
　　FilterInputStream、FilterOutputStream 仅仅是对InputStream、OutputStream中所有方法进行了重写，并且只是调用传入的InputStream、OutputStream子类的方法，话句话说就是没有对传入的低级字节输入流进行任何的装饰，它们的作用是为所有字节输入流的装饰类提供一个标准、一个类似于接口的作用，具体的装饰功能由FilterInputStream、FilterOutputStream的子类来完成。
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FilterInputStream
类声明：public class FilterInputStream extends InputStream
位于java.io包下
官方对其说明：
　　A FilterInputStream contains some other input stream, which it uses as its basic source of data, possibly transforming the data along the way or providing additional functionality. The class FilterInputStream itself simply overrides all methods of InputStream with versions that pass all requests to the contained input stream. Subclasses of FilterInputStream may further override some of these methods and may also provide additional methods and fields.
　　(简单翻译：FilterInputStream包含其他一些输入流,它将这些流用作其基本数据源,它可以直接传输数据或提供一些额外的功能。FilterInputStream类本身只是简单地重写那些将所有请求传递给所包含输入流的InputStream的所有方法。FilterInputStream的子类可进一步重写这些方法中的一些方法，并且还可以提供一些额外的方法和字段。)

主要字段：
　　protected InputStream in; //要过滤的输入流

构造方法：
　　protected FilterInputStream(InputStream in)

主要方法:
　　- int available(): 返回输入流中还可以读取的字节个数.
　　- void close(): 关闭此输入流并释放与该流有关的系统资源.
　　- void mark(int readlimit): 在此输入流中标记当前的位置.
　　- boolean markSupported(): 检测此输入流是否支持mark和reset.
　　- int read(): 从输入流中读取数据的下一个字节.
　　- int read(byte[] b): 从输入流中读取一定数量的字节,并将其存储在字节数组b中
　　- int read(byte[] b,int off,int len): 从输入流中读取len个字节，并将其存储在字节数组b中off位置开始的地方
　　- void reset(): 将此流重新定位到最后一次对此输入流调用mark方法时的位置.
　　- long skip(long n): 跳过和丢弃此输入流中n个字节的数据.

源代码如下:

 package java.io;

 /**
  * A <code>FilterInputStream</code> contains
  * some other input stream, which it uses as
  * its  basic source of data, possibly transforming
  * the data along the way or providing  additional
  * functionality. The class <code>FilterInputStream</code>
  * itself simply overrides all  methods of
  * <code>InputStream</code> with versions that
  * pass all requests to the contained  input
  * stream. Subclasses of <code>FilterInputStream</code>
  * may further override some of  these methods
  * and may also provide additional methods
  * and fields.
  *
  * @author  Jonathan Payne
  * @since   JDK1.0
  */
 public
 class FilterInputStream extends InputStream {
     /**
      * The input stream to be filtered.
      */
     protected volatile InputStream in;

     /**
      * Creates a <code>FilterInputStream</code>
      * by assigning the  argument <code>in</code>
      * to the field <code>this.in</code> so as
      * to remember it for later use.
      *
      * @param   in   the underlying input stream, or <code>null</code> if
      *          this instance is to be created without an underlying stream.
      */
     protected FilterInputStream(InputStream in) {
         this.in = in;
     }

     /**
      * Reads the next byte of data from this input stream. The value
      * 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
      * simply performs <code>in.read()</code> and returns the result.
      *
      * @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.
      * @see        java.io.FilterInputStream#in
      */
     public int read() throws IOException {
         return in.read();
     }

     /**
      * Reads up to <code>byte.length</code> bytes of data from this
      * input stream into an array of bytes. This method blocks until some
      * input is available.
      * <p>
      * This method simply performs the call
      * <code>read(b, 0, b.length)</code> and returns
      * the  result. It is important that it does
      * <i>not</i> do <code>in.read(b)</code> instead;
      * certain subclasses of  <code>FilterInputStream</code>
      * depend on the implementation strategy actually
      * used.
      *
      * @param      b   the buffer into which the data is read.
      * @return     the total number of bytes read into the buffer, or
      *             <code>-1</code> if there is no more data because the end of
      *             the stream has been reached.
      * @exception  IOException  if an I/O error occurs.
      * @see        java.io.FilterInputStream#read(byte[], int, int)
      */
     public int read(byte b[]) throws IOException {
         return read(b, 0, b.length);
     }

     /**
      * 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 some input is available; otherwise, no
      * bytes are read and <code>0</code> is returned.
      * <p>
      * This method simply performs <code>in.read(b, off, len)</code>
      * and returns the result.
      *
      * @param      b     the buffer into which the data is read.
      * @param      off   the start offset in the destination array <code>b</code>
      * @param      len   the maximum number of bytes read.
      * @return     the total number of bytes read into the buffer, or
      *             <code>-1</code> if there is no more data because the end of
      *             the stream has been reached.
      * @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.
      * @see        java.io.FilterInputStream#in
      */
     public int read(byte b[], int off, int len) throws IOException {
         return in.read(b, off, len);
     }

