counterimpl.java

jdk-6u10-docs java开发宝典

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package examples.activation; 

import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.rmi.MarshalledObject;
import java.rmi.activation.Activatable;
import java.rmi.activation.ActivationID;

public class CounterImpl implements Counter {
    private RandomAccessFile raf;
    private int count;
    private final Object countLock = new Object();
    
    /**
     * Constructs a <code>CounterImpl</code> instance with the specified
     * activation ID and data.  This constructor is called during
     * activation to construct the object.  The data should contain the
     * name of a file used in keeping track of this object's invocation
     * count.
     **/
    public CounterImpl(ActivationID id, MarshalledObject data) 
	throws Exception
    {
	/*
	 * Obtain the file name from the supplied marshalled data, and
	 * initialize the count value, read from the file.  Updating
	 * the count value must be in a synchronized block because multiple
	 * threads may need to access it.
	 */
	if (data != null) {
	    String filename = (String) data.get();
	    synchronized (countLock) {
		count = openFile(filename);
	    }
	    System.err.println("count upon activation = " + count);
	}
	
	/*
	 * Export the activatable object on an anonymous port.  The object
	 * is exported after the count is read to prevent clients from
	 * invoking the 'increment' method before initialization completes.
	 */
	Activatable.exportObject(this, id, 0);
    }

    /**
     * Opens the specified file, and reads and returns the current count
     * value.  If the file does not exist, the file is created with a count
     * value of 0x00.  Any exceptions encountered while using the file are
     * propagated to the caller.
     **/
    private int openFile(String filename) throws IOException {
	
	if (filename != null && !filename.equals("")) {
	    File file = new File(filename);
	    boolean fileExists = file.exists();
	    raf = new RandomAccessFile(file, "rws");
	    return (fileExists) ? raf.readInt() : writeCount(0);
	    
	} else {
	    throw new IOException("invalid filename");
	}
    }

    /**
     * Writes the specified count value to the file, and returns the value
     * written.
     *
     * Implementation note: this simple implementation could lose the count
     * value if a crash occurs after the call to 'setLength' succeeds, but
     * before the 'writeInt' call completes.  A more robust implementation
     * would save the previous count value so that it could be recovered in
     * the event of a crash.
     **/
    private int writeCount(int value) throws IOException {
	raf.setLength(0);
	raf.writeInt(value);
	return value;
    }
	    
    /**
     * Increments the count value, stores it persistently, and returns the
     * result.
     *
     * @throws IOException if an exception occurs while storing the count
     * value 
     **/
    public int increment() throws IOException {
	synchronized (countLock) {
	    return writeCount(++count);
	}
    }
}