RandomAccessFile.java

/*
 * Copyright (c) 1994, 2017, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

package java.io;

import java.nio.channels.FileChannel;
import java.util.concurrent.atomic.AtomicBoolean;
import jdk.internal.misc.JavaIORandomAccessFileAccess;
import jdk.internal.misc.SharedSecrets;
import sun.nio.ch.FileChannelImpl;

/**
 * Instances of this class support both reading and writing to a
 * random access file. A random access file behaves like a large
 * array of bytes stored in the file system. There is a kind of cursor,
 * or index into the implied array, called the <em>file pointer</em>;
 * input operations read bytes starting at the file pointer and advance
 * the file pointer past the bytes read. If the random access file is
 * created in read/write mode, then output operations are also available;
 * output operations write bytes starting at the file pointer and advance
 * the file pointer past the bytes written. Output operations that write
 * past the current end of the implied array cause the array to be
 * extended. The file pointer can be read by the
 * {@code getFilePointer} method and set by the {@code seek}
 * method.
 * <p>
 * It is generally true of all the reading routines in this class that
 * if end-of-file is reached before the desired number of bytes has been
 * read, an {@code EOFException} (which is a kind of
 * {@code IOException}) is thrown. If any byte cannot be read for
 * any reason other than end-of-file, an {@code IOException} other
 * than {@code EOFException} is thrown. In particular, an
 * {@code IOException} may be thrown if the stream has been closed.
 *
 * @author unascribed
 * @since 1.0
 */
// 支持随机访问的文件
public class RandomAccessFile implements DataOutput, DataInput, Closeable {
    
    // 文件访问模式
    private static final int O_RDONLY = 1;   // r    只读模式
    private static final int O_RDWR = 2;   // rw   读写模式，如果文件不存在，则尝试创建文件
    private static final int O_SYNC = 4;   // rws  读写模式，与rw不同的是，文件内容或元数据的每个更新都实时同步到底层设备
    private static final int O_DSYNC = 8;   // rwd  读写模式，与rw不同的是，文件内容的每个更新都实时同步到底层设备
    private static final int O_TEMPORARY = 16;  // 创建临时文件，流关闭后删除
    
    /**
     * The path of the referenced file
     * (null if the stream is created with a file descriptor)
     */
    // 文件路径
    private final String path;
    
    // 文件描述符
    private FileDescriptor fd;
    
    // 是否使用读写模式
    private boolean rw;
    
    // 文件是否已关闭
    private final AtomicBoolean closed = new AtomicBoolean(false);
    
    // 当前文件关联的通道
    private volatile FileChannel channel;
    
    
    static {
        initIDs();
        
        SharedSecrets.setJavaIORandomAccessFileAccess(new JavaIORandomAccessFileAccess() {
            // This is for j.u.z.ZipFile.OPEN_DELETE. The O_TEMPORARY flag is only implemented/supported on windows.
            public RandomAccessFile openAndDelete(File file, String mode) throws IOException {
                // 为ZipFile类准备，第三个参数true指示该zip解压后，原zip压缩包会被删除
                return new RandomAccessFile(file, mode, true);
            }
        });
    }
    
    
    
    /*▼ 构造器 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Creates a random access file stream to read from, and optionally
     * to write to, a file with the specified name. A new
     * {@link FileDescriptor} object is created to represent the
     * connection to the file.
     *
     * <p> The {@code mode} argument specifies the access mode with which the
     * file is to be opened.  The permitted values and their meanings are as
     * specified for the <a
     * href="#mode">{@code RandomAccessFile(File,String)}</a> constructor.
     *
     * <p>
     * If there is a security manager, its {@code checkRead} method
     * is called with the {@code name} argument
     * as its argument to see if read access to the file is allowed.
     * If the mode allows writing, the security manager's
     * {@code checkWrite} method
     * is also called with the {@code name} argument
     * as its argument to see if write access to the file is allowed.
     *
     * @param name the system-dependent filename
     * @param mode the access <a href="#mode">mode</a>
     *
     * @throws IllegalArgumentException if the mode argument is not equal
     *                                  to one of {@code "r"}, {@code "rw"}, {@code "rws"}, or
     *                                  {@code "rwd"}
     * @throws FileNotFoundException    if the mode is {@code "r"} but the given string does not
     *                                  denote an existing regular file, or if the mode begins with
     *                                  {@code "rw"} but the given string does not denote an
     *                                  existing, writable regular file and a new regular file of
     *                                  that name cannot be created, or if some other error occurs
     *                                  while opening or creating the file
     * @throws SecurityException        if a security manager exists and its
     *                                  {@code checkRead} method denies read access to the file
     *                                  or the mode is {@code "rw"} and the security manager's
     *                                  {@code checkWrite} method denies write access to the file
     * @revised 1.4
     * @spec JSR-51
     * @see java.lang.SecurityException
     * @see java.lang.SecurityManager#checkRead(java.lang.String)
     * @see java.lang.SecurityManager#checkWrite(java.lang.String)
     */
    // 打开指定的文件，访问结束后不删除文件
    public RandomAccessFile(String name, String mode) throws FileNotFoundException {
        this(name != null ? new File(name) : null, mode);
    }
    
