以前看java书，都将IO作为一个大的章节甚至模块来编写，可见IO在java语言中的重要性。
java的流按功能和处理的目标数据不同可以分为字节流和字符流。字符流处理的基本数据单元是字符；字节流处理的基本数据单元是字节。类关系结构图如下：
图片转自：https://blog.csdn.net/weixin_44411569/article/details/88788085
IO本身就是输入输出的意思，从上图可以看出，无论字符还是字节流，都是分为输入和输出两大块。
并且不管是输入还是输出流、字符流还是字节流，都实现了接口 java.io.Closeable ，该接口只有一个 close 方法。官方文档是这样说的：A {@code Closeable} is a source or destination of data that can be closed.The close method is invoked to release resources that the object is holding (such as open files).意思是说：
一个Closeable对象是一个可以关闭的数据源或者数据目标地，可以通过调用close方法来释放该对象所占用的资源（比如打开的文件）。显然，所谓数据源就是对应输入流，而数据目标代表输出流。
值得注意的是，从jdk1.7开始，Closeable 接口新增了一个 java.lang.AutoCloseable 父类接口，该接口也是只有一个 close 方法。看接口命名就知道，这个 close 方法是可以自动调用的。官方文档：The {@link #close()} method of an {@code AutoCloseable} object is called automatically when exiting a {@code try}-with-resources block for which the object has been declared in the resource specification header.意思是在退出 try 代码块的时候自动调用 AutoCloseable.close 方法，这些AutoCloseable对象在try代码块中声明。也就是说以后在使用 Closeable 对象的时候，不用再使用 finally 调用 close 方法了，只需要把代码写进try代码块。
1. 字符输入流：
字符输入流的基类是 java.io.Reader，这是一个抽象类，除了 Closeable 接口还继承了 java.lang.Readable，该接口只有一个 public int read(java.nio.CharBuffer cb)throws IOException; 方法，Reader实现了该方法。Reader有4个read重载方法：
public int read(java.nio.CharBuffer target) throws IOException { int len = target.remaining(); char[] cbuf = new char[len]; int n = read(cbuf, 0, len); if (n > 0) target.put(cbuf, 0, n); return n; } public int read() throws IOException { char cb[] = new char[1]; if (read(cb, 0, 1) == -1) return -1; else return cb[0]; } public int read(char cbuf[]) throws IOException { return read(cbuf, 0, cbuf.length); } abstract public int read(char cbuf[], int off, int len) throws IOException;
可以看出，第4个为核心方法，其它3个都调用它，它也是抽象方法，需要子类实现。不过几种已知子类都做了各自的重写。带参数的read方法作用是把内容读入参数指定的字符数组或者CharBuffer对象。
2. 字符输出流：
字符输出流的基类是 java.io.Writer，也是抽象类，除了 Closeable 接口还继承了 java.lang.Appendable 和 java.io.Flushable，分别提供 append() 和 flush() 方法，其中 append 方法是重载的。
2.1. 既然集成了 Flushable，当然是需要设计缓存的，Writer 中预置了 private char[] writeBuffer; 来作为缓存空间，并且设置了 private static final int WRITE_BUFFER_SIZE = 1024; 作为默认缓存大小。
2.2. Writer重载了5个 write() 方法分别针对单字符、字符数组或字符串的参数场景，以及3个 append() 方法：
/** * Writes a single character. The character to be written is contained in * the 16 low-order bits of the given integer value; the 16 high-order bits * are ignored. * * <p> Subclasses that intend to support efficient single-character output * should override this method. * * @param c * int specifying a character to be written * * @throws IOException * If an I/O error occurs */ public void write(int c) throws IOException { synchronized (lock) { if (writeBuffer == null){ writeBuffer = new char[WRITE_BUFFER_SIZE]; } writeBuffer[0] = (char) c; write(writeBuffer, 0, 1); } } /** * Writes an array of characters. * * @param cbuf * Array of characters to be written * * @throws IOException * If an I/O error occurs */ public void write(char cbuf[]) throws IOException { write(cbuf, 0, cbuf.length); } /** * Writes a portion of an array of characters. * * @param cbuf * Array of characters * * @param off * Offset from which to start writing characters * * @param len * Number of characters to write * * @throws IOException * If an I/O error occurs */ abstract public void write(char cbuf[], int off, int len) throws IOException; /** * Writes a string. * * @param str * String to be written * * @throws IOException * If an I/O error occurs */ public void write(String str) throws IOException { write(str, 0, str.length()); } /** * Writes a portion of a string. * * @param str * A String * * @param off * Offset from which to start writing characters * * @param len * Number of characters to write * * @throws IndexOutOfBoundsException * If <tt>off</tt> is negative, or <tt>len</tt> is negative, * or <tt>off+len</tt> is negative or greater than the length * of the given string * * @throws IOException * If an I/O error occurs */ public void write(String str, int off, int len) throws IOException { synchronized (lock) { char cbuf[]; if (len <= WRITE_BUFFER_SIZE) { if (writeBuffer == null) { writeBuffer = new char[WRITE_BUFFER_SIZE]; } cbuf = writeBuffer; } else { // Don't permanently allocate very large buffers. cbuf = new char[len]; } str.getChars(off, (off + len), cbuf, 0); write(cbuf, 0, len); } } /** * Appends the specified character sequence to this writer. * * <p> An invocation of this method of the form <tt>out.append(csq)</tt> * behaves in exactly the same way as the invocation * * <pre> * out.write(csq.toString()) </pre> * * <p> Depending on the specification of <tt>toString</tt> for the * character sequence <tt>csq</tt>, the entire sequence may not be * appended. For instance, invoking the <tt>toString</tt> method of a * character buffer will return a subsequence whose content depends upon * the buffer's position and limit. * * @param csq * The character sequence to append. If <tt>csq</tt> is * <tt>null</tt>, then the four characters <tt>"null"</tt> are * appended to this writer. * * @return This writer * * @throws IOException * If an I/O error occurs * * @since 1.5 */ public Writer append(CharSequence csq) throws IOException { if (csq == null) write("null"); else write(csq.toString()); return this; } /** * Appends a subsequence of the specified character sequence to this writer. * <tt>Appendable</tt>. * * <p> An invocation of this method of the form <tt>out.append(csq, start, * end)</tt> when <tt>csq</tt> is not <tt>null</tt> behaves in exactly the * same way as the invocation * * <pre> * out.write(csq.subSequence(start, end).toString()) </pre> * * @param csq * The character sequence from which