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Java > Open Source Codes > org > apache > turbine > modules > ScreenLoader


1 package org.apache.turbine.modules;
2
3 /*
4  * Copyright 2001-2004 The Apache Software Foundation.
5  *
6  * Licensed under the Apache License, Version 2.0 (the "License")
7  * you may not use this file except in compliance with the License.
8  * You may obtain a copy of the License at
9  *
10  * http://www.apache.org/licenses/LICENSE-2.0
11  *
12  * Unless required by applicable law or agreed to in writing, software
13  * distributed under the License is distributed on an "AS IS" BASIS,
14  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
15  * See the License for the specific language governing permissions and
16  * limitations under the License.
17  */
18
19 import java.util.List JavaDoc;
20
21 import org.apache.commons.logging.Log;
22 import org.apache.commons.logging.LogFactory;
23
24 import org.apache.ecs.ConcreteElement;
25
26 import org.apache.turbine.Turbine;
27 import org.apache.turbine.TurbineConstants;
28 import org.apache.turbine.services.assemblerbroker.AssemblerBrokerService;
29 import org.apache.turbine.services.assemblerbroker.TurbineAssemblerBroker;
30 import org.apache.turbine.util.ObjectUtils;
31 import org.apache.turbine.util.RunData;
32
33 /**
34  * The purpose of this class is to allow one to load and execute
35  * Screen modules.
36  *
37  * @author <a HREF="mailto:mbryson@mont.mindspring.com">Dave Bryson</a>
38  * @author <a HREF="mailto:hps@intermeta.de">Henning P. Schmiedehausen</a>
39  * @version $Id: ScreenLoader.java,v 1.10.2.3 2004/08/16 22:57:50 henning Exp $
40  */
41 public class ScreenLoader
42     extends GenericLoader
43     implements Loader
44 {
45     /** Logging */
46     private static Log log = LogFactory.getLog(ScreenLoader.class);
47
48     /** The single instance of this class. */
49     private static ScreenLoader instance =
50         new ScreenLoader(Turbine.getConfiguration()
51                          .getInt(TurbineConstants.SCREEN_CACHE_SIZE_KEY,
52                                  TurbineConstants.SCREEN_CACHE_SIZE_DEFAULT));
53
54     /** The Assembler Broker Service */
55     private static AssemblerBrokerService ab = TurbineAssemblerBroker.getService();
56
57     /**
58      * These ctor's are private to force clients to use getInstance()
59      * to access this class.
60      */
61     private ScreenLoader()
62     {
63         super();
64     }
65
66     /**
67      * These ctor's are private to force clients to use getInstance()
68      * to access this class.
69      */
70     private ScreenLoader(int i)
71     {
72         super(i);
73     }
74
75     /**
76      * Adds an instance of an object into the hashtable.
77      *
78      * @param name Name of object.
79      * @param screen Screen to be associated with name.
80      */
81     private void addInstance(String JavaDoc name, Screen screen)
82     {
83         if (cache())
84         {
85             this.put(name, (Screen) screen);
86         }
87     }
88
89     /**
90      * Attempts to load and execute the external Screen. This is used
91      * when you want to execute a Screen which returns its output via
92      * a MultiPartElement instead of out the data.getPage() value.
93      * This allows you to easily chain the execution of Screen modules
94      * together.
95      *
96      * @param data Turbine information.
97      * @param name Name of object that will execute the screen.
98      * @exception Exception a generic exception.
99      */
100     public ConcreteElement eval(RunData data, String JavaDoc name)
101             throws Exception JavaDoc
102     {
103         // Execute screen
104 return getInstance(name).build(data);
105     }
106
107     /**
108      * Attempts to load and execute the Screen. This is used when you
109      * want to execute a Screen which returns its output via the
110      * data.getPage() object.
111      *
112      * @param data Turbine information.
113      * @param name Name of object that will execute the screen.
114      * @exception Exception a generic exception.
115      */
116     public void exec(RunData data, String JavaDoc name)
117             throws Exception JavaDoc
118     {
119         this.eval(data, name);
120     }
121
122     /**
123      * Pulls out an instance of the object by name. Name is just the
124      * single name of the object. This is equal to getInstance but
125      * returns an Assembler object and is needed to fulfil the Loader
126      * interface.
127      *
128      * @param name Name of object instance.
129      * @return A Screen with the specified name, or null.
130      * @exception Exception a generic exception.
131      */
132     public Assembler getAssembler(String JavaDoc name)
133         throws Exception JavaDoc
134     {
135         return getInstance(name);
136     }
137
138     /**
139      * Pulls out an instance of the Screen by name. Name is just the
140      * single name of the Screen.
141      *
142      * @param name Name of requested Screen.
143      * @return A Screen with the specified name, or null.
144      * @exception Exception a generic exception.
145      */
146     public Screen getInstance(String JavaDoc name)
147             throws Exception JavaDoc
148     {
149         Screen screen = null;
150
151         // Check if the screen is already in the cache
152 if (cache() && this.containsKey(name))
153         {
154             screen = (Screen) this.get(name);
155             log.debug("Found Screen " + name + " in the cache!");
156         }
157         else
158         {
159             log.debug("Loading Screen " + name + " from the Assembler Broker");
160
161             try
162             {
163                 if (ab != null)
164                 {
165                     // Attempt to load the screen
166 screen = (Screen) ab.getAssembler(
167                         AssemblerBrokerService.SCREEN_TYPE, name);
168                 }
169             }
170             catch (ClassCastException JavaDoc cce)
171             {
172                 // This can alternatively let this exception be thrown
173 // So that the ClassCastException is shown in the
174 // browser window. Like this it shows "Screen not Found"
175 screen = null;
176             }
177
178             if (screen == null)
179             {
180                 // If we did not find a screen we should try and give
181 // the user a reason for that...
182 // FIX ME: The AssemblerFactories should each add it's
183 // own string here...
184 List JavaDoc packages = Turbine.getConfiguration()
185                     .getList(TurbineConstants.MODULE_PACKAGES);
186
187                 ObjectUtils.addOnce(packages,
188                         GenericLoader.getBasePackage());
189
190                 throw new ClassNotFoundException JavaDoc(
191                         "\n\n\tRequested Screen not found: " + name +
192                         "\n\tTurbine looked in the following " +
193                         "modules.packages path: \n\t" + packages.toString() + "\n");
194             }
195             else if (cache())
196             {
197                 // The new instance is added to the cache
198 addInstance(name, screen);
199             }
200         }
201         return screen;
202     }
203
204     /**
205      * The method through which this class is accessed.
206      *
207      * @return The single instance of this class.
208      */
209     public static ScreenLoader getInstance()
210     {
211         return instance;
212     }
213 }
214
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