What is rt.jar file in Java?

'rt' stands for runtime. 'rt.jar' contains all the RunTime classes that comprise the Java SE platform’s core API’s and is located at $JAVA_HOME/jre/lib. 'rt.jar' contains the class files like String, Object etc., The source code for these API’s can be found in src.zip file in JAVA_HOME.

Whenever new JVM starts, (You can start new JVM by typing 'java ClassName'), the bootstrap class loader(also known as Primordial class loader) is responsible for loading key classes like Object class. This class loader loads the core system classes from the boot classpath, which is normally the JAR files located in the jre/lib directory. 

Related Links

https://self-learning-java-tutorial.blogspot.com/2014/08/class-loaders.html