输出流需要关闭，因此需要Closeable接口
缓冲流需要刷新，因此需要Flushable接口

输入流的架构

输出流的基类

UML概述

重要方法

输入流将具体如何读，如何关闭留给了子类

read

从输入流中读取下一个字节的数据。值字节以 0 到 255 范围内的 int 形式返回（即int只有低八位有效，和写操作相对应）。如果由于已到达流的末尾而没有可用字节，则返回值 -1。此方法会阻塞，直到输入数据可用、检测到流结束或抛出异常为止。子类必须提供此方法的实现。

    /**
     * Reads the next byte of data from the 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> A subclass must provide an implementation of this method.
     *
     * @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 abstract int read() throws IOException;

将输入流中最多 len 个数据字节读入 byte 数组。尝试读取 len 个字节，但读取的字节也可能小于该值。以整数形式返回实际读取的字节数。在输入数据可用、检测到流末尾或者抛出异常前，此方法一直阻塞。如果 len 为 0，则不读取任何字节并返回 0；否则，尝试读取至少一个字节。如果因为流位于文件末尾而没有可用的字节，则返回值 -1；否则，至少读取一个字节并将其存储在 b 中。

将读取的第一个字节存储在元素 b[off] 中，下一个存储在 b[off+1] 中，依次类推。读取的字节数最多等于 len。设 k 为实际读取的字节数；这些字节将存储在 b[off] 到 b[off+k-1] 的元素中，不影响 b[off+k] 到 b[off+len-1] 的元素。在任何情况下，b[0] 到 b[off] 的元素以及 b[off+len] 到 b[b.length-1] 的元素都不会受到影响。

类 InputStream 的 read(b, off, len) 方法重复调用方法 read()。如果第一次这样的调用导致 IOException，则从对 read(b, off, len) 方法的调用中返回该异常。如果对 read() 的任何后续调用导致 IOException，则捕获该异常并将其视为到达文件末尾；到达该点时读取的字节存储在 b 中，并返回发生异常之前读取的字节数。千言万语一句话，就是中途抛异常不影响已经读到的数据。

在已读取输入数据 len 的请求数量、检测到文件结束标记、抛出异常前，此方法的默认实现将一直阻塞。建议子类提供此方法更为有效的实现。

    /**
     * Reads up to <code>len</code> bytes of data from the input stream into
     * an array of bytes.  An attempt is made to read as many as
     * <code>len</code> bytes, but a smaller number may be read.
     * The number of bytes actually read is returned as an integer.
     *
     * <p> This method blocks until input data is available, end of file is
     * detected, or an exception is thrown.
     *
     * <p> If <code>len</code> is zero, then no bytes are read and
     * <code>0</code> is returned; otherwise, there is an attempt to read at
     * least one byte. If no byte is available because the stream is at end of
     * file, the value <code>-1</code> is returned; otherwise, at least one
     * byte is read and stored into <code>b</code>.
     *
     * <p> The first byte read is stored into element <code>b[off]</code>, the
     * next one into <code>b[off+1]</code>, and so on. The number of bytes read
     * is, at most, equal to <code>len</code>. Let <i>k</i> be the number of
     * bytes actually read; these bytes will be stored in elements
     * <code>b[off]</code> through <code>b[off+</code><i>k</i><code>-1]</code>,
     * leaving elements <code>b[off+</code><i>k</i><code>]</code> through
     * <code>b[off+len-1]</code> unaffected.
     *
     * <p> In every case, elements <code>b[0]</code> through
     * <code>b[off]</code> and elements <code>b[off+len]</code> through
     * <code>b[b.length-1]</code> are unaffected.
     *
     * <p> The <code>read(b,</code> <code>off,</code> <code>len)</code> method
     * for class <code>InputStream</code> simply calls the method
     * <code>read()</code> repeatedly. If the first such call results in an
     * <code>IOException</code>, that exception is returned from the call to
     * the <code>read(b,</code> <code>off,</code> <code>len)</code> method.  If
     * any subsequent call to <code>read()</code> results in a
     * <code>IOException</code>, the exception is caught and treated as if it
     * were end of file; the bytes read up to that point are stored into
     * <code>b</code> and the number of bytes read before the exception
     * occurred is returned. The default implementation of this method blocks
     * until the requested amount of input data <code>len</code> has been read,
     * end of file is detected, or an exception is thrown. Subclasses are encouraged
     * to provide a more efficient implementation of this method.
     *
     * @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 to 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 the first byte cannot be read for any reason
     * other than end of file, or if the input stream has been closed, or if
     * some other I/O error occurs.
     * @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>
     * @see        java.io.InputStream#read()
     */
    public int read(byte b[], int off, int len) throws IOException {
        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 c = read();
        if (c == -1) {
            return -1;
        }
        b[off] = (byte)c;
        int i = 1;
        try {
            for (; i < len ; i++) {
                c = read();
                if (c == -1) {
                    break;
                }
                b[off + i] = (byte)c;
            }
        } catch (IOException ee) {
        }
        return i;
    }

