How to Prevent Simultaneous Access

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

If you’re concerned about simultaneous access by independent processes,you’ll have to rely on the host OS to prevent this. Perhaps thismechanism can be invoked by a native function that can be imported intoyour Java program. (It is also possible to set the security manager objectassociated with a particular virtual machine to prevent access to a fileby 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, andsynchronized methods for accessing that resource. A synchronizedmethod can only be invoked by one thread at a time. Other threadswait in an associated queue.

Returning to your file example, you could create a file monitorcontaining 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 {         f.write(b);      }      // etc.   }Alternatively, you could create a secure extension of theFileOutputStream class:   class SecureFileOutputStream extends FileOutputStream {      public SecureFileOutputStream(String fname) { super(fname); }      public synchronized void write(byte b[]) throws IOException {         super.write(b);      }      // etc.   } 

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