File 类源代码解析

1. 分隔符

    // 名称分隔符，Windows系统为反斜杠'\\'，Linux系统为斜杠'/'
    public static final char separatorChar = fs.getSeparator();

    // 名称分隔字符串
    public static final String separator = "" + separatorChar;

    // 路径分隔符，Windows系统为分号';'，Linux系统为冒号':'
    public static final char pathSeparatorChar = fs.getPathSeparator();

    // 路径分隔字符串
    public static final String pathSeparator = "" + pathSeparatorChar;

2. 文件路径

    // 标准的符号完全解析的绝对路径，具有唯一性，去除了绝对路径中可能存在的..或.，符号链接(Linux)，标准化盘符字母大小写。
    public String getCanonicalPath() throws IOException {
        return fs.canonicalize(fs.resolve(this));
    }

    // 文件的绝对路径
    public String getAbsolutePath() {
        return fs.resolve(this);
    }

    // ile构造方法里的路径，可以是相对或绝对路径
    public String getPath() {
        return path;
    }

    // 父路径
    public String getParent() {
        int index = path.lastIndexOf(separatorChar); // 最后一个名称分隔符位置
        if (index < prefixLength) {
            if ((prefixLength > 0) && (path.length() > prefixLength))
                return path.substring(0, prefixLength); // 返回前缀
            return null;
        }
        return path.substring(0, index); // 返回分隔符之前的字符串
    }

    // 文件名称
    public String getName() {
        int index = path.lastIndexOf(separatorChar);
        if (index < prefixLength) return path.substring(prefixLength);
        return path.substring(index + 1);
    }

测试：

    System.out.println(System.getProperty("user.dir"));
    
    System.out.println("-----Relative path: different return paths-----");
    File file1 = new File("..\\src\\test1.txt");
    System.out.println(file1.getPath());
    System.out.println(file1.getAbsolutePath());
    System.out.println(file1.getCanonicalPath());
    
    System.out.println("-----Relative path: different return paths-----");
    File file2 = new File(".\\test1.txt");
    System.out.println(file2.getPath());
    System.out.println(file2.getAbsolutePath());
    System.out.println(file2.getCanonicalPath());
    
    System.out.println("-----Absolute path: the same return paths-----");
    File file3 = new File("D:\\workspace\\perfume\\src\\test1.txt");
    System.out.println(file3.getPath());
    System.out.println(file3.getAbsolutePath());
    System.out.println(file3.getCanonicalPath());

输出：
D:\workspace\perfume
-----Relative path: different return paths-----
..\src\test1.txt
D:\workspace\perfume\..\src\test1.txt
D:\workspace\src\test1.txt
-----Relative path: different return paths-----
.\test1.txt
D:\workspace\perfume\.\test1.txt
D:\workspace\perfume\test1.txt
-----Absolute path: the same return paths-----
D:\workspace\perfume\src\test1.txt
D:\workspace\perfume\src\test1.txt
D:\workspace\perfume\src\test1.txt

3. 文件属性

    // 是否可读
    public boolean canRead() {
        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            security.checkRead(path);
        }
        return fs.checkAccess(this, FileSystem.ACCESS_READ);
    }

    // 是否可写
    public boolean canWrite() {
        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            security.checkWrite(path);
        }
        return fs.checkAccess(this, FileSystem.ACCESS_WRITE);
    }

    // 文件是否存在
    public boolean exists() {
        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            security.checkRead(path);
        }
        return ((fs.getBooleanAttributes(this) & FileSystem.BA_EXISTS) != 0);
    }

    // 是否是目录
    public boolean isDirectory() {
        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            security.checkRead(path);
        }
        return ((fs.getBooleanAttributes(this) & FileSystem.BA_DIRECTORY)
                != 0);
    }

    // 是否是文件
    public boolean isFile() {
        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            security.checkRead(path);
        }
        return ((fs.getBooleanAttributes(this) & FileSystem.BA_REGULAR) != 0);
    }

