java.nio.channels.FileChannel源码解读
版本：JDK7
package java.nio.channels;
import java.io.*;
import java.nio.ByteBuffer;
import java.nio.MappedByteBuffer;
import java.nio.channels.spi.AbstractInterruptibleChannel;
import java.nio.file.*;
import java.nio.file.attribute.FileAttribute;
import java.nio.file.spi.*;
import java.util.Set;
import java.util.HashSet;
import java.util.Collections;
public abstract class FileChannel extends AbstractInterruptibleChannel
implements SeekableByteChannel, GatheringByteChannel, ScatteringByteChannel {
protected FileChannel() { }
/**
* Opens or creates a file, returning a file channel to access the file.
*
* The {@code options} parameter determines how the file is opened.
* The {@link StandardOpenOption#READ READ} and {@link StandardOpenOption#WRITE
* WRITE} options determine if the file should be opened for reading and/or
* writing. If neither option (or the {@link StandardOpenOption#APPEND APPEND}
* option) is contained in the array then the file is opened for reading.
* By default reading or writing commences at the beginning of the file.
* @since 1.7
*/
public static FileChannel open(Path path, Set options, FileAttribute... attrs) throws IOException {
FileSystemProvider provider = path.getFileSystem().provider();
return provider.newFileChannel(path, options, attrs);
}
private static final FileAttribute[] NO_ATTRIBUTES = new FileAttribute[0];
/**
* Opens or creates a file, returning a file channel to access the file.
* @since 1.7
*/
public static FileChannel open(Path path, OpenOption... options) throws IOException {
Set set = new HashSet(options.length);
Collections.addAll(set, options);
return open(path, set, NO_ATTRIBUTES);
}
// -- Channel operations --
/**
* Reads a sequence of bytes from this channel into the given buffer.
*/
public abstract int read(ByteBuffer dst) throws IOException;
/**
* Reads a sequence of bytes from this channel into a subsequence of the
* given buffers.
*/
public abstract long read(ByteBuffer[] dsts, int offset, int length)
throws IOException;
/**
* Reads a sequence of bytes from this channel into the given buffers.
*/
public final long read(ByteBuffer[] dsts) throws IOException {
return read(dsts, 0, dsts.length);
}
/**
* Writes a sequence of bytes to this channel from the given buffer.
*/
public abstract int write(ByteBuffer src) throws IOException;
/**
* Writes a sequence of bytes to this channel from a subsequence of the given buffers.
*/
public abstract long write(ByteBuffer[] srcs, int offset, int length)
throws IOException;
/**
* Writes a sequence of bytes to this channel from the given buffers.
*/
public final long write(ByteBuffer[] srcs) throws IOException {
return write(srcs, 0, srcs.length);
}
/**
* Returns this channel's file position.
*/
public abstract long position() throws IOException;
/**
* Sets this channel's file position.
*
* Setting the position to a value that is greater than the file's
* current size is legal but does not change the size of the file. 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 file to be grown to accommodate the new bytes;
* the values of any bytes between the previous end-of-file and the
* newly-written bytes are unspecified.
*
* 如果将位置设置在文件结束符之后，然后试图从文件通道中读取数据，读方法将返回-1 —— 文件结束标志。
* 如果将位置设置在文件结束符之后，然后向通道中写数据，文件将撑大到当前位置并(从当前位置开始)写入数据。这可能导致“文件空洞”，磁盘上物理文件中写入的数据间有空隙。
*
* @param newPosition
* The new position, a non-negative integer counting
* the number of bytes from the beginning of the file
*/
public abstract FileChannel position(long newPosition) throws IOException;
/**
* Returns the current size of this channel's file.
*/
public abstract long size() throws IOException;
/**
* Truncates this channel's file to the given size. 截取一个文件
*
* If the given size is less than the file's current size then the file
* is truncated, discarding any bytes beyond the new end of the file. If
* the given size is greater than or equal to the file's current size then
* the file is not modified. In either case, if this channel's file
* position is greater than the given size then it is set to that size.
*/
public abstract FileChannel truncate(long size) throws IOException;
/**
* Forces any updates to this channel's file to be written to the storage
* device that contains it.
* 将通道里尚未写入磁盘的数据强制写到磁盘上。
* 说明：
* 1)操作系统将数据缓存在内存中，所以无法保证写入到FileChannel里的数据一定会即时写到磁盘上。
* 2)如果要保证写入到FileChannel里的数据即时写到磁盘上，需要调用force()方法，参数表示是否将文件的元数据写到磁盘上。
*/
public abstract void force(boolean metaData) throws IOException;
// 通道之间的数据传输
/**
* Transfers bytes from this channel's file to the given writable byte channel.
*/
public abstract long transferTo(long position, long count, WritableByteChannel target) throws IOException;
/**
* Transfers bytes into this channel's file from the given readable byte channel.