    /**
     * Creates a random access file stream to read from, and optionally to
     * write to, the file specified by the {@link File} argument.  A new {@link
     * FileDescriptor} object is created to represent this file connection.
     *
     * <p>The <a id="mode">{@code mode}</a> argument specifies the access mode
     * in which the file is to be opened.  The permitted values and their
     * meanings are:
     *
     * <table class="striped">
     * <caption style="display:none">Access mode permitted values and meanings</caption>
     * <thead>
     * <tr><th scope="col" style="text-align:left">Value</th><th scope="col" style="text-align:left">Meaning</th></tr>
     * </thead>
     * <tbody>
     * <tr><th scope="row" style="vertical-align:top">{@code "r"}</th>
     * <td> Open for reading only. Invoking any of the {@code write}
     * methods of the resulting object will cause an
     * {@link java.io.IOException} to be thrown.</td></tr>
     * <tr><th scope="row" style="vertical-align:top">{@code "rw"}</th>
     * <td> Open for reading and writing.  If the file does not already
     * exist then an attempt will be made to create it.</td></tr>
     * <tr><th scope="row" style="vertical-align:top">{@code "rws"}</th>
     * <td> Open for reading and writing, as with {@code "rw"}, and also
     * require that every update to the file's content or metadata be
     * written synchronously to the underlying storage device.</td></tr>
     * <tr><th scope="row" style="vertical-align:top">{@code "rwd"}</th>
     * <td> Open for reading and writing, as with {@code "rw"}, and also
     * require that every update to the file's content be written
     * synchronously to the underlying storage device.</td></tr>
     * </tbody>
     * </table>
     *
     * The {@code "rws"} and {@code "rwd"} modes work much like the {@link
     * java.nio.channels.FileChannel#force(boolean) force(boolean)} method of
     * the {@link java.nio.channels.FileChannel} class, passing arguments of
     * {@code true} and {@code false}, respectively, except that they always
     * apply to every I/O operation and are therefore often more efficient.  If
     * the file resides on a local storage device then when an invocation of a
     * method of this class returns it is guaranteed that all changes made to
     * the file by that invocation will have been written to that device.  This
     * is useful for ensuring that critical information is not lost in the
     * event of a system crash.  If the file does not reside on a local device
     * then no such guarantee is made.
     *
     * <p>The {@code "rwd"} mode can be used to reduce the number of I/O
     * operations performed.  Using {@code "rwd"} only requires updates to the
     * file's content to be written to storage; using {@code "rws"} requires
     * updates to both the file's content and its metadata to be written, which
     * generally requires at least one more low-level I/O operation.
     *
     * <p>If there is a security manager, its {@code checkRead} method is
     * called with the pathname of the {@code file} argument as its
     * argument to see if read access to the file is allowed.  If the mode
     * allows writing, the security manager's {@code checkWrite} method is
     * also called with the path argument to see if write access to the file is
     * allowed.
     *
     * @param file the file object
     * @param mode the access mode, as described
     *             <a href="#mode">above</a>
     *
     * @throws IllegalArgumentException if the mode argument is not equal
     *                                  to one of {@code "r"}, {@code "rw"}, {@code "rws"}, or
     *                                  {@code "rwd"}
     * @throws FileNotFoundException    if the mode is {@code "r"} but the given file object does
     *                                  not denote an existing regular file, or if the mode begins
     *                                  with {@code "rw"} but the given file object does not denote
     *                                  an existing, writable regular file and a new regular file of
     *                                  that name cannot be created, or if some other error occurs
     *                                  while opening or creating the file
     * @throws SecurityException        if a security manager exists and its
     *                                  {@code checkRead} method denies read access to the file
     *                                  or the mode is {@code "rw"} and the security manager's
     *                                  {@code checkWrite} method denies write access to the file
     * @revised 1.4
     * @spec JSR-51
     * @see java.lang.SecurityManager#checkRead(java.lang.String)
     * @see java.lang.SecurityManager#checkWrite(java.lang.String)
     * @see java.nio.channels.FileChannel#force(boolean)
     */
    // 打开指定的文件，访问结束后不删除文件
    public RandomAccessFile(File file, String mode) throws FileNotFoundException {
        this(file, mode, false);
    }
    
    /*
     * file         : 文件对象
     * mode         : 访问模式，参考字段中的描述
     * openAndDelete: 对文件访问结束后，是否删除原文件
     */
    private RandomAccessFile(File file, String mode, boolean openAndDelete) throws FileNotFoundException {
        String name = (file != null ? file.getPath() : null);
        
        int imode = -1;
        
        /*
         * "rwd"模式可用于减少I/O次数
         * 使用"rwd"更新时只要写入文件内容;
         * 使用"rws"更新时不仅要写入文件内容，还要写入文件元数据(文件大小、文件名等信息)
         * 文件元数据的更新，通常需要至少一个底层IO操作
         */
        
        if(mode.equals("r")) {
            imode = O_RDONLY;
        } else if(mode.startsWith("rw")) {
            imode = O_RDWR;
            rw = true;
            if(mode.length()>2) {
                if(mode.equals("rws")) {
                    imode |= O_SYNC;
                } else if(mode.equals("rwd")) {
                    imode |= O_DSYNC;
                } else {
                    imode = -1;
                }
            }
        }
        
        // 关键关闭后需要删除它
        if(openAndDelete) {
            imode |= O_TEMPORARY;
        }
        
        if(imode<0) {
            throw new IllegalArgumentException("Illegal mode \"" + mode + "\" must be one of " + "\"r\", \"rw\", \"rws\"," + " or \"rwd\"");
        }
        
        SecurityManager security = System.getSecurityManager();
        if(security != null) {
            security.checkRead(name);
            if(rw) {
                security.checkWrite(name);
            }
        }
        
        if(name == null) {
            throw new NullPointerException();
        }
        
        if(file.isInvalid()) {
            throw new FileNotFoundException("Invalid file path");
        }
        
        fd = new FileDescriptor();
        
