When editing Java code in Eclipse I can modify the code while it’s running in various ways and have those changes take effect without restarting my program.
What is the technology underlying this feature? Is it part of Eclipse, or is it something I can do with the command line tools and the tools that come with the JDK?
Its part of the JVM and its called HotSwap.
This feature is usually called “hot code replace” and was introduced in Java 1.4 – it’s part of the Java Virtual Machine Debug Interface and various APIs on top of that.
So it’s not specific to Eclipse, but I don’t think any of the command line tools that come with the JDK support it.