Source Code Cross Referenced for Context.java in  » Science » javolution-5.2 » javolution » context » Java Source Code / Java DocumentationJava Source Code and Java Documentation

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Java Source Code / Java Documentation » Science » javolution 5.2 » javolution.context 
Source Cross Referenced  Class Diagram Java Document (Java Doc) 


001:        /*
002:         * Javolution - Java(TM) Solution for Real-Time and Embedded Systems
003:         * Copyright (C) 2007 - Javolution (http://javolution.org/)
004:         * All rights reserved.
005:         * 
006:         * Permission to use, copy, modify, and distribute this software is
007:         * freely granted, provided that this notice is preserved.
008:         */
009:        package javolution.context;
010:
011:        import javolution.xml.XMLSerializable;
012:        import j2me.lang.IllegalStateException;
013:        import j2me.lang.UnsupportedOperationException;
014:        import j2me.lang.ThreadLocal;
015:
016:        /**
017:         * <p> This class represents an execution context; they can be associated to 
018:         *     particular threads or objects.</p>
019:         *     
020:         * <p> Context-aware applications may extend the context base class or any 
021:         *     predefined contexts in order to facilitate <a 
022:         *     href="package-summary.html#package_description">
023:         *     separation of concerns</a>.</p>
024:         *     
025:         * <p> The scope of a {@link Context} should be surrounded by a <code>try, 
026:         *     finally</code> block statement to ensure correct behavior in case 
027:         *     of exceptions being raised. For example:[code]
028:         *     LocalContext.enter(); // Current thread enter a local context.
029:         *     try 
030:         *         ModuloInteger.setModulus(m); // No impact on other threads!
031:         *         z = x.times(y); // Multiplication modulo m.
032:         *     } finally {
033:         *         LocalContext.exit();
034:         *     }[/code]</p>
035:         *      
036:         * @author  <a href="mailto:jean-marie@dautelle.com">Jean-Marie Dautelle</a>
037:         * @version 5.0, April 15, 2007
038:         */
039:        public abstract class Context implements  XMLSerializable {
040:
041:            /**
042:             * Holds the root context (top context of all threads).
043:             */
044:            public static final Context ROOT = new Root();
045:
046:            /**
047:             * Holds the current context (thread-local).
048:             */
049:            private static final ThreadLocal CURRENT = new ThreadLocal() {
050:                protected Object initialValue() {
051:                    return ROOT;
052:                }
053:            };
054:
055:            /**
056:             * Holds the current owner of this context or <code>null</code> if global 
057:             * context.
058:             */
059:            private Thread _owner;
060:
061:            /**
062:             * Holds the outer context or <code>null</code> if none (root context).
063:             */
064:            private Context _outer;
065:
066:            /**
067:             * Holds the factory having produced this context if any (for recycling 
068:             * purpose upon exit).
069:             */
070:            private ObjectFactory _factory;
071:
072:            /**
073:             * Holds the inherited allocator context or <code>null</code>
074:             */
075:            private AllocatorContext _allocator;
076:
077:            /**
078:             * Default constructor. 
079:             */
080:            protected Context() {
081:            }
082:
083:            /**
084:             * Returns the current context for the current thread. 
085:             *
086:             * @return the current context.
087:             */
088:            public static Context getCurrent() {
089:                return (Context) Context.CURRENT.get();
090:            }
091:
092:            /**
093:             * Returns the current owner of this context. The owner of a
094:             * context is the thread which {@link #enter(Context) entered}
095:             * the context and has not yet {@link #exit(Context) exited}.
096:             * A context can only have one owner at any given time, although
097:             * contexts can be shared by {@link ConcurrentContext concurrent} threads.
098:             *
099:             * @return the thread owner of this context or <code>null</code>.
100:             */
101:            public final Thread getOwner() {
102:                return _owner;
103:            }
104:
105:            /**
106:             * Returns the outer context of this context or <code>null</code>
107:             * if {@link #ROOT}.
108:             *
109:             * @return the outer context or <code>null</code>.
110:             */
111:            public final Context getOuter() {
112:                return _outer;
113:            }
114:
115:            /**
116:             * Returns the string representation of this context (default 
117:             * <code>"Instance of " + this.getClass().getName()</code>).
