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Java > Open Source Codes > org > apache > avalon > framework > context > DefaultContext


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55 package org.apache.avalon.framework.context;
56
57 import java.io.Serializable JavaDoc;
58 import java.util.Hashtable JavaDoc;
59 import java.util.Map JavaDoc;
60
61 /**
62  * Default implementation of Context.
63  * This implementation is a static hierarchial store.
64  *
65  * @author <a HREF="mailto:dev@avalon.apache.org">Avalon Development Team</a>
66  * @version CVS $Revision: 1.25 $ $Date: 2003/02/11 15:58:40 $
67  */
68 public class DefaultContext
69     implements Context
70 {
71     private static final class Hidden implements Serializable JavaDoc
72     {
73     }
74
75     private static final Hidden HIDDEN_MAKER = new Hidden();
76
77     private final Map JavaDoc m_contextData;
78     private final Context m_parent;
79     private boolean m_readOnly;
80
81     /**
82      * Create a Context with specified data and parent.
83      *
84      * @param contextData the context data
85      * @param parent the parent Context (may be null)
86      */
87     public DefaultContext( final Map JavaDoc contextData, final Context parent )
88     {
89         m_parent = parent;
90         m_contextData = contextData;
91     }
92
93     /**
94      * Create a Context with specified data.
95      *
96      * @param contextData the context data
97      */
98     public DefaultContext( final Map JavaDoc contextData )
99     {
100         this( contextData, null );
101     }
102
103     /**
104      * Create a Context with specified parent.
105      *
106      * @param parent the parent Context (may be null)
107      */
108     public DefaultContext( final Context parent )
109     {
110         this( new Hashtable JavaDoc(), parent );
111     }
112
113     /**
114      * Create a Context with no parent.
115      *
116      */
117     public DefaultContext()
118     {
119         this( (Context)null );
120     }
121
122     /**
123      * Retrieve an item from the Context.
124      *
125      * @param key the key of item
126      * @return the item stored in context
127      * @throws ContextException if item not present
128      */
129     public Object JavaDoc get( final Object JavaDoc key )
130         throws ContextException
131     {
132         final Object JavaDoc data = m_contextData.get( key );
133
134         if( null != data )
135         {
136             if( data instanceof Hidden )
137             {
138                 // Always fail.
139 final String JavaDoc message = "Unable to locate " + key;
140                 throw new ContextException( message );
141             }
142
143             if( data instanceof Resolvable )
144             {
145                 return ( (Resolvable)data ).resolve( this );
146             }
147
148             return data;
149         }
150
151         // If data was null, check the parent
152 if( null == m_parent )
153         {
154             // There was no parent, and no data
155 final String JavaDoc message =
156                 "Unable to resolve context key: " + key;
157             throw new ContextException( message );
158         }
159
160         return m_parent.get( key );
161     }
162
163     /**
164      * Helper method fo adding items to Context.
165      *
166      * @param key the items key
167      * @param value the item
168      * @throws IllegalStateException if context is read only
169      */
170     public void put( final Object JavaDoc key, final Object JavaDoc value )
171         throws IllegalStateException JavaDoc
172     {
173         checkWriteable();
174         if( null == value )
175         {
176             m_contextData.remove( key );
177         }
178         else
179         {
180             m_contextData.put( key, value );
181         }
182     }
183
184     /**
185      * Hides the item in the context.
186      * After remove(key) has been called, a get(key)
187      * will always fail, even if the parent context
188      * has such a mapping.
189      *
190      * @param key the items key
191      * @throws IllegalStateException if context is read only
192      */
193     public void hide( final Object JavaDoc key )
194         throws IllegalStateException JavaDoc
195     {
196         checkWriteable();
197         m_contextData.put( key, HIDDEN_MAKER );
198     }
199
200     /**
201      * Utility method to retrieve context data.
202      *
203      * @return the context data
204      */
205     protected final Map JavaDoc getContextData()
206     {
207         return m_contextData;
208     }
209
210     /**
211      * Get parent context if any.
212      *
213      * @return the parent Context (may be null)
214      */
215     protected final Context getParent()
216     {
217         return m_parent;
218     }
219
220     /**
221      * Make the context read-only.
222      * Any attempt to write to the context via put()
223      * will result in an IllegalStateException.
224      */
225     public void makeReadOnly()
226     {
227         m_readOnly = true;
228     }
229
230     /**
231      * Utility method to check if context is writeable and if not throw exception.
232      *
233      * @throws IllegalStateException if context is read only
234      */
235     protected final void checkWriteable()
236         throws IllegalStateException JavaDoc
237     {
238         if( m_readOnly )
239         {
240             final String JavaDoc message =
241                 "Context is read only and can not be modified";
242             throw new IllegalStateException JavaDoc( message );
243         }
244     }
245 }
246
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