通过初始化操作建立一个模拟外存空间的虚拟磁盘文件，在该文件中保存目录和文件内容。创建该文件时应创建初始的根目录内容、文件分配表。根目录实为一特殊文件，其开始内容为空，大小为一个块。Java代码实现

以下是一个简单的 Java 代码示例，用于创建一个模拟外存空间的虚拟磁盘文件，并在其中保存目录和文件内容。该代码主要通过文件流来实现文件的读写操作。

import java.io.*;

public class VirtualDisk {
    private static final int BLOCK_SIZE = 1024; // 块大小为 1024 字节
    private static final int BLOCK_NUM = 1024; // 总块数为 1024 个

    private static final int ROOT_BLOCK_INDEX = 0; // 根目录所在块的索引为 0

    private static final int FAT_SIZE = BLOCK_NUM * 4; // 文件分配表的大小为 4 字节 * 总块数

    private static final int DIRECTORY_ENTRY_SIZE = 64; // 目录项大小为 64 字节

    private RandomAccessFile diskFile;

    public VirtualDisk(String fileName) throws IOException {
        diskFile = new RandomAccessFile(fileName, "rw");

        // 初始化文件分配表
        byte[] fat = new byte[FAT_SIZE];
        diskFile.write(fat);

        // 初始化根目录
        byte[] root = new byte[BLOCK_SIZE];
        diskFile.write(root);

        // 将根目录的起始块号写入文件分配表的第一个块中
        int rootBlockNumber = ROOT_BLOCK_INDEX;
        diskFile.seek(rootBlockNumber * 4);
        diskFile.writeInt(rootBlockNumber);
    }

    public void createFile(String path, byte[] content) throws IOException {
        // 拆分路径
        String[] dirs = path.split("/");

        // 从根目录开始查找目录
        int currentBlockNumber = ROOT_BLOCK_INDEX;
        for (int i = 1; i < dirs.length - 1; i++) {
            currentBlockNumber = findDirectoryEntry(currentBlockNumber, dirs[i]);
            if (currentBlockNumber == -1) {
                // 目录不存在，创建目录
                currentBlockNumber = createDirectoryEntry(currentBlockNumber, dirs[i]);
            }
        }

        // 在最后一个目录中创建文件
        createFileEntry(currentBlockNumber, dirs[dirs.length - 1], content);
    }

    private int findDirectoryEntry(int directoryBlockNumber, String name) throws IOException {
        byte[] block = readBlock(directoryBlockNumber);
        for (int i = 0; i < BLOCK_SIZE; i += DIRECTORY_ENTRY_SIZE) {
            String entryName = new String(block, i, 32).trim();
            if (entryName.equals(name)) {
                // 找到目录项，返回起始块号
                return byteArrayToInt(block, i + 32);
            }
        }
        // 目录项不存在，返回 -1
        return -1;
    }

    private int createDirectoryEntry(int directoryBlockNumber, String name) throws IOException {
        byte[] block = readBlock(directoryBlockNumber);
        for (int i = 0; i < BLOCK_SIZE; i += DIRECTORY_ENTRY_SIZE) {
            String entryName = new String(block, i, 32).trim();
            if (entryName.length() == 0) {
                // 找到空闲目录项，创建新目录
                int newBlockNumber = allocateBlock();
                writeBlock(newBlockNumber, new byte[BLOCK_SIZE]);
                writeDirectoryEntry(block, i, name, newBlockNumber);
                return newBlockNumber;
            }
        }
        // 目录已满，无法创建新目录
        return -1;
    }

    private void createFileEntry(int directoryBlockNumber, String name, byte[] content) throws IOException {
        byte[] block = readBlock(directoryBlockNumber);
        for (int i = 0; i < BLOCK_SIZE; i += DIRECTORY_ENTRY_SIZE) {
            String entryName = new String(block, i, 32).trim();
            if (entryName.length() == 0) {
                // 找到空闲目录项，创建新文件
                int newBlockNumber = allocateBlocks(content.length);
                writeBlocks(newBlockNumber, content);
                writeDirectoryEntry(block, i, name, newBlockNumber);
                return;
            }
        }
        // 目录已满，无法创建新文件
    }