        // 将当前文件(流)记录到关联的文件描述符fd中
        fd.attach(this);
        
        path = name;
        
        open(name, imode);
        
        // 注册文件描述符fd到清理器
        FileCleanable.register(fd);   // open sets the fd, register the cleanup
    }
    
    /*▲ 构造器 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 读 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Reads a {@code boolean} from this file. This method reads a
     * single byte from the file, starting at the current file pointer.
     * A value of {@code 0} represents
     * {@code false}. Any other value represents {@code true}.
     * This method blocks until the byte is read, the end of the stream
     * is detected, or an exception is thrown.
     *
     * @return the {@code boolean} value read.
     *
     * @throws EOFException if this file has reached the end.
     * @throws IOException  if an I/O error occurs.
     */
    // 从当前文件读取boolean
    public final boolean readBoolean() throws IOException {
        int ch = this.read();
        if(ch<0) {
            throw new EOFException();
        }
        return (ch != 0);
    }
    
    /**
     * Reads a character from this file. This method reads two
     * bytes from the file, starting at the current file pointer.
     * If the bytes read, in order, are
     * {@code b1} and {@code b2}, where
     * <code>0&nbsp;&lt;=&nbsp;b1,&nbsp;b2&nbsp;&lt;=&nbsp;255</code>,
     * then the result is equal to:
     * <blockquote><pre>
     *     (char)((b1 &lt;&lt; 8) | b2)
     * </pre></blockquote>
     * <p>
     * This method blocks until the two bytes are read, the end of the
     * stream is detected, or an exception is thrown.
     *
     * @return the next two bytes of this file, interpreted as a
     * {@code char}.
     *
     * @throws EOFException if this file reaches the end before reading
     *                      two bytes.
     * @throws IOException  if an I/O error occurs.
     */
    // 从当前文件读取char值（按大端法读取）
    public final char readChar() throws IOException {
        int ch1 = this.read();
        int ch2 = this.read();
        if((ch1 | ch2)<0) {
            throw new EOFException();
        }
        return (char) ((ch1 << 8) + (ch2 << 0));
    }
    
    /**
     * Reads a signed eight-bit value from this file. This method reads a
     * byte from the file, starting from the current file pointer.
     * If the byte read is {@code b}, where
     * <code>0&nbsp;&lt;=&nbsp;b&nbsp;&lt;=&nbsp;255</code>,
     * then the result is:
     * <blockquote><pre>
     *     (byte)(b)
     * </pre></blockquote>
     * <p>
     * This method blocks until the byte is read, the end of the stream
     * is detected, or an exception is thrown.
     *
     * @return the next byte of this file as a signed eight-bit
     * {@code byte}.
     *
     * @throws EOFException if this file has reached the end.
     * @throws IOException  if an I/O error occurs.
     */
    // 从当前文件读取byte值
    public final byte readByte() throws IOException {
        int ch = this.read();
        if(ch<0) {
            throw new EOFException();
        }
        return (byte) (ch);
    }
    
    /**
     * Reads an unsigned eight-bit number from this file. This method reads
     * a byte from this file, starting at the current file pointer,
     * and returns that byte.
     * <p>
     * This method blocks until the byte is read, the end of the stream
     * is detected, or an exception is thrown.
     *
     * @return the next byte of this file, interpreted as an unsigned
     * eight-bit number.
     *
     * @throws EOFException if this file has reached the end.
     * @throws IOException  if an I/O error occurs.
     */
    // 从当前文件读取无符号byte
    public final int readUnsignedByte() throws IOException {
        int ch = this.read();
        if(ch<0) {
            throw new EOFException();
        }
        return ch;
    }
    
    /**
     * Reads a signed 16-bit number from this file. The method reads two
     * bytes from this file, starting at the current file pointer.
     * If the two bytes read, in order, are
     * {@code b1} and {@code b2}, where each of the two values is
     * between {@code 0} and {@code 255}, inclusive, then the
     * result is equal to:
     * <blockquote><pre>
     *     (short)((b1 &lt;&lt; 8) | b2)
     * </pre></blockquote>
     * <p>
     * This method blocks until the two bytes are read, the end of the
     * stream is detected, or an exception is thrown.
     *
     * @return the next two bytes of this file, interpreted as a signed
     * 16-bit number.
     *
     * @throws EOFException if this file reaches the end before reading
     *                      two bytes.
     * @throws IOException  if an I/O error occurs.
     */
    // 从当前文件读取short（按大端法读取）
    public final short readShort() throws IOException {
        int ch1 = this.read();
        int ch2 = this.read();
        if((ch1 | ch2)<0) {
            throw new EOFException();
        }
        return (short) ((ch1 << 8) + (ch2 << 0));
    }
    
    /**
     * Reads an unsigned 16-bit number from this file. This method reads
     * two bytes from the file, starting at the current file pointer.
     * If the bytes read, in order, are
     * {@code b1} and {@code b2}, where
     * <code>0&nbsp;&lt;=&nbsp;b1, b2&nbsp;&lt;=&nbsp;255</code>,
     * then the result is equal to:
     * <blockquote><pre>
     *     (b1 &lt;&lt; 8) | b2
     * </pre></blockquote>
     * <p>
     * This method blocks until the two bytes are read, the end of the
     * stream is detected, or an exception is thrown.
     *
     * @return the next two bytes of this file, interpreted as an unsigned
     * 16-bit integer.
     *
     * @throws EOFException if this file reaches the end before reading
     *                      two bytes.
     * @throws IOException  if an I/O error occurs.
     */
    // 从当前文件读取无符号short（按大端法读取）
    public final int readUnsignedShort() throws IOException {
        int ch1 = this.read();
        int ch2 = this.read();
        if((ch1 | ch2)<0) {
            throw new EOFException();
        }
        return (ch1 << 8) + (ch2 << 0);
    }
    
