1.方法： new File(path);
我們知道根據輸入的路徑path的不同 ,File可以根據path的不同格式，來訪問文件。那么，path的形式有幾種呢？
根據源碼
可以知道，輸入的路徑path其實是在類FileSystem中處理的。FileSystem是一個抽象類，所以，其實是在其實現類WinNTFileSystem中處理。
設定一個目標，我們要得到文件的絕對地址！
由如下代碼
可以看出，我們是從WinNTFileSystem 的resolve得到絕對路徑
@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); }
我們需要兩個參數，即String path = f.getPath();和int pl = f.getPrefixLength();
得到path
注意：windows 把"\\"當成"\"處理，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 < n; i++) { char c = path.charAt(i); if (c == altSlash) // 1：包含“/” return normalize(path, n, (prev == slash) ? i - 1 : i); if ((c == slash) && (prev == slash) && (i > 1)) /:2：包含 "\"，並且"\\"，並且i>1 return normalize(path, n, i - 1); if ((c == ':') && (i > 1)) //3：包含 ":",並且i>1 return normalize(path, n, 0); prev = c; } if (prev == slash) return normalize(path, n, n - 1); return path; //4:若是絕對路徑，則直接返回絕對路徑
//5:"\\" 或 "\" 開頭，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 < 3) off = 0; /* Avoid fencepost cases with UNC pathnames */ int src; char slash = this.slash; StringBuffer sb = new StringBuffer(len); if (off == 0) { /* Complete normalization, including prefix */ src = normalizePrefix(path, len, sb); } else { /* Partial normalization */ src = off; sb.append(path.substring(0, off)); } /* Remove redundant slashes from the remainder of the path, forcing all slashes into the preferred slash */ while (src < len) { char c = path.charAt(src++); if (isSlash(c)) { while ((src < len) && isSlash(path.charAt(src))) src++; if (src == len) { /* Check for trailing separator */ int sn = sb.length(); if ((sn == 2) && (sb.charAt(1) == ':')) { /* "z:\\" */ sb.append(slash); break; } if (sn == 0) { /* "\\" */ sb.append(slash); break; } if ((sn == 1) && (isSlash(sb.charAt(0)))) { /* "\\\\" is not collapsed to "\\" because "\\\\" marks the beginning of a UNC pathname. Even though it is not, by itself, a valid UNC pathname, we leave it as is in order to be consistent with the win32 APIs, which treat this case as an invalid UNC pathname rather than as an alias for the root directory of the current drive. */ sb.append(slash); break; } /* Path does not denote a root directory, so do not append trailing slash */ break; } else { sb.append(slash); } } else { sb.append(c); } } String rv = sb.toString(); return rv; }
路徑類型的分類：
@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的絕對路徑 return 1; /* Drive-relative "\\foo" */ 與驅動盤相對路徑 } if (isLetter(c0) && (c1 == ':')) { if ((n > 2) && (path.charAt(2) == slash)) return 3; /* Absolute local pathname "z:\\foo" */ 本地絕對路徑 return 2; /* Directory-relative "z:foo" */ 目錄相對路徑 } return 0; /* Completely relative */ 相對路徑 }
File(path)各種輸入的path 及其 絕對路徑
| 輸入的path | /long | lo/ng | \/long | long/k | long | D:\JACK\OK | \\long(或者\long) | //long |
| this.path | \long | lo\ng | X非法輸入 | long\k | long | D:\JACK\OK | \long | \\long |
| this.PrefixLength | 1 | 0 | X | 0 | 0 | 3 | 1 | 2 |
| 相關類型 | 盤相關 | 項目路徑相關 | 項目路徑相關 | 項目路徑相關 | 完全絕對路徑 | 盤相關 | ||
| absolutePath | D:\long | D:\javaTest\long | X | D:\javaTest\long\k | D:\javaTest\long | D:\JACK\OK | D:\long | \\long |
總結： 1:輸入path 以'/' 或者 ’\\‘ 開頭的 ，是以項目所在的硬盤位基礎路徑
2:輸入path 以 字母開頭 的,是以項目的路徑為基礎路徑 即： System.getProperty("user.dir")
3.輸入絕對路徑的，就是以該絕對路徑做為路徑咯