    private void writeDirectoryEntry(byte[] block, int offset, String name, int startBlockNumber) throws IOException {
        byte[] nameBytes = name.getBytes();
        System.arraycopy(nameBytes, 0, block, offset, nameBytes.length);
        writeIntToByteArray(startBlockNumber, block, offset + 32);
        diskFile.seek(getFatOffset(startBlockNumber));
        diskFile.writeInt(startBlockNumber);
        diskFile.seek(getFatOffset(startBlockNumber) + 4);
        diskFile.writeInt(-1);
    }

    private byte[] readBlock(int blockNumber) throws IOException {
        byte[] block = new byte[BLOCK_SIZE];
        diskFile.seek(blockNumber * BLOCK_SIZE);
        diskFile.read(block);
        return block;
    }

    private void writeBlock(int blockNumber, byte[] block) throws IOException {
        diskFile.seek(blockNumber * BLOCK_SIZE);
        diskFile.write(block);
    }

    private void writeBlocks(int startBlockNumber, byte[] data) throws IOException {
        int dataOffset = 0;
        int blockNumber = startBlockNumber;
        while (dataOffset < data.length) {
            byte[] block = new byte[BLOCK_SIZE];
            int bytesToWrite = Math.min(BLOCK_SIZE, data.length - dataOffset);
            System.arraycopy(data, dataOffset, block, 0, bytesToWrite);
            writeBlock(blockNumber, block);
            if (dataOffset + BLOCK_SIZE < data.length) {
                int nextBlockNumber = allocateBlock();
                writeIntToByteArray(nextBlockNumber, block, BLOCK_SIZE - 4);
                diskFile.seek(getFatOffset(blockNumber) + 4);
                diskFile.writeInt(nextBlockNumber);
                blockNumber = nextBlockNumber;
            } else {
                diskFile.seek(getFatOffset(blockNumber) + 4);
                diskFile.writeInt(-1);
            }
            dataOffset += BLOCK_SIZE;
        }
    }

    private int allocateBlock() throws IOException {
        byte[] fat = new byte[4];
        diskFile.seek(getFatOffset(0));
        for (int i = 0; i < BLOCK_NUM; i++) {
            diskFile.read(fat);
            if (byteArrayToInt(fat, 0) == 0) {
                // 找到空闲块
                diskFile.seek(getFatOffset(i));
                diskFile.writeInt(-1);
                return i;
            }
        }
        // 没有空闲块了
        return -1;
    }

    private int allocateBlocks(int size) throws IOException {
        int numBlocks = (size + BLOCK_SIZE - 1) / BLOCK_SIZE;
        int[] blockNumbers = new int[numBlocks];
        for (int i = 0; i < numBlocks; i++) {
            blockNumbers[i] = allocateBlock();
            if (blockNumbers[i] == -1) {
                // 没有足够的空闲块
                return -1;
            }
        }
        return blockNumbers[0];
    }

    private int getFatOffset(int blockNumber) {
        return FAT_SIZE + blockNumber * 8;
    }

    private void writeIntToByteArray(int value, byte[] array, int offset) {
        array[offset] = (byte) (value & 0xff);
        array[offset + 1] = (byte) ((value >> 8) & 0xff);
        array[offset + 2] = (byte) ((value >> 16) & 0xff);
        array[offset + 3] = (byte) ((value >> 24) & 0xff);
    }

    private int byteArrayToInt(byte[] array, int offset) {
        return (array[offset] & 0xff) |
                ((array[offset + 1] & 0xff) << 8) |
                ((array[offset + 2] & 0xff) << 16) |
                ((array[offset + 3] & 0xff) << 24);
    }

    public static void main(String[] args) throws IOException {
        VirtualDisk disk = new VirtualDisk("disk.bin");
        disk.createFile("/dir1/dir2/file1", "Hello, world!".getBytes());
        disk.createFile("/dir1/file2", "12345".getBytes());
        disk.createFile("/file3", "abcdefg".getBytes());
    }
}

该程序通过创建一个名为 `disk.bin` 的文件来模拟外存空间的虚拟磁盘。在 `main` 方法中，我们创建了三个文件 `/dir1/dir2/file1`、`/dir1/file2` 和 `/file3`，每个文件都包含不同的内容。在创建文件的过程中，程序会自动创建目录和分配块。