Why do we use rt.jar in a java project?

Question

What is the necessity of rt.jar ??

Answer 1

The runtime (rt.jar) holds all the (most of the..) java classes that form the Java SE. It is added to the classpath automatically.

Answer 2

rt = Run Time

It contains all the java runtime libraries. (Essential)

Answer 3

The rt.jar is where all the java packages reside. For example, if a class file calls for the java.util package, then the JVM can look for it inside the rt.jar, thus enabling it to run correctly.

On a side note: Don't mess around with it.