Usage of custom freemarker template - freemarker

Question

I was wondering if anyone can help me with Apache FreeMarker? I'm trying to use a custom model but I can't figure it out.
Imagine I want to dump the result of a query (java ResultSet in a FreeMarker template). What is the best approach?
I have found on Google the class: ResultSetTemplateModel
import java.sql.ResultSet;
import freemarker.template.SimpleScalar;
import freemarker.template.TemplateHashModel;
import freemarker.template.TemplateModel;
import freemarker.template.TemplateModelException;
import freemarker.template.TemplateSequenceModel;
public class ResultSetTemplateModel implements TemplateSequenceModel {
private ResultSet rs = null;
public ResultSetTemplateModel(ResultSet rs) {
this.rs = rs;
}
public TemplateModel get(int i) throws TemplateModelException {
try {
rs.next();
} catch(Exception e) {
throw new TemplateModelException(e.toString());
}
TemplateModel model = new Row(rs);
return model;
}
public int size() throws TemplateModelException {
int size=0;
try {
rs.last();
size = rs.getRow();
rs.beforeFirst();
} catch (Exception e ) {
throw new TemplateModelException( e.toString());
}
return size;
}
class Row implements TemplateHashModel {
private ResultSet rs = null;
public Row(ResultSet rs) {
this.rs = rs;
}
public TemplateModel get(String s) throws TemplateModelException {
TemplateModel model = null;
try {
model = new SimpleScalar( rs.getString(s) );
} catch (Exception e) { e.printStackTrace(); }
return model;
}
public boolean isEmpty() throws TemplateModelException {
boolean isEmpty = false;
if ( rs == null ) { isEmpty = true; }
return isEmpty;
}
}
}
And I have a very simple class (I even made it easier than previous):
public static void main(String[] args) {
try {
Configuration cfg = new Configuration(Configuration.VERSION_2_3_27);
cfg.setTemplateExceptionHandler(TemplateExceptionHandler.RETHROW_HANDLER);
cfg.setClassForTemplateLoading(MyCLASS.class, "/");
StringWriter out = new StringWriter();
Map<String, Object> parameters = new TreeMap<>();
ResultSet rs = getResultSet("Select foo, bar FROM my_table");
parameters.put("hello", "World");
parameters.put("result", rs);
Template temp = cfg.getTemplate("template.txt");
temp.process(parameters, out);
System.out.println("out = " + out);
} catch (IOException | TemplateException e) {
e.printStackTrace();
}
}
My template
Hello ${hello}
<#-- how do I specify ResultSet columns here ?? -->
How can I use the custom template?? Any advice?? I know how to load the template file. But I don't know how to specify that it is a custom model in the template.
THank you guys for the support :)

Answer 1

There are two ways of using ResultSetTemplateModel for wrapping ResultSet-s:
Either extend DefaultObjectWrapper by overriding handleUnknownType, where you return new ResultSetTemplateModel((ResultSet) obj) if obj is a ResultSet, otherwise call super. Then use Configuration.setObjectWrapper to actually use it.
Or, add new ResultSetTemplate(rs) to parameters instead of rs; if something is already a TempalteModel, it will not be wrapped again. Note that if you get a ResultSet from somewhere else in the template, this approach will not work as it avoids your manual wrapping, so extending the DefaultObjectWrapper is what you want generally.
Note that the ResultSetTemplateModel implementation shown is quite limited. The ObjectWrapper should be passed to the constructor as well, and stored in a final field. Then, instead of new SimpleScalar( rs.getString(s) ) it should do objectWrapper.wrap(rs.getObject(s)).