为文件存储预分配驱动器空间

Question

是否有Java方法为应用程序中的独占使用预先分配驱动器空间？

不要求此空间是单独的文件系统或现有文件系统的一部分(因此很容易成为数据库),但它应该允许保留指定的空间量并允许随机读取/写入具有足够高的吞吐量.

Answer 1

这是我基于JNA的fallocate解决方案的精简版.主要技巧是获取本机文件描述符.到目前为止,我只在Linux上进行了测试,但它应该适用于所有现代POSIX /非Windows系统.在Windows上没有必要,因为Windows默认情况下不会创建稀疏文件(仅限于StandardOpenOption.SPARSE),因此RandomAccessFile.setLength(size)或者FileChannel.write(ByteBuffer.allocate(1), size - 1)足够.

/**
 * Provides access to operating system-specific {@code fallocate} and
 * {@code posix_fallocate} functions.
 */
public final class Fallocate {

    private static final boolean IS_LINUX = Platform.isLinux();
    private static final boolean IS_POSIX = !Platform.isWindows();

    private static final int FALLOC_FL_KEEP_SIZE = 0x01;

    private final int fd;
    private int mode;
    private long offset;
    private final long length;

    private Fallocate(int fd, long length) {
        if (!isSupported()) {
            throwUnsupported("fallocate");
        }
        this.fd = fd;
        this.length = length;
    }

    public static boolean isSupported() {
        return IS_POSIX;
    }

    public static Fallocate forChannel(FileChannel channel, long length) {
        return new Fallocate(getDescriptor(channel), length);
    }

    public static Fallocate forDescriptor(FileDescriptor descriptor, long length) {
        return new Fallocate(getDescriptor(descriptor), length);
    }

    public Fallocate fromOffset(long offset) {
        this.offset = offset;
        return this;
    }

    public Fallocate keepSize() {
        requireLinux("fallocate keep size");
        mode |= FALLOC_FL_KEEP_SIZE;
        return this;
    }

    private void requireLinux(String feature) {
        if (!IS_LINUX) {
            throwUnsupported(feature);
        }
    }

    private void throwUnsupported(String feature) {
        throw new UnsupportedOperationException(feature +
                " is not supported on this operating system");
    }

    public void execute() throws IOException {
        final int errno;
        if (IS_LINUX) {
            final int result = FallocateHolder.fallocate(fd, mode, offset, length);
            errno = result == 0 ? 0 : Native.getLastError();
        } else {
            errno = PosixFallocateHolder.posix_fallocate(fd, offset, length);
        }
        if (errno != 0) {
            throw new IOException("fallocate returned " + errno);
        }
    }

    private static class FallocateHolder {

        static {
            Native.register(Platform.C_LIBRARY_NAME);
        }

        private static native int fallocate(int fd, int mode, long offset, long length);
    }

    private static class PosixFallocateHolder {

        static {
            Native.register(Platform.C_LIBRARY_NAME);
        }

        private static native int posix_fallocate(int fd, long offset, long length);
    }

    private static int getDescriptor(FileChannel channel) {
        try {
            // sun.nio.ch.FileChannelImpl declares private final java.io.FileDescriptor fd
            final Field field = channel.getClass().getDeclaredField("fd");
            field.setAccessible(true);
            return getDescriptor((FileDescriptor) field.get(channel));
        } catch (final Exception e) {
            throw new UnsupportedOperationException("unsupported FileChannel implementation", e);
        }
    }

    private static int getDescriptor(FileDescriptor descriptor) {
        try {
            // Oracle java.io.FileDescriptor declares private int fd
            final Field field = descriptor.getClass().getDeclaredField("fd");
            field.setAccessible(true);
            return (int) field.get(descriptor);
        } catch (final Exception e) {
            throw new UnsupportedOperationException("unsupported FileDescriptor implementation", e);
        }
    }
}

Answer 2

您可以尝试使用RandomAccessFile对象并使用setLength()方法.

例:

File file = ... //Create a temporary file on the filesystem your trying to reserve.
long bytes = ... //number of bytes you want to reserve.

RandomAccessFile rf = null;
try{
    rf = new RandomAccessFile(file, "rw"); //rw stands for open in read/write mode.
    rf.setLength(bytes); //This will cause java to "reserve" memory for your application by inflating/truncating the file to the specific size.

    //Do whatever you want with the space here...
}catch(IOException ex){
    //Handle this...
}finally{
    if(rf != null){
        try{
            rf.close(); //Lets be nice and tidy here.
        }catch(IOException ioex){
            //Handle this if you want...
        }
    }
}

注意:在创建RandomAccessFile对象之前,该文件必须存在.

然后可以使用RandomAccessFile对象读取/写入文件.确保目标文件系统有足够的可用空间.空间可能不是"独占",但您可以随时使用文件锁来执行此操作.

PS:如果您最终意识到硬盘驱动器速度慢且无用(或者从一开始就意味着使用RAM),您可以使用java.nio中的ByteBuffer对象.allocate()和allocateDirect()方法应该足够了.字节缓冲区将被分配到RAM(和可能的SwapFile)中,并且将独占于此java程序.可以通过改变缓冲区的位置来进行随机访问.由于这些缓冲区使用有符号整数来引用位置,因此最大大小限制为2 ^ 31 - 1.

在此处阅读有关RandomAccessFile的更多信息.

在这里阅读有关FileLock(java对象)的更多信息.

在这里阅读更多关于ByteBuffer的信息.