你可以运行JAR的外部工具启动配置，在运行>外部工具>外部工具配置中找到。

I tried to run a jar file by going to Run configurations and then creating a new Java App, but Eclipse wants me to give it a main class and reference to the project. Can't I just give Eclipse a jar file?

You could run the JAR with an "External Tool" launch configuration, found at Run > External Tools > External tools Configurations.