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When I start the client from eclipse it automaticly sets the user Playerxxx (xxx is a number) and I cannot join servers with it (because theres no accessToken or something). How can I make it so it will launch the client with my account data?
You should be able to configure your program arguments of your Minecraft Launch Configuration. You can find that by clicking the downards facing arrow right to the button you click when launching Minecraft from eclipse. Click on "Run Configurations..." and switch to the "Arguments" tab. In the first text field labeled "Program arguments:", add these arguments at the end of the line: --username <name> --password <password>, where <name> is your login name or E-Mail address, depending wether you have migrated your account or not, and <password> is the password to your Minecraft/Mojang account.