自己找到了，DroidGap.java中有hack，重载了WebviewClient的onJsPrompt方法，然后执行了自己的逻辑。
/**
* Tell the client to display a prompt dialog to the user.
* If the client returns true, WebView will assume that the client will
* handle the prompt dialog and call the appropriate JsPromptResult method.
*
* Since we are hacking prompts for our own purposes, we should not be using them for
* this purpose, perhaps we should hack console.log to do this instead!
@
Override public boolean onJsPrompt(WebView view, String url, String message, String defaultValue, JsPromptResult result) {
...
}