Why is Synchronize called in vertx's Handlebars library?

Question

I am trying to understand why a synchronized block is used in this method from the vertx handlebars library io.vertx.ext.web.templ.handlebars.impl.HandlebarsTemplateEngineImpl class:

@Override
public void render(Map<String, Object> context, String templateFile, Handler<AsyncResult<Buffer>> handler) {
    try {
        int idx = templateFile.lastIndexOf('/');
        String prefix = "";
        String basename = templateFile;
        if (idx != -1) {
            prefix = templateFile.substring(0, idx);
            basename = templateFile.substring(idx + 1);
        }
        Template template = isCachingEnabled() ? cache.get(templateFile) : null;
        if (template == null) {
            synchronized (this) {
                loader.setPrefix(prefix);
                // Strip leading slash from Utils##normalizePath
                template = handlebars.compile(basename);
                if (isCachingEnabled()) {
                    cache.put(templateFile, template);
                }
            }
        }
        Context engineContext = Context.newBuilder(context).resolver(getResolvers()).build();
        handler.handle(Future.succeededFuture(Buffer.buffer(template.apply(engineContext))));
    } catch (Exception ex) {
        handler.handle(Future.failedFuture(ex));
    }
}

Please explain it to me like I'm an idiot!

Answer 1

First, synchronization questions are never "idiot" questions.
I spent some time looking at this code too, and still not 100% sure it's totally correct.

The main reason to have synchronized block here is to protect the following two methods from executing out of order:

loader.setPrefix(prefix);
...
template = handlebars.compile(basename);

You see, Handlebars has a reference to loader:

loader = new Loader(vertx);
...
handlebars = new Handlebars(loader);

Possible scenario without sync block would be

T1 sets prefix to A and switches
T2 sets prefix to B and switches
T1 compiles template with prefix set to B, while thinking it's still A