/**
* This file Copyright (c) 2011-2012 Magnolia International
* Ltd. (http://www.magnolia-cms.com). All rights reserved.
*
*
* This file is dual-licensed under both the Magnolia
* Network Agreement and the GNU General Public License.
* You may elect to use one or the other of these licenses.
*
* This file is distributed in the hope that it will be
* useful, but AS-IS and WITHOUT ANY WARRANTY; without even the
* implied warranty of MERCHANTABILITY or FITNESS FOR A
* PARTICULAR PURPOSE, TITLE, or NONINFRINGEMENT.
* Redistribution, except as permitted by whichever of the GPL
* or MNA you select, is prohibited.
*
* 1. For the GPL license (GPL), you can redistribute and/or
* modify this file under the terms of the GNU General
* Public License, Version 3, as published by the Free Software
* Foundation. You should have received a copy of the GNU
* General Public License, Version 3 along with this program;
* if not, write to the Free Software Foundation, Inc., 51
* Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* 2. For the Magnolia Network Agreement (MNA), this file
* and the accompanying materials are made available under the
* terms of the MNA which accompanies this distribution, and
* is available at http://www.magnolia-cms.com/mna.html
*
* Any modifications to this file must keep this entire header
* intact.
*
*/
package info.magnolia.cms.filters;
import java.io.IOException;
import java.util.concurrent.locks.ReentrantReadWriteLock;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
/**
* Wrapper for a MgnlFilter that using a read/write lock ensures that the destroy method waits for requests to complete
* before destroying the target filter. All use of the wrapper must be done while holding the read lock. The exception
* to this is the destroy method that MUST NOT be called while holding the read lock, doing so would result in dead
* lock.
*
* @version $Id$
*/
public class SafeDestroyMgnlFilterWrapper implements MgnlFilter {
private final MgnlFilter target;
private final ReentrantReadWriteLock lock = new ReentrantReadWriteLock(true);
public SafeDestroyMgnlFilterWrapper(MgnlFilter target) {
if (target == null) {
throw new NullPointerException("Target filter must not be null");
}
this.target = target;
}
public void acquireReadLock() {
lock.readLock().lock();
}
public void releaseReadLock() {
lock.readLock().unlock();
}
@Override
public void init(FilterConfig filterConfig) throws ServletException {
target.init(filterConfig);
}
/**
* Read lock must be held when invoking this method.
*/
@Override
public String getName() {
return target.getName();
}
/**
* Read lock must be held when invoking this method.
*/
@Override
public void setName(String name) {
target.setName(name);
}
/**
* Read lock must be held when invoking this method.
*/
@Override
public boolean matches(HttpServletRequest request) {
return target.matches(request);
}
/**
* Read lock must be held when invoking this method.
*/
@Override
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
target.doFilter(request, response, chain);
}
/**
* Destroys the target filter after waiting for all requests to complete. Calling this method while holding the read
* lock will result in dead lock. Therefore a request that has passed through this filter MUST NEVER call this
* method.
*/
@Override
public void destroy() {
// We could employ a ThreadLocal to throw an exception instead of dead locking. There is no other way
// to see if the calling thread is holding the read lock.
lock.writeLock().lock();
try {
target.destroy();
} finally {
lock.writeLock().unlock();
}
}
public MgnlFilter getTargetFilter() {
return target;
}
/**
* Construct for keeping a reference to a {@link SafeDestroyMgnlFilterWrapper} and change it with the guarantee that
* the returned previous reference will not get more read locks. It's then safe to destroy the returned reference.
*
* @version $Id$
*/
public static class Switcher {
private SafeDestroyMgnlFilterWrapper filter;
/**
* Replaces the current filter with a new one and returns the previous filter. The returned filter is possibly still
* in use by currently executing requests but it will not be used by any more requests after this method returns.
* <p/>
* Notes about the returned filter:
* <ul>
* <li>No methods other than destroy() should be called on the returned filter</li>
* <li>The destroy() method will wait for all requests to complete before it is destroyed</li>
* <li>Calling the destroy() method from a request that has entered the filter WILL RESULT IN DEAD LOCK.</li>
* </ul>
*/
public synchronized SafeDestroyMgnlFilterWrapper replaceFilter(SafeDestroyMgnlFilterWrapper newFilter) {
SafeDestroyMgnlFilterWrapper oldFilter = filter;
filter = newFilter;
return oldFilter;
}
/**
* Returns the current filter with a read lock held for the current thread. After use the thread must relinquish the
* lock by calling releaseReadLock(). If no filter has been set this method returns null and no lock is taken.
*/
public synchronized SafeDestroyMgnlFilterWrapper getFilterAndAcquireReadLock() {
if (filter == null) {
return null;
}
filter.acquireReadLock();
return filter;
}
/**
* This method is provided for introspection only. It is inherently UNSAFE as using this method and then
* acquiring the read lock on the returned filter breaks the guarantee that a replaced reference wont get more
* read locks.
*/
public synchronized SafeDestroyMgnlFilterWrapper getFilter() {
return filter;
}
}
}