    /**
     * Reads a signed 32-bit integer from this file. This method reads 4
     * bytes from the file, starting at the current file pointer.
     * If the bytes read, in order, are {@code b1},
     * {@code b2}, {@code b3}, and {@code b4}, where
     * <code>0&nbsp;&lt;=&nbsp;b1, b2, b3, b4&nbsp;&lt;=&nbsp;255</code>,
     * then the result is equal to:
     * <blockquote><pre>
     *     (b1 &lt;&lt; 24) | (b2 &lt;&lt; 16) + (b3 &lt;&lt; 8) + b4
     * </pre></blockquote>
     * <p>
     * This method blocks until the four bytes are read, the end of the
     * stream is detected, or an exception is thrown.
     *
     * @return the next four bytes of this file, interpreted as an
     * {@code int}.
     *
     * @throws EOFException if this file reaches the end before reading
     *                      four bytes.
     * @throws IOException  if an I/O error occurs.
     */
    // 从当前文件读取int（按大端法读取）
    public final int readInt() throws IOException {
        int ch1 = this.read();
        int ch2 = this.read();
        int ch3 = this.read();
        int ch4 = this.read();
        if((ch1 | ch2 | ch3 | ch4)<0) {
            throw new EOFException();
        }
        return ((ch1 << 24) + (ch2 << 16) + (ch3 << 8) + (ch4 << 0));
    }
    
    /**
     * Reads a signed 64-bit integer from this file. This method reads eight
     * bytes from the file, starting at the current file pointer.
     * If the bytes read, in order, are
     * {@code b1}, {@code b2}, {@code b3},
     * {@code b4}, {@code b5}, {@code b6},
     * {@code b7}, and {@code b8,} where:
     * <blockquote><pre>
     *     0 &lt;= b1, b2, b3, b4, b5, b6, b7, b8 &lt;=255,
     * </pre></blockquote>
     * <p>
     * then the result is equal to:
     * <blockquote><pre>
     *     ((long)b1 &lt;&lt; 56) + ((long)b2 &lt;&lt; 48)
     *     + ((long)b3 &lt;&lt; 40) + ((long)b4 &lt;&lt; 32)
     *     + ((long)b5 &lt;&lt; 24) + ((long)b6 &lt;&lt; 16)
     *     + ((long)b7 &lt;&lt; 8) + b8
     * </pre></blockquote>
     * <p>
     * This method blocks until the eight bytes are read, the end of the
     * stream is detected, or an exception is thrown.
     *
     * @return the next eight bytes of this file, interpreted as a
     * {@code long}.
     *
     * @throws EOFException if this file reaches the end before reading
     *                      eight bytes.
     * @throws IOException  if an I/O error occurs.
     */
    // 从当前文件读取long（按大端法读取）
    public final long readLong() throws IOException {
        return ((long) (readInt()) << 32) + (readInt() & 0xFFFFFFFFL);
    }
    
    /**
     * Reads a {@code float} from this file. This method reads an
     * {@code int} value, starting at the current file pointer,
     * as if by the {@code readInt} method
     * and then converts that {@code int} to a {@code float}
     * using the {@code intBitsToFloat} method in class
     * {@code Float}.
     * <p>
     * This method blocks until the four bytes are read, the end of the
     * stream is detected, or an exception is thrown.
     *
     * @return the next four bytes of this file, interpreted as a
     * {@code float}.
     *
     * @throws EOFException if this file reaches the end before reading
     *                      four bytes.
     * @throws IOException  if an I/O error occurs.
     * @see java.io.RandomAccessFile#readInt()
     * @see java.lang.Float#intBitsToFloat(int)
     */
    // 从当前文件读取float（按大端法读取）
    public final float readFloat() throws IOException {
        return Float.intBitsToFloat(readInt());
    }
    
    /**
     * Reads a {@code double} from this file. This method reads a
     * {@code long} value, starting at the current file pointer,
     * as if by the {@code readLong} method
     * and then converts that {@code long} to a {@code double}
     * using the {@code longBitsToDouble} method in
     * class {@code Double}.
     * <p>
     * This method blocks until the eight bytes are read, the end of the
     * stream is detected, or an exception is thrown.
     *
     * @return the next eight bytes of this file, interpreted as a
     * {@code double}.
     *
     * @throws EOFException if this file reaches the end before reading
     *                      eight bytes.
     * @throws IOException  if an I/O error occurs.
     * @see java.io.RandomAccessFile#readLong()
     * @see java.lang.Double#longBitsToDouble(long)
     */
    // 从当前文件读取double（按大端法读取）
    public final double readDouble() throws IOException {
        return Double.longBitsToDouble(readLong());
    }
    
    
    /**
     * Reads {@code b.length} bytes from this file into the byte
     * array, starting at the current file pointer. This method reads
     * repeatedly from the file until the requested number of bytes are
     * read. This method blocks until the requested number of bytes are
     * read, the end of the stream is detected, or an exception is thrown.
     *
     * @param b the buffer into which the data is read.
     *
     * @throws NullPointerException if {@code b} is {@code null}.
     * @throws EOFException         if this file reaches the end before reading
     *                              all the bytes.
     * @throws IOException          if an I/O error occurs.
     */
    // 从当前文件读取足量的字节填充给定的字节数组，填不满不返回
    public final void readFully(byte[] b) throws IOException {
        readFully(b, 0, b.length);
    }
    
