/*

* Copyright 2009 Weaver authors

*

* This code is part of the Weaver tutorial and may be freely used.

*/

import java.util.concurrent.locks.ReentrantReadWriteLock;

/**

* Caches an avatar corresponding to a given user. This is a test class,

* used to illustrate issues with read-write locks. See class example for

* {@link java.util.concurrent.locks.ReentrantReadWriteLock}.

*

* NOTE: This test will fail. It was written to demonstrate a fault in the class

* under test.

*

* @author alasdair.mackintosh@gmail.com (Alasdair Mackintosh)

*/

public class UserCache {

private final UserDb db;

private final String username;

private String avatar;

private RenderingContext context;

volatile boolean cacheValid;

ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();

UserCache(UserDb db, String username, RenderingContext context) {

this.db = db;

this.username = username;

this.context = context;

updateCache();

cacheValid = true;

}

void updateCache() {

avatar = db.getAvatar(username);

}

public void drawUserAvatar() {

rwl.readLock().lock();

if (!cacheValid) {

// upgrade lock manually

rwl.readLock().unlock(); // must unlock first to obtain writelock

rwl.writeLock().lock();

if (!cacheValid) { // recheck

updateCache();

cacheValid = true;

}

// downgrade lock

rwl.readLock().lock(); // reacquire read without giving up write lock

rwl.writeLock().unlock(); // unlock write, still hold read

}

context.draw(avatar);

rwl.readLock().unlock();

}

/** Invoked when the user changes. Marks the cache as dirty. */

public void updateUser() {

db.update(username);

cacheValid = false;

}

}