velocity 开发篇

先来看看velocity是怎么工作的？

在应用中使用velocity，一般需要以下的几个步骤：

• 初始化Velocity,可以使用单例，或者运行期实例
• 创建context对象，用于包括相应的变量
• 在context中增加相应的数据
• 选择模板
• 合并模板，产生输出

如下的例子：

<!--<br><br>Code highlighting produced by Actipro CodeHighlighter (freeware)<br>http://www.CodeHighlighter.com/<br><br>--> import java.io.StringWriter;
import org.apache.velocity.VelocityContext;
import org.apache.velocity.Template;
import org.apache.velocity.app.Velocity;
import org.apache.velocity.exception.ResourceNotFoundException;
import org.apache.velocity.exception.ParseErrorException;
import org.apache.velocity.exception.MethodInvocationException;

Velocity.init(); 1

VelocityContext context = new VelocityContext(); 2

context.put( " name " , new String( " Velocity " ) ); 3

Template template = null ;

try
{
template = Velocity.getTemplate( " mytemplate.vm " ); 4
}
catch ( ResourceNotFoundException rnfe )
{
// couldn't find the template
}
catch ( ParseErrorException pee )
{
// syntax error: problem parsing the template
}
catch ( MethodInvocationException mie )
{
// something invoked in the template
// threw an exception
}
catch ( Exception e )
{}

StringWriter sw = new StringWriter();

template.merge( context, sw ); 5

上面的例子使用的是单例模式，可以使用运行期实例：

<!--<br><br>Code highlighting produced by Actipro CodeHighlighter (freeware)<br>http://www.CodeHighlighter.com/<br><br>--> VelocityEngine ve = new VelocityEngine();
ve.setProperty(
VelocityEngine.RUNTIME_LOG_LOGSYSTEM, this );
ve.init();

关于context
context，类似于map环境，包括两个主要的方法：

<!--<br><br>Code highlighting produced by Actipro CodeHighlighter (freeware)<br>http://www.CodeHighlighter.com/<br><br>--> public Object put(String key, Object value);
public Object get(String key);

而默认的VelocityContext是使用map封装，保存相应的变量

当然，如果想和其他环境合并，如FacesContext中的Elcontext，需要定义自己的实现类。

Context chaining，
context支持多个context串，如下：

<!--<br><br>Code highlighting produced by Actipro CodeHighlighter (freeware)<br>http://www.CodeHighlighter.com/<br><br>--> VelocityContext context1 = new VelocityContext();

context1.put( " name " , " Velocity " );
context1.put( " project " , " Jakarta " );
context1.put( " duplicate " , " I am in context1 " );

VelocityContext context2 = new VelocityContext( context1 );

context2.put( " lang " , " Java " );
context2.put( " duplicate " , " I am in context2 " );

template.merge( context2, writer );

Velocity不仅可以用于提供模板输出，而且可以直接对字符串进行评估：

<!--<br><br>Code highlighting produced by Actipro CodeHighlighter (freeware)<br>http://www.CodeHighlighter.com/<br><br>--> StringWriter w = new StringWriter();
Velocity.mergeTemplate( " testtemplate.vm " , context, w );

String s = " We are using $project $name to render this. " ;
w = new StringWriter();
Velocity.evaluate( context, w, " mystring " , s );

 

转：http://www.blogjava.net/zyl/archive/2007/05/17/117957.html