001 package org.apache.turbine.services;
002
003
004 /*
005 * Licensed to the Apache Software Foundation (ASF) under one
006 * or more contributor license agreements. See the NOTICE file
007 * distributed with this work for additional information
008 * regarding copyright ownership. The ASF licenses this file
009 * to you under the Apache License, Version 2.0 (the
010 * "License"); you may not use this file except in compliance
011 * with the License. You may obtain a copy of the License at
012 *
013 * http://www.apache.org/licenses/LICENSE-2.0
014 *
015 * Unless required by applicable law or agreed to in writing,
016 * software distributed under the License is distributed on an
017 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
018 * KIND, either express or implied. See the License for the
019 * specific language governing permissions and limitations
020 * under the License.
021 */
022
023
024 import java.util.Hashtable;
025 import java.util.Stack;
026
027 import org.apache.commons.logging.Log;
028 import org.apache.commons.logging.LogFactory;
029
030 /**
031 * A generic implementation of <code>InitableBroker</code>.
032 * Functionality provided by the broker includes:
033 *
034 * <ul>
035 *
036 * <li>Maintaining single instance of each <code>Initable</code> in
037 * the system.</li>
038 *
039 * <li>Early initialization of <code>Initables</code> during system
040 * startup.</li>
041 *
042 * <li>Late initialization of <code>Initables</code> before they are
043 * used.</li>
044 *
045 * <li>Providing instances of <code>Initables</code> to requesting
046 * parties.</li>
047 *
048 * <li>Maintaining dependencies between <code>Initables</code> during
049 * early initalization phases, including circular dependencies
050 * detection.</li>
051 *
052 * </ul>
053 *
054 * @author <a href="mailto:burton@apache.org">Kevin Burton</a>
055 * @author <a href="mailto:krzewski@e-point.pl">Rafal Krzewski</a>
056 * @author <a href="mailto:hps@intermeta.de">Henning P. Schmiedehausen</a>
057 * @version $Id: BaseInitableBroker.java 1078552 2011-03-06 19:58:46Z tv $
058 */
059 public abstract class BaseInitableBroker
060 implements InitableBroker
061 {
062 /** A repository of Initable instances. */
063 protected Hashtable<String, Initable> initables = new Hashtable<String, Initable>();
064
065 /**
066 * Names of classes being early-initialized are pushed onto this
067 * stack. A name appearing twice indicates a circular dependency
068 * chain.
069 */
070 protected Stack<String> stack = new Stack<String>();
071
072 /** Logging */
073 private final Log log = LogFactory.getLog(this.getClass());
074
075 /**
076 * Default constructor of InitableBroker.
077 *
078 * This constructor does nothing. Your brokers should be
079 * singletons, therefore their constructors should be
080 * private. They should also have public YourBroker getInstance()
081 * methods.
082 */
083 protected BaseInitableBroker()
084 {
085 // empty
086 }
087
088 /**
089 * Performs early initialization of an Initable class.
090 *
091 * @param className The name of the class to be initialized.
092 * @param data An Object to be used for initialization activities.
093 * @exception InitializationException Initialization was not successful.
094 */
095 public void initClass(String className, Object data)
096 throws InitializationException
097 {
098 // make sure that only one thread calls this method recursively
099 synchronized (stack)
100 {
101 int pos = stack.search(className);
102 if (pos != -1)
103 {
104 StringBuffer msg = new StringBuffer().append(className)
105 .append(" couldn't be initialized because of circular depency chain:\n");
106 for (int i = pos; i > 0; i--)
107 {
108 msg.append(stack.elementAt(stack.size() - i - 1) + "->");
109 }
110 msg.append(className).append('\n');
111
112 throw new InitializationException(msg.toString());
113 }
114 try
115 {
116 stack.push(className);
117 Initable instance = getInitableInstance(className);
118 if (!instance.getInit())
119 {
120 // this call might result in an indirect recursion
121 instance.init(data);
122 }
123 }
124 finally
125 {
126 // Succeeded or not, make sure the name gets off the stack.
