之前的博客总是不是从书中摘录就是从别人的文章中copy点，加点自己轻微的思考，就认为，哇，我又完成一篇。现在要尝试，尽量（有些类啊API啊还是要copy滴）不拷贝任何文章书籍，自己写！
java io在编程中或多或少都遇到，比如读取文件了，比如接口返回的是一堆流信息。那么针对这些流怎么处理呢?这时候就是io上场的时候了。jdk1.4之后加入了新的io叫nio，之前我总是不明白，既然都有了更好的nio了，那为何还不淘汰掉旧io呢？随着书籍和文章的阅读，代码的查阅，对这个问题终于知道了一些。两者的适用场景不同。正如他们的实现方式不一样。一个阻塞，一个非阻塞。
首先呢，我先来记录下近期对io的理解。
网上搜索关于java io总会出现一大堆的类的继承图，看得人昏昏欲睡，一点学习的念头都没，那么多类，那么多继承关系。其实呢这么多累的关系之所以看起来那么复杂，是因为没有明白一个设计模式，叫做装饰器模式。接口InputStream不用说，定义功能的。他下面的继承类中只需要关注这么几个（我个人觉得）ByteArrayInputStream（比特数组流）,StringInputStream（字符串流）,PipedInputStream（管道流）,FileInputStream（文件流），DataInputStream,FilterInputStream（装饰器）。根据他们的名字，差不多就能知道他们的适用场景。具体的每个流的demo要不要贴，后面再思考吧。先讲讲FilterInputStream，FilterInputStream就是传说中的装饰器，下面让我们来个他的源码
/* * Copyright (c) 1994, 2010, Oracle and/or its affiliates. All rights reserved. * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. * * * * * * * * * * * * * * * * * * * * */ 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(); } }
仔细观察，你会发现，这家伙在开始就有这么个定义
/** * The input stream to be filtered. */ protected volatile InputStream in;
这是啥？我的第一感觉就是。这不是策略模式的形状吗？
这里的FilterInputStream其实什么都不做的，真正干活的是传给他的in，而他的子类就是利用那些传进来的in做一些功能处理。比如我最常用的BufferedInputStream就是增加了缓存这一步骤，从而实现效率，不过看BufferedInputStream的源码，你会发现，真正读取流信息的动作还是传给他的in！，下面上BufferedInputStream的代码。
/* * Copyright (c) 1994, 2010, Oracle and/or its affiliates. All rights reserved. * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. * * * * * * * * * * * * * * * * * * * * */ package java.io; import java.util.concurrent.atomic.AtomicReferenceFieldUpdater; /** * A <code>BufferedInputStream</code> adds * functionality to another input stream-namely, * the ability to buffer the input and to * support the <code>mark</code> and <code>reset</code> * methods. When the <code>BufferedInputStream</code> * is created, an internal buffer array is * created. As bytes from the stream are read * or skipped, the internal buffer is refilled * as necessary from the contained input stream, * many bytes at a time. The <code>mark</code> * operation remembers a point in the input * stream and the <code>reset</code> operation * causes all the bytes read since the most * recent <code>mark</code> operation to be * reread before new bytes are taken from * the contained input stream. * * @author Arthur van Hoff * @since JDK1.0 */ public class BufferedInputStream extends FilterInputStream { private static int defaultBufferSize = 8192; /** * The internal buffer array where the data is stored. When necessary, * it may be replaced by another array of * a different size. */ protected volatile byte buf[]; /** * Atomic updater to provide compareAndSet for buf. This is * necessary because closes can be asynchronous. We use nullness * of buf[] as primary indicator that this stream is closed. (The * "in" field is also nulled out on close.) */ private static final AtomicReferenceFieldUpdater<BufferedInputStream, byte[]> bufUpdater = AtomicReferenceFieldUpdater.newUpdater (BufferedInputStream.class, byte[].class, "buf"); /** * The index one greater than the index of the last valid byte in * the buffer. * This value is always * in the range <code>0</code> through <code>buf.length</code>; * elements <code>buf[0]</code> through <code>buf[count-1] * </code>contain buffered input data obtained * from the underlying input stream. */ protected int count; /** * The current position in the buffer. This is the index of the next * character to be read from the <code>buf</code> array. * <p> * This value is always in the range <code>0</code> * through <code>count</code>. If it is less * than <code>count</code>, then <code>buf[pos]</code> * is the next byte to be supplied as input; * if it is equal to <code>count</code>, then * the next <code>read</code> or <code>skip</code> * operation will require more bytes to be * read from the contained input stream. * * @see java.io.BufferedInputStream#buf */ protected int pos; /** * The value of the <code>pos</code> field at the time the last * <code>mark</code> method was called. * <p> * This