Java NIO 2.0：异步网络IO与API实战

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"Java NIO 2.0 异步IO和网络IO API的使用与设计" Java NIO 2，也称为JSR 203，是Java平台对非阻塞I/O（Non-blocking I/O）的一次重大升级，其中最显著的改进就是引入了异步I/O操作，包括对文件和网络的支持。这个特性极大地提升了Java在处理大量并发I/O请求时的性能和效率，因为它允许程序在等待I/O操作完成时，不必被阻塞，而是可以继续执行其他任务。 1. **Path类**：Path类是Java NIO 2的一个核心组件，它作为文件系统路径的抽象表示，提供了丰富的操作方法。例如，可以创建、解析、比较和操作路径，同时支持跨不同文件系统的路径操作。Path类使得与文件系统的交互更加规范和方便，是进行文件操作的首选入口点。 2. **元数据和文件属性**：在Java NIO 2中，可以获取文件的元数据，如文件大小、最后修改时间、访问权限等。这通过`FileAttribute`接口和`Files`类的方法实现，使得读取和设置文件属性更加简单。 3. **符号链接和硬链接管理**：NIO 2增加了对符号链接和硬链接的支持，允许程序创建、查询和遍历这些特殊类型的文件，这在处理文件系统结构时非常有用。 4. **文件和目录操作**：`Files`类提供了大量用于处理文件和目录的方法，包括创建、删除、移动、复制文件，以及遍历目录树等，这比传统的Java IO API更加强大和灵活。 5. **递归操作和文件遍历**：`Files.walk()`方法允许程序递归地遍历目录结构，查找匹配特定条件的文件，这对于批量处理或搜索文件系统中的文件非常有用。 6. **WatchService API**：这个API允许程序监听文件系统事件，如文件创建、删除或修改，从而实现实时监控和响应文件系统的变化。 7. **随机访问文件**：`RandomAccessFile`类在Java NIO 2中得到了改进，提供了对文件的随机读写操作，支持在文件的任意位置进行读写，而不仅仅是从头到尾。 8. **Sockets API**：Java NIO 2扩展了Socket API，增加了非阻塞的套接字操作，如`SocketChannel`和`ServerSocketChannel`，它们支持异步I/O，可以高效处理多个客户端连接。 9. **异步通道API**：`AsynchronousChannelGroup`和`AsynchronousSocketChannel`等类实现了异步I/O的核心功能。异步通道允许程序注册监听器来处理I/O事件，而不是等待I/O操作完成，从而提高了系统吞吐量。 10. **重要事项**：在使用Java NIO 2时，需要注意线程安全、异常处理、资源管理等问题，确保程序的健壮性和性能。 Java NIO 2的这些新特性不仅提升了I/O操作的效率，还简化了开发人员处理复杂I/O任务的难度，使得Java在处理大数据、高并发场景时表现更加出色。通过熟练掌握这些知识点，开发者可以构建出更加高效和可靠的文件系统和网络应用程序。

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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