Pro Java 7 NIO 2读书笔记

转自

花了几天把这本书初略的看了一下， 之所以初略的看了一下，是因为这书里确实没啥内容。如果去了NIO 1的内容和大段大段的代码， 基本上压缩到50页应该没问题。

下面简单的罗列一下看到的内容：

Path

这个类在java.nio.file，在NIO里对文件系统进行了进一步的抽象。 是用来替换原来的java.io.File。其中 FileSystems, Files, Path, PathMatcher 成为一个体系。

在java7里File和Path可以相互转换： File.toPath(), Path.toFile()
原来的File里包含了文件引用和文件，在Path系统里， Path里是文件的引用，而文件操作都放到了Files的静态方法里。这种方式究竟好不好用，我没啥感觉。不过我个人偏向于把操作放到另外一个类里面。

获得Path实例的方式：
File.toPath(), Paths.get(), FileSystem.getPath()

注意： Path和File一样，能创建出实例不代表着这个文件一定在文件系统里真是存在。

Path和File相比使用上方便了的地方：

不用特意指定路径的separator：
Paths.get("C:","folder1","subfolder","aa.txt")

在windows系统了是C:\folder1\subfolder\aa.txt
在Mac下是C:/folder/subfolder/aa.txt
Java会根据当前操作系统来决定separator

重复利用基本路径：

//define the fixed path
Path base = Paths.get("C:/rafaelnadal/tournaments/2009");
//resolve BNP.txt file
Path path_1 = base.resolve("BNP.txt");
//output: C:\rafaelnadal\tournaments\2009\BNP.txt
System.out.println(path_1.toString());
//resolve AEGON.txt file
Path path_2 = base.resolve("AEGON.txt");
//output: C:\rafaelnadal\tournaments\2009\AEGON.txt
System.out.println(path_2.toString());

而resolveSibling方法更加好用：可以直接获取相同目录下的其他文件：

//define the fixed path
Path base = Paths.get("C:/rafaelnadal/tournaments/2009/BNP.txt");
//resolve sibling AEGON.txt file
Path path = base.resolveSibling("AEGON.txt");
//output: C:\rafaelnadal\tournaments\2009\AEGON.txt
System.out.println(path.toString());

取得相对路径：
假设BNP.txt和AEGON.txt在同一目录下

Path path01 = Paths.get("BNP.txt");
Path path02 = Paths.get("AEGON.txt");

//output: ..\AEGON.txt
Path path01_to_path02 = path01.relativize(path02);
System.out.println(path01_to_path02);
//output: ..\BNP.txt
Path path02_to_path01 = path02.relativize(path01);
System.out.println(path02_to_path01);

假设：

Path path01 = Paths.get("/tournaments/2009/BNP.txt");
Path path02 = Paths.get("/tournaments/2011");

那么：

//output: ..\..\2011
Path path01_to_path02 = path01.relativize(path02);
System.out.println(path01_to_path02);
//output: ..\2009\BNP.txt
Path path02_to_path01 = path02.relativize(path01);
System.out.println(path02_to_path01);

遍历：
Path实现了Iterable接口，也就是说我们可以这样：

Path path = Paths.get("C:", "rafaelnadal/tournaments/2009", "BNP.txt");
for (Path name : path) {
    System.out.println(name);
}

猜猜结果是什么？
我一开始以为是吧目录下的文件遍历出来呢。结果是这样的：
rafaelnadal
tournaments
2009
BNP.txt
说实话，我觉得这个比较坑爹！

文件属性

java.nio.file.attribute包下的类提供了获取文件属性的类，针对不同操作系统使用的类不太一样，当然也有所有操作系统通用的属性。

属性分类有一些几种：
BasicFileAttributeView
DosFileAttributeView
PosixFileAttributeView
FileOwnerAttributeView
AclFileAttributeView
UserDefinedFileAttributeView

获取属性：

BasicFileAttributes attr = null;
Path path = Paths.get("C:/rafaelnadal/tournaments/2009", "BNP.txt");
try {
    attr = Files.readAttributes(path, BasicFileAttributes.class);
} catch (IOException e) {
    System.err.println(e);
}
System.out.println("File size: " + attr.size());
System.out.println("File creation time: " + attr.creationTime());
System.out.println("File was last accessed at: " + attr.lastAccessTime());
System.out.println("File was last modified at: " + attr.lastModifiedTime());
System.out.println("Is directory? " + attr.isDirectory());
System.out.println("Is regular file? " + attr.isRegularFile());
System.out.println("Is symbolic link? " + attr.isSymbolicLink());
System.out.println("Is other? " + attr.isOther());

或

long size = (Long)Files.getAttribute(path, "basic:size", NOFOLLOW_LINKS);