*/
public abstract long transferFrom(ReadableByteChannel src, long position, long count) throws IOException;
/**
* Reads a sequence of bytes from this channel into the given buffer, starting at the given file position.
*
* This method does not modify this channel's position.
* If the given position is greater than the file's current size then no bytes are read.
*/
public abstract int read(ByteBuffer dst, long position) throws IOException;
/**
* Writes a sequence of bytes to this channel from the given buffer, starting at the given file position.
*
* This method does not modify this channel's position.
* If the given position is greater than the file's current size then the file will be grown to accommodate the new bytes;
* the values of any bytes between the previous end-of-file and the newly-written bytes are unspecified.
*/
public abstract int write(ByteBuffer src, long position) throws IOException;
// -- Memory-mapped buffers --
/**
* A typesafe enumeration for file-mapping modes.
*
* @since 1.4
*
* @see java.nio.channels.FileChannel#map
*/
public static class MapMode {
/**
* Mode for a read-only mapping.
*/
public static final MapMode READ_ONLY = new MapMode("READ_ONLY");
/**
* Mode for a read/write mapping.
*/
public static final MapMode READ_WRITE = new MapMode("READ_WRITE");
/**
* Mode for a private (copy-on-write) mapping.
*/
public static final MapMode PRIVATE = new MapMode("PRIVATE");
private final String name;
private MapMode(String name) {
this.name = name;
}
public String toString() {
return name;
}
}
/**
* Maps a region of this channel's file directly into memory.
*
* A region of a file may be mapped into memory in one of three modes:
*
* Read-only、Read/write、Private
*
* @param mode
* One of the constants {@link MapMode#READ_ONLY READ_ONLY}, {@link
* MapMode#READ_WRITE READ_WRITE}, or {@link MapMode#PRIVATE
* PRIVATE} defined in the {@link MapMode} class, according to
* whether the file is to be mapped read-only, read/write, or
* privately (copy-on-write), respectively
*
* @param position
* The position within the file at which the mapped region
* is to start; must be non-negative
*
* @param size
* The size of the region to be mapped; must be non-negative and
* no greater than {@link java.lang.Integer#MAX_VALUE}
*
*
* @see java.nio.channels.FileChannel.MapMode
* @see java.nio.MappedByteBuffer
*/
public abstract MappedByteBuffer map(MapMode mode, long position, long size) throws IOException;
// -- Locks --
/**
* Acquires a lock on the given region of this channel's file.
*
*
An invocation of this method will block until the region can be
* locked, this channel is closed, or the invoking thread is interrupted,
* whichever comes first.
*
*
If this channel is closed by another thread during an invocation of
* this method then an {@link AsynchronousCloseException} will be thrown.
*
*
If the invoking thread is interrupted while waiting to acquire the
* lock then its interrupt status will be set and a {@link
* FileLockInterruptionException} will be thrown. If the invoker's
* interrupt status is set when this method is invoked then that exception
* will be thrown immediately; the thread's interrupt status will not be
* changed.
*
*
The region specified by the position and size
* parameters need not be contained within, or even overlap, the actual
* underlying file. Lock regions are fixed in size; if a locked region
* initially contains the end of the file and the file grows beyond the
* region then the new portion of the file will not be covered by the lock.
* If a file is expected to grow in size and a lock on the entire file is
* required then a region starting at zero, and no smaller than the
* expected maximum size of the file, should be locked. The zero-argument
* {@link #lock()} method simply locks a region of size {@link
* Long#MAX_VALUE}.
*
*
Some operating systems do not support shared locks, in which case a
* request for a shared lock is automatically converted into a request for
* an exclusive lock. Whether the newly-acquired lock is shared or
* exclusive may be tested by invoking the resulting lock object's {@link
* FileLock#isShared() isShared} method.
*
*
File locks are held on behalf of the entire Java virtual machine.
* They are not suitable for controlling access to a file by multiple
* threads within the same virtual machine.
*
* @param position
* The position at which the locked region is to start; must be
* non-negative
*
* @param size
* The size of the locked region; must be non-negative, and the sum
* position + size must be non-negative
*
* @param shared
* true to request a shared lock, in which case this
* channel must be open for reading (and possibly writing);
* false to request an exclusive lock, in which case this
* channel must be open for writing (and possibly reading)
*
* @return A lock object representing the newly-acquired lock
*
* @throws IllegalArgumentException
* If the preconditions on the parameters do not hold
*
* @throws ClosedChannelException
* If this channel is closed
*
* @throws AsynchronousCloseException
* If another thread closes this channel while the invoking
* thread is blocked in this method
*
* @throws FileLockInterruptionException
* If the invoking thread is interrupted while blocked in this
* method
*
* @throws OverlappingFileLockException
* If a lock that overlaps the requested region is already held by
* this Java virtual machine, or if another thread is already
* blocked in this method and is attempting to lock an overlapping
* region
*
* @throws NonReadableChannelException
* If shared is true this channel was not
* opened for reading
*
* @throws NonWritableChannelException
* If shared is false but this channel was not
* opened for writing
*
* @throws IOException
* If some other I/O error occurs
*
* @see #lock()
* @see #tryLock()
* @see #tryLock(long,long,boolean)
*/
public abstract FileLock lock(long position, long size, boolean shared) throws IOException;
/**
* Acquires an exclusive lock on this channel's file.