a subsequence will be * appended. If <tt>csq</tt> is <tt>null</tt>, then characters * will be appended as if <tt>csq</tt> contained the four * characters <tt>"null"</tt>. * * @param start * The index of the first character in the subsequence * * @param end * The index of the character following the last character in the * subsequence * * @return This writer * * @throws IndexOutOfBoundsException * If <tt>start</tt> or <tt>end</tt> are negative, <tt>start</tt> * is greater than <tt>end</tt>, or <tt>end</tt> is greater than * <tt>csq.length()</tt> * * @throws IOException * If an I/O error occurs * * @since 1.5 */ public Writer append(CharSequence csq, int start, int end) throws IOException { CharSequence cs = (csq == null ? "null" : csq); write(cs.subSequence(start, end).toString()); return this; } /** * Appends the specified character to this writer. * * <p> An invocation of this method of the form <tt>out.append(c)</tt> * behaves in exactly the same way as the invocation * * <pre> * out.write(c) </pre> * * @param c * The 16-bit character to append * * @return This writer * * @throws IOException * If an I/O error occurs * * @since 1.5 */ public Writer append(char c) throws IOException { write(c); return this; }
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3. 字节输入流：
字节输入流的基类是 java.io.InputStream，抽象类，继承 java.io.Closeable。InputStream 有3个重载的 read 方法，其中不带参的是抽象方法，供其它两个调用。此外，该类还提供了 skip（跳过）、mark（标记）、reset（充值）等实用方法。
/** * 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; /** * Reads some number of bytes from the input stream and stores them into * the buffer array <code>b</code>. The number of bytes actually read is * returned as an integer. This method blocks until input data is * available, end of file is detected, or an exception is thrown. * * <p> If the length of <code>b</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 the * end of the 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[0]</code>, the * next one into <code>b[1]</code>, and so on. The number of bytes read is, * at most, equal to the length of <code>b</code>. Let <i>k</i> be the * number of bytes actually read; these bytes will be stored in elements * <code>b[0]</code> through <code>b[</code><i>k</i><code>-1]</code>, * leaving elements <code>b[</code><i>k</i><code>]</code> through * <code>b[b.length-1]</code> unaffected. * * <p> The <code>read(b)</code> method for class <code>InputStream</code> * has the same effect as: <pre><code> read(b, 0, b.length) </code></pre> * * @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 the first byte cannot be read for any reason * other than the end of the file, 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>. * @see java.io.InputStream#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 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; }
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4. 字节输出流：
字节输出流的基类是 java.io.OutputStream，抽象类，继承 java.io,Closeable 和 java.io.Flushable接口。OutputStream 有3个重载的 write 方法，其中不带参的为抽象方法。
/** * Writes the specified byte to this output stream. The general * contract for <code>write</code> is that one byte is written * to the output stream. The byte to be written is the eight * low-order bits of the argument <code>b</code>. The 24 * high-order bits of <code>b</code> are ignored. * <p> * Subclasses of <code>OutputStream</code> must provide an * implementation for this method. * * @param b the <code>byte</code>. * @exception IOException if an I/O error occurs. In particular, * an <code>IOException</code> may be thrown if the * output stream has been closed. */ public abstract void write(int b) throws IOException; /** * Writes <code>b.length</code> bytes from the specified byte array * to this output stream. The general contract for <code>write(b)</code> * is that it should have exactly the same effect as the call * <code>write(b, 0, b.length)</code>. * * @param b the data. * @exception IOException if an I/O error occurs. * @see java.io.OutputStream#write(byte[], int, int) */ public void write(byte b[]) throws IOException { write(b, 0, b.length); } /** * Writes <code>len</code> bytes from the specified byte array * starting at offset <code>off</code> to this output stream. * The general contract for <code>write(b, off, len)</code> is that * some of the bytes in the array <code>b</code> are written to the * output stream in order; element <code>b[off]</code> is the first * byte written and <code>b[off+len-1]</code> is the last byte written * by this operation. * <p> * The <code>write</code> method of <code>OutputStream</code> calls * the write method of one argument on each of the bytes to be * written out. Subclasses are encouraged to override this method and * provide a more efficient implementation. * <p> * If <code>b</code> is <code>null</code>, a * <code>NullPointerException</code> is thrown. * <p> * If <code>off</code> is negative, or <code>len</code> is negative, or * <code>off+len</code> is greater than the length of the array * <code>b</code>, then an <tt>IndexOutOfBoundsException</tt> is thrown. * * @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. In particular, * an <code>IOException</code> is thrown if the output * stream is closed. */ public void write(byte b[], int off, int len) throws IOException { if (b == null) { throw new NullPointerException(); } else if ((off < 0) || (off > b.length) || (len < 0) || ((off + len) > b.length) || ((off + len) < 0)) { throw new IndexOutOfBoundsException(); } else if (len == 0) { return; } for (int i = 0 ; i < len ; i++) { write(b[off + i]); } }
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