java File源码理解，探索File路径

1.方法： new File(path);

我们知道根据输入的路径path的不同 ,File可以根据path的不同格式，来访问文件。那么，path的形式有几种呢？

根据源码

可以看出，我们是从WinNTFileSystem 的resolve得到绝对路径

@Override
    public String resolve(File f) {
        String path = f.getPath();
        int pl = f.getPrefixLength();
        if ((pl == 2) && (path.charAt(0) == slash))
            return path;                        /* UNC */
        if (pl == 3)
            return path;                        /* Absolute local */
        if (pl == 0)
            return getUserPath() + slashify(path); /* Completely relative */
        if (pl == 1) {                          /* Drive-relative */
            String up = getUserPath();
            String ud = getDrive(up);
            if (ud != null) return ud + path;
            return up + path;                   /* User dir is a UNC path */
        }
        if (pl == 2) {                          /* Directory-relative */
            String up = getUserPath();
            String ud = getDrive(up);
            if ((ud != null) && path.startsWith(ud))
                return up + slashify(path.substring(2));
            char drive = path.charAt(0);
            String dir = getDriveDirectory(drive);
            String np;
            if (dir != null) {
                /* When resolving a directory-relative path that refers to a
                   drive other than the current drive, insist that the caller
                   have read permission on the result */
                String p = drive + (':' + dir + slashify(path.substring(2)));
                SecurityManager security = System.getSecurityManager();
                try {
                    if (security != null) security.checkRead(p);
                } catch (SecurityException x) {
                    /* Don't disclose the drive's directory in the exception */
                    throw new SecurityException("Cannot resolve path " + path);
                }
                return p;
            }
            return drive + ":" + slashify(path.substring(2)); /* fake it */
        }
        throw new InternalError("Unresolvable path: " + path);
    }

我们需要两个参数，即String path = f.getPath();和int pl = f.getPrefixLength();

得到path

注意：windows 把"\\"当成"\"处理，即 "\\".length() ==1

@Override
    public String normalize(String path) {
        int n = path.length();
        char slash = this.slash;
        char altSlash = this.altSlash;
        char prev = 0;
        for (int i = 0; i  1)) /:2：包含  "\"，并且"\\"，并且i>1
                return normalize(path, n, i - 1);
            if ((c == ':') && (i > 1))   //3：包含 ":",并且i>1
                return normalize(path, n, 0);
            prev = c;
        }
        if (prev == slash) return normalize(path, n, n - 1);
        return path;  //4:若是绝对路径，则直接返回绝对路径
                      //5:"\\" 或 "\" 开头，直接返回
    }

/* Normalize the given pathname, whose length is len, starting at the given
       offset; everything before this offset is already normal. */
    private String normalize(String path, int len, int off) {
        if (len == 0) return path;
        if (off 
路径类型的分类：
@Override
    public int prefixLength(String path) {
        char slash = this.slash;
        int n = path.length();
        if (n == 0) return 0;
        char c0 = path.charAt(0);
        char c1 = (n > 1) ? path.charAt(1) : 0;
        if (c0 == slash) {
            if (c1 == slash) return 2;  /* Absolute UNC pathname "\\\\foo" */ UNC的绝对路径
            return 1;                   /* Drive-relative "\\foo" */ 与驱动盘相对路径
        }
        if (isLetter(c0) && (c1 == ':')) {
            if ((n > 2) && (path.charAt(2) == slash))
                return 3;               /* Absolute local pathname "z:\\foo" */  本地绝对路径
            return 2;                   /* Directory-relative "z:foo" */  目录相对路径
        }
        return 0;                       /* Completely relative */   相对路径
    }
File(path)各种输入的path 及其 绝对路径
总结： 1:输入path 以'/' 或者 ’\\‘ 开头的 ，是以项目所在的硬盘位基础路径
          2:输入path 以 字母开头 的,是以项目的路径为基础路径 即： System.getProperty("user.dir")
          3.输入绝对路径的，就是以该绝对路径做为路径咯