    /**
     * Reads exactly {@code len} bytes from this file into the byte
     * array, starting at the current file pointer. This method reads
     * repeatedly from the file until the requested number of bytes are
     * read. This method blocks until the requested number of bytes are
     * read, the end of the stream is detected, or an exception is thrown.
     *
     * @param b   the buffer into which the data is read.
     * @param off the start offset into the data array {@code b}.
     * @param len the number of bytes to read.
     *
     * @throws NullPointerException      if {@code b} is {@code null}.
     * @throws IndexOutOfBoundsException if {@code off} is negative,
     *                                   {@code len} is negative, or {@code len} is greater than
     *                                   {@code b.length - off}.
     * @throws EOFException              if this file reaches the end before reading
     *                                   all the bytes.
     * @throws IOException               if an I/O error occurs.
     */
    // 从当前文件读取len个字节插入到字节数组b的off处，读不够len个字节不返回
    public final void readFully(byte[] b, int off, int len) throws IOException {
        int n = 0;
        do {
            int count = this.read(b, off + n, len - n);
            if(count<0) {
                throw new EOFException();
            }
            n += count;
        } while(n<len);
    }
    
    
    /**
     * Reads in a string from this file. The string has been encoded
     * using a
     * <a href="DataInput.html#modified-utf-8">modified UTF-8</a>
     * format.
     * <p>
     * The first two bytes are read, starting from the current file
     * pointer, as if by
     * {@code readUnsignedShort}. This value gives the number of
     * following bytes that are in the encoded string, not
     * the length of the resulting string. The following bytes are then
     * interpreted as bytes encoding characters in the modified UTF-8 format
     * and are converted into characters.
     * <p>
     * This method blocks until all the bytes are read, the end of the
     * stream is detected, or an exception is thrown.
     *
     * @return a Unicode string.
     *
     * @throws EOFException           if this file reaches the end before
     *                                reading all the bytes.
     * @throws IOException            if an I/O error occurs.
     * @throws UTFDataFormatException if the bytes do not represent
     *                                valid modified UTF-8 encoding of a Unicode string.
     * @see java.io.RandomAccessFile#readUnsignedShort()
     */
    // 从当前文件读取UTF8格式的字节，将其解码为String
    public final String readUTF() throws IOException {
        return DataInputStream.readUTF(this);
    }
    
    
    /**
     * Reads the next line of text from this file.  This method successively
     * reads bytes from the file, starting at the current file pointer,
     * until it reaches a line terminator or the end
     * of the file.  Each byte is converted into a character by taking the
     * byte's value for the lower eight bits of the character and setting the
     * high eight bits of the character to zero.  This method does not,
     * therefore, support the full Unicode character set.
     *
     * <p> A line of text is terminated by a carriage-return character
     * ({@code '\u005Cr'}), a newline character ({@code '\u005Cn'}), a
     * carriage-return character immediately followed by a newline character,
     * or the end of the file.  Line-terminating characters are discarded and
     * are not included as part of the string returned.
     *
     * <p> This method blocks until a newline character is read, a carriage
     * return and the byte following it are read (to see if it is a newline),
     * the end of the file is reached, or an exception is thrown.
     *
     * @return the next line of text from this file, or null if end
     * of file is encountered before even one byte is read.
     *
     * @throws IOException if an I/O error occurs.
     */
    // 从当前文件读取一行数据
    public final String readLine() throws IOException {
        StringBuilder input = new StringBuilder();
        int c = -1;
        boolean eol = false;
    
        while(!eol) {
            switch(c = read()) {
                case -1:
                case '\n':
                    eol = true;
                    break;
                case '\r':
                    eol = true;
                    long cur = getFilePointer();
                    if((read()) != '\n') {
                        seek(cur);
                    }
                    break;
                default:
                    input.append((char) c);
                    break;
            }
        }
    
        if((c == -1) && (input.length() == 0)) {
            return null;
        }
    
        return input.toString();
    }
    
    
    /**
     * Reads a byte of data from this file. The byte is returned as an
     * integer in the range 0 to 255 ({@code 0x00-0x0ff}). This
     * method blocks if no input is yet available.
     * <p>
     * Although {@code RandomAccessFile} is not a subclass of
     * {@code InputStream}, this method behaves in exactly the same
     * way as the {@link InputStream#read()} method of
     * {@code InputStream}.
     *
     * @return the next byte of data, or {@code -1} if the end of the
     * file has been reached.
     *
     * @throws IOException if an I/O error occurs. Not thrown if
     *                     end-of-file has been reached.
     */
    // 从当前文件读取一个字节，并将其返回；返回-1表示没有可读数据
    public int read() throws IOException {
        return read0();
    }
    
    /**
     * Reads up to {@code b.length} bytes of data from this file
     * into an array of bytes. This method blocks until at least one byte
     * of input is available.
     * <p>
     * Although {@code RandomAccessFile} is not a subclass of
     * {@code InputStream}, this method behaves in exactly the
     * same way as the {@link InputStream#read(byte[])} method of
     * {@code InputStream}.
     *
     * @param b the buffer into which the data is read.
     *
     * @return the total number of bytes read into the buffer, or
     * {@code -1} if there is no more data because the end of
     * this file has been reached.
     *
     * @throws IOException          If the first byte cannot be read for any reason
     *                              other than end of file, or if the random access file has been closed, or if
     *                              some other I/O error occurs.
     * @throws NullPointerException If {@code b} is {@code null}.
     */
    // 从当前文件读取足量的字节填充字节数组b，返回实际填充的字节数
    public int read(byte[] b) throws IOException {
        return readBytes(b, 0, b.length);
    }
    