skip

MAX_SKIP_BUFFER_SIZE 用于确定跳过时要使用的最大缓冲区大小。skip本身也是读操作，只不过不返回读到的数据，读到的数据也是通过读缓冲区来保存，只不过这里叫skipBuffer

跳过和丢弃此输入流中数据的 n 个字节。出于各种原因， skip 方法结束时跳过的字节数可能小于该数，也可能为 0。导致这种情况的原因很多，跳过 n 个字节之前已到达文件末尾只是其中一种可能。返回跳过的实际字节数。如果 n 为负，则不跳过任何字节。此类的 skip 方法创建一个 byte 数组，然后重复将字节读入其中，直到读够 n 个字节或已到达流末尾为止。建议子类提供此方法更为有效的实现。例如，可依赖搜索能力的实现。

    // MAX_SKIP_BUFFER_SIZE is used to determine the maximum buffer size to
    // use when skipping.
    private static final int MAX_SKIP_BUFFER_SIZE = 2048;
    /**
     * Skips over and discards <code>n</code> bytes of data from this 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>.
     * This may result from any of a number of conditions; reaching end of file
     * before <code>n</code> bytes have been skipped is only one possibility.
     * The actual number of bytes skipped is returned. If {@code n} is
     * negative, the {@code skip} method for class {@code InputStream} always
     * returns 0, and no bytes are skipped. Subclasses may handle the negative
     * value differently.
     *
     * <p> The <code>skip</code> method of this class creates a
     * byte array and then repeatedly reads into it until <code>n</code> bytes
     * have been read or the end of the stream has been reached. Subclasses are
     * encouraged to provide a more efficient implementation of this method.
     * For instance, the implementation may depend on the ability to seek.
     *
     * @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 {
        long remaining = n;
        int nr;
        if (n <= 0) {
            return 0;
        }
        int size = (int)Math.min(MAX_SKIP_BUFFER_SIZE, remaining);
        byte[] skipBuffer = new byte[size];
        while (remaining > 0) {
            nr = read(skipBuffer, 0, (int)Math.min(size, remaining));
            if (nr < 0) {
                break;
            }
            remaining -= nr;
        }
        return n - remaining;
    }

available

返回此输入流下一个方法调用可以不受阻塞地从此输入流读取（或跳过）的估计字节数。下一个调用可能是同一个线程，也可能是另一个线程。一次读取或跳过此估计数个字节不会受阻塞，但读取或跳过的字节数可能小于该数。注意，有些 InputStream 的实现将返回流中的字节总数，但也有很多实现不会这样做。试图使用此方法的返回值分配缓冲区，以保存此流所有数据的做法是不正确的。

如果已经调用 close() 方法关闭了此输入流，那么此方法的子类实现可以选择抛出 IOException。类 InputStream 的 available 方法总是返回 0。此方法应该由子类重写。

