Java 学习笔记- File I/O - java.nio.file.Path(Jdk7) 与java.io.File比较

Path 类是jdk7新增加的特性之一，用来代替java.io.File类。

之所以新增这个类，是由于java.io.File类有很多缺陷：

 

1.java.io.File类里面很多方法失败时没有异常处理，或抛出异常，例如：

             

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

 java.io.File.delete()方法返回一个布尔值指示成功或失败但是没有失败原因。而java.nio.file.Files.delete(Path):

 

 

public static void delete(Path path) throws IOException {
        provider(path).delete(path);
    }

 这个方法会抛出：NoSuchFileException，DirectoryNotEmptyException，IOException，SecurityException这样当删除一个文件失败时可以根据异常来查找失败原因。

 

例如：

 

package io.path;

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;

public class DeleteFile {

    public static void main(String[] args) throws IOException {
        Path path = Paths.get("H://afile");//This path does not exsit in file system.
        Files.delete(path);
    }
}

 运行结果：

 

Exception in thread "main" java.nio.file.NoSuchFileException: H:\afile
at sun.nio.fs.WindowsException.translateToIOException(WindowsException.java:79)
at sun.nio.fs.WindowsException.rethrowAsIOException(WindowsException.java:97)
at sun.nio.fs.WindowsException.rethrowAsIOException(WindowsException.java:102)
at sun.nio.fs.WindowsFileSystemProvider.implDelete(WindowsFileSystemProvider.java:268)
at sun.nio.fs.AbstractFileSystemProvider.delete(AbstractFileSystemProvider.java:103)
at java.nio.file.Files.delete(Files.java:1077)
at io.path.DeleteFile.main(DeleteFile.java:12)

 

2.java.io.File.rename(File file)方法在不同平台中运行时可能会有问题,这个方法不能将一个文件从一个文件系统移到另一个文件系统，这个方法的操作也不是原子性的，如果参数指定的文件名已经存在这个方法也可能执行失败：

 

package io.path;

import java.io.File;
import java.io.IOException;
import java.nio.file.CopyOption;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.StandardCopyOption;

public class FileRename {

    /**
     * Expected Result: rename beforeRename1.txt to rename1.txt; beforeRename1.txt disappears; and the content of
     * rename1.txt is the same with beforeRename1.txt
     */
    public static void rename1() {

        File file = new File("D:\\360云盘\\javase\\src\\io\\beforeRename1.txt");// The content of beforeRename1.txt is a
                                                                              // string
                                                                              // with value "1111"

        File destinationfile1 = new File("D:\\360云盘\\javase\\src\\io\\rename1.txt");// rename1.txt has already
                                                                                    // existed. and the content is null.
        file.renameTo(destinationfile1);
    }

    /**
     * Expected Result: rename beforeRename2.txt to rename2.txt ;beforeRename2.txt disappears; and the content of
     * rename2.txt is the same with beforeRename2.txt,that is "2222"
     */
    public static void rename2() {

        File file = new File("D:\\360云盘\\javase\\src\\io\\beforeRename2.txt");// The content of beforeRename2.txt is a
                                                                              // string
                                                                              // with value "2222"

        File destinationfile2 = new File("D:\\360云盘\\javase\\src\\io\\rename2.txt");// rename2.txt does not exist.
        file.renameTo(destinationfile2);
    }

    
    /**
     * Expected Result: rename beforeRename3.txt to rename3.txt ;beforeRename3.txt disappears; and the content of
     * rename3.txt is the same with beforeRename3.txt ,that is "3333"
     */
    public static void rename3() throws IOException {
        Path path = Paths.get("D:\\360云盘\\javase\\src\\io\\beforeRename3.txt");// The content of beforeRename3.txt is a
                                                                               // string with value of "3333"

        Path pathRename = Paths.get("D:\\360云盘\\javase\\src\\io\\rename3.txt");// rename3.txt has already existed,and
                                                                               // the content is null.

        Files.move(path, pathRename, StandardCopyOption.REPLACE_EXISTING);
    }

    public static void main(String[] args) throws IOException {
//        rename1();
//        rename2();
        rename3();
    }
}

 

上面这个类中，执行之后会发现，beforeRename1.txt依然还在，rename1.txt的文件内容也米有改变，而beforeRename2.txt文件已经被重命名为rename2.txt，而且rename2.txt的内容与beforeRename2.txt的内容一样 为“2222”。但是如果使用java.nio.Files.move(Path source, Path target, CopyOption... options)可以发现即使 rename3已经存在，也可以把beforeRename3.txt成功的重命名为rename3.txt

 

 

3. 读取文件属性相关

File类中读取文件属性都是一个方法返回一个属性值，而没有能够直接一次返回很多属性的方法，造成访问文件属性时效率的问题。

但是对于jkd7新增的api中可以批量读取文件属性，而且可以访问到文件更详细的属性。

package io.path;

import java.io.File;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.LinkOption;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.attribute.BasicFileAttributes;
import java.util.Map;

public class FileAttribute {

    public static void main(String[] args) throws IOException {
        // ==============Get attribute by File=========================
        File file = new File("D:\\360云盘\\javase\\src\\io\\path.txt");
        System.out.println("isDirectory:" + file.isDirectory());
        System.out.println("isHidden:" + file.isHidden());
        System.out.println("canRead:" + file.canRead());
        System.out.println("canWrite:" + file.canWrite());
        System.out.println("lastModified:" + file.lastModified());
        System.out.println("length:" + file.length());

        // ==============Get attribute by Path=========================

        Path path = Paths.get("D:\\360云盘\\javase\\src\\io\\path.txt");
        readAttributes(path, "*", LinkOption.NOFOLLOW_LINKS);
        System.out.println(Files.getOwner(path, LinkOption.NOFOLLOW_LINKS).getName());

        BasicFileAttributes attributes = Files.readAttributes(path, BasicFileAttributes.class,
                LinkOption.NOFOLLOW_LINKS);
        System.out.println(attributes.fileKey());
        System.out.println(attributes.isOther());
    }

    /**
     * 
     * @param path
     * @param attributes
     * @param linkOption
     * @throws IOException
     */
    public static void readAttributes(Path path, String attributes, LinkOption... linkOption) throws IOException {
        Map<String, Object> map = Files.readAttributes(path, attributes, linkOption);
        for (String s : map.keySet()) {
            System.out.println(s + ":" + map.get(s));
        }
    }

}

 

除了以上还有其它一些问题，例如file.list();方法在处理比较大的目录时效率很低,但是为什么效率低？而java.nio.file是怎样优化的呢？没搞清楚.........

 

 

评论

1 楼 3GQQ2012 2013-09-14  

io.File只能通过listFile一次性全部获取File,但是这样对大目录处理时很耗内存, java.nio.file通过visitor模式和steam进行迭代 每次只处理一个 来改进性能