Multi-Threadable Headless Browser with good Javascript support
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I'm aware of many headless browsers but I need to know those that can be used safely in multiple threads (New instances can be created from multiple threads), because I'm programming a web crawler.

HtmlUnit does not have solid javascript support.

QtWebKit QWebPage cannot be created from multiple thread.

PhantomJS is command-line based, will have to start new processes which I don't want.

Awesomium is also not runnable in multiple thread.

Do you know of any headless browser with solid javascript support and that can be run from multiple threads without problem?

I don't mind the programming language.

Thank you.