    /**
     * 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> Note that while some implementations of {@code InputStream} will return
     * the total number of bytes in the stream, many will not.  It is
     * never correct to use the return value of this method to allocate
     * a buffer intended to hold all data in this stream.
     *
     * <p> A subclass' implementation of this method may choose to throw an
     * {@link IOException} if this input stream has been closed by
     * invoking the {@link #close()} method.
     *
     * <p> The {@code available} method for class {@code InputStream} always
     * returns {@code 0}.
     *
     * <p> This method should be overridden by subclasses.
     *
     * @return     an estimate of the number of bytes that can be read (or skipped
     *             over) from this input stream without blocking or {@code 0} when
     *             it reaches the end of the input stream.
     * @exception  IOException if an I/O error occurs.
     */
    public int available() throws IOException {
        return 0;
    }

mark

很多读数据的操作都会提供类似mark的方法，用来作为游标，在数据或记录中游走。并且会匹配reset的设计。

    在此输入流中标记当前的位置。对 reset 方法的后续调用会在最后标记的位置重新定位此流，**以便后续回到该位置重新读取相同的字节**。readlimit 参数告知此输入流在标记位置失效之前允许读取的字节数。mark 的常规协定是：如果方法 markSupported 返回 true，那么输入流总是在调用 mark 之后记录所有读取的字节，并时刻准备在调用方法 reset 时（无论何时），再次提供这些相同的字节。但是，如果在调用 reset 之前可以从流中读取多于 readlimit 的字节，则不需要该流记录任何数据。**标记已关闭的流对其无效。InputStream 的 mark 方法不执行任何操作。**

    /**
     * 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> arguments tells this input stream to
     * allow that many bytes to be read before the mark position gets
     * invalidated.
     *
     * <p> The general contract of <code>mark</code> is that, if the method
     * <code>markSupported</code> returns <code>true</code>, the stream somehow
     * remembers all the bytes read after the call to <code>mark</code> and
     * stands ready to supply those same bytes again if and whenever the method
     * <code>reset</code> is called.  However, the stream is not required to
     * remember any data at all if more than <code>readlimit</code> bytes are
     * read from the stream before <code>reset</code> is called.
     *
     * <p> Marking a closed stream should not have any effect on the stream.
     *
     * <p> The <code>mark</code> method of <code>InputStream</code> 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) {}

markSupported

并非所有流都支持设置标记。测试此输入流是否支持 mark 和 reset 方法。是否支持 mark 和 reset 是特定输入流实例的不变属性。 InputStream 的 markSupported 方法返回 false。

    /**
     * Tests if this input stream supports the <code>mark</code> and
     * <code>reset</code> methods. Whether or not <code>mark</code> and
     * <code>reset</code> are supported is an invariant property of a
     * particular input stream instance. The <code>markSupported</code> method
     * of <code>InputStream</code> returns <code>false</code>.
     *
     * @return  <code>true</code> if this stream instance supports the mark
     *          and reset methods; <code>false</code> otherwise.
     * @see     java.io.InputStream#mark(int)
     * @see     java.io.InputStream#reset()
     */
    public boolean markSupported() {
        return false;
    }

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。reset 的常规协定是：

如果方法 markSupported 返回 true，那么：
- 如果创建流以后未调用方法 mark，或最后调用 mark 以后从该流读取的字节数大于最后调用 mark 时的参数，则可能抛出 IOException。
- 如果未抛出这样的 IOException，则将该流重新设置为这种状态：最近一次调用 mark 以后（如果未调用过 mark，则从文件开头开始）读取的所有字节将重新提供给 read 方法的后续调用者，后跟任何从调用 reset 时起将作为下一输入数据的字节。
如果方法 markSupported 返回 false，那么：
- 对 reset 的调用可能抛出 IOException。
- 如果未抛出 IOException，则将该流重新设置为一种固定状态，该状态取决于输入流的特定类型及其创建方式。提供给 read 方法后续调用者的字节取决于特定类型的输入流。

