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Java 基础之 IO 和 NIO 补完

本贴最后更新于 2545 天前，其中的信息可能已经东海扬尘

Java Stream，File，IO

java 包之 java.io

java 包之 java.nio

15 Java NIO Path

  • 翻译自：15Java NIO Path
  • Java 的 java.nio.file.Path 接口是 JavaNIO 2 update 的一部分，在 Java6 和 Java7 中得到更新。Java path 接口在 java7 中被加入到 Java NIO 中。Path 接口位于 java.nio.file 包中，所以 Java Path 接口的全名是：java.nio.file.Path。
  • 一个 Java Path 接口代表了一个文件系统中的 path 路径。path 路径可以指向文件或目录。一个路径可以是绝对路径也可以是相对路径。一个绝对路径是从文件系统根目录指向该文件或路径的全路径。一个相对路径是指相对于其他路径一个文件或目录的相对路径。相对路径可能比较费解，不用担心，我会在该文中详细介绍。
  • 对于操作系统中的系统路径这样的环境变量不要疑惑，java Path 接口跟它们没有关系。
  • 在很多方面 java.nio.file.Path 和 java.io.File 类是类似的，在一些地方你可以使用 Path 接口来代替 File 类，但是也有一些不同。

1. 创建 Path 实例

  • 你可以使用 java.nio.file.Paths 类的静态方法 Paths.get()来创建一个 java.nio.file.Path 的实例:
import java.nio.file.Path;
import java.nio.file.Paths;

public class PathExample {

    public static void main(String[] args) {

        Path path = Paths.get("c:\\data\\myfile.txt");

    }
}
  • Paths.get()方法是创建 Path 实例的工厂方法。

2. 创建一个绝对路径 Path 实例

  • Windows 文件系统下的绝对路径：
Path path = Paths.get("c:\\data\\myfile.txt");
  • 类 Unix 操作系统下的绝对路径：
Path path = Paths.get("/home/jakobjenkov/myfile.txt");
//如果你在windows上这么使用，会表示当前系统分区下的路径，如：C:/home/jakobjenkov/myfile.txt

3. 创建一个相对路径 Path 实例

  • 相对路径需要一个 basepath 和 relativepath 来创建：
Path projects = Paths.get("d:\\data", "projects");

Path file     = Paths.get("d:\\data", "projects\\a-project\\myfile.txt");
  • 可以使用'.','..'来创建路径实例：
Path currentDir = Paths.get(".");
System.out.println(currentDir.toAbsolutePath());

4. Path.normalize()

String originalPath =
        "d:\\data\\projects\\a-project\\..\\another-project";

Path path1 = Paths.get(originalPath);
System.out.println("path1 = " + path1);

Path path2 = path1.normalize();
System.out.println("path2 = " + path2);
path1 = d:\data\projects\a-project\..\another-project
path2 = d:\data\projects\another-project

16. Java NIO Files

  • 翻译自：16Java NIO Files
  • java.nio.file.Files 类提供了多种方法来操作文件。

1. Files.exists()

  • LinkOption.NOFOLLOW_LINKS 表示文件存在，但是不能是链接形式的。
Path path = Paths.get("data/logging.properties");

boolean pathExists =
        Files.exists(path,
            new LinkOption[]{ LinkOption.NOFOLLOW_LINKS});

2. Files.createDirectory()

Path path = Paths.get("data/subdir");

try {
    Path newDir = Files.createDirectory(path);
} catch(FileAlreadyExistsException e){
    // the directory already exists.
} catch (IOException e) {
    //something else went wrong
    e.printStackTrace();
}

3. Files.copy()

Path sourcePath      = Paths.get("data/logging.properties");
Path destinationPath = Paths.get("data/logging-copy.properties");

try {
    Files.copy(sourcePath, destinationPath);
} catch(FileAlreadyExistsException e) {
    //destination file already exists
} catch (IOException e) {
    //something else went wrong
    e.printStackTrace();
}

