深入探索Java 7 NIO.2 API

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"本书主要探讨Java 7中的NIO2特性，通过深入解析Path类、文件元数据、符号链接管理、文件与目录操作、递归操作、WatchService API、随机访问文件、套接字API以及异步通道API等多个关键主题，帮助读者全面掌握Java平台上的新I/O APIs。" 在Java 7中引入的NIO2（也称为JSR203）对Java的I/O功能进行了重大扩展，提供了更高效、更灵活的文件系统操作。其中，`java.nio.file.Path`类是进入NIO2世界的门户，它是一个抽象类，用于表示文件系统中的路径。这个类具有里程碑式的意义，几乎所有的I/O操作都会用到Path类的强大功能。 Path类提供了一系列方法来处理文件系统的路径，包括创建、解析、比较和转换路径。例如，你可以使用`Paths.get()`静态方法创建一个Path对象，或者使用`Path.toString()`获取路径字符串。此外，Path还支持相对路径和绝对路径之间的转换，以及与其他Path对象的组合。在深入学习Path类之后，书中接下来讨论了文件元数据和文件属性，如权限、所有权、时间戳等。这些信息对于检查或修改文件状态非常有用。同时，Path类还支持处理符号链接和硬链接，这在处理文件系统结构时具有很高的灵活性。接着，书中介绍了对文件和目录的操作，包括创建、删除、移动和复制。这些操作都提供了原子性和异常处理，确保了文件系统的稳定。递归操作章节则关注如何遍历目录树，`Files.walkFileTree()`方法是一个重要的工具，可以方便地实现深度优先或广度优先的文件树遍历。 WatchService API是NIO2中的另一个亮点，它允许程序注册对文件系统事件的兴趣，如文件创建、修改或删除。当事件发生时，WatchService会通知注册的观察者，这样就可以实现文件系统的实时监控。随机访问文件章节涵盖了如何使用NIO2进行高效的读写操作，特别是在处理大文件时。这包括使用`FileChannel`进行内存映射和直接缓冲区操作，以提高性能。此外，NIO2还引入了套接字API的增强，如`SocketChannel`和`ServerSocketChannel`，它们提供了非阻塞I/O，适用于构建高性能的网络应用程序。最后，异步通道API允许异步地执行I/O操作，这对于并发处理大量I/O请求非常有用。书末，作者总结了一些使用NIO2时的重要注意事项，帮助读者避免常见的陷阱和优化代码实践。这本书是Java开发者深入理解并充分利用NIO2功能的宝贵资源，无论你是从事文件系统操作、网络编程还是其他涉及I/O的领域，都能从中受益。通过详尽的示例和讲解，你将能够熟练地运用NIO2提升应用程序的效率和可靠性。

CHAPTER 2 ■

METADATA FILE ATTRIBUTES

You can test a particular view on a file store by calling the FileStore.supportsFileAttributeView()

method. You can pass the desired view as a String or as a class name. The following code checks

whether the basic view is supported by all the available file stores:

import java.nio.file.FileStore;

import java.nio.file.FileSystem;

import java.nio.file.FileSystems;

import java.nio.file.attribute.BasicFileAttributeView;

…

FileSystem fs = FileSystems.getDefault();

for (FileStore store : fs.getFileStores()) {

boolean supported = store.supportsFileAttributeView(BasicFileAttributeView.class);

System.out.println(store.name() + " ---" + supported);

}

Moreover, you can check if a file store in which a particular file resides supports a single view, as

shown in this example:

import java.io.IOException;

import java.nio.file.FileStore;

import java.nio.file.Files;

import java.nio.file.Path;

import java.nio.file.Paths;

…

Path path = Paths.get("C:/rafaelnadal/tournaments/2009", "BNP.txt");

try {

FileStore store = Files.getFileStore(path);

boolean supported = store.supportsFileAttributeView("basic");

System.out.println(store.name() + " ---" + supported);

} catch (IOException e) {

System.err.println(e);

}

Now that you can determine which views are supported on your file system, it is time to dig deeper

and explore each view’s attributes, starting with the basic view.

Basic View

Most file system implementations support a set of common attributes (size, creation time, last accessed

time, last modified time, etc.). These attributes are grouped into a view named BasicFileAttributeView

and can be extracted and set as described in the following subsections.

Get Bulk Attributes with readAttributes()

You can extract attributes in bulk using the readAttributes() method as follows (the varargs argument

currently supports the LinkOption.NOFOLLOW_LINKS enum—do not follow symbolic links):

import java.io.IOException;

import java.nio.file.Files;

import java.nio.file.Path;

CHAPTER 2 ■ METADATA FILE ATTRIBUTES

Updating the file’s last modified time can also be accomplished with the setAttribute() method.

Actually, this method may be used to update the file’s last modified time, last access time, or create time

attributes as if by invoking the setTimes() method:

import static java.nio.file.LinkOption.NOFOLLOW_LINKS;

…

try {

Files.setAttribute(path, "basic:lastModifiedTime", fileTime, NOFOLLOW_LINKS);

Files.setAttribute(path, "basic:creationTime", fileTime, NOFOLLOW_LINKS);

Files.setAttribute(path, "basic:lastAccessTime", fileTime, NOFOLLOW_LINKS);

} catch (IOException e) {

System.err.println(e);

}

Obviously, now you have to extract the three attributes’ values to see the changes. You can do so by

using the getAttribute() method:

try {

FileTime lastModifiedTime = (FileTime)Files.getAttribute(path,

"basic:lastModifiedTime", NOFOLLOW_LINKS);

FileTime creationTime = (FileTime)Files.getAttribute(path,

"basic:creationTime", NOFOLLOW_LINKS);

FileTime lastAccessTime = (FileTime)Files.getAttribute(path,

"basic:lastAccessTime", NOFOLLOW_LINKS);

System.out.println("New last modified time: " + lastModifiedTime);

System.out.println("New creation time: " + creationTime);

System.out.println("New last access time: " + lastAccessTime);

} catch (IOException e) {

System.err.println(e);

}

DOS View

Specific to the DOS file system (or Samba), the DosFileAttributeView view extends the basic view with

the DOS attributes (which means that you can access the basic view directly from DOS view). There are

four attributes, which are mapped by the following methods:

• isReadOnly(): Returns the readonly attribute’s value (if true, the file can’t be

deleted or updated)

• isHidden(): Returns the hidden attribute’s value (if true, the file is not visible to the

users)

• isArchive(): Returns the archive attribute’s value (specific to backup programs)

• isSystem(): Returns the system attribute’s value (if true, the file belongs to the

operating system)

The following listing extracts in bulk the preceding four attributes for a given path:

import java.io.IOException;

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