Docker container vs Java Virtual Machine

Generally Docker containers cannot be done "within Java" because Docker serves to encapsulate the application, and "within Java" is the code being loaded after the JVM launches.

The JVM is already running when it parses the class it will search for the main method. So encapsulation at the process level can't be done because the process (the JVM) is already running.

Java has encapsulation techniques which do provide protection between various Java elements (see class loader hierarchies in Tomcat, for an example); but those only isolate "application plugins" from each other, the main process that is running them all is Tomcat, which is really a program loaded into an already running JVM.

This doesn't mean you can't combine the two to achieve some object, it just means that the types of isolation provided between the two products isn't interchangeable.

what is now done in docker containers can also be done in java with jvm someone wanted to

Short Answer: No. You could wrap a docker container around your JVM, but you cannot wrap a JVM around a docker container, non-trivially.

docker basically do what Java has been doing with its virtual machine for ages ? i.e. it provides executable environment separate from the underlying