SeekableByteChannel接口定义

ByteChannel，分散聚集通道接口的定义（SocketChannel）:http://donald-draper.iteye.com/blog/2371065

package java.nio.channels;

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

/**
 * A byte channel that maintains a current <i>position</i> and allows the
 * position to be changed.
 *SeekableByteChannel维护是一个当前position，运行修改position。
 * <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>}.
 *SeekableByteChannel连接一个实体，典型为一个包含可变长度的字节序列文件，可以读取
 和写。可通过#position方法获取当前位置，通过#position(long)修改当前position。通道
 提供了访问通道连接实体的当前size。当字节序列被写到实体时，实体size增加；当压缩时，
 实体size减少。
 * <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.
 *#position(long)和#truncate方法，在调用时，返回的值是不确定。这些方法可以链式调用。
 接口的实现，将该使这些方法返回一个精确的子，以便链式调用。
 * @since 1.7
 * @see java.nio.file.Files#newByteChannel
 */

public interface SeekableByteChannel
    extends ByteChannel
{
    /**
     * Reads a sequence of bytes from this channel into the given buffer.
     *从通道读取字节序列，写到buffer中
     * <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.
     从当前通道的postion位置开始读取字节，当实际字节被读取时，则更细positon。其他
     的和ReadableByteChannel接口相同
     */
    @Override
    int read(ByteBuffer dst) throws IOException;

    /**
     * Writes a sequence of bytes to this channel from the given buffer.
     *从给定的buffer，读取字节序列写到通道中
     * <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.
     从通道当前的位置写字节序列，除非通道连接一个实体，比如以StandardOpenOption#APPEND
     配置选项打开的文件，在这种情况下，position首先定位到文件的尾部。通道连接的实际将会
     增加，如果需要，计算写的字节数，更新position。其他方法与WritableByteChannel相同。
     */
    @Override
    int write(ByteBuffer src) throws IOException;

    /**
     * Returns this channel's position.
     *返回当前通道的位置
     * @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
     */
    long position() throws IOException;

    /**
     * Sets this channel's position.
     *设置通道的位置
     * <p> 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.
     *如果设置position大于当前实体size，是合法的，不会改变实体的size。尝试从设置后的
     position位置读取字节，position将会立即定位到文件的末端。此时尝试写字节序列，
     将会引起实体的增加。
     * <p> 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.
     *当通道连接一个实体时，不建议设置通道的position，特别是一个以StandardOpenOption#APPEND
     配置选项打开的文件，在这种情况下，在写之前，首先定位position的位置到文件的末尾。
     * @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
     */
    SeekableByteChannel position(long newPosition) throws IOException;

    /**
     * Returns the current size of entity to which this channel is connected.
     *返回通道连接实体的size
     * @return  The current size, measured in bytes
     *
     * @throws  ClosedChannelException
     *          If this channel is closed
     * @throws  IOException
     *          If some other I/O error occurs
     */
    long size() throws IOException;

    /**
     * Truncates the entity, to which this channel is connected, to the given
     * size.
     *压缩通道连接的实体到指定的size
     * <p> 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.
     *如果给定的size小于实体当前的size，将会压缩实体，丢弃超过新末端的字节。如果
     给定的size大于或等于当前size，实际将不会修改。在其他一些情况下，如果当前的position
     大于给定的size，那么将会设置position。
     * <p> 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.
     *当连接一个实体时，具体接口的实现也许会禁止压缩实体，比如以StandardOpenOption#APPEND
     配置选项打开的文件。
     * @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
     */
    SeekableByteChannel truncate(long size) throws IOException;
}