     /**
      * Skips over and discards <code>n</code> bytes of data from the
      * input stream. The <code>skip</code> method may, for a variety of
      * reasons, end up skipping over some smaller number of bytes,
      * possibly <code>0</code>. The actual number of bytes skipped is
      * returned.
      * <p>
      * This method simply performs <code>in.skip(n)</code>.
      *
      * @param      n   the number of bytes to be skipped.
      * @return     the actual number of bytes skipped.
      * @exception  IOException  if the stream does not support seek,
      *                          or if some other I/O error occurs.
      */
     public long skip(long n) throws IOException {
         return in.skip(n);
     }

     /**
      * Returns an estimate of the number of bytes that can be read (or
      * skipped over) from this input stream without blocking by the next
      * caller of a method for this input stream. The next caller 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 returns the result of {@link #in in}.available().
      *
      * @return     an estimate of the number of bytes that can be read (or skipped
      *             over) from this input stream without blocking.
      * @exception  IOException  if an I/O error occurs.
      */
     public int available() throws IOException {
         return in.available();
     }

     /**
      * Closes this input stream and releases any system resources
      * associated with the stream.
      * This
      * method simply performs <code>in.close()</code>.
      *
      * @exception  IOException  if an I/O error occurs.
      * @see        java.io.FilterInputStream#in
      */
     public void close() throws IOException {
         in.close();
     }

     /**
      * Marks the current position in this input stream. A subsequent
      * call to the <code>reset</code> method repositions this stream at
      * the last marked position so that subsequent reads re-read the same bytes.
      * <p>
      * The <code>readlimit</code> argument tells this input stream to
      * allow that many bytes to be read before the mark position gets
      * invalidated.
      * <p>
      * This method simply performs <code>in.mark(readlimit)</code>.
      *
      * @param   readlimit   the maximum limit of bytes that can be read before
      *                      the mark position becomes invalid.
      * @see     java.io.FilterInputStream#in
      * @see     java.io.FilterInputStream#reset()
      */
     public synchronized void mark(int readlimit) {
         in.mark(readlimit);
     }

     /**
      * Repositions this stream to the position at the time the
      * <code>mark</code> method was last called on this input stream.
      * <p>
      * This method
      * simply performs <code>in.reset()</code>.
      * <p>
      * Stream marks are intended to be used in
      * situations where you need to read ahead a little to see what's in
      * the stream. Often this is most easily done by invoking some
      * general parser. If the stream is of the type handled by the
      * parse, it just chugs along happily. If the stream is not of
      * that type, the parser should toss an exception when it fails.
      * If this happens within readlimit bytes, it allows the outer
      * code to reset the stream and try another parser.
      *
      * @exception  IOException  if the stream has not been marked or if the
      *               mark has been invalidated.
      * @see        java.io.FilterInputStream#in
      * @see        java.io.FilterInputStream#mark(int)
      */
     public synchronized void reset() throws IOException {
         in.reset();
     }

     /**
      * Tests if this input stream supports the <code>mark</code>
      * and <code>reset</code> methods.
      * This method
      * simply performs <code>in.markSupported()</code>.
      *
      * @return  <code>true</code> if this stream type supports the
      *          <code>mark</code> and <code>reset</code> method;
      *          <code>false</code> otherwise.
      * @see     java.io.FilterInputStream#in
      * @see     java.io.InputStream#mark(int)
      * @see     java.io.InputStream#reset()
      */
     public boolean markSupported() {
         return in.markSupported();
     }
 }

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FilterOutputStream
类声明：public class FilterOutputStream extends OutputStream
位于java.io包下
官方对其说明：
　　This class is the superclass of all classes that filter output streams. These streams sit on top of an already existing output stream (the underlying output stream) which it uses as its basic sink of data, but possibly transforming the data along the way or providing additional functionality.
　　(简单翻译:此类是过滤输出流的所有类的超累。这些流位于已存在的输出流之上，它们将已存在的输出流作为其基本数据接收器，但可能直接传输数据或提供一些额外的功能。)
　　The class FilterOutputStream itself simply overrides all methods of OutputStream with versions that pass all requests to the underlying output stream. Subclasses of FilterOutputStream may further override some of these methods as well as provide additional methods and fields.
　　(简单翻译：FilterOutputStream 类本身只是简单地重写那些将所请求传递给所包含输出流的 OutputStream 的所方法。FilterOutputStream 的子类可进一步地重写这些方法中的一些方法，并且还可以提供一些额外的方法和字段。)

主要字段：
　　protected OutputStream out; //要过滤的基础输出流

构造方法：
　　protected FilterOutputStream(OutputStream out)

主要方法:
　　- void close(): 关闭此输出流并释放与该流有关的系统资源.
　　- void flush(): 刷新此输出流并强制写出所有缓冲的输出字节.
　　- void write(byte[] b): 将b.length个字节从指定的byte数组写入此输出流.
　　- void write(byte[] b,int off,int len): 将byte数组中从off位置开始的len个字节写入此输出流.
　　- void write(int b): 将指定的字节写入此输出流.

