1 /**
2 * This file Copyright (c) 2011 Magnolia International
3 * Ltd. (http://www.magnolia-cms.com). All rights reserved.
4 *
5 *
6 * This file is dual-licensed under both the Magnolia
7 * Network Agreement and the GNU General Public License.
8 * You may elect to use one or the other of these licenses.
9 *
10 * This file is distributed in the hope that it will be
11 * useful, but AS-IS and WITHOUT ANY WARRANTY; without even the
12 * implied warranty of MERCHANTABILITY or FITNESS FOR A
13 * PARTICULAR PURPOSE, TITLE, or NONINFRINGEMENT.
14 * Redistribution, except as permitted by whichever of the GPL
15 * or MNA you select, is prohibited.
16 *
17 * 1. For the GPL license (GPL), you can redistribute and/or
18 * modify this file under the terms of the GNU General
19 * Public License, Version 3, as published by the Free Software
20 * Foundation. You should have received a copy of the GNU
21 * General Public License, Version 3 along with this program;
22 * if not, write to the Free Software Foundation, Inc., 51
23 * Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
24 *
25 * 2. For the Magnolia Network Agreement (MNA), this file
26 * and the accompanying materials are made available under the
27 * terms of the MNA which accompanies this distribution, and
28 * is available at http://www.magnolia-cms.com/mna.html
29 *
30 * Any modifications to this file must keep this entire header
31 * intact.
32 *
33 */
34 package info.magnolia.cms.filters;
35
36 import java.io.IOException;
37 import java.util.concurrent.locks.ReentrantReadWriteLock;
38 import javax.servlet.FilterChain;
39 import javax.servlet.FilterConfig;
40 import javax.servlet.ServletException;
41 import javax.servlet.ServletRequest;
42 import javax.servlet.ServletResponse;
43 import javax.servlet.http.HttpServletRequest;
44
45
46 /**
47 * Wrapper for a MgnlFilter that using a read/write lock ensures that the destroy method waits for requests to complete
48 * before destroying the target filter. All use of the wrapper must be done while holding the read lock. The exception
49 * to this is the destroy method that MUST NOT be called while holding the read lock, doing so would result in dead
50 * lock.
51 *
52 * @version $Id$
53 */
54 public class SafeDestroyMgnlFilterWrapper implements MgnlFilter {
55
56 private final MgnlFilter target;
57 private final ReentrantReadWriteLock lock = new ReentrantReadWriteLock(true);
58
59 public SafeDestroyMgnlFilterWrapper(MgnlFilter target) {
60 if (target == null) {
61 throw new NullPointerException("Target filter must not be null");
62 }
63 this.target = target;
64 }
65
66 public void acquireReadLock() {
67 lock.readLock().lock();
68 }
69
70 public void releaseReadLock() {
71 lock.readLock().unlock();
72 }
73
74 @Override
75 public void init(FilterConfig filterConfig) throws ServletException {
76 target.init(filterConfig);
77 }
78
79 /**
80 * Read lock must be held when invoking this method.
81 */
82 @Override
83 public String getName() {
84 return target.getName();
85 }
86
87 /**
88 * Read lock must be held when invoking this method.
89 */
90 @Override
91 public void setName(String name) {
92 target.setName(name);
93 }
94
95 /**
96 * Read lock must be held when invoking this method.
97 */
98 @Override
99 public boolean matches(HttpServletRequest request) {
100 return target.matches(request);
101 }
102
103 /**
104 * Read lock must be held when invoking this method.
105 */
106 @Override
107 public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
108 target.doFilter(request, response, chain);
109 }
110
111 /**
112 * Destroys the target filter after waiting for all requests to complete. Calling this method while holding the read
113 * lock will result in dead lock. Therefore a request that has passed through this filter MUST NEVER call this
114 * method.
115 */
116 @Override
117 public void destroy() {
118
119 // We could employ a ThreadLocal to throw an exception instead of dead locking. There is no other way
120 // to see if the calling thread is holding the read lock.
121
122 lock.writeLock().lock();
123 try {
124 target.destroy();
125 } finally {
126 lock.writeLock().unlock();
127 }
128 }
129
130 public MgnlFilter getTargetFilter() {
131 return target;
132 }
133
134 /**
135 * Construct for keeping a reference to a {@link SafeDestroyMgnlFilterWrapper} and change it with the guarantee that
136 * the returned previous reference will not get more read locks. It's then safe to destroy the returned reference.
137 *
138 * @version $Id$
139 */
140 public static class Switcher {
141
142 private SafeDestroyMgnlFilterWrapper filter;
143
144 /**
145 * Replaces the current filter with a new one and returns the previous filter. The returned filter is possibly still
146 * in use by currently executing requests but it will not be used by any more requests after this method returns.
147 * <p/>
148 * Notes about the returned filter:
149 * <ul>
150 * <li>No methods other than destroy() should be called on the returned filter</li>
151 * <li>The destroy() method will wait for all requests to complete before it is destroyed</li>
152 * <li>Calling the destroy() method from a request that has entered the filter WILL RESULT IN DEAD LOCK.</li>
153 * </ul>
154 */
155 public synchronized SafeDestroyMgnlFilterWrapper replaceFilter(SafeDestroyMgnlFilterWrapper newFilter) {
156 SafeDestroyMgnlFilterWrapper oldFilter = filter;
157 filter = newFilter;
158 return oldFilter;
159 }
160
161 /**
162 * Returns the current filter with a read lock held for the current thread. After use the thread must relinquish the
163 * lock by calling releaseReadLock(). If no filter has been set this method returns null and no lock is taken.
164 */
165 public synchronized SafeDestroyMgnlFilterWrapper getFilterAndAcquireReadLock() {
166 if (filter == null) {
167 return null;
168 }
169 filter.acquireReadLock();
170 return filter;
171 }
172
173 /**
174 * This method is provided for introspection only. It is inherently UNSAFE as using this method and then
175 * acquiring the read lock on the returned filter breaks the guarantee that a replaced reference wont get more
176 * read locks.
177 */
178 public synchronized SafeDestroyMgnlFilterWrapper getFilter() {
179 return filter;
180 }
181 }
182 }