设置属性：

Path path = Paths.get("C:/rafaelnadal/tournaments/2009", "BNP.txt");
long time = System.currentTimeMillis();
FileTime fileTime = FileTime.fromMillis(time);
try {
    Files.getFileAttributeView(path,
        BasicFileAttributeView.class).setTimes(fileTime, fileTime, fileTime);
} catch (IOException e) {
    System.err.println(e);
}

或

long time = System.currentTimeMillis();
FileTime fileTime = FileTime.fromMillis(time);
try {
    Files.setLastModifiedTime(path, fileTime);
} catch (IOException e) {
    System.err.println(e);
}

Symbolic和Hard Links
相同于用Java程序实现linux下的 ln命令。

Path link = FileSystems.getDefault().getPath("rafael.nadal.1");
Path target= FileSystems.getDefault().getPath("C:/rafaelnadal/photos", "rafa_winner.jpg");
Files.createSymbolicLink(link, target);

DirectoryStream

这也是一个比较有用的类：用来遍历路径下的子路径或文件，而且支持通配符过滤。

Path path = Paths.get("C:/rafaelnadal/tournaments/2009");
…
//glob pattern applied
System.out.println("\nGlob pattern applied:");
try (DirectoryStream<Path> ds = Files.newDirectoryStream(path, "*.{png,jpg,bmp}")) {
    for (Path file : ds) {
        System.out.println(file.getFileName());
    }
} catch (IOException e) {
    System.err.println(e);
}

FileVisitor和Files.walkFileTree

FileVisitor是一个接口，Files.walkFileTree是一个方法。 通过两者的配合可以遍历整个某个路径下的所有子路径和文件。没有这个之前我们用递归方法也能实现，有了这个不能说是现实更加容易， 只能说是现实更加规范， 如果大家都用这个，代码的可维护性会更好。我觉得仅此而已。

FileVisitor有四个方法

• FileVisitResult postVisitDirectory(T dir, IOException exc)
  Invoked for a directory after entries in the directory, and all of their descendants, have been visited.
• FileVisitResult preVisitDirectory(T dir, BasicFileAttributes attrs)
  Invoked for a directory before entries in the directory are visited.
  +FileVisitResult visitFile(T file, BasicFileAttributes attrs)
  Invoked for a file in a directory.
• FileVisitResult visitFileFailed(T file, IOException exc)
  Invoked for a file that could not be visited.

FileVisitResult枚举类型：
CONTINUE，SKIP_SIBLINGS，SKIP_SUBTREE，TERMINATE

Watch Service API

这是NIO2里比较重要的一个新增功能， 以前直接用java监视文件系统的变化是不可能的，只能通过jni的方式调用操作系统的api来对文件系统进行监视。在java7里这部分被加到了标准库里，这样我们就不能在去寻找jni的结果方案了。但是事实上为了保持java的扩平台特性，监控的功能范围被定为各个操作系统的交集，所以没有特殊的情况还是需要直接调用操作系统的api来实现。

public void watchRNDir(Path path) throws IOException, InterruptedException {
    try (WatchService watchService = FileSystems.getDefault().newWatchService()) {
        path.register(watchService, StandardWatchEventKinds.ENTRY_CREATE,
        StandardWatchEventKinds.ENTRY_MODIFY, StandardWatchEventKinds.ENTRY_DELETE);

        //start an infinite loop
        while (true) {
            //retrieve and remove the next watch key
            final WatchKey key = watchService.take();
            //get list of pending events for the watch key
            for (WatchEvent<?> watchEvent : key.pollEvents()) {
                //get the kind of event (create, modify, delete)
                final Kind<?> kind = watchEvent.kind();
                //handle OVERFLOW event
                if (kind == StandardWatchEventKinds.OVERFLOW) {
                    continue;
                }
                //get the filename for the event
                final WatchEvent<Path> watchEventPath = (WatchEvent<Path>) watchEvent;
                final Path filename = watchEventPath.context();
                //print it out
                System.out.println(kind + " -> " + filename);
            }
            //reset the key
            boolean valid = key.reset();
            //exit loop if the key is not valid (if the directory was deleted, for example)
            if (!valid) {
                break;
            }
        }
    }
}