除了抛出 IOException 之外，类 InputStream 的方法 reset 不执行任何操作。

    /**
     * Repositions this stream to the position at the time the
     * <code>mark</code> method was last called on this input stream.
     *
     * <p> The general contract of <code>reset</code> is:
     *
     * <ul>
     * <li> If the method <code>markSupported</code> returns
     * <code>true</code>, then:
     *
     *     <ul><li> If the method <code>mark</code> has not been called since
     *     the stream was created, or the number of bytes read from the stream
     *     since <code>mark</code> was last called is larger than the argument
     *     to <code>mark</code> at that last call, then an
     *     <code>IOException</code> might be thrown.
     *
     *     <li> If such an <code>IOException</code> is not thrown, then the
     *     stream is reset to a state such that all the bytes read since the
     *     most recent call to <code>mark</code> (or since the start of the
     *     file, if <code>mark</code> has not been called) will be resupplied
     *     to subsequent callers of the <code>read</code> method, followed by
     *     any bytes that otherwise would have been the next input data as of
     *     the time of the call to <code>reset</code>. </ul>
     *
     * <li> If the method <code>markSupported</code> returns
     * <code>false</code>, then:
     *
     *     <ul><li> The call to <code>reset</code> may throw an
     *     <code>IOException</code>.
     *
     *     <li> If an <code>IOException</code> is not thrown, then the stream
     *     is reset to a fixed state that depends on the particular type of the
     *     input stream and how it was created. The bytes that will be supplied
     *     to subsequent callers of the <code>read</code> method depend on the
     *     particular type of the input stream. </ul></ul>
     *
     * <p>The method <code>reset</code> for class <code>InputStream</code>
     * does nothing except throw an <code>IOException</code>.
     *
     * @exception  IOException  if this stream has not been marked or if the
     *               mark has been invalidated.
     * @see     java.io.InputStream#mark(int)
     * @see     java.io.IOException
     */
    public synchronized void reset() throws IOException {
        throw new IOException("mark/reset not supported");
    }

close

关闭此输入流并释放与该流关联的所有系统资源。InputStream 的 close 方法不执行任何操作。

    /**
     * Closes this input stream and releases any system resources associated
     * with the stream.
     *
     * <p> The <code>close</code> method of <code>InputStream</code> does
     * nothing.
     *
     * @exception  IOException  if an I/O error occurs.
     */
    public void close() throws IOException {}

输入流的子类

ByteArrayInputStream

这个类是一个字节数组输入流，数据是被读入内存中的字节数组。ByteArrayInputStream 包含一个内部缓冲区，该缓冲区包含从流中读取的字节。内部计数器跟踪 read 方法要提供的下一个字节。关闭 ByteArrayInputStream 无效。此类中的方法在关闭此流后仍可被调用，而不会产生任何 IOException。

UML概述

重要方法

构造函数

buf[] - 由流的创建者提供的字节数组。元素 buf[0] 到 buf[count-1] 是唯一可以从流中读取的字节；元素 buf[pos] 是要读取的下一个字节。
pos - 要从输入流缓冲区读取的下一个字符的索引。该值应始终为非负且不大于计数值。要从输入流缓冲区读取的下一个字节将是 buf[pos]
mark - 流中当前标记的位置。 ByteArrayInputStream 对象在构造时默认标记在位置零。它们可以通过 mark() 方法在缓冲区内的另一个位置进行标记。当前缓冲区位置由 reset() 方法设置到这一点。如果未设置标记，则标记的值是传递给构造函数的偏移量（如果未提供偏移量，则为 0 )
count - 比输入流缓冲区中最后一个有效字符大一的索引。该值应始终为非负且不大于 buf 的长度。它比 buf 中可以从输入流缓冲区读取的最后一个字节的位置大 1。

