Java基础之读文件——使用通道复制文件（FileBackup）

控制台程序，除了使用Files类中使用copy()方法将文件复制外，还可以使用FileChannel对象复制文件，连接到输入文件的FileChannel对象能直接将数据传输到连接到输出文件的FileChannel对象中而不涉及显式的缓冲区。

本例用来对命令行参数设定的文件进行复制。文件被复制到一个类似在原始目录下创建的备份文件中。新文件的名称通过在原始文件名称的后面附加多次“_backup”以获得唯一文件名。

 import static java.nio.file.StandardOpenOption.*;
 import java.nio.file.*;
 import java.nio.channels.*;
 import java.io.IOException;
 import java.util.EnumSet;

 public class FileBackup {
   public static void main(String[] args) {
     if(args.length==0) {
       System.out.println("No file to copy. Application usage is:\n" + "java -classpath . FileCopy \"filepath\"" );
       System.exit(1);
     }
     Path fromFile = Paths.get(args[0]);
     fromFile.toAbsolutePath();

     if(Files.notExists(fromFile)) {
       System.out.printf("File to copy, %s, does not exist.", fromFile);
       System.exit(1);
     }

     Path toFile = createBackupFilePath(fromFile);
     try (FileChannel inCh = (FileChannel)(Files.newByteChannel(fromFile));
           WritableByteChannel outCh = Files.newByteChannel( toFile, EnumSet.of(WRITE,CREATE_NEW))){
       int bytesWritten = 0;
       long byteCount = inCh.size();
       while(bytesWritten < byteCount) {
         bytesWritten += inCh.transferTo(bytesWritten, byteCount-bytesWritten, outCh);
       }

       System.out.printf("File copy complete. %d bytes copied to %s%n", byteCount, toFile);
     } catch(IOException e) {
       e.printStackTrace();
     }
   }

   // Method to create a unique backup Path object under MS Windows
   public static Path createBackupFilePath(Path file) {
      Path parent = file.getParent();
      String name = file.getFileName().toString();                      // Get the file name
      int period = name.indexOf('.');                                   // Find the extension separator
      if(period == -1) {                                                // If there isn't one
        period = name.length();                                         // set it to the end of the string
      }
      String nameAdd = "_backup";                                       // String to be appended

      // Create a Path object that is a unique
       Path backup = parent.resolve(
                 name.substring(0,period) + nameAdd + name.substring(period));
      while(Files.exists(backup)) {                                     // If the path already exists...
         name = backup.getFileName().toString();                        // Get the current file name
         backup = parent.resolve(name.substring(0,period) +             // add _backup
                                 nameAdd + name.substring(period));
        period += nameAdd.length();                                     // Increment separator index
      }
      return backup;
   }
 }