Processing non-strings in Velocity email template with Apache Camel/ActiveMQ - activemq

Question

I've inherited an application which sends emails using Apache Camel and ActiveMQ, integrated with Velocity email templates. The ProducerTemplate.sendBodyAndHeaders() method is called to send a message body and headers to a Queue endpoint. The Camel context XML file defines a route that first sends the message to a Velocity template file, then sends the result thereof to a previously-defined mail server and off goes the email.
As I acquaint myself with the application, it seems that things are set up not very differently from these examples:
http://camel.apache.org/velocity.html
http://camel.apache.org/tutorial-example-reportincident-part2.html
The problem is that these examples (and the current uses in my application) all have the template accessing only simple string values. The "body" passed to ProducerTemplate.sendBodyAndHeaders() is a string whose value is accessed in the template as "$body". The headers passed to ProducerTemplate.sendBodyAndHeaders() are in the form of a Map of string keys to string values, with those values accessed in the template as "$headers.keyName1", "$headers.keyName2", etc.
All well and good, but now I would like to have my header keys map not to strings, but to lists of strings. (Preferably, it would actually be a list of POJO's, though I could get by if it were a list of strings.) So, in other words, the exact number of strings sent to the template with a given header key could vary from one invocation to the next, and the template would iterate over the list to construct the email as is appropriate for however many came in the list.
I see in the VTL documentation how to write a template that could process a list in that way … if I could get such a list passed to the template. But how do I get it there? I tried just putting a Java ArrayList into the Map, rather than a String, but that doesn't seem to work. When I try to access $headers.list it just gives me the literal string "$headers.list". So what's the correct way to do this?

Answer 1

With the org.apache.velocity.VelocityContext class we will be able
to access the list in the template as shown below
Message message = exchange.getIn();
List<Name> nameList = new ArrayList<Name>()
Name testName = new Name();
testName.setFirstName("testFirstName");
testName.setLastName("testLastName");
nameList.add(testName)
Map<String, Object> model = new HashMap<>();
model.put("nameList", nameList);
VelocityContext velocityContext = new VelocityContext(model);
message.setHeader("camelVelocityContext", velocityContext);
In the template file we should use internalGet method of org.apache.velocity.VelocityContext inorder to access list as shown below
#if(${headers.camelVelocityContext.internalGet("nameList").size()}!=0)
<table>
<thead>
<tr>
<th class="td-border">FirstName</th>
<th class="td-border">LastName</th>
</tr>
</thead>
<tbody>
#foreach($entry in ${headers.camelVelocityContext.internalGet("nameList")} )
<tr>
<td class="td-border">$entry.firstName</td>
<td class="td-border">$entry.lastName</td>
</tr>
#end
</tbody>
</table>
#end
Name POJO class
public class Name {
private String firstName;
private String lastName;
public String getFirstName() {
return firstName;
}
public void setFirstName(String firstName) {
this.firstName = firstName;
}
public String getLastName() {
return lastName;
}
public void setLastName(String lastName) {
this.lastName = lastName;
}
}