public
class ByteArrayInputStream extends InputStream {
    /**
     * An array of bytes that was provided
     * by the creator of the stream. Elements <code>buf[0]</code>
     * through <code>buf[count-1]</code> are the
     * only bytes that can ever be read from the
     * stream;  element <code>buf[pos]</code> is
     * the next byte to be read.
     */
    protected byte buf[];
    /**
     * The index of the next character to read from the input stream buffer.
     * This value should always be nonnegative
     * and not larger than the value of <code>count</code>.
     * The next byte to be read from the input stream buffer
     * will be <code>buf[pos]</code>.
     */
    protected int pos;
    /**
     * The currently marked position in the stream.
     * ByteArrayInputStream objects are marked at position zero by
     * default when constructed.  They may be marked at another
     * position within the buffer by the <code>mark()</code> method.
     * The current buffer position is set to this point by the
     * <code>reset()</code> method.
     * <p>
     * If no mark has been set, then the value of mark is the offset
     * passed to the constructor (or 0 if the offset was not supplied).
     *
     * @since   JDK1.1
     */
    protected int mark = 0;
    /**
     * The index one greater than the last valid character in the input
     * stream buffer.
     * This value should always be nonnegative
     * and not larger than the length of <code>buf</code>.
     * It  is one greater than the position of
     * the last byte within <code>buf</code> that
     * can ever be read  from the input stream buffer.
     */
    protected int count;
    /**
     * Creates <code>ByteArrayInputStream</code>
     * that uses <code>buf</code> as its
     * buffer array. The initial value of <code>pos</code>
     * is <code>offset</code> and the initial value
     * of <code>count</code> is the minimum of <code>offset+length</code>
     * and <code>buf.length</code>.
     * The buffer array is not copied. The buffer's mark is
     * set to the specified offset.
     *
     * @param   buf      the input buffer.
     * @param   offset   the offset in the buffer of the first byte to read.
     * @param   length   the maximum number of bytes to read from the buffer.
     */
    public ByteArrayInputStream(byte buf[], int offset, int length) {
        this.buf = buf;
        this.pos = offset;
        this.count = Math.min(offset + length, buf.length);
        this.mark = offset;
    }

read()

从此输入流中读取下一个数据字节。返回一个 0 到 255 范围内的 int 字节值。如果因为到达流末尾而没有可用的字节，则返回值 -1。此 read 方法不会阻塞。

    /**
     * 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.
     * <p>
     * This <code>read</code> method
     * cannot block.
     *
     * @return  the next byte of data, or <code>-1</code> if the end of the
     *          stream has been reached.
     */
    public synchronized int read() {
        return (pos < count) ? (buf[pos++] & 0xff) : -1;
    }

读取一个字节的时候有些特别，需要将字节转成一个int值返回，并且是一个 0 到 255 范围内的无符号的数值。因此通过位运算 buf[pos++] & 0xff 进行无符号int的转换。
在byte 到 int 的无符号转换中，int 的高 24 位必须为零，低 8 位继续等于 byte 参数的位。

public class Main {
    public static void main(String[] args) {
        byte b1 = (byte) 1;
        int i1 = (int) b1;
        System.out.println(i1); // 1
        System.out.println(Integer.toBinaryString(b1)); // 1
        byte b2 = (byte) 127;
        int i2 = (int) b2;
        System.out.println(i2); // 127
        System.out.println(Integer.toBinaryString(b2)); // 1111111
        byte b3 = (byte) 128;
        int i3 = (int) b3;
        System.out.println(i3); // -128
        System.out.println(Integer.toBinaryString(i3)); // 11111111111111111111111110000000
        // Integer.MAX_VALUE = 2147483647 = 2^31
        byte b4 = (byte) 128;
        int i4 = (int) (b4 & 0xff);
        System.out.println(i4); // 128
    }
}

read(byte b[], int off, int len)

将最多 len 个数据字节从此输入流读入 byte 数组。如果 pos 等于 count，则返回 -1 指示文件结束。否则，读取的字节数 k 等于 len 和 count-pos 中的较小者。如果 k 是正数，则以 System.arraycopy 执行的方式将 buf[pos] 到 buf[pos+k-1] 的字节复制到 b[off] 到 b[off+k-1] 中。将值 k 与 pos 相加并返回 k。此 read 方法不会阻塞。

    /**
     * Reads up to <code>len</code> bytes of data into an array of bytes
     * from this input stream.
     * If <code>pos</code> equals <code>count</code>,
     * then <code>-1</code> is returned to indicate
     * end of file. Otherwise, the  number <code>k</code>
     * of bytes read is equal to the smaller of
     * <code>len</code> and <code>count-pos</code>.
     * If <code>k</code> is positive, then bytes
     * <code>buf[pos]</code> through <code>buf[pos+k-1]</code>
     * are copied into <code>b[off]</code>  through
     * <code>b[off+k-1]</code> in the manner performed
     * by <code>System.arraycopy</code>. The
     * value <code>k</code> is added into <code>pos</code>
     * and <code>k</code> is returned.
     * <p>
     * This <code>read</code> method cannot block.
     *
     * @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>
     */
    public synchronized int read(byte b[], int off, int len) {
        if (b == null) {
            throw new NullPointerException();
        } else if (off < 0 || len < 0 || len > b.length - off) {
            throw new IndexOutOfBoundsException();
        }
        if (pos >= count) {
            return -1;
        }
        int avail = count - pos;
        if (len > avail) {
            len = avail;
        }
        if (len <= 0) {
            return 0;
        }
        System.arraycopy(buf, pos, b, off, len);
        pos += len;
        return len;
    }