Random Access Files

主要是提供了一个SeekableByteChannel接口，配合ByteBuffer使随机访问文件更加方便。

Path path = Paths.get("C:/rafaelnadal/tournaments/2009", "MovistarOpen.txt");
ByteBuffer buffer = ByteBuffer.allocate(1);
String encoding = System.getProperty("file.encoding");
try (SeekableByteChannel seekableByteChannel = (Files.newByteChannel(path, EnumSet.of(StandardOpenOption.READ)))) {
    //the initial position should be 0 anyway
    seekableByteChannel.position(0);
    System.out.println("Reading one character from position: " + seekableByteChannel.position());
    seekableByteChannel.read(buffer);
    buffer.flip();
    System.out.print(Charset.forName(encoding).decode(buffer));
    buffer.rewind();
    //get into the middle
    seekableByteChannel.position(seekableByteChannel.size()/2);
    System.out.println("\nReading one character from position: " + seekableByteChannel.position());
    seekableByteChannel.read(buffer);
    buffer.flip();
    System.out.print(Charset.forName(encoding).decode(buffer));
    buffer.rewind();
    //get to the end
    seekableByteChannel.position(seekableByteChannel.size()-1);
    System.out.println("\nReading one character from position: " + seekableByteChannel.position());
    seekableByteChannel.read(buffer);
    buffer.flip();
    System.out.print(Charset.forName(encoding).decode(buffer));
    buffer.clear();
} catch (IOException ex) {
    System.err.println(ex);
}

Asynchronous Channel API

这个是NIO2的较大的变化，有原来的Selecor方法变成方法回调模式。使用上更加方便。并且文件的读写也可以异步的方式实现了。

异步读取文件 ：

ByteBuffer buffer = ByteBuffer.allocate(100);
Path path = Paths.get("C:/rafaelnadal/grandslam/RolandGarros", "story.txt");
try (AsynchronousFileChannel asynchronousFileChannel = AsynchronousFileChannel.open(path, StandardOpenOption.READ)) {
    current = Thread.currentThread();
    asynchronousFileChannel.read(buffer, 0, "Read operation status ...", new CompletionHandler<Integer, Object>() {
        @Override
        public void completed(Integer result, Object attachment) {
            System.out.println(attachment);
            System.out.print("Read bytes: " + result);
            current.interrupt();
        }
        @Override
        public void failed(Throwable exc, Object attachment) {
            System.out.println(attachment);
            System.out.println("Error:" + exc);
            current.interrupt();
        }
    });
    System.out.println("\nWaiting for reading operation to end ...\n");
    try {
        current.join();
    } catch (InterruptedException e) {
    }
    //now the buffer contains the read bytes
    System.out.println("\n\nClose everything and leave! Bye, bye ...");
} catch (Exception ex) {
    System.err.println(ex);
}

异步socket服务器

public class AsyncEchoServer {

    private AsynchronousServerSocketChannel serverChannel;

    public void start() throws IOException {
        System.out.println(String.format("start: name: %s", Thread.currentThread().getName()));
        serverChannel = AsynchronousServerSocketChannel.open();
        serverChannel.setOption(StandardSocketOptions.SO_REUSEADDR, true);
        serverChannel.bind(new InetSocketAddress(8000));
        serverChannel.accept(serverChannel, new Acceptor());
    }
    
    class Acceptor implements CompletionHandler<AsynchronousSocketChannel, AsynchronousServerSocketChannel> {
        
        private final ByteBuffer buffer = ByteBuffer.allocate(1024);
        
        public Acceptor(){
            System.out.println("an acceptor has created.");
        }
        
        public void completed(final AsynchronousSocketChannel channel, AsynchronousServerSocketChannel serverChannel) {
            System.out.println(String.format("write: name: %s", Thread.currentThread().getName()));
            channel.read(buffer, channel, new Reader(buffer));
            serverChannel.accept(serverChannel, new Acceptor());
        }
        
        public void failed(Throwable exception, AsynchronousServerSocketChannel serverChannel) {
            throw new RuntimeException(exception);
        }
    }
    
    class Reader implements CompletionHandler<Integer, AsynchronousSocketChannel> {
        
        private ByteBuffer buffer;

        public Reader(ByteBuffer buffer){
            this.buffer = buffer;
        }
        
        public void completed(Integer result, AsynchronousSocketChannel channel){
            System.out.println(String.format("read: name: %s", Thread.currentThread().getName()));
            if(result != null && result < 0){
                try{
                  channel.close();
                  return;
                }catch(IOException ignore){}
            }
            buffer.flip();
            channel.write(buffer, channel, new Writer(buffer));
        }
        public void failed(Throwable exception, AsynchronousSocketChannel channel){
            throw new RuntimeException(exception);
        }                    
    }
    
    class Writer implements CompletionHandler<Integer, AsynchronousSocketChannel> {
    
        private ByteBuffer buffer;
        
        public Writer(ByteBuffer buffer){
            this.buffer = buffer;
        }
        
        public void completed(Integer result, AsynchronousSocketChannel channel) {
            System.out.println(String.format("write: name: %s", Thread.currentThread().getName()));
            buffer.clear();
            channel.read(buffer, channel, new Reader(buffer));
        }
        
        public void failed(Throwable exception, AsynchronousSocketChannel channel) {
            throw new RuntimeException(exception);
        }
    }
    
    public static void main(String[] args) throws IOException, InterruptedException{
        new AsyncEchoServer().start();
        while(true){
            Thread.sleep(1000L);
        }
    }
}