Basically, you want to reopen the same page (refresh it ?) in the browser ? You can set settings in the project properties > run configuration, then chaange the index file or project url. But apart from that I am afraid that you can't do anything to prevent opening a new window on run. When you click 'run', netbeans says to your OS : "Hey, let's open that page (your_project_URL/index_file) now". The OS thinks and execute a command like "default_browser url", where the argument "url" is here your_project_URL/index_file. Then the browser thinks it's a new page to open, and BAM, opens a new page.
Hope it helps. Also, not entirely sure why you would need a netbeans plugin? When your application is running, it will do so in its own JVM, so unless you are planning on packaging the plugin with your tool, I don't see how you can use it.
To set this JDK as the default for all projects, you can run the IDE with the --jdkhome switch on the command line, or by entering the path to the JDK in the netbeans_j2sdkhome property of your INSTALLATION_DIRECTORY/etc/netbeans.conf file.
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