About the Javadoc.jar and Soures.jar in the jar bag downloaded from the official website __JDK

Generally downloaded from the official web jar package will have three packages. Jars, Sources.jar and Javadoc.jar, what's the use of these three?

1. Jar bag Everyone should know what to do, go to the official website is nothing but for this.
2, Sources.jar is the entire jar package source code, when you use the method in the jar bag, you want to pass "CTRL + Mouse left button" to see the source words will be associated with this file.
3. Javadoc.jar is the help document for the entire jar package.