Is there a way to get the generated HTML as a String from a UIComponent object?

Question

I have a UIComponent object. I would like to get the HTML code generated by this component at runtime so I can analyze it.

Is there a way to achieve this?

I

Answer 1

Just do the same what JSF does under the covers: invoke UIComponent#encodeAll(). To capture the output, set the response writer to a local buffer by FacesContext#setResponseWriter().

E.g. (assuming that you're sitting in invoke application phase; when sitting in render response phase, this needs to be done differently):

FacesContext context = FacesContext.getCurrentInstance();
ResponseWriter originalWriter = context.getResponseWriter();
StringWriter writer = new StringWriter();

try {
    context.setResponseWriter(context.getRenderKit().createResponseWriter(writer, "text/html", "UTF-8"));
    component.encodeAll(context);
} finally {
    if (originalWriter != null) {
        context.setResponseWriter(originalWriter);
    }
}

String output = writer.toString();
// ...

Answer 2

Solution by BalusC to invoke UIComponent#encodeAll()generally works, but I had a problem with unicode characters when using utf-8 encoding. All non-ascii characters in ajax response were damaged after I modified current context's response writer.

Instead of modifying response writer on current context retrieved by FacesContext.getCurrentInstance(), I created a wrapper over current context by extending FacesContextWrapper, so that original context is left unmodified:

StringWriter writer = new StringWriter();
FacesContext context = new FacesContextWrapper() {
    private ResponseWriter internalWriter = getWrapped()
        .getRenderKit().createResponseWriter(writer, "text/html", "UTF-8");

    @Override
    public FacesContext getWrapped() {
        return FacesContext.getCurrentInstance();
    }

    @Override
    public ResponseWriter getResponseWriter() {
        return internalWriter;
    }

};

component.encodeAll(context);
String output = writer.toString();