*
* An invocation of this method of the form fc.lock() behaves
* in exactly the same way as the invocation
*
*
* fc.{@link #lock(long,long,boolean) lock}(0L, Long.MAX_VALUE, false)
*
* @return A lock object representing the newly-acquired lock
*
* @throws ClosedChannelException
* If this channel is closed
*
* @throws AsynchronousCloseException
* If another thread closes this channel while the invoking
* thread is blocked in this method
*
* @throws FileLockInterruptionException
* If the invoking thread is interrupted while blocked in this
* method
*
* @throws OverlappingFileLockException
* If a lock that overlaps the requested region is already held by
* this Java virtual machine, or if another thread is already
* blocked in this method and is attempting to lock an overlapping
* region of the same file
*
* @throws NonWritableChannelException
* If this channel was not opened for writing
*
* @throws IOException
* If some other I/O error occurs
*
* @see #lock(long,long,boolean)
* @see #tryLock()
* @see #tryLock(long,long,boolean)
*/
public final FileLock lock() throws IOException {
return lock(0L, Long.MAX_VALUE, false);
}
/**
* Attempts to acquire a lock on the given region of this channel's file.
*
* This method does not block. An invocation always returns
* immediately, either having acquired a lock on the requested region or
* having failed to do so. If it fails to acquire a lock because an
* overlapping lock is held by another program then it returns
* null. If it fails to acquire a lock for any other reason then
* an appropriate exception is thrown.
*
*
The region specified by the position and size
* parameters need not be contained within, or even overlap, the actual
* underlying file. Lock regions are fixed in size; if a locked region
* initially contains the end of the file and the file grows beyond the
* region then the new portion of the file will not be covered by the lock.
* If a file is expected to grow in size and a lock on the entire file is
* required then a region starting at zero, and no smaller than the
* expected maximum size of the file, should be locked. The zero-argument
* {@link #tryLock()} method simply locks a region of size {@link
* Long#MAX_VALUE}.
*
*
Some operating systems do not support shared locks, in which case a
* request for a shared lock is automatically converted into a request for
* an exclusive lock. Whether the newly-acquired lock is shared or
* exclusive may be tested by invoking the resulting lock object's {@link
* FileLock#isShared() isShared} method.
*
*
File locks are held on behalf of the entire Java virtual machine.
* They are not suitable for controlling access to a file by multiple
* threads within the same virtual machine.
*
* @param position
* The position at which the locked region is to start; must be
* non-negative
*
* @param size
* The size of the locked region; must be non-negative, and the sum
* position + size must be non-negative
*
* @param shared
* true to request a shared lock,
* false to request an exclusive lock
*
* @return A lock object representing the newly-acquired lock,
* or null if the lock could not be acquired
* because another program holds an overlapping lock
*
* @throws IllegalArgumentException
* If the preconditions on the parameters do not hold
*
* @throws ClosedChannelException
* If this channel is closed
*
* @throws OverlappingFileLockException
* If a lock that overlaps the requested region is already held by
* this Java virtual machine, or if another thread is already
* blocked in this method and is attempting to lock an overlapping
* region of the same file
*
* @throws IOException
* If some other I/O error occurs
*
* @see #lock()
* @see #lock(long,long,boolean)
* @see #tryLock()
*/
public abstract FileLock tryLock(long position, long size, boolean shared)
throws IOException;
/**
* Attempts to acquire an exclusive lock on this channel's file.
*
* An invocation of this method of the form fc.tryLock()
* behaves in exactly the same way as the invocation
*
*
* fc.{@link #tryLock(long,long,boolean) tryLock}(0L, Long.MAX_VALUE, false)
*
* @return A lock object representing the newly-acquired lock,
* or null if the lock could not be acquired
* because another program holds an overlapping lock
*
* @throws ClosedChannelException
* If this channel is closed
*
* @throws OverlappingFileLockException
* If a lock that overlaps the requested region is already held by
* this Java virtual machine, or if another thread is already
* blocked in this method and is attempting to lock an overlapping
* region
*
* @throws IOException
* If some other I/O error occurs
*
* @see #lock()
* @see #lock(long,long,boolean)
* @see #tryLock(long,long,boolean)
*/
public final FileLock tryLock() throws IOException {
return tryLock(0L, Long.MAX_VALUE, false);
}
}