127 stack.pop();
128 }
129 }
130 }
131
132 /**
133 * Shuts down an <code>Initable</code>.
134 *
135 * This method is used to release resources allocated by an
136 * <code>Initable</code>, and return it to its initial (uninitailized)
137 * state.
138 *
139 * @param className The name of the class to be uninitialized.
140 */
141 public void shutdownClass(String className)
142 {
143 try
144 {
145 Initable initable = getInitableInstance(className);
146 if (initable.getInit())
147 {
148 initable.shutdown();
149 ((BaseInitable) initable).setInit(false);
150 }
151 }
152 catch (InstantiationException e)
153 {
154 // Shutdown of a nonexistent class was requested.
155 // This does not hurt anything, so we log the error and continue.
156 log.error("Shutdown of a nonexistent class " +
157 className + " was requested", e);
158 }
159 }
160
161 /**
162 * Provides an instance of Initable class ready to work.
163 *
164 * If the requested class couldn't be instatiated or initialized,
165 * an InstantiationException will be thrown. You needn't handle
166 * this exception in your code, since it indicates fatal
167 * misconfigurtion of the system.
168 *
169 * @param className The name of the Initable requested.
170 * @return An instance of the requested Initable.
171 * @exception InstantiationException if there was a problem
172 * during instantiation or initialization of the Initable.
173 */
174 public Initable getInitable(String className)
175 throws InstantiationException
176 {
177 Initable initable;
178 try
179 {
180 initable = getInitableInstance(className);
181 if (!initable.getInit())
182 {
183 synchronized (initable.getClass())
184 {
185 if (!initable.getInit())
186 {
187 initable.init();
188 }
189 if (!initable.getInit())
190 {
191 // this exception will be caught & rethrown by this
192 // very method. getInit() returning false indicates
193 // some initialization issue, which in turn prevents
194 // the InitableBroker from passing a working
195 // instance of the initable to the client.
196 throw new InitializationException(
197 "init() failed to initialize class "
198 + className);
199 }
200 }
201 }
202 return initable;
203 }
204 catch (InitializationException e)
205 {
206 throw new InstantiationException("Class " + className +
207 " failed to initialize", e);
208 }
209 }
210
211 /**
212 * Retrieves an instance of an Initable from the repository.
213 *
214 * If the requested class is not present in the repository, it is
215 * instantiated and passed a reference to the broker, saved and
216 * then returned.
217 *
218 * @param className The name of the class to be instantiated.
219 * @exception InstantiationException if the requested class can't
220 * be instantiated.
221 */
222 protected Initable getInitableInstance(String className)
223 throws InstantiationException
224 {
225 Initable initable = initables.get(className);
226
227 if (initable == null)
228 {
229 try
230 {
231 initable = (Initable) Class.forName(className).newInstance();
232 }
233
234 // those two errors must be passed to the VM
235 catch (ThreadDeath t)
236 {
237 throw t;
238 }
239 catch (OutOfMemoryError t)
240 {
241 throw t;
242 }
243
244 catch (Throwable t)
245 {
246 // Used to indicate error condition.
247 String msg = null;
248
249 if (t instanceof NoClassDefFoundError)
250 {
251 msg = "A class referenced by " + className +
252 " is unavailable. Check your jars and classes.";
253 }
254 else if (t instanceof ClassNotFoundException)
255 {
256 msg = "Class " + className +
257 " is unavailable. Check your jars and classes.";
258 }
259 else if (t instanceof ClassCastException)
260 {
261 msg = "Class " + className +
262 " doesn't implement Initable.";
263 }
264 else
265 {
266 msg = "Failed to instantiate " + className;
267 }
268
269 throw new InstantiationException(msg, t);
270 }
271
272 initable.setInitableBroker(this);
273 initables.put(className, initable);
274 }
275
276 return initable;
277 }
278
279 }