    /**
     * Reads up to {@code len} bytes of data from this file into an
     * array of bytes. This method blocks until at least one byte of input
     * is available.
     * <p>
     * Although {@code RandomAccessFile} is not a subclass of
     * {@code InputStream}, this method behaves in exactly the
     * same way as the {@link InputStream#read(byte[], int, int)} method of
     * {@code InputStream}.
     *
     * @param b   the buffer into which the data is read.
     * @param off the start offset in array {@code b}
     *            at which the data is written.
     * @param len the maximum number of bytes read.
     *
     * @return the total number of bytes read into the buffer, or
     * {@code -1} if there is no more data because the end of
     * the file has been reached.
     *
     * @throws IOException               If the first byte cannot be read for any reason
     *                                   other than end of file, or if the random access file has been closed, or if
     *                                   some other I/O error occurs.
     * @throws NullPointerException      If {@code b} is {@code null}.
     * @throws IndexOutOfBoundsException If {@code off} is negative,
     *                                   {@code len} is negative, or {@code len} is greater than
     *                                   {@code b.length - off}
     */
    // 从当前文件读取len个字节填充到字节数组b的off处，返回实际填充的字节数
    public int read(byte[] b, int off, int len) throws IOException {
        return readBytes(b, off, len);
    }
    
    /*▲ 读 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 写 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Writes a {@code boolean} to the file as a one-byte value. The
     * value {@code true} is written out as the value
     * {@code (byte)1}; the value {@code false} is written out
     * as the value {@code (byte)0}. The write starts at
     * the current position of the file pointer.
     *
     * @param v a {@code boolean} value to be written.
     *
     * @throws IOException if an I/O error occurs.
     */
    // 向当前文件写入boolean值
    public final void writeBoolean(boolean v) throws IOException {
        write(v ? 1 : 0);
        //written++;
    }
    
    /**
     * Writes a {@code char} to the file as a two-byte value, high
     * byte first. The write starts at the current position of the
     * file pointer.
     *
     * @param v a {@code char} value to be written.
     *
     * @throws IOException if an I/O error occurs.
     */
    // 向当前文件写入char值（按大端法写入）
    public final void writeChar(int v) throws IOException {
        write((v >>> 8) & 0xFF);
        write((v >>> 0) & 0xFF);
        //written += 2;
    }
    
    /**
     * Writes a {@code byte} to the file as a one-byte value. The
     * write starts at the current position of the file pointer.
     *
     * @param v a {@code byte} value to be written.
     *
     * @throws IOException if an I/O error occurs.
     */
    // 向当前文件写入byte值
    public final void writeByte(int v) throws IOException {
        write(v);
        //written++;
    }
    
    /**
     * Writes a {@code short} to the file as two bytes, high byte first.
     * The write starts at the current position of the file pointer.
     *
     * @param v a {@code short} to be written.
     *
     * @throws IOException if an I/O error occurs.
     */
    // 向当前文件写入short值（按大端法写入）
    public final void writeShort(int v) throws IOException {
        write((v >>> 8) & 0xFF);
        write((v >>> 0) & 0xFF);
        //written += 2;
    }
    
    /**
     * Writes an {@code int} to the file as four bytes, high byte first.
     * The write starts at the current position of the file pointer.
     *
     * @param v an {@code int} to be written.
     *
     * @throws IOException if an I/O error occurs.
     */
    // 向当前文件写入int值（按大端法写入）
    public final void writeInt(int v) throws IOException {
        write((v >>> 24) & 0xFF);
        write((v >>> 16) & 0xFF);
        write((v >>> 8) & 0xFF);
        write((v >>> 0) & 0xFF);
        //written += 4;
    }
    
    /**
     * Writes a {@code long} to the file as eight bytes, high byte first.
     * The write starts at the current position of the file pointer.
     *
     * @param v a {@code long} to be written.
     *
     * @throws IOException if an I/O error occurs.
     */
    // 向当前文件写入long值（按大端法写入）
    public final void writeLong(long v) throws IOException {
        write((int) (v >>> 56) & 0xFF);
        write((int) (v >>> 48) & 0xFF);
        write((int) (v >>> 40) & 0xFF);
        write((int) (v >>> 32) & 0xFF);
        write((int) (v >>> 24) & 0xFF);
        write((int) (v >>> 16) & 0xFF);
        write((int) (v >>> 8) & 0xFF);
        write((int) (v >>> 0) & 0xFF);
        //written += 8;
    }
    
    /**
     * Converts the float argument to an {@code int} using the
     * {@code floatToIntBits} method in class {@code Float},
     * and then writes that {@code int} value to the file as a
     * four-byte quantity, high byte first. The write starts at the
     * current position of the file pointer.
     *
     * @param v a {@code float} value to be written.
     *
     * @throws IOException if an I/O error occurs.
     * @see java.lang.Float#floatToIntBits(float)
     */
    // 向当前文件写入float值（先转为二进制，再按大端法写入）
    public final void writeFloat(float v) throws IOException {
        writeInt(Float.floatToIntBits(v));
    }
    
    /**
     * Converts the double argument to a {@code long} using the
     * {@code doubleToLongBits} method in class {@code Double},
     * and then writes that {@code long} value to the file as an
     * eight-byte quantity, high byte first. The write starts at the current
     * position of the file pointer.
     *
     * @param v a {@code double} value to be written.
     *
     * @throws IOException if an I/O error occurs.
     * @see java.lang.Double#doubleToLongBits(double)
     */
    // 向当前文件写入double值（先转为二进制，按大端法写入）
    public final void writeDouble(double v) throws IOException {
        writeLong(Double.doubleToLongBits(v));
    }
    