value is always * in the range <code>-1</code> through <code>pos</code>. * If there is no marked position in the input * stream, this field is <code>-1</code>. If * there is a marked position in the input * stream, then <code>buf[markpos]</code> * is the first byte to be supplied as input * after a <code>reset</code> operation. If * <code>markpos</code> is not <code>-1</code>, * then all bytes from positions <code>buf[markpos]</code> * through <code>buf[pos-1]</code> must remain * in the buffer array (though they may be * moved to another place in the buffer array, * with suitable adjustments to the values * of <code>count</code>, <code>pos</code>, * and <code>markpos</code>); they may not * be discarded unless and until the difference * between <code>pos</code> and <code>markpos</code> * exceeds <code>marklimit</code>. * * @see java.io.BufferedInputStream#mark(int) * @see java.io.BufferedInputStream#pos */ protected int markpos = -1; /** * The maximum read ahead allowed after a call to the * <code>mark</code> method before subsequent calls to the * <code>reset</code> method fail. * Whenever the difference between <code>pos</code> * and <code>markpos</code> exceeds <code>marklimit</code>, * then the mark may be dropped by setting * <code>markpos</code> to <code>-1</code>. * * @see java.io.BufferedInputStream#mark(int) * @see java.io.BufferedInputStream#reset() */ protected int marklimit; /** * Check to make sure that underlying input stream has not been * nulled out due to close; if not return it; */ private InputStream getInIfOpen() throws IOException { InputStream input = in; if (input == null) throw new IOException("Stream closed"); return input; } /** * Check to make sure that buffer has not been nulled out due to * close; if not return it; */ private byte[] getBufIfOpen() throws IOException { byte[] buffer = buf; if (buffer == null) throw new IOException("Stream closed"); return buffer; } /** * Creates a <code>BufferedInputStream</code> * and saves its argument, the input stream * <code>in</code>, for later use. An internal * buffer array is created and stored in <code>buf</code>. * * @param in the underlying input stream. */ public BufferedInputStream(InputStream in) { this(in, defaultBufferSize); } /** * Creates a <code>BufferedInputStream</code> * with the specified buffer size, * and saves its argument, the input stream * <code>in</code>, for later use. An internal * buffer array of length <code>size</code> * is created and stored in <code>buf</code>. * * @param in the underlying input stream. * @param size the buffer size. * @exception IllegalArgumentException if size <= 0. */ public BufferedInputStream(InputStream in, int size) { super(in); if (size <= 0) { throw new IllegalArgumentException("Buffer size <= 0"); } buf = new byte[size]; } /** * Fills the buffer with more data, taking into account * shuffling and other tricks for dealing with marks. * Assumes that it is being called by a synchronized method. * This method also assumes that all data has already been read in, * hence pos > count. */ private void fill() throws IOException { byte[] buffer = getBufIfOpen(); if (markpos < 0) pos = 0; /* no mark: throw away the buffer */ else if (pos >= buffer.length) /* no room left in buffer */ if (markpos > 0) { /* can throw away early part of the buffer */ int sz = pos - markpos; System.arraycopy(buffer, markpos, buffer, 0, sz); pos = sz; markpos = 0; } else if (buffer.length >= marklimit) { markpos = -1; /* buffer got too big, invalidate mark */ pos = 0; /* drop buffer contents */ } else { /* grow buffer */ int nsz = pos * 2; if (nsz > marklimit) nsz = marklimit; byte nbuf[] = new byte[nsz]; System.arraycopy(buffer, 0, nbuf, 0, pos); if (!bufUpdater.compareAndSet(this, buffer, nbuf)) { // Can't replace buf if there was an async close. // Note: This would need to be changed if fill() // is ever made accessible to multiple threads. // But for now, the only way CAS can fail is via close. // assert buf == null; throw