http://hi.baidu.com/ly_dayu/item/828b09c5c3c5e547a8ba9409
velocity使用基本來說比較簡單，但在加載模板時老出問題，很多初學者經常會遇到找不到模板這種異常。本文就針對目前常用的三種模板加載方式做以說明。


一、velocity默認的加載方式(文件加載方式)

1. package com.velocity.test;
3. import java.io.StringWriter;
4. import java.util.Properties;
6. import org.apache.velocity.VelocityContext;
7. import org.apache.velocity.app.VelocityEngine;
9. /**
10. * 從文件中加載模板文件,即velocity默認的模板文件加載方式
11. * @author welcome
12. *
13. */
14. public class LoaderFromFile {
16. public static void main(String[] args) throws Exception{
17. //初始化參數
18. Properties properties=new Properties();
19. //設置velocity資源加載方式為file
20. properties.setProperty("resource.loader", "file");
21. //設置velocity資源加載方式為file時的處理類
22. properties.setProperty("file.resource.loader.class", "org.apache.velocity.runtime.resource.loader.FileResourceLoader");
23. //實例化一個VelocityEngine對象
24. VelocityEngine velocityEngine=new VelocityEngine(properties);
26. //實例化一個VelocityContext
27. VelocityContext context=new VelocityContext();
28. //向VelocityContext中放入鍵值
29. context.put("username", "張三");
30. context.put("password", "123456789");
31. context.put("age", "20");
32. context.put("address", "陜西西安");
33. context.put("blog", "http://blogjava.net/sxyx2008");
34. //實例化一個StringWriter
35. StringWriter writer=new StringWriter();
36. //從vm目錄下加載hello.vm模板,在eclipse工程中該vm目錄與src目錄平級
37. velocityEngine.mergeTemplate("vm/hello.vm", "gbk", context, writer);
38. System.out.println(writer.toString());
40. }
41. }

二、從類路徑加載模板文件

1. package com.velocity.test;
3. import java.io.StringWriter;
4. import java.util.Properties;
6. import org.apache.velocity.VelocityContext;
7. import org.apache.velocity.app.VelocityEngine;
9. /**
10. * 從class(類路徑)中加載模板文件
11. * @author welcome
12. *
13. */
14. public class LoaderFromClass {
16. public static void main(String[] args) throws Exception{
17. //初始化參數
18. Properties properties=new Properties();
19. //設置velocity資源加載方式為class
20. properties.setProperty("resource.loader", "class");
21. //設置velocity資源加載方式為file時的處理類
22. properties.setProperty("class.resource.loader.class", "org.apache.velocity.runtime.resource.loader.ClasspathResourceLoader");
23. //實例化一個VelocityEngine對象
24. VelocityEngine velocityEngine=new VelocityEngine(properties);
26. //實例化一個VelocityContext
27. VelocityContext context=new VelocityContext();
28. //向VelocityContext中放入鍵值
29. context.put("username", "張三");
30. context.put("password", "123456789");
31. context.put("age", "20");
32. context.put("address", "陜西西安");
33. context.put("blog", "http://blogjava.net/sxyx2008");
34. //實例化一個StringWriter
35. StringWriter writer=new StringWriter();
37. //從src目錄下加載hello.vm模板
38. //假若在com.velocity.test包下有一個hello.vm文件,那麽加載路徑為com/velocity/test/hello.vm
39. velocityEngine.mergeTemplate("com/velocity/test/hello.vm", "gbk", context, writer);
41. //velocityEngine.mergeTemplate("hello.vm", "gbk", context, writer);
42. System.out.println(writer.toString());
43. }
44. }