    /**
     * Writes the string to the file as a sequence of bytes. Each
     * character in the string is written out, in sequence, by discarding
     * its high eight bits. The write starts at the current position of
     * the file pointer.
     *
     * @param s a string of bytes to be written.
     *
     * @throws IOException if an I/O error occurs.
     */
    // 向当前文件写入String值(按字节写入)
    @SuppressWarnings("deprecation")
    public final void writeBytes(String s) throws IOException {
        int len = s.length();
        byte[] b = new byte[len];
        s.getBytes(0, len, b, 0);
        writeBytes(b, 0, len);
    }
    
    /**
     * Writes a string to the file as a sequence of characters. Each
     * character is written to the data output stream as if by the
     * {@code writeChar} method. The write starts at the current
     * position of the file pointer.
     *
     * @param s a {@code String} value to be written.
     *
     * @throws IOException if an I/O error occurs.
     * @see java.io.RandomAccessFile#writeChar(int)
     */
    // 向当前文件写入String值(按字符写入)
    public final void writeChars(String s) throws IOException {
        int clen = s.length();
        int blen = 2 * clen;
        byte[] b = new byte[blen];
        char[] c = new char[clen];
        s.getChars(0, clen, c, 0);
        for(int i = 0, j = 0; i<clen; i++) {
            b[j++] = (byte) (c[i] >>> 8);
            b[j++] = (byte) (c[i] >>> 0);
        }
        writeBytes(b, 0, blen);
    }
    
    /**
     * Writes a string to the file using
     * <a href="DataInput.html#modified-utf-8">modified UTF-8</a>
     * encoding in a machine-independent manner.
     * <p>
     * First, two bytes are written to the file, starting at the
     * current file pointer, as if by the
     * {@code writeShort} method giving the number of bytes to
     * follow. This value is the number of bytes actually written out,
     * not the length of the string. Following the length, each character
     * of the string is output, in sequence, using the modified UTF-8 encoding
     * for each character.
     *
     * @param str a string to be written.
     *
     * @throws IOException if an I/O error occurs.
     */
    // 将String按UTF8编码后写入当前文件
    public final void writeUTF(String str) throws IOException {
        DataOutputStream.writeUTF(str, this);
    }
    
    
    /**
     * Writes the specified byte to this file. The write starts at
     * the current file pointer.
     *
     * @param b the {@code byte} to be written.
     *
     * @throws IOException if an I/O error occurs.
     */
    // 向当前文件写入一个byte
    public void write(int b) throws IOException {
        write0(b);
    }
    
    /**
     * Writes {@code b.length} bytes from the specified byte array
     * to this file, starting at the current file pointer.
     *
     * @param b the data.
     *
     * @throws IOException if an I/O error occurs.
     */
    // 向当前文件写入b中全部字节
    public void write(byte[] b) throws IOException {
        writeBytes(b, 0, b.length);
    }
    
    /**
     * Writes {@code len} bytes from the specified byte array
     * starting at offset {@code off} to this file.
     *
     * @param b   the data.
     * @param off the start offset in the data.
     * @param len the number of bytes to write.
     *
     * @throws IOException if an I/O error occurs.
     */
    // 向当前文件写入b中off处起的len个byte
    public void write(byte[] b, int off, int len) throws IOException {
        writeBytes(b, off, len);
    }
    
    /*▲ 写 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 杂项 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns the opaque file descriptor object associated with this
     * stream.
     *
     * @return the file descriptor object associated with this stream.
     *
     * @throws IOException if an I/O error occurs.
     * @see java.io.FileDescriptor
     */
    // 返回当前文件的文件描述符
    public final FileDescriptor getFD() throws IOException {
        if(fd != null) {
            return fd;
        }
        
        throw new IOException();
    }
    
    /**
     * Returns the unique {@link java.nio.channels.FileChannel FileChannel}
     * object associated with this file.
     *
     * <p> The {@link java.nio.channels.FileChannel#position()
     * position} of the returned channel will always be equal to
     * this object's file-pointer offset as returned by the {@link
     * #getFilePointer getFilePointer} method.  Changing this object's
     * file-pointer offset, whether explicitly or by reading or writing bytes,
     * will change the position of the channel, and vice versa.  Changing the
     * file's length via this object will change the length seen via the file
     * channel, and vice versa.
     *
     * @return the file channel associated with this file
     *
     * @spec JSR-51
     * @since 1.4
     */
    // 返回当前文件关联的文件通道
    public final FileChannel getChannel() {
        FileChannel fc = this.channel;
        if(fc != null) {
            return fc;
        }
        
        synchronized(this) {
            fc = this.channel;
            if(fc != null) {
                return fc;
            }
            
            // 获取文件通道
            this.channel = fc = FileChannelImpl.open(fd, path, true, rw, false, this);
            
            // 如果文件已关闭
            if(closed.get()) {
                try {
                    // 关闭文件通道
                    fc.close();
                } catch(IOException ioe) {
                    throw new InternalError(ioe); // should not happen
                }
            }
        }
        
        return fc;
    }
    
    /**
     * Returns the current offset in this file.
     *
     * @return the offset from the beginning of the file, in bytes,
     * at which the next read or write occurs.
     *
     * @throws IOException if an I/O error occurs.
     */
    // 返回当前文件的游标位置
    public native long getFilePointer() throws IOException;
    
