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Tip of the Day
Language: Java Language
Expertise: Beginner
Mar 20, 1997



How to Help Your Business Become an AI Early Adopter

How to Prevent Simultaneous Access

How do you lock a file in Java to prevent simultaneous multiple accesses?

If you're concerned about simultaneous access by independent processes, you'll have to rely on the host OS to prevent this. Perhaps this mechanism can be invoked by a native function that can be imported into your Java program. (It is also possible to set the security manager object associated with a particular virtual machine to prevent access to a file by all threads running in that virtual machine.)

If you're concerned about simultaneous access by independent threads, Java offers an elegant mechanism called a monitor. It's an object containing a shared resource, such as a file, and synchronized methods for accessing that resource. A synchronized method can only be invoked by one thread at a time. Other threads wait in an associated queue.

Returning to your file example, you could create a file monitor containing the file as a member:

   class FileMonitor {

      private FileOutputStream f;

      public FileMonitor(String fname) {
         f = FileOutputStream(fname);

      public synchronized void write(byte b[]) throws IOException {

      // etc.

Alternatively, you could create a secure extension of the
FileOutputStream class:

   class SecureFileOutputStream extends FileOutputStream {

      public SecureFileOutputStream(String fname) { super(fname); }

      public synchronized void write(byte b[]) throws IOException {

      // etc.
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