skip

从此输入流中跳过 n 个输入字节。如果已到达输入流末尾，则可能会跳过较少的字节。实际跳过的字节数 k 等于 n 和 count-pos 中的较小者。将值 k 与 pos 相加并返回 k。

千言万语一句话，向后跳过n个位置，但是不要越界，如果会越界就跳到末尾就可以了。

    /**
     * Skips <code>n</code> bytes of input from this input stream. Fewer
     * bytes might be skipped if the end of the input stream is reached.
     * The actual number <code>k</code>
     * of bytes to be skipped is equal to the smaller
     * of <code>n</code> and  <code>count-pos</code>.
     * The value <code>k</code> is added into <code>pos</code>
     * and <code>k</code> is returned.
     *
     * @param   n   the number of bytes to be skipped.
     * @return  the actual number of bytes skipped.
     */
    public synchronized long skip(long n) {
        long k = count - pos;
        if (n < k) {
            k = n < 0 ? 0 : n;
        }
        pos += k;
        return k;
    }

available

返回可从此输入流读取（或跳过）的剩余字节数。返回值是 count - pos，它是要从输入缓冲区中读取的剩余字节数。

    /**
     * Returns the number of remaining bytes that can be read (or skipped over)
     * from this input stream.
     * <p>
     * The value returned is <code>count&nbsp;- pos</code>,
     * which is the number of bytes remaining to be read from the input buffer.
     *
     * @return  the number of remaining bytes that can be read (or skipped
     *          over) from this input stream without blocking.
     */
    public synchronized int available() {
        return count - pos;
    }

markSupported

测试此 InputStream 是否支持 mark/reset。 ByteArrayInputStream 的 markSupported 方法始终返回 true。

    /**
     * Tests if this <code>InputStream</code> supports mark/reset. The
     * <code>markSupported</code> method of <code>ByteArrayInputStream</code>
     * always returns <code>true</code>.
     *
     * @since   JDK1.1
     */
    public boolean markSupported() {
        return true;
    }

mark

设置流中的当前标记位置。构造时默认将 ByteArrayInputStream 对象标记在位置零处。通过此方法可将其标记在缓冲区内的另一个位置处。如果尚未设置标记，则标记值是传递给构造方法的偏移量（如果未提供偏移量，则标记值为 0）。注：readAheadLimit 对于此类没有意义。

    /**
     * Set the current marked position in the stream.
     * ByteArrayInputStream objects are marked at position zero by
     * default when constructed.  They may be marked at another
     * position within the buffer by this method.
     * <p>
     * If no mark has been set, then the value of the mark is the
     * offset passed to the constructor (or 0 if the offset was not
     * supplied).
     *
     * <p> Note: The <code>readAheadLimit</code> for this class
     *  has no meaning.
     *
     * @since   JDK1.1
     */
    public void mark(int readAheadLimit) {
        mark = pos;
    }

reset

将缓冲区的位置重置为标记位置。除非已标记了另一个位置，或者在构造方法中指定了一个偏移量，否则该标记位置是 0。

    /**
     * Resets the buffer to the marked position.  The marked position
     * is 0 unless another position was marked or an offset was specified
     * in the constructor.
     */
    public synchronized void reset() {
        pos = mark;
    }

FilterInputStream

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。

千言万语一句话就是用来对数据进行过滤拦截的用的，在数据的处理过程中对数据做一些额外的操作，数据经过一层一层的过滤器，被逐层处理，比如被缓冲，被压缩，被加密，或者什么都不做只是打印一下log，这是一种很常见的设计方式，比如MINA中的Filter等等。

