VelocityLayoutView


1   /*
2    * Copyright 2002-2006 the original author or authors.
3    *
4    * Licensed under the Apache License, Version 2.0 (the "License");
5    * you may not use this file except in compliance with the License.
6    * You may obtain a copy of the License at
7    *
8    *      http://www.apache.org/licenses/LICENSE-2.0
9    *
10   * Unless required by applicable law or agreed to in writing, software
11   * distributed under the License is distributed on an "AS IS" BASIS,
12   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13   * See the License for the specific language governing permissions and
14   * limitations under the License.
15   */
16  
17  package org.springframework.web.servlet.view.velocity;
18  
19  import java.io.StringWriter  ;
20  
21  import javax.servlet.http.HttpServletResponse  ;
22  
23  import org.apache.velocity.Template;
24  import org.apache.velocity.context.Context;
25  import org.apache.velocity.exception.ResourceNotFoundException;
26  
27  import org.springframework.context.ApplicationContextException;
28  
29  /**
30   * VelocityLayoutView emulates the functionality offered by Velocity's
31   * VelocityLayoutServlet to ease page composition from different templates.
32   *
33   * <p>The <code>url</code> property should be set to the content template
34   * for the view, and the layout template location should be specified as
35   * <code>layoutUrl</code> property. A view can override the configured
36   * layout template location by setting the appropriate key (the default
37   * is "layout") in the content template.
38   *
39   * <p>When the view is rendered, the VelocityContext is first merged with
40   * the content template (specified by the <code>url</code> property) and
41   * then merged with the layout template to produce the final output.
42   *
43   * <p>The layout template can include the screen content through a
44   * VelocityContext variable (the default is "screen_content").
45   * At runtime, this variable will contain the rendered content template.
46   *
47   * @author Darren Davison
48   * @author Juergen Hoeller
49   * @since 1.2
50   * @see #setLayoutUrl
51   * @see #setLayoutKey
52   * @see #setScreenContentKey
53   */
54  public class VelocityLayoutView extends VelocityToolboxView {
55  
56      /**
57       * The default {@link #setLayoutUrl(String) layout url}.
58       */
59      public static final String   DEFAULT_LAYOUT_URL = "layout.vm";
60  
61      /**
62       * The default {@link #setLayoutKey(String) layout key}.
63       */
64      public static final String   DEFAULT_LAYOUT_KEY = "layout";
65  
66      /**
67       * The default {@link #setScreenContentKey(String) screen content key}.
68       */
69      public static final String   DEFAULT_SCREEN_CONTENT_KEY = "screen_content";
70  
71  
72      private String   layoutUrl = DEFAULT_LAYOUT_URL;
73  
74      private String   layoutKey = DEFAULT_LAYOUT_KEY;
75  
76      private String   screenContentKey = DEFAULT_SCREEN_CONTENT_KEY;
77  
78  
79      /**
80       * Set the layout template to use. Default is {@link #DEFAULT_LAYOUT_URL "layout.vm"}.
81       * @param layoutUrl the template location (relative to the template
82       * root directory)
83       */
84      public void setLayoutUrl(String   layoutUrl) {
85          this.layoutUrl = layoutUrl;
86      }
87  
88      /**
89       * Set the context key used to specify an alternate layout to be used instead
90       * of the default layout. Screen content templates can override the layout
91       * template that they wish to be wrapped with by setting this value in the
92       * template, for example:<br>
93       * <code>#set( $layout = "MyLayout.vm" )</code>
94       * <p>Default key is {@link #DEFAULT_LAYOUT_KEY "layout"}, as illustrated above.
95       * @param layoutKey the name of the key you wish to use in your
96       * screen content templates to override the layout template
97       */
98      public void setLayoutKey(String   layoutKey) {
99          this.layoutKey = layoutKey;
100     }
101 
102     /**
103      * Set the name of the context key that will hold the content of
104      * the screen within the layout template. This key must be present
105      * in the layout template for the current screen to be rendered.
106      * <p>Default is {@link #DEFAULT_SCREEN_CONTENT_KEY "screen_content"}:
107      * accessed in VTL as <code>$screen_content</code>.
108      * @param screenContentKey the name of the screen content key to use
109      */
110     public void setScreenContentKey(String   screenContentKey) {
111         this.screenContentKey = screenContentKey;
112     }
113 
114 
115     /**
116      * Overrides <code>VelocityView.checkTemplate()</code> to additionally check
117      * that both the layout template and the screen content template can be loaded.
118      * Note that during rendering of the screen content, the layout template
119      * can be changed which may invalidate any early checking done here.
120      */
121     protected void checkTemplate() throws ApplicationContextException {
122         super.checkTemplate();
123 
124         try {
125             // Check that we can get the template, even if we might subsequently get it again.
126             getTemplate(this.layoutUrl);
127         }
128         catch (ResourceNotFoundException ex) {
129             throw new ApplicationContextException("Cannot find Velocity template for URL [" + this.layoutUrl +
130                     "]: Did you specify the correct resource loader path?", ex);
131         }
132         catch (Exception   ex) {
133             throw new ApplicationContextException(
134                     "Could not load Velocity template for URL [" + this.layoutUrl + "]", ex);
135         }
136     }
137 
138     /**
139      * Overrides the normal rendering process in order to pre-process the Context,
140      * merging it with the screen template into a single value (identified by the
141      * value of screenContentKey). The layout template is then merged with the
142      * modified Context in the super class.
143      */
144     protected void doRender(Context context, HttpServletResponse   response) throws Exception   {
145         renderScreenContent(context);
146 
147         // Velocity context now includes any mappings that were defined
148         // (via #set) in screen content template.
149         // The screen template can overrule the layout by doing
150         // #set( $layout = "MyLayout.vm" )
151         String   layoutUrlToUse = (String  ) context.get(this.layoutKey);
152         if (layoutUrlToUse != null) {
153             if (logger.isDebugEnabled()) {
154                 logger.debug("Screen content template has requested layout [" + layoutUrlToUse + "]");
155             }
156         }
157         else {
158             // No explicit layout URL given -> use default layout of this view.
159             layoutUrlToUse = this.layoutUrl;
160         }
161 
162         mergeTemplate(getTemplate(layoutUrlToUse), context, response);
163     }
164 
165     /**
166      * The resulting context contains any mappings from render, plus screen content.
167      */
168     private void renderScreenContent(Context velocityContext) throws Exception   {
169         if (logger.isDebugEnabled()) {
170             logger.debug("Rendering screen content template [" + getUrl() + "]");
171         }
172 
173         StringWriter   sw = new StringWriter  ();
174         Template screenContentTemplate = getTemplate(getUrl());
175         screenContentTemplate.merge(velocityContext, sw);
176 
177         // Put rendered content into Velocity context.
178         velocityContext.put(this.screenContentKey, sw.toString());
179     }
180 
181 }
182
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