Using Thymeleaf 3 pre-processor to remove whitespace

With Thymeleaf 3.0 onwards it is possible to use pre/post processors to manage output, prior to content being rendered, and are applied to the entire template as opposed to a single fragment. As an example, it is possible to strip out whitespace from within HTML elements. Looking at the source of a rendered Thymeleaf HTML page, it is clear that there is typically a substantial level of empty space in, for example, a paragraph or a span. This is not to be confused with inter-HTML element space, already managed by https://github.com/connect-group/thymeleaf-extras.

As mentioned above the removal of unwanted/unnecessarry whitespace is a good example to demonstrate processor usage. The first step is to create the worker, and typically, an extension of AbstractTemplateHandler will suffice.

public final class StripElementWhitespaceHandler extends AbstractTemplateHandler
{

    public StripElementWhitespaceHandler()
    {
        super();
    }

    @Override
    public void handleText(final IText text)
    {

        if (getNext() == null)
        {
            return;
        }

        if (StringUtils.hasText(text.getText()))
        {
            this.getNext().handleText(text);
        }

    }

}

The next step is to create a dialect for this handler, that implements IPreProcessorDialect

public class StripElementWhitespaceDialect implements IPreProcessorDialect
{
    @Override
    public String getName()
    {
        return "your_dialect_name";
    }

    @Override
    public int getDialectPreProcessorPrecedence()
    {
        return 1000;
    }

    @Override
    public Set<IPreProcessor> getPreProcessors()
    {
        return ImmutableSet.of(new PreProcessor(TemplateMode.HTML, 
                          new StripElementWhitespaceHandler().getClass(),
                getDialectPreProcessorPrecedence()));
    }

}

and then finally, add the dialect to the template engine. Using Springboot, as below

@Bean
    public SpringTemplateEngine templateEngine(.....)
    {
        SpringTemplateEngine engine = new SpringTemplateEngine()
        
        ......
        
        engine.addDialect(new StripElementWhitespaceDialect());
        return engine;
}

An alternative example makes use of AbstractTextProcessor where there is a requirement to do more than just remove whitespace e.g. modify text.

public class StripElementWhiteSpace extends AbstractTextProcessor
{

    StripElementWhiteSpace(TemplateMode templateMode, int precedence)
    {
        super(templateMode, precedence);
    }

    @Override
    public void doProcess(ITemplateContext context, IText text, 
                          ITextStructureHandler structureHandler)
    {
        if (!StringUtils.hasText(text.getText()))
        {
            structureHandler.removeText();
        }
    }
}

public class StripElementWhiteSpaceDialect 
       extends AbstractProcessorDialect
{

    private final TemplateMode templateMode;

    StripElementWhiteSpaceDialect(String name, String prefix, 
                                  int precedence, 
                                  TemplateMode templateMode) {
      super(name, prefix, precedence);
      this.templateMode = templateMode;
    }

    StripElementWhiteSpaceDialect() {
      this("strip-element-whitespace-dialect", 
           "strip-html-element-white-space", 100, TemplateMode.HTML);
    }

    @Override
    public Set<IProcessor> getProcessors(String dialectPrefix) {
return ImmutableSet.of(new StriplElementWhiteSpace(templateMode, 
                           getDialectProcessorPrecedence()));
    }
}

//add to template engine
engine.addDialect(new StripElementWhitespaceDialect());

A starting point for more detailed information can be found at https://www.thymeleaf.org/doc/tutorials/3.0/extendingthymeleaf.html.