java - 为什么 `Paths.get` 方法的行为不同？

Question

此代码片段打印出来 \\home\cay\

Path absolute = Paths.get("/", "home", "cay");
System.out.println(absolute);

然而这个打印出来 myprog\conf\user.properties

Path relative = Paths.get("myprog", "conf", "user.properties");
System.out.println(relative);

我有两个问题:

为什么有两个领先\\在第一种情况下？

为什么会有尾随 \在第一种情况下？

ps:我在Windows系统上。

Answer 1

您的第一个路径被解释为 UNC path没有对象名称。

怎么发生的？
深入实现，我们可以发现以下内容:

sun.nio.fs.WindowsPathParser

/**
  * Parses the given input as a Windows path.
  * 
  * @param   input - 1st case: '/\home\cay' ; 2nd case: 'myprog\conf\user.properties'
  * @param   requireToNormalize - true for both cases
  */
private static Result parse(String input, boolean requireToNormalize) {
  String root = "";
  WindowsPathType type = null;

  int len = input.length(); // 1st case: len = 10 , 2nd case: len = 27
  int off = 0;
  if (len > 1) {
    char c0 = input.charAt(0); // 1st case: c0 = '/' , 2nd case: c0 = 'm'
    char c1 = input.charAt(1); // 1st case: c1 = '\' , 2nd case: c1 = 'y'
    int next = 2;
    if (isSlash(c0) && isSlash(c1)) // this condition is true ONLY for 1st case
    {
       type = WindowsPathType.UNC;
       off = nextNonSlash(input, next, len); // 1st case: off = 2
       next = nextSlash(input, off, len);    // 1st case: next = 6
       if (off == next)
         throw new InvalidPathException(input, "UNC path is missing hostname");

       String host = input.substring(off, next); // 1st case: host = home
       off = nextNonSlash(input, next, len);     // 1st case: off = 7
       next = nextSlash(input, off, len);        // 1st case: next = 10
       if (off == next)
         throw new InvalidPathException(input, "UNC path is missing sharename");

       root = "\\\\" + host + "\\" + input.substring(off, next) + "\\"; // 1st case: root = \\home\cay\
       off = next; // 1st case: off = next = 10
    } else {
      if (isLetter(c0) && c1 == ':') { // this condition is false for 2nd case
        ...
      }
    }
 }
 if (off == 0) { // 1st case: off = 10, 2nd case: off = 0
    if (len > 0 && isSlash(input.charAt(0))) { // 2nd case: false
      type = WindowsPathType.DIRECTORY_RELATIVE;
      root = "\\";
    } else {
       type = WindowsPathType.RELATIVE;
    }
  }

  if (requireToNormalize) { // true for both cases
    StringBuilder sb = new StringBuilder(input.length());
    sb.append(root);
    /* 
      1st case: type = UNC, root = '\\home\cay\' , off = 10 , normalize(...) = '\\home\cay\'
      2nd case: type = RELATIVE, root = '' , off = 0 , normalize(...) = 'myprog\conf\user.properties'
     */
     return new Result(type, root, normalize(sb, input, off));
   } else {
     ...
   }
}

private static final boolean isSlash(char c) {
  return (c == '\\') || (c == '/');
}

和 sun.nio.fs.WindowsPathType

因此，对于第一种情况，您将拥有 UNC 路径类型，对于第二种情况， 亲戚路径类型。

如果您尝试运行以下代码段，您将得到稍微不同的结果(没有最终的 \ ，因为 对象名称 将出现):

Path uncPath = Paths.get("/", "my-host", "share", "obj");
System.out.println(uncPath); //  \\my-host\share\obj

如果您尝试运行以下代码段:

Path uncPath = Paths.get("/", "my-host");

你会得到:

java.nio.file.InvalidPathException: UNC path is missing sharename: /\my-host
    at sun.nio.fs.WindowsPathParser.parse(WindowsPathParser.java:118)
    at sun.nio.fs.WindowsPathParser.parse(WindowsPathParser.java:77)
    at sun.nio.fs.WindowsPath.parse(WindowsPath.java:94)
    at sun.nio.fs.WindowsFileSystem.getPath(WindowsFileSystem.java:255)
    at java.nio.file.Paths.get(Paths.java:84)

因为根据 this :

In a UNC path used to access files and directories in an SMB share, for example, object-name can be the name of a file or a directory. The host-name, share-name, and object-name are referred to as "pathname components" or "path components". A valid UNC path consists of two or more path components.