文件操作

Determining If a File or Directory Exists

boolean exists = (new File("filename")).exists();

    if (exists) {

        // File or directory exists

    } else {

        // File or directory does not exist

    }

Creating a File

try {

        File file = new File("filename");

   

        // Create file if it does not exist

        boolean success = file.createNewFile();

        if (success) {

            // File did not exist and was created

        } else {

            // File already exists

        }

    } catch (IOException e) {

    }

Copying One File to Another

This example uses file streams to copy the contents of one file to another file.

    // Copies src file to dst file.

    // If the dst file does not exist, it is created

    void copy(File src, File dst) throws IOException {

        InputStream in = new FileInputStream(src);

        OutputStream out = new FileOutputStream(dst);

   

        // Transfer bytes from in to out

        byte[] buf = new byte[1024];

        int len;

        while ((len = in.read(buf)) > 0) {

            out.write(buf, 0, len);

        }

        in.close();

        out.close();

    }
Getting the Size of a File

     File file = new File("infilename");

   

    // Get the number of bytes in the file

    long length = file.length();

Deleting a File

     boolean success = (new File("filename")).delete();

    if (!success) {

        // Deletion failed

    }

Creating a Temporary File

try {

        // Create temp file.

        File temp = File.createTempFile("pattern", ".suffix");

   

        // Delete temp file when program exits.

        temp.deleteOnExit();

   

        // Write to temp file

        BufferedWriter out = new BufferedWriter(new FileWriter(temp));

        out.write("aString");

        out.close();

    } catch (IOException e) {

    }

Renaming a File or Directory

    // File (or directory) with old name

    File file = new File("oldname");

   

    // File (or directory) with new name

    File file2 = new File("newname");

   

    // Rename file (or directory)

    boolean success = file.renameTo(file2);

    if (!success) {

        // File was not successfully renamed

    }
Moving a File or Directory to Another Directory

// File (or directory) to be moved

    File file = new File("filename");

   

    // Destination directory

    File dir = new File("directoryname");

   

    // Move file to new directory

    boolean success = file.renameTo(new File(dir, file.getName()));

    if (!success) {

        // File was not successfully moved

    }

Getting and Setting the Modification Time of a File or Directory

This example gets the last modified time of a file or directory and then sets it to the current time.

    File file = new File("filename");

   

    // Get the last modified time

    long modifiedTime = file.lastModified();

    // 0L is returned if the file does not exist

   

    // Set the last modified time

    long newModifiedTime = System.currentTimeMillis();

    boolean success = file.setLastModified(newModifiedTime);

    if (!success) {

        // operation failed.

    }
Forcing Updates to a File to the Disk

In some applications, such as transaction processing, it is necessary to ensure that an update has been made to the disk. FileDescriptor.sync() blocks until all changes to a file are written to disk.

    try {

        // Open or create the output file

        FileOutputStream os = new FileOutputStream("outfilename");

        FileDescriptor fd = os.getFD();

   

        // Write some data to the stream

        byte[] data = new byte[]{(byte)0xCA, (byte)0xFE, (byte)0xBA, (byte)0xBE};

        os.write(data);

   

        // Flush the data from the streams and writers into system buffers.

        // The data may or may not be written to disk.

        os.flush();

   

        // Block until the system buffers have been written to disk.

        // After this method returns, the data is guaranteed to have

        // been written to disk.

        fd.sync();

    } catch (IOException e) {

    }