UML概述

重要方法

构造函数

用一个外部的输入流来包装或者说是装饰一个内部输入流。大家可能知道IO是基于装饰器模式。而装饰器模式和代理模式看上去很相似，一般来说代理模式中代理类和被代理类之间的角色定义很明确。而装饰器模式中装饰器的装饰顺序并没有明确或固定的叠加顺序，非常灵活，甚至可以重复叠加，比如一个计算折扣的装饰器，被连续嵌套两次，就相当于折上折。当然也有一些装饰器需要遵循顺序，比如先缓冲区装饰器需要先写入缓冲区，再写入底层文件系统。

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;
    }

read

从此输入流中读取下一个数据字节。返回一个 0 到 255 范围内的 int 字节值。如果因为已经到达流末尾而没有字节可用，则返回 -1。在输入数据可用、检测到流末尾或抛出异常之前，此方法将一直阻塞。此方法只执行 in.read() 并返回结果。

    /**
     * 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();
    }

read(byte b[], int off, int len)

从此输入流中将 len 个字节的数据读入一个 byte 数组中。如果 len 不为 0，则在输入可用前，此方法将阻塞；否则，不读取任何字节并且返回 0。此方法只执行 in.read(b, off, len) 并返回结果。

    /**
     * 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);
    }

skip

跳过和丢弃此输入流中数据的 n 个字节。出于各种原因， skip 方法结束时跳过的字节数可能小于该数，也可能为 0。导致这种情况的原因很多，跳过 n 个字节之前已到达文件末尾只是其中一种可能。返回跳过的实际字节数。如果 n 为负，则不跳过任何字节。
此类的 skip 方法创建一个 byte 数组，然后重复将字节读入其中，直到读够 n 个字节或已到达流末尾为止。建议子类提供此方法更为有效的实现。例如，可依赖搜索能力的实现。
此方法只执行 in.skip(n)。

    /**
     * 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);
    }

available

返回下一次对此输入流调用的方法可以不受阻塞地从此输入流读取（或跳过）的估计剩余字节数。下一个调用者可能是同一个线程，也可能是另一个线程。一次读取或跳过此数量个字节不会发生阻塞，但读取或跳过的字节可能小于该数。此方法返回 [in](https://tool.oschina.net/uploads/apidocs/jdk-zh/java/io/FilterInputStream.html#in).available() 的结果。

    /**
     * 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();
    }

close

关闭此输入流并释放与此流关联的所有系统资源。此方法只执行 in.close()。

    /**
     * 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();
    }

mark

在输入流中的当前位置上作标记。 reset 方法的后续调用将此流重新定位在最后标记的位置上，以便后续读取操作重新读取相同的字节。readlimit 参数告知此输入流在标记位置无效之前允许读取的字节数。此方法只执行 in.mark(readlimit)。

    /**
     * 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);
    }

reset

将此流重新定位到对此输入流最后调用 mark 方法时的位置。此方法只执行 in.reset()。在需要提前读取一小部分数据以查看流中有什么的情况下，可以使用流的标记。通过调用通用解析器常常最容易做到这一点。如果流属于通过解析处理的类型，那么解析起来就很容易。如果流不属于那种类型，那么解析器应该在解析失败时抛出一个异常。如果这发生在 readlimit 个字节内，那么它允许外部代码重置流，并尝试另一种解析器。

    /**
     * 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();
    }

markSupported

测试此输入流是否支持 mark 和 reset 方法。此方法只执行 in.markSupported()。

    /**
     * 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();
    }

BufferedInputStream

BufferedInputStream 为另一个输入流添加一些功能，即缓冲输入以及支持 mark 和 reset 方法的能力。在创建 BufferedInputStream 时，会创建一个内部缓冲区数组。在读取或跳过流中的字节时，可根据需要从包含的输入流再次填充该内部缓冲区，一次填充多个字节。mark 操作记录输入流中的某个点，reset 操作使得在从包含的输入流中获取新字节之前，再次读取自最后一次 mark 操作后读取的所有字节。