    /**
     * Sets the file-pointer offset, measured from the beginning of this
     * file, at which the next read or write occurs.  The offset may be
     * set beyond the end of the file. Setting the offset beyond the end
     * of the file does not change the file length.  The file length will
     * change only by writing after the offset has been set beyond the end
     * of the file.
     *
     * @param pos the offset position, measured in bytes from the
     *            beginning of the file, at which to set the file
     *            pointer.
     *
     * @throws IOException if {@code pos} is less than
     *                     {@code 0} or if an I/O error occurs.
     */
    // 将文件游标移动到指定的文职
    public void seek(long pos) throws IOException {
        if(pos<0) {
            throw new IOException("Negative seek offset");
        } else {
            seek0(pos);
        }
    }
    
    /**
     * Attempts to skip over {@code n} bytes of input discarding the
     * skipped bytes.
     * <p>
     *
     * This method may skip over some smaller number of bytes, possibly zero.
     * This may result from any of a number of conditions; reaching end of
     * file before {@code n} bytes have been skipped is only one
     * possibility. This method never throws an {@code EOFException}.
     * The actual number of bytes skipped is returned.  If {@code n}
     * is negative, no bytes are skipped.
     *
     * @param n the number of bytes to be skipped.
     *
     * @return the actual number of bytes skipped.
     *
     * @throws IOException if an I/O error occurs.
     */
    // 跳过n个字节，返回实际跳过的字节数
    public int skipBytes(int n) throws IOException {
        long pos;
        long len;
        long newpos;
        
        if(n<=0) {
            return 0;
        }
        
        // 获取当前文件游标位置
        pos = getFilePointer();
        len = length();
        
        // 计算新游标位置
        newpos = pos + n;
        if(newpos>len) {
            newpos = len;
        }
        
        // 将游标移动到新位置
        seek(newpos);
        
        /* return the actual number of bytes skipped */
        return (int) (newpos - pos);
    }
    
    /**
     * Returns the length of this file.
     *
     * @return the length of this file, measured in bytes.
     *
     * @throws IOException if an I/O error occurs.
     */
    // 返回文件长度
    public native long length() throws IOException;
    
    /**
     * Sets the length of this file.
     *
     * <p> If the present length of the file as returned by the
     * {@code length} method is greater than the {@code newLength}
     * argument then the file will be truncated.  In this case, if the file
     * offset as returned by the {@code getFilePointer} method is greater
     * than {@code newLength} then after this method returns the offset
     * will be equal to {@code newLength}.
     *
     * <p> If the present length of the file as returned by the
     * {@code length} method is smaller than the {@code newLength}
     * argument then the file will be extended.  In this case, the contents of
     * the extended portion of the file are not defined.
     *
     * @param newLength The desired length of the file
     *
     * @throws IOException If an I/O error occurs
     * @since 1.2
     */
    // 设置文件长度，文件会被扩大/缩小
    public native void setLength(long newLength) throws IOException;
    
    /**
     * Closes this random access file stream and releases any system
     * resources associated with the stream. A closed random access
     * file cannot perform input or output operations and cannot be
     * reopened.
     *
     * <p> If this file has an associated channel then the channel is closed
     * as well.
     *
     * @throws IOException if an I/O error occurs.
     * @revised 1.4
     * @spec JSR-51
     */
    // 关闭文件
    public void close() throws IOException {
        // 原子地更改关闭标记
        if(!closed.compareAndSet(false, true)) {
            // if compareAndSet() returns false closed was already true
            return;
        }
        
        // 关闭文件通道
        FileChannel fc = channel;
        if(fc != null) {
            fc.close();
        }
        
        fd.closeAll(new Closeable() {
            public void close() throws IOException {
                fd.close();
            }
        });
    }
    
    /*▲ 杂项 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    private static native void initIDs();
    
    /**
     * Opens a file and returns the file descriptor.  The file is
     * opened in read-write mode if the O_RDWR bit in {@code mode}
     * is true, else the file is opened as read-only.
     * If the {@code name} refers to a directory, an IOException
     * is thrown.
     *
     * @param name the name of the file
     * @param mode the mode flags, a combination of the O_ constants
     *             defined above
     */
    private native void open0(String name, int mode) throws FileNotFoundException;
    
    /**
     * Opens a file and returns the file descriptor.  The file is
     * opened in read-write mode if the O_RDWR bit in {@code mode}
     * is true, else the file is opened as read-only.
     * If the {@code name} refers to a directory, an IOException
     * is thrown.
     *
     * @param name the name of the file
     * @param mode the mode flags, a combination of the O_ constants
     *             defined above
     */
    // wrap native call to allow instrumentation
    private void open(String name, int mode) throws FileNotFoundException {
        open0(name, mode);
    }
    
    private native void write0(int b) throws IOException;
    
    /**
     * Writes a sub array as a sequence of bytes.
     *
     * @param b   the data to be written
     * @param off the start offset in the data
     * @param len the number of bytes that are written
     *
     * @throws IOException If an I/O error has occurred.
     */
    private native void writeBytes(byte[] b, int off, int len) throws IOException;
    
    private native int read0() throws IOException;
    
    /**
     * Reads a sub array as a sequence of bytes.
     *
     * @param b   the buffer into which the data is read.
     * @param off the start offset of the data.
     * @param len the number of bytes to read.
     *
     * @throws IOException If an I/O error has occurred.
     */
    private native int readBytes(byte[] b, int off, int len) throws IOException;
    
    private native void seek0(long pos) throws IOException;
    
}