4. Overwriting Existing Files

  • 通过复制选项来覆盖已经存在的 Files
Path sourcePath      = Paths.get("data/logging.properties");
Path destinationPath = Paths.get("data/logging-copy.properties");

try {
    Files.copy(sourcePath, destinationPath,
            StandardCopyOption.REPLACE_EXISTING);
} catch(FileAlreadyExistsException e) {
    //destination file already exists
} catch (IOException e) {
    //something else went wrong
    e.printStackTrace();
}

5. Files.move()

  • java.io.File 有方法 renameTo() ,Files 可以 Files.move()
Path sourcePath      = Paths.get("data/logging-copy.properties");
Path destinationPath = Paths.get("data/subdir/logging-moved.properties");

try {
    Files.move(sourcePath, destinationPath,
            StandardCopyOption.REPLACE_EXISTING);
} catch (IOException e) {
    //moving file failed.
    e.printStackTrace();
}

6. Files.delete()

Path path = Paths.get("data/subdir/logging-moved.properties");

try {
    Files.delete(path);
} catch (IOException e) {
    //deleting file failed
    e.printStackTrace();
}

7. Files.walkFileTree()

  • Files.walkFileTree()用来递归遍历文件目录，有 Path 实例和 FileVisitor 来做参数。
  • FileVisitor 接口有如下形式：
public interface FileVisitor {

    public FileVisitResult preVisitDirectory(
        Path dir, BasicFileAttributes attrs) throws IOException;

    public FileVisitResult visitFile(
        Path file, BasicFileAttributes attrs) throws IOException;

    public FileVisitResult visitFileFailed(
        Path file, IOException exc) throws IOException;

    public FileVisitResult postVisitDirectory(
        Path dir, IOException exc) throws IOException {

}
  • FileVisitor 有一个简单实现 SimpleFileVisitor 类：
Files.walkFileTree(path, new FileVisitor<Path>() {
  @Override
  public FileVisitResult preVisitDirectory(Path dir, BasicFileAttributes attrs) throws IOException {
    System.out.println("pre visit dir:" + dir);
    return FileVisitResult.CONTINUE;
  }

  @Override
  public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) throws IOException {
    System.out.println("visit file: " + file);
    return FileVisitResult.CONTINUE;
  }

  @Override
  public FileVisitResult visitFileFailed(Path file, IOException exc) throws IOException {
    System.out.println("visit file failed: " + file);
    return FileVisitResult.CONTINUE;
  }

  @Override
  public FileVisitResult postVisitDirectory(Path dir, IOException exc) throws IOException {
    System.out.println("post visit directory: " + dir);
    return FileVisitResult.CONTINUE;
  }
});
FileVisitResult的集合有：
CONTINUE 表示继续
TERMINATE 表示停止
SKIP_SIBLINGS 表示继续但是不再便利兄弟目录
SKIP_SUBTREE 表示继续但是不再便利子目录

8. Searching For Files

  • 下面可以实现 SimpleFileVisitor 来查找文件：
Path rootPath = Paths.get("data");
String fileToFind = File.separator + "README.txt";

try {
  Files.walkFileTree(rootPath, new SimpleFileVisitor<Path>() {
    
    @Override
    public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) throws IOException {
      String fileString = file.toAbsolutePath().toString();
      //System.out.println("pathString = " + fileString);

      if(fileString.endsWith(fileToFind)){
        System.out.println("file found at path: " + file.toAbsolutePath());
        return FileVisitResult.TERMINATE;
      }
      return FileVisitResult.CONTINUE;
    }
  });
} catch(IOException e){
    e.printStackTrace();
}

9. Deleting Directories Recursively

Path rootPath = Paths.get("data/to-delete");

try {
  Files.walkFileTree(rootPath, new SimpleFileVisitor<Path>() {
    @Override
    public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) throws IOException {
      System.out.println("delete file: " + file.toString());
      Files.delete(file);
      return FileVisitResult.CONTINUE;
    }