源代码如下:

 package java.io;

 /**
  * This class is the superclass of all classes that filter output
  * streams. These streams sit on top of an already existing output
  * stream (the <i>underlying</i> output stream) which it uses as its
  * basic sink of data, but possibly transforming the data along the
  * way or providing additional functionality.
  * <p>
  * The class <code>FilterOutputStream</code> itself simply overrides
  * all methods of <code>OutputStream</code> with versions that pass
  * all requests to the underlying output stream. Subclasses of
  * <code>FilterOutputStream</code> may further override some of these
  * methods as well as provide additional methods and fields.
  *
  * @author  Jonathan Payne
  * @since   JDK1.0
  */
 public
 class FilterOutputStream extends OutputStream {
     /**
      * The underlying output stream to be filtered.
      */
     protected OutputStream out;

     /**
      * Creates an output stream filter built on top of the specified
      * underlying output stream.
      *
      * @param   out   the underlying output stream to be assigned to
      *                the field <tt>this.out</tt> for later use, or
      *                <code>null</code> if this instance is to be
      *                created without an underlying stream.
      */
     public FilterOutputStream(OutputStream out) {
         this.out = out;
     }

     /**
      * Writes the specified <code>byte</code> to this output stream.
      * <p>
      * The <code>write</code> method of <code>FilterOutputStream</code>
      * calls the <code>write</code> method of its underlying output stream,
      * that is, it performs <tt>out.write(b)</tt>.
      * <p>
      * Implements the abstract <tt>write</tt> method of <tt>OutputStream</tt>.
      *
      * @param      b   the <code>byte</code>.
      * @exception  IOException  if an I/O error occurs.
      */
     public void write(int b) throws IOException {
         out.write(b);
     }

     /**
      * Writes <code>b.length</code> bytes to this output stream.
      * <p>
      * The <code>write</code> method of <code>FilterOutputStream</code>
      * calls its <code>write</code> method of three arguments with the
      * arguments <code>b</code>, <code>0</code>, and
      * <code>b.length</code>.
      * <p>
      * Note that this method does not call the one-argument
      * <code>write</code> method of its underlying stream with the single
      * argument <code>b</code>.
      *
      * @param      b   the data to be written.
      * @exception  IOException  if an I/O error occurs.
      * @see        java.io.FilterOutputStream#write(byte[], int, int)
      */
     public void write(byte b[]) throws IOException {
         write(b, 0, b.length);
     }

     /**
      * Writes <code>len</code> bytes from the specified
      * <code>byte</code> array starting at offset <code>off</code> to
      * this output stream.
      * <p>
      * The <code>write</code> method of <code>FilterOutputStream</code>
      * calls the <code>write</code> method of one argument on each
      * <code>byte</code> to output.
      * <p>
      * Note that this method does not call the <code>write</code> method
      * of its underlying input stream with the same arguments. Subclasses
      * of <code>FilterOutputStream</code> should provide a more efficient
      * implementation of this method.
      *
      * @param      b     the data.
      * @param      off   the start offset in the data.
      * @param      len   the number of bytes to write.
      * @exception  IOException  if an I/O error occurs.
      * @see        java.io.FilterOutputStream#write(int)
      */
     public void write(byte b[], int off, int len) throws IOException {
         if ((off | len | (b.length - (len + off)) | (off + len)) < 0)
             throw new IndexOutOfBoundsException();

         for (int i = 0 ; i < len ; i++) {
             write(b[off + i]);
         }
     }

     /**
      * Flushes this output stream and forces any buffered output bytes
      * to be written out to the stream.
      * <p>
      * The <code>flush</code> method of <code>FilterOutputStream</code>
      * calls the <code>flush</code> method of its underlying output stream.
      *
      * @exception  IOException  if an I/O error occurs.
      * @see        java.io.FilterOutputStream#out
      */
     public void flush() throws IOException {
         out.flush();
     }

     /**
      * Closes this output stream and releases any system resources
      * associated with the stream.
      * <p>
      * The <code>close</code> method of <code>FilterOutputStream</code>
      * calls its <code>flush</code> method, and then calls the
      * <code>close</code> method of its underlying output stream.
      *
      * @exception  IOException  if an I/O error occurs.
      * @see        java.io.FilterOutputStream#flush()
      * @see        java.io.FilterOutputStream#out
      */
     public void close() throws IOException {
         try {
           flush();
         } catch (IOException ignored) {
         }
         out.close();
     }
 }