Java Stream，File，IO
java包之java.io
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
Path path = Paths.get("c:\\data\\myfile.txt");
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");
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
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
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();
}
});
