为什么在Vertx的车把库中调用Synchronize?

我想知道为什么 synchronized 此方法中使用了来自Vertx手把库的块 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));
    }
}

请跟我解释一下,好像我是个白痴!