new IOException("Stream closed"); } buffer = nbuf; } count = pos; int n = getInIfOpen().read(buffer, pos, buffer.length - pos); if (n > 0) count = n + pos; } /** * See * the general contract of the <code>read</code> * method of <code>InputStream</code>. * * @return the next byte of data, or <code>-1</code> if the end of the * stream is 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.FilterInputStream#in */ public synchronized int read() throws IOException { if (pos >= count) { fill(); if (pos >= count) return -1; } return getBufIfOpen()[pos++] & 0xff; } /** * Read characters into a portion of an array, reading from the underlying * stream at most once if necessary. */ private int read1(byte[] b, int off, int len) throws IOException { int avail = count - pos; if (avail <= 0) { /* If the requested length is at least as large as the buffer, and if there is no mark/reset activity, do not bother to copy the bytes into the local buffer. In this way buffered streams will cascade harmlessly. */ if (len >= getBufIfOpen().length && markpos < 0) { return getInIfOpen().read(b, off, len); } fill(); avail = count - pos; if (avail <= 0) return -1; } int cnt = (avail < len) ? avail : len; System.arraycopy(getBufIfOpen(), pos, b, off, cnt); pos += cnt; return cnt; } /** * Reads bytes from this byte-input stream into the specified byte array, * starting at the given offset. * * <p> This method implements the general contract of the corresponding * <code>{@link InputStream#read(byte[], int, int) read}</code> method of * the <code>{@link InputStream}</code> class. As an additional * convenience, it attempts to read as many bytes as possible by repeatedly * invoking the <code>read</code> method of the underlying stream. This * iterated <code>read</code> continues until one of the following * conditions becomes true: <ul> * * <li> The specified number of bytes have been read, * * <li> The <code>read</code> method of the underlying stream returns * <code>-1</code>, indicating end-of-file, or * * <li> The <code>available</code> method of the underlying stream * returns zero, indicating that further input requests would block. * * </ul> If the first <code>read</code> on the underlying stream returns * <code>-1</code> to indicate end-of-file then this method returns * <code>-1</code>. Otherwise this method returns the number of bytes * actually read. * * <p> Subclasses of this class are encouraged, but not required, to * attempt to read as many bytes as possible in the same fashion. * * @param b destination buffer. * @param off offset at which to start storing bytes. * @param len maximum number of bytes to read. * @return the number of bytes read, or <code>-1</code> 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. */ public synchronized int read(byte b[], int off, int len) throws IOException { getBufIfOpen(); // Check for closed stream if ((off | len | (off + len) | (b.length - (off + len))) < 0) { throw new IndexOutOfBoundsException(); } else if (len == 0) { return 0; } int n = 0; for (;;) { int nread = read1(b, off + n, len - n); if (nread <= 0) return (n == 0) ? nread : n; n += nread; if (n >= len) return n; // if not closed but no bytes available, return InputStream input = in; if (input != null && input.available() <= 0) return n; } } /** * See the general contract of the <code>skip</code> * method of <code>InputStream</code>. * * @exception IOException if the stream does not support seek, * or if this input stream has been closed by * invoking its {@link #close()} method, or an * I/O error occurs. */ public synchronized long skip(long n) throws IOException { getBufIfOpen(); // Check for closed stream if (n <= 0) { return 0; } long avail = count - pos; if (avail <= 0) { // If no mark position set then don't keep in buffer if (markpos <0) return getInIfOpen().skip(n); // Fill in buffer to save bytes for reset fill(); avail = count - pos; if (avail <= 0) return 0; } long skipped = (avail < n) ? avail : n; pos += skipped; return