Java virtual machine (JVM)

 Java virtual machine (JVM) -

• A Java virtual machine (JVM) is an abstract computing machine. 
• There are three notions of the JVM specification, implementation ,and instance.
• The specification is a book that formally describes what is required of a JVM implementation. 
• Having a single specification ensures all implementation are interoperable. 
• A JVM implementation is a computer program that meets the requirements of JVM specification in a compliant and preferably performant manner. 
• An instance of the JVM is a process that executes a computer program complied into Java bytecode.
• Java virtual machine (JVM) an implementation of the Java virtual machine specification, interprets compiled Java binary code (called bytecode) for a computer's processor or (hardware platform) so that it can perform a Java.

Program's instructions. 

1. Logically we can say that threads run simultaneously but practically it's not true, only one thread can run at a time in such a ways that user feels that concurrent environment. 
2. Fixed priority scheduling algorithm is used to select one thread for execution based on priority.