    // 是否隐藏
    public boolean isHidden() {
        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            security.checkRead(path);
        }
        return ((fs.getBooleanAttributes(this) & FileSystem.BA_HIDDEN) != 0);
    }

    // 最近访问时间
    public long lastModified() {
        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            security.checkRead(path);
        }
        return fs.getLastModifiedTime(this);
    }

    // 文件长度，如果是目录，返回值未指定
    public long length() {
        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            security.checkRead(path);
        }
        return fs.getLength(this);
    }

4. 文件操作

    // 创建文件
    public boolean createNewFile() throws IOException {
        SecurityManager security = System.getSecurityManager();
        if (security != null) security.checkWrite(path);
        return fs.createFileExclusively(path);
    }

    // 删除文件
    public boolean delete() {
        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            security.checkDelete(path);
        }
        return fs.delete(this);
    }

    // 在虚拟机正常终止的时候删除文件，删除文件的顺序和注册的时候相反，已经注册过的再次调用此方法没有影响
    public void deleteOnExit() {
        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            security.checkDelete(path);
        }
        DeleteOnExitHook.add(path);
    }

    // 获取当前路径所代表的目录下的文件和目录列表，如果不是目录，返回null。
    // 返回的字符串是文件名，不是完整路径。
    // 列表根据alphabetical来排序。
    public String[] list() {
        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            security.checkRead(path);
        }
        return fs.list(this);
    }

    // 同上，加了个文件名称过滤
    public String[] list(FilenameFilter filter) {
        String names[] = list();
        if ((names == null) || (filter == null)) {
            return names;
        }
        List<String> v = new ArrayList<>();
        for (int i = 0 ; i < names.length ; i++) {
            if (filter.accept(this, names[i])) {
                v.add(names[i]);
            }
        }
        return v.toArray(new String[v.size()]);
    }

    // 同上，将路径名转换为文件
    public File[] listFiles() {
        String[] ss = list();
        if (ss == null) return null;
        int n = ss.length;
        File[] fs = new File[n];
        for (int i = 0; i < n; i++) {
            fs[i] = new File(ss[i], this); // 根据路径实例化文件对象
        }
        return fs;
    }

    // 创建目录，如果父目录不存在，抛异常
    public boolean mkdir() {
        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            security.checkWrite(path);
        }
        return fs.createDirectory(this);
    }

    // 创建目录，如果父目录不存在，先创建父目录
    public boolean mkdirs() {
        if (exists()) {
            return false;
        }
        if (mkdir()) {
            return true;
        }
        File canonFile = null;
        try {
            canonFile = getCanonicalFile();
        } catch (IOException e) {
            return false;
        }

        File parent = canonFile.getParentFile();
        return (parent != null && (parent.mkdirs() || parent.exists()) &&
        canonFile.mkdir()); // 当且仅当父目录不为null，父目录创建成功或存在，当前目录创建成功时返回true
    }

    // 重命名文件
    public boolean renameTo(File dest) {
        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            security.checkWrite(path);
            security.checkWrite(dest.path);
        }
        return fs.rename(this, dest);
    }

设置属性

    // 设置文件为只读，直到设置为允许写或删除
    public boolean setReadOnly() {
        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            security.checkWrite(path);
        }
        return fs.setReadOnly(this);
    }

某些设置可能不起作用，如在Windows平台上，setReadable(false)，文件的canRead()还是true：

    String fileName = "D:/spark.txt";
    File file = new File(fileName);
    System.out.println("canRead: " + file.canRead());
    System.out.println("canWrite: " + file.canWrite());
    System.out.println("canExecute: " + file.canExecute());
    System.out.println("-----After changing-----");
    file.setReadable(false); // It doesn't take effect on Windows platform
    System.out.println("canRead: " + file.canRead());
    file.setWritable(false);
    System.out.println("canWrite: " + file.canWrite());
    file.setExecutable(false); // It doesn't take effect on Windows platform
    System.out.println("canExecute: " + file.canExecute());

输出：
canRead: true
canWrite: true
canExecute: true
-----After changing-----
canRead: true
canWrite: false
canExecute: true