Java NIO —— 用 Path 取代 File

川长思鸟来 2022-07-12 13:27 200阅读 0赞

# Java7中文件IO发生了很大的变化，专门引入了很多新的类： #

import java.nio.file.DirectoryStream;
    import java.nio.file.FileSystem;
    import java.nio.file.FileSystems;
    import java.nio.file.Files;
    import java.nio.file.Path;
    import java.nio.file.Paths;
    import java.nio.file.attribute.FileAttribute;
    import java.nio.file.attribute.PosixFilePermission;
    import java.nio.file.attribute.PosixFilePermissions;

……等等，来取代原来的基于java.io.File的文件IO操作方式.

# Path #

Path 用于来表示文件路径和文件。

## construct method（构造方法） ##

*  Paths.get(String first, String… more)

/* 源码
    public static Path get(String first, String... more) { return FileSystems.getDefault().getPath(first, more); }
    */
    Path path1 = Paths.get("C:\\Users\\Administrator\\Desktop\\test");
    Path path2 = Paths.get("C:\\Users\\Administrator\\Desktop\\", "test.txt");

*  Paths.get(URI uri)

/* 源码
    public static Path get(URI uri) {
        String scheme =  uri.getScheme();
        if (scheme == null)
            throw new IllegalArgumentException("Missing scheme");
    
        // check for default provider to avoid loading of installed providers
        if (scheme.equalsIgnoreCase("file"))
            return FileSystems.getDefault().provider().getPath(uri);
    
        // try to find provider
        for (FileSystemProvider provider: FileSystemProvider.installedProviders()) {
            if (provider.getScheme().equalsIgnoreCase(scheme)) {
                return provider.getPath(uri);
            }
        }
    
        throw new FileSystemNotFoundException("Provider \"" + scheme + "\" not installed");
    }
    */
    URI uri = URI.create("file:///C:/Users/Administrator/Desktop/test")

*  FileSystems.getDefault().getPath(String first, String… more)  
    从源码可以看出Path.get本质上是调用该方法的

Path path = FileSystems.getDefault().getPath("C:\\Users\\Administrator\\Desktop\\log.txt");

*  file.toPath()

File file = new File("C:/Users/Administrator/Desktop/log.txt");
    Path path = file.toPath();
    File f = path.toFile();
    URI uri = file.toURI();

## path.isAbsolute() & path.toAbsolutePath() ##

> An absolute path is complete in that it doesn’t need to be combined with other path information in order to locate a file.

一个绝对路径是完整的，因为它不需要与其他路径信息合并来定位一个文件。

也就是说，如果它是相对当前目录进行定位的，那就是相对路径，否则就是绝对路径。

Path path1 = Paths.get("C:\\Users\\Administrator\\Desktop");
    Path path2 = Paths.get("C:\\Users\\..\\Windows");
    Path path3 = Paths.get("..\\");
    
    System.out.println(path1.toAbsolutePath() + " isAbsolute " + path1.isAbsolute());
    System.out.println(path2.toAbsolutePath() + " isAbsolute " + path2.isAbsolute());
    System.out.println(path3.toAbsolutePath() + " isAbsolute " + path3.isAbsolute());

输出

C:\Users\Administrator\Desktop isAbsolute true
    C:\Users\..\Windows isAbsolute true
    D:\workspace\Test\.. isAbsolute false

## path.getRoot() ##

> Returns the root component of this path as a Path object, or null if this path does not have a root component.

返回该路径的根组件作为一个路径对象，或者返回空，如果该路径没有一个根组件。

那么什么样的路径没有根组件呢？就是上面所说的相对路径。

Path path1 = Paths.get("C:\\Users\\Administrator\\Desktop");
    Path path2 = Paths.get("C:\\Users\\..\\Windows");
    Path path3 = Paths.get("..\\");
    Path path4 = Paths.get("..\\workspace");
    
    System.out.println(path1 + " getRoot=> " + path1.getRoot());
    System.out.println(path2 + " getRoot=> " + path2.getRoot());
    System.out.println(path3 + " getRoot=> " + path3.getRoot());
    System.out.println(path4 + " getRoot=> " + path4.getRoot());

输出

C:\Users\Administrator\Desktop getRoot=> C:\ C:\Users\..\Windows getRoot=> C:\ .. getRoot=> null
    ..\workspace getRoot=> null

## path.getFileName() ##

文件名一般是路径的最后一个名字组件。

注意以下两点：

*  如果该路径是绝对路径的根，那么它不存在文件名。
 *  相对路径一定存在文件名，即使是“..”也可以当作文件名的。

Path path1 = Paths.get("C:\\Users\\Administrator\\Desktop");
    Path path2 = Paths.get("C:\\Users\\..\\Windows");
    Path path3 = Paths.get("C:\\");
    Path path4 = Paths.get("C:");
    Path path5 = Paths.get("..\\");
    
    System.out.println(path1 + " getFileName=> " + path1.getFileName());
    System.out.println(path2 + " getFileName=> " + path2.getFileName());
    System.out.println(path3 + " getFileName=> " + path3.getFileName());
    System.out.println(path4 + " getFileName=> " + path4.getFileName());
    System.out.println(path5 + " getFileName=> " + path5.getFileName());

输出

C:\Users\Administrator\Desktop getFileName=> Desktop
    C:\Users\..\Windows getFileName=> Windows
    C:\ getFileName=> null
    C: getFileName=> null
    .. getFileName=> ..