    @Override
    public FileVisitResult postVisitDirectory(Path dir, IOException exc) throws IOException {
      Files.delete(dir);
      System.out.println("delete dir: " + dir.toString());
      return FileVisitResult.CONTINUE;
    }
  });
} catch(IOException e){
  e.printStackTrace();
}

10. Additional Methods in the Files Class

  • Files 类有很多其他方法，如 creating symbolic links, determining the file size, setting file permissions 等。参考 java 文档。

17. Java NIO AsynchronousFileChannel

  • java7 中 AsynchronousFileChannel 加入到了 java NIO 的包中。AsynchronousFileChannels 使得异步读写文件实现。

1. Creating an AsynchronousFileChannel

Path path = Paths.get("data/test.xml");

AsynchronousFileChannel fileChannel =
    AsynchronousFileChannel.open(path, StandardOpenOption.READ);

2. Reading Data

  • Reading Data Via a Future
Future<Integer> operation = fileChannel.read(buffer, 0);
AsynchronousFileChannel fileChannel = 
    AsynchronousFileChannel.open(path, StandardOpenOption.READ);

ByteBuffer buffer = ByteBuffer.allocate(1024);
long position = 0;

Future<Integer> operation = fileChannel.read(buffer, position);
// Of course, this is not a very efficient use of the CPU - but somehow you need to wait until the read operation has completed.

while(!operation.isDone());

buffer.flip();
byte[] data = new byte[buffer.limit()];
buffer.get(data);
System.out.println(new String(data));
buffer.clear();
  • Reading Data Via a CompletionHandler
fileChannel.read(buffer, position, buffer, new CompletionHandler<Integer, ByteBuffer>() {
//Once the read operation finishes the CompletionHandler's completed() method will be called.
    @Override
    public void completed(Integer result, ByteBuffer attachment) {
        System.out.println("result = " + result);

        attachment.flip();
        byte[] data = new byte[attachment.limit()];
        attachment.get(data);
        System.out.println(new String(data));
        attachment.clear();
    }
//If the read operation fails, the failed() method of the CompletionHandler will get called instead.
    @Override
    public void failed(Throwable exc, ByteBuffer attachment) {

    }
});

3. Writing Data

  • Writing Data Via a Future

Path path = Paths.get("data/test-write.txt");
//If the file does not exist the write() method will throw a java.nio.file.NoSuchFileException .
if(!Files.exists(path)){
    Files.createFile(path);
}
AsynchronousFileChannel fileChannel = 
    AsynchronousFileChannel.open(path, StandardOpenOption.WRITE);

ByteBuffer buffer = ByteBuffer.allocate(1024);
long position = 0;

buffer.put("test data".getBytes());
buffer.flip();

Future<Integer> operation = fileChannel.write(buffer, position);
buffer.clear();

while(!operation.isDone());

System.out.println("Write done");
  • Writing Data Via a CompletionHandler
Path path = Paths.get("data/test-write.txt");
if(!Files.exists(path)){
    Files.createFile(path);
}
AsynchronousFileChannel fileChannel = 
    AsynchronousFileChannel.open(path, StandardOpenOption.WRITE);

ByteBuffer buffer = ByteBuffer.allocate(1024);
long position = 0;

buffer.put("test data".getBytes());
buffer.flip();

fileChannel.write(buffer, position, buffer, new CompletionHandler<Integer, ByteBuffer>() {

    @Override
    public void completed(Integer result, ByteBuffer attachment) {
        System.out.println("bytes written: " + result);
    }

    @Override
    public void failed(Throwable exc, ByteBuffer attachment) {
        System.out.println("Write failed");
        exc.printStackTrace();
    }
});
  • Java

    Java 是一种可以撰写跨平台应用软件的面向对象的程序设计语言，是由 Sun Microsystems 公司于 1995 年 5 月推出的。Java 技术具有卓越的通用性、高效性、平台移植性和安全性。

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