skipped; } /** * 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. * <p> * This method returns the sum of the number of bytes remaining to be read in * the buffer (<code>count - pos</code>) and the result of calling the * {@link java.io.FilterInputStream#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 this input stream has been closed by * invoking its {@link #close()} method, * or an I/O error occurs. */ public synchronized int available() throws IOException { int n = count - pos; int avail = getInIfOpen().available(); return n > (Integer.MAX_VALUE - avail) ? Integer.MAX_VALUE : n + avail; } /** * See the general contract of the <code>mark</code> * method of <code>InputStream</code>. * * @param readlimit the maximum limit of bytes that can be read before * the mark position becomes invalid. * @see java.io.BufferedInputStream#reset() */ public synchronized void mark(int readlimit) { marklimit = readlimit; markpos = pos; } /** * See the general contract of the <code>reset</code> * method of <code>InputStream</code>. * <p> * If <code>markpos</code> is <code>-1</code> * (no mark has been set or the mark has been * invalidated), an <code>IOException</code> * is thrown. Otherwise, <code>pos</code> is * set equal to <code>markpos</code>. * * @exception IOException if this stream has not been marked or, * if the mark has been invalidated, or the stream * has been closed by invoking its {@link #close()} * method, or an I/O error occurs. * @see java.io.BufferedInputStream#mark(int) */ public synchronized void reset() throws IOException { getBufIfOpen(); // Cause exception if closed if (markpos < 0) throw new IOException("Resetting to invalid mark"); pos = markpos; } /** * Tests if this input stream supports the <code>mark</code> * and <code>reset</code> methods. The <code>markSupported</code> * method of <code>BufferedInputStream</code> returns * <code>true</code>. * * @return a <code>boolean</code> indicating if this stream type supports * the <code>mark</code> and <code>reset</code> methods. * @see java.io.InputStream#mark(int) * @see java.io.InputStream#reset() */ public boolean markSupported() { return true; } /** * Closes this input stream and releases any system resources * associated with the stream. * Once the stream has been closed, further read(), 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 void close() throws IOException { byte[] buffer; while ( (buffer = buf) != null) { if (bufUpdater.compareAndSet(this, buffer, null)) { InputStream input = in; in = null; if (input != null) input.close(); return; } // Else retry in case a new buf was CASed in fill() } } }
BufferedInputStream的实现呢是先将流中的一部分数据读取到自己成员变量protected volatile byte buf[];中去，然后再从这个byte数组中去读。而读取到byte数组中的实现就需要依赖传给他的in了，可能是FileInputStream,也可能是ByteArrayInputStream。可以看看fill()方法，就会明白了。
//这行代码就是利用传进来的流进行读取，读取的byte数组存在buffer中 int n = getInIfOpen().read(buffer, pos, buffer.length - pos); 下面是getInIfOpen()方法 private InputStream getInIfOpen() throws IOException { InputStream input = in; if (input == null) throw new IOException("Stream closed"); return input; }
BufferedInputStream还实现了一个功能，就是标记和重读。，对了这里需要强调一点，java的原生态io流是不可以回读的，就像个管道一样，我取出了管道前面的水，那么他就不在管道中了。
具体的用法呢，我还没完全搞明白，先说说我理解明白的部分。。。
首先我们要关注BufferedInputStream定义的这么几个成员变量
//从流中读取的总量 protected int count; //当前读取的位置 protected int pos; //标记的位置 protected int markpos = -1; //这个我暂时没想好怎么解释他，不好用官方的解释，和功能不对应 protected int marklimit;
以数组{1,2,3,4,5,6}为例
byte[] b = {1,2,3,4,5,6}; InputStream out = new BufferedInputStream(new ByteArrayInputStream(b),3);//读取b数组的信息，其中设置BufferedInputStream的缓冲buffer为3 System.out.println(out.read());//调用read之后pos由0变为1，count由0变为3 out.mark(1);//此刻marklimit = readlimit;markpos = pos = 1; System.out.println(out.read()); System.out.println(out.read()); out.reset();//pos = markpos = 1; System.out.println(out.read());//所以又从数组1的位置开始读了， System.out.println(out.read()); System.out.println(out.read()); System.out.println(out.read()); out.close();
以上只用了常规的写法，关于marklimit 字段的讨论放到以后去，今天就写到这