## path.getParent() ##

如果路径中只有一个名字组件，那么它没有父目录，否则有父目录。

要注意：如果路径的最后一个名字组件是“..”，那么getParent() 返回的结果与实际预期的会不一样。要将该路径转化为一个实际路径后（path.toRealPath(LinkOption… options)）再使用该方法。

Path path1 = Paths.get("C:\\Users\\Administrator\\Desktop");
    Path path2 = Paths.get("C:\\Users\\..\\Windows");
    Path path3 = Paths.get("C:\\Users\\..");
    Path path4 = Paths.get("C:\\");
    Path path5 = Paths.get("..\\");
    Path path6 = Paths.get("..\\workspace");
    
    System.out.println(path1 + " getParent=> " + path1.getParent());
    System.out.println(path2 + " getParent=> " + path2.getParent());
    System.out.println(path3 + " getParent=> " + path3.getParent());
    System.out.println(path4 + " getParent=> " + path4.getParent());
    System.out.println(path5 + " getParent=> " + path5.getParent());
    System.out.println(path6 + " getParent=> " + path6.getParent());

输出

C:\Users\Administrator\Desktop getParent=> C:\Users\Administrator
    C:\Users\..\Windows getParent=> C:\Users\..
    C:\Users\.. getParent=> C:\Users
    C:\ getParent=> null
    .. getParent=> null
    ..\workspace getParent=> ..

## path.getNameCount() & path.getName(int) & path.subpath(int beginIndex, int endIndex) ##

Path path1 = Paths.get("C:\\Users\\Administrator\\Desktop");
    Path path2 = Paths.get("C:\\Users\\..\\Windows");
    Path path3 = Paths.get("..\\workspace");
    
    System.out.println(path1 + " getNameCount=> " + path1.getNameCount());
    System.out.println(path2 + " getNameCount=> " + path2.getNameCount());
    System.out.println(path3 + " getNameCount=> " + path3.getNameCount());
    System.out.println();
    System.out.println(path1 + " getName(1)=> " + path1.getName(1));
    System.out.println(path2 + " getName(1)=> " + path2.getName(1));
    System.out.println(path3 + " getName(1)=> " + path3.getName(1));
    System.out.println();
    System.out.println(path1 + " subpath(1,2)=> " + path1.subpath(1,2));
    System.out.println(path2 + " subpath(1,2)=> " + path2.subpath(1,2));
    System.out.println(path3 + " subpath(1,2)=> " + path3.subpath(1,2));

输出

C:\Users\Administrator\Desktop getNameCount=> 3
    C:\Users\..\Windows getNameCount=> 3
    ..\workspace getNameCount=> 2
    
    C:\Users\Administrator\Desktop getName(1)=> Administrator
    C:\Users\..\Windows getName(1)=> ..
    ..\workspace getName(1)=> workspace
    
    C:\Users\Administrator\Desktop subpath(1,2)=> Administrator
    C:\Users\..\Windows subpath(1,2)=> ..
    ..\workspace subpath(1,2)=> workspace

## path.normalize() ##

> Returns a path that is this path with redundant name elements eliminated.

返回一个消除了累赘的名字元素的路径。

该方法与path.toRealPath()的区别是：该方法不会解析以“..”打头的路径，即它利用的只是当前给定的路径信息。

Path path1 = Paths.get("C:\\Users\\..\\Windows");
    Path path2 = Paths.get("C:\\Users\\..");
    Path path3 = Paths.get("..\\");
    Path path4 = Paths.get("..\\workspace");
    
    System.out.println(path1 + " normalize=> " + path1.normalize());
    System.out.println(path2 + " normalize=> " + path2.normalize());
    System.out.println(path3 + " normalize=> " + path3.normalize());
    System.out.println(path4 + " normalize=> " + path4.normalize());

输出

C:\Users\..\Windows normalize=> C:\Windows
    C:\Users\.. normalize=> C:\ .. normalize=> ..
    ..\workspace normalize=> ..\workspace

## path.toRealPath(LinkOption… options) ##

> Returns the real path of an existing file.

将相对路径转化为其实际路径。

Path path1 = Paths.get("C:\\Users\\..\\Windows");
    Path path2 = Paths.get("..\\");
    try {
        System.out.println(path1 + " toRealPath=> " + path1.toRealPath());
        System.out.println(path2 + " toRealPath=> " + path2.toRealPath());
    } catch (IOException e) {
        e.printStackTrace();
    }

输出

C:\Users\..\Windows toRealPath=> C:\Windows
    .. toRealPath=> D:\workspace

## path.resolve & path.resolveSibling ##

path.resolve(Path) 和 path.resolve(String) 在当前目录下拼接给定路径。  
path.resolveSibling(Path) 和 path.resolveSibling(String) 在当前路径的父目录下拼接给定路径（即拼接在其兄弟节点上）

和 Node.js 上的 url 拼接一样用的都是 resolve 方法

Path path1 = Paths.get("C:\\Users");
    Path path2 = Paths.get("Administrator\\Desktop");
    Path path3 = path1.resolve(path2);
    Path path4 = path1.resolve("Administrator\\Desktop");
    Path path5 = path1.resolveSibling(path2);
    Path path6 = path1.resolveSibling("Administrator\\Desktop");
    System.out.println(path3);
    System.out.println(path4);
    System.out.println(path5);
    System.out.println(path6);

输出

C:\Users.txt\Administrator\Desktop
    C:\Users.txt\Administrator\Desktop
    C:\Administrator\Desktop
    C:\Administrator\Desktop
    ##path.toString()

输出Path指向的路径，分隔符用System.getProperty(“file.separator”)的值。

Path path1 = Paths.get("C:\\Users\\Administrator\\Desktop");
    Path path2 = Paths.get("C:/Users/Administrator/Desktop");
    System.out.println(path1.toString());
    System.out.println(path2.toString());
    System.out.println(System.getProperty("file.separator"));

输出

C:\Users\Administrator\Desktop
    C:\Users\Administrator\Desktop
    \