UML概述

重要方法

构造函数

创建具有指定缓冲区大小的 BufferedInputStream 并保存其参数，即输入流 in，以便将来使用。创建一个长度为 size 的内部缓冲区数组并将其存储在 buf 中。

public
class BufferedInputStream extends FilterInputStream {
    private static int DEFAULT_BUFFER_SIZE = 8192;
    /**
     * The maximum size of array to allocate.
     * Some VMs reserve some header words in an array.
     * Attempts to allocate larger arrays may result in
     * OutOfMemoryError: Requested array size exceeds VM limit
     */
    private static int MAX_BUFFER_SIZE = Integer.MAX_VALUE - 8;
    /**
     * 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 {@code size <= 0}.
     */
    public BufferedInputStream(InputStream in, int size) {
        super(in);
        if (size <= 0) {
            throw new IllegalArgumentException("Buffer size <= 0");
        }
        buf = new byte[size];
    }

read

返回缓冲区中的下一个数据字节，如果到达流末尾，则返回 -1。

    /**
     * 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;
    }

read1(byte[] b, int off, int len)

从此字节输入流中给定偏移量处开始将各字节读取到指定的 byte 数组中。
此方法实现了 [InputStream](https://tool.oschina.net/uploads/apidocs/jdk-zh/java/io/InputStream.html) 类相应 [read](https://tool.oschina.net/uploads/apidocs/jdk-zh/java/io/InputStream.html#read(byte[],%20int,%20int)) 方法的常规协定。另一个便捷之处在于，它将通过重复地调用底层流的 read 方法，尝试读取尽可能多的字节。这种迭代的 read 会一直继续下去，直到满足以下条件之一：

已经读取了指定的字节数，
底层流的 read 方法返回 -1，指示文件末尾（end-of-file），或者
底层流的 available 方法返回 0，指示将阻塞后续的输入请求。

如果第一次对底层流调用 read 返回 -1（指示文件末尾），则此方法返回 -1。否则此方法返回实际读取的字节数。鼓励（但不是必须）此类的各个子类以相同的方式尝试读取尽可能多的字节。

    /**
     * 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;
        }
    }

available

返回可以从此输入流读取（或跳过）、且不受此输入流接下来的方法调用阻塞的估计字节数。接下来的调用可能是同一个线程，也可能是不同的线程。一次读取或跳过这么多字节将不会受阻塞，但可以读取或跳过数量更少的字节。此方法返回缓冲区中剩余的待读取字节数 (count - pos) 与调用 in.available() 的结果之和。

千言万语一句话就是 available 等于当前输入流自身缓冲区的available（count - pos） + 底层输入流的available 就是整体的available。包装类和被包装类各管各的available，重量就是它们之和。

    /**
     * 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&nbsp;- 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;
    }

mark

设置一个mark，rest的时候可以回到这里。readlimit - 在mark位置变为无效之前可以读取字节的最大限制。

    /**
     * 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;
    }

reset

如果 markpos 为 -1（尚未设置标记，或者标记已失效），则抛出 IOException。否则将 pos 设置为与 markpos 相等。

    /**
     * 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;
    }

markSupported

测试此输入流是否支持 mark 和 reset 方法。 BufferedInputStream 的 markSupported 方法返回 true。

    /**
     * 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;
    }

close

关闭此输入流并释放与该流关联的所有系统资源。关闭了该流之后，后续的 read()、available()、reset() 或 skip() 调用都将抛出 IOException。关闭之前已关闭的流不会产生任何效果。

    /**
     * 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()
        }
    }

BIO的基本架构-InputStream

输入流的架构

输出流的基类

UML概述

重要方法

read

skip

available

mark

markSupported

reset

close

输入流的子类

ByteArrayInputStream

UML概述

重要方法

构造函数

read()

read(byte b[], int off, int len)

skip

available

markSupported

mark

reset

FilterInputStream

UML概述

重要方法

构造函数

read

read(byte b[], int off, int len)

skip

available

close

mark

reset

markSupported

BufferedInputStream

UML概述

重要方法

构造函数

read

read1(byte[] b, int off, int len)

available

mark

reset

markSupported

close