118:             * 
119:             * @return the string representation of this context.
120:             */
121:            public String toString() {
122:                return "Instance of " + this .getClass().getName();
123:            }
124:
125:            /**
126:             * The action to be performed after this context becomes the current 
127:             * context.
128:             */
129:            protected abstract void enterAction();
130:
131:            /**
132:             * The action to be performed before this context is no more the current 
133:             * context.
134:             */
135:            protected abstract void exitAction();
136:
137:            /**
138:             * Enters the specified context.
139:             *
140:             * @param context the context being entered.
141:             * @return the specified context.
142:             * @throws IllegalStateException if this context is currently in use.
143:             */
144:            public static final/*<T extends Context>*//*T*/Context enter(
145:                    /*T*/Context context) {
146:                if (context._owner != null)
147:                    throw new IllegalStateException(
148:                            "Context is currently in use");
149:                Context current = Context.getCurrent();
150:                context._outer = current;
151:                context._owner = Thread.currentThread();
152:                context._allocator = context instanceof  AllocatorContext ? (AllocatorContext) context
153:                        : current._allocator;
154:                Context.CURRENT.set(context);
155:                context.enterAction();
156:                return (/*T*/Context) context;
157:            }
158:
159:            /**
160:             * Enters a factory produced context of specified type to be recycled 
161:             * after {@link #exit exiting}.
162:             * This method is called by the static <code>enter()</code> method 
163:             * of specialized contexts. If the context class has no public 
164:             * no-arg constructor accessible, then the factory for the class should 
165:             * be {@link ObjectFactory#setInstance explicitely set} (typically 
166:             * in a static initializer).  
167:             *
168:             * @param contextType the type of context being entered.
169:             * @return the context being entered.
170:             * @see ObjectFactory#getInstance(Class)
171:             */
172:            public static final/*<T extends Context>*//*T*/Context enter(
173:                    Class/*<T>*/contextType) {
174:                ObjectFactory factory = ObjectFactory.getInstance(contextType);
175:                Context context = (Context) factory.object();
176:                context._factory = factory;
177:                Context.enter(context);
178:                return (/*T*/Context) context;
179:            }
180:
181:            /**
182:             * Exits the current context (the {@link #getOuter outer} context
183:             * becomes the current context).
184:             * 
185:             * @return the context which has been exited.
186:             * @throws IllegalStateException if this context is the {@link #ROOT} 
187:             *         context.
188:             */
189:            public static Context exit() {
190:                Context context = Context.getCurrent();
191:                Context outer = context._outer;
192:                if (outer == null)
193:                    throw new IllegalStateException(Thread.currentThread()
194:                            + " Cannot exit instance of " + context.getClass());
195:                try {
196:                    context.exitAction();
197:                } finally {
198:                    Context.CURRENT.set(outer);
199:                    context._outer = null;
200:                    context._owner = null;
201:                    context._allocator = null;
202:                    if (context._factory != null) { // Factory produced.
203:                        context._factory.recycle(context);
204:                        context._factory = null;
205:                    }
206:                }
207:                return context;
208:            }
209:
210:            /**
211:             * Sets the current context, used by {@link ConcurrentContext}
212:             * exclusively.
213:             * 
214:             * @param context the concurrent context.
215:             */
216:            protected static void setCurrent(ConcurrentContext context) {
217:                Context.CURRENT.set(context);
218:            }
219:
220:            /**
221:             * Returns the allocator context used while in this context (shortcut).
222:             * 
223:             * @return the allocator context for this context.
224:             */
225:            final AllocatorContext getAllocatorContext() {
226:                return (_allocator == null) ? AllocatorContext.getDefault()
227:                        : _allocator;
228:            }
229:
230:            // Holds the root context definition.
231:            private static final class Root extends Context {
232:
233:                protected void enterAction() {
234:                    throw new UnsupportedOperationException(
235:                            "Cannot enter the root context");
236:                }
237:
238:                protected void exitAction() {
239:                    throw new UnsupportedOperationException(
240:                            "Cannot enter the root context");
241:                }
242:
243:            };
244:
245:            /**
246:             * @deprecated {@link #exit()} should be used.
247:             */
248:            public static final void exit(Context ctx) {
249:                Context.exit();
250:            }
251:
252:        }
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