三、從jar文件中加載模板文件

1. package com.velocity.test;
3. import java.io.StringWriter;
4. import java.util.Properties;
6. import org.apache.velocity.VelocityContext;
7. import org.apache.velocity.app.VelocityEngine;
9. /**
10. * 從jar文件中加載模板文件
11. * @author welcome
12. *
13. */
14. public class LoaderFromJar {
16. public static void main(String[] args) throws Exception{
17. //初始化參數
18. Properties properties=new Properties();
19. //設置velocity資源加載方式為jar
20. properties.setProperty("resource.loader", "jar");
21. //設置velocity資源加載方式為file時的處理類
22. properties.setProperty("jar.resource.loader.class", "org.apache.velocity.runtime.resource.loader.JarResourceLoader");
23. //設置jar包所在的位置
24. properties.setProperty("jar.resource.loader.path", "jar:file:WebRoot/WEB-INF/lib/vm.jar");
25. //實例化一個VelocityEngine對象
26. VelocityEngine velocityEngine=new VelocityEngine(properties);
28. //實例化一個VelocityContext
29. VelocityContext context=new VelocityContext();
30. //向VelocityContext中放入鍵值
31. context.put("username", "張三");
32. context.put("password", "123456789");
33. context.put("age", "20");
34. context.put("address", "陜西西安");
35. context.put("blog", "http://blogjava.net/sxyx2008");
36. //實例化一個StringWriter
37. StringWriter writer=new StringWriter();
38. //從/WebRoot/WEB-INF/lib/vm.jar中加載hello.vm模板 vm.jar的目錄結構為vm/hello.vm
39. velocityEngine.mergeTemplate("vm/hello.vm", "gbk", context, writer);
40. System.out.println(writer.toString());
41. }
42. }

velocity模板路徑又一解http://www.blogjava.net/patterns/archive/2006/11/28/velocity_template_path_another_method.html
研究hibernatesynchronizer的源碼，看到他將velocity模板和編譯的類一起打包在jar包中，在獲得模板時使用

1. Xobject.class.getClassLoader().getResourceAsStream("/templates/xx.vm")

獲得流，然後再將轉變成字符串

1. public static String getStringFromStream(InputStream is) throws IOException {
2. if (null == is)
3. return null;
4. try {
5. InputStreamReader reader = new InputStreamReader(is);
6. char[] buffer = new char[1024];
7. StringWriter writer = new StringWriter();
8. int bytes_read;
9. while ((bytes_read = reader.read(buffer)) != -1) {
10. writer.write(buffer, 0, bytes_read);
11. }
12. return (writer.toString());
13. } finally {
14. if (null != is)
15. is.close();
16. }
17. }

最後調用velocity的方法

1. Velocity.evaluate(Context context, java.io.Writer out, java.lang.String logTag, java.lang.String instring)

從而生成文件。居然不知道velocity有這樣的方法，挺無知的，為了路徑焦頭爛額，終於得解了。總結一下技巧：
1、Xobject.class.getClassLoader().getResourceAsStream("/templates/xx.vm")相對路徑獲得流；
2、Velocity.evaluate(...)方法使用；


velocity模板路徑: http://zhyt710.iteye.com/blog/235250
遇到的velocity加載模板時的路徑問題。
於是查閱資料解決。最後綜合velocity自己帶的例子的example1和example2，改寫了一個例子。怎樣解決的在例子的註釋中已經說的很明確。對於初學velocity的同誌來說，這個例子可以是你參照學習的良好實例

1. /*
2. * Licensed to the Apache Software Foundation (ASF) under one
3. * or more contributor license agreements. See the NOTICE file
4. * distributed with this work for additional information
5. * regarding copyright ownership. The ASF licenses this file
6. * to you under the Apache License, Version 2.0 (the
7. * "License"); you may not use this file except in compliance
8. * with the License. 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,
13. * software distributed under the License is distributed on an
14. * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
15. * KIND, either express or implied. See the License for the
16. * specific language governing permissions and limitations
17. * under the License.
18. */
20. import java.io.BufferedWriter;
21. import java.io.OutputStreamWriter;
22. import java.io.StringWriter;
23. import java.util.ArrayList;
24. import java.util.Properties;
26. import org.apache.velocity.Template;
27. import org.apache.velocity.VelocityContext;
28. import org.apache.velocity.app.Velocity;
29. import org.apache.velocity.app.VelocityEngine;
30. import org.apache.velocity.exception.MethodInvocationException;
31. import org.apache.velocity.exception.ParseErrorException;
33. /**
34. * This class is a simple demonstration of how the Velocity Template Engine
35. * can be used in a standalone application using the Velocity utility class.
36. *
37. * It demonstrates two of the ‘helper‘ methods found in the org.apache.velocity.util.Velocity
38. * class, mergeTemplate() and evaluate().
39. *
40. *
41. * @author <a href="mailto:[email protected]">Geir Magnusson Jr.</a>
42. * @version $Id: Example2.java 463298 2006-10-12 16:10:32Z henning $
43. */
45. public class Example2
46. {
48. public static ArrayList getNames()
49. {
50. ArrayList list = new ArrayList();
52. list.add("ArrayList element 1");
53. list.add("ArrayList element 2");
54. list.add("ArrayList element 3");
55. list.add("ArrayList element 4");
57. return list;
58. }
60. public static void main( String args[] )
61. {
62. /* first, we init the runtime engine. Defaults are fine. */
64. Properties p = new Properties();
65. //設置輸入輸出編碼類型。和這次說的解決的問題無關
66. p.setProperty(Velocity.INPUT_ENCODING, "UTF-8");
67. p.setProperty(Velocity.OUTPUT_ENCODING, "UTF-8");
68. //這裏加載類路徑裏的模板而不是文件系統路徑裏的模板
69. p.setProperty("file.resource.loader.class", "org.apache.velocity.runtime.resource.loader.ClasspathResourceLoader");
70. //也可以用下面方法指定一個絕對路徑，不過這樣要求你所有的模板都放在該路徑下，是有局限的
71. //p.setProperty(Velocity.FILE_RESOURCE_LOADER_PATH, "模板路徑");
72. try
73. {
74. Velocity.init(p);
75. }
76. catch(Exception e)
77. {
78. System.out.println("Problem initializing Velocity : " + e );
79. return;
80. }
82. /* lets make a Context and put data into it */
84. VelocityContext context = new VelocityContext();
86. context.put("name", "Velocity");
87. context.put("project", "阿帕奇");
89. context.put("list", getNames());
91. /* lets render a template */
93. StringWriter w = new StringWriter();
95. try
96. {
97. Velocity.mergeTemplate("example2.vm", "UTF-8", context, w );
98. }
99. catch (Exception e )
100. {
101. System.out.println("Problem merging template : " + e );
102. }
104. System.out.println(" template : " + w );
106. /*
107. * lets dynamically ‘create‘ our template
108. * and use the evaluate() method to render it
109. */
111. //這個例子也同時告訴我們可以先從文件系統讀取一個文件到字符串，然後進行我們想要的操作
112. String s = "We are using $project $name to render this.";
113. w = new StringWriter();
115. try
116. {
117. Velocity.evaluate( context, w, "mystring", s );
118. }
119. catch( ParseErrorException pee )
120. {
121. /*
122. * thrown if something is wrong with the
123. * syntax of our template string
124. */
125. System.out.println("ParseErrorException : " + pee );
126. }
127. catch( MethodInvocationException mee )
128. {
129. /*
130. * thrown if a method of a reference
131. * called by the template
132. * throws an exception. That won‘t happen here
133. * as we aren‘t calling any methods in this
134. * example, but we have to catch them anyway
135. */
136. System.out.println("MethodInvocationException : " + mee );
137. }
138. catch( Exception e )
139. {
140. System.out.println("Exception : " + e );
141. }
143. System.out.println(" string : " + w );
145. ///////////////////////////////////////////////////////
146. //其他方法: 1分別指定路徑，此方法可以設定不同的路徑 （也可是相對的。在eclipse下是工程目錄）
147. try {
148. VelocityEngine velocityEngine = new VelocityEngine();
149. Properties properties = new Properties();
150. //也可以在這裏指定絕對路徑。當指定相對路徑時， 在不同的環境下是有區別的。
151. //比如把程序部署到tomcat以後，相對路徑相對到哪裏是個很惡心的事情。
152. String basePath = "vm";
153. //可設置絕對路徑
154. //String basePath = "F:/";
155. properties.setProperty(Velocity.FILE_RESOURCE_LOADER_PATH, basePath);
156. velocityEngine.init(properties);
157. Template template = velocityEngine.getTemplate("example2.vm");
158. BufferedWriter writer = new BufferedWriter(
159. new OutputStreamWriter(System.out));
160. template.merge(context, writer);
161. writer.flush();
162. writer.close();
163. } catch (Exception e) {
164. e.printStackTrace();
165. }
166. }
167. }

velocity模板加載