SeekableByteChannel.java

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package java.nio.channels;

import java.io.IOException;
import java.nio.ByteBuffer;

/**
 * A byte channel that maintains a current <i>position</i> and allows the
 * position to be changed.
 *
 * <p> A seekable byte channel is connected to an entity, typically a file,
 * that contains a variable-length sequence of bytes that can be read and
 * written. The current position can be {@link #position() <i>queried</i>} and
 * {@link #position(long) <i>modified</i>}. The channel also provides access to
 * the current <i>size</i> of the entity to which the channel is connected. The
 * size increases when bytes are written beyond its current size; the size
 * decreases when it is {@link #truncate <i>truncated</i>}.
 *
 * <p> The {@link #position(long) position} and {@link #truncate truncate} methods
 * which do not otherwise have a value to return are specified to return the
 * channel upon which they are invoked. This allows method invocations to be
 * chained. Implementations of this interface should specialize the return type
 * so that method invocations on the implementation class can be chained.
 *
 * @see java.nio.file.Files#newByteChannel
 * @since 1.7
 */
// 支持随机读/写的字节通道（常用于file通道）
public interface SeekableByteChannel extends ByteChannel {
    
    /**
     * Reads a sequence of bytes from this channel into the given buffer.
     *
     * <p> Bytes are read starting at this channel's current position, and
     * then the position is updated with the number of bytes actually read.
     * Otherwise this method behaves exactly as specified in the {@link
     * ReadableByteChannel} interface.
     */
    /*
     * 从当前通道（关联的文件）中起始处读取，读到的内容存入dst后，返回读到的字节数量
     * 该方法是一次性地，即已经读完的流不可以重复读取
     */
    @Override
    int read(ByteBuffer dst) throws IOException;
    
    /**
     * Writes a sequence of bytes to this channel from the given buffer.
     *
     * <p> Bytes are written starting at this channel's current position, unless
     * the channel is connected to an entity such as a file that is opened with
     * the {@link java.nio.file.StandardOpenOption#APPEND APPEND} option, in
     * which case the position is first advanced to the end. The entity to which
     * the channel is connected is grown, if necessary, to accommodate the
     * written bytes, and then the position is updated with the number of bytes
     * actually written. Otherwise this method behaves exactly as specified by
     * the {@link WritableByteChannel} interface.
     */
    /*
     * 从缓冲区src读取，读到的内容向当前通道（关联的文件）中追加写入后，返回写入的字节数量
     * 待写入内容从fd中上次position==-1时写完的末尾追加内容（不支持随机写入）
     */
    @Override
    int write(ByteBuffer src) throws IOException;
    
    /**
     * Returns this channel's position.
     *
     * @return This channel's position, a non-negative integer counting the number of bytes
     * from the beginning of the entity to the current position
     *
     * @throws ClosedChannelException If this channel is closed
     * @throws IOException            If some other I/O error occurs
     */
    // 返回此通道（文件）的游标位置
    long position() throws IOException;
    
    /**
     * Sets this channel's position.
     *
     * Setting the position to a value that is greater than the current size
     * is legal but does not change the size of the entity.  A later attempt to
     * read bytes at such a position will immediately return an end-of-file
     * indication.  A later attempt to write bytes at such a position will cause
     * the entity to grow to accommodate the new bytes; the values of any bytes
     * between the previous end-of-file and the newly-written bytes are
     * unspecified.
     *
     * Setting the channel's position is not recommended when connected to
     * an entity, typically a file, that is opened with the {@link
     * java.nio.file.StandardOpenOption#APPEND APPEND} option. When opened for
     * append, the position is first advanced to the end before writing.
     *
     * @param newPosition The new position, a non-negative integer counting
     *                    the number of bytes from the beginning of the entity
     *
     * @return This channel
     *
     * @throws ClosedChannelException   If this channel is closed
     * @throws IllegalArgumentException If the new position is negative
     * @throws IOException              If some other I/O error occurs
     */
    // 设置此通道（文件）的游标位置
    SeekableByteChannel position(long newPosition) throws IOException;
    
    /**
     * Returns the current size of entity to which this channel is connected.
     *
     * @return The current size, measured in bytes
     *
     * @throws ClosedChannelException If this channel is closed
     * @throws IOException            If some other I/O error occurs
     */
    // 返回此通道（文件）的字节数量
    long size() throws IOException;
    
    /**
     * Truncates the entity, to which this channel is connected, to the given size.
     *
     * If the given size is less than the current size then the entity is
     * truncated, discarding any bytes beyond the new end. If the given size is
     * greater than or equal to the current size then the entity is not modified.
     * In either case, if the current position is greater than the given size
     * then it is set to that size.
     *
     * An implementation of this interface may prohibit truncation when
     * connected to an entity, typically a file, opened with the {@link
     * java.nio.file.StandardOpenOption#APPEND APPEND} option.
     *
     * @param size The new size, a non-negative byte count
     *
     * @return This channel
     *
     * @throws NonWritableChannelException If this channel was not opened for writing
     * @throws ClosedChannelException      If this channel is closed
     * @throws IllegalArgumentException    If the new size is negative
     * @throws IOException                 If some other I/O error occurs
     */
    // 用指定的新尺寸size截断通道（文件）
    SeekableByteChannel truncate(long size) throws IOException;
    
}