What is Java Virtual Machine(JVM)?

A Java Virtual Machine is a runtime environment required for execution of a Java application. Every Java application runs inside a runtime instance of some concrete implementation of abstract specifications of JVM. It is JVM which is 'platform independent'.

A runtime instance of the Java virtual machine has a clear mission in life: to run one Java application. When a Java application starts, a runtime instance is born. When the application completes, the instance dies. If you start three Java applications at the same time, on the same computer, using the same concrete implementation, you'll get three Java virtual machine instances. Each Java application runs inside its own Java virtual machine.

A Java virtual machine instance starts running its application by invoking the main() method of some initial class. The main() method must be public, static, return void, and accept one parameter: a String array. Any class with such a main() method can be used as the starting point for a Java application.

 public class SampleProgram {  
      /**  
       * @param args  
       */  
      public static void main(String[] args) {  
           System.out.println("Hello World");  
      }       
 }  

The main() method of an application's initial class serves as the starting point for that application's initial thread. Inside the Java virtual machine, threads come in two flavours: daemon and non- daemon. A daemon thread is ordinarily a thread used by the virtual machine itself, such as a thread that performs garbage collection. The application, however, can mark any threads it creates as daemon threads. The initial thread of an application, the one that begins at main() is a non- daemon thread.

A Java application continues to execute (the virtual machine instance continues to live) as long as any non-daemon threads are still running. When all non-daemon threads of a Java application terminate, the virtual machine instance will exit. If permitted by the security manager, the application can also cause its own demise by invoking the exit() method of class Runtime or System.

In the SampleProgram application previous, the main() method doesn't invoke any other threads. After it prints out the command line arguments, main() returns. This terminates the application's only non-daemon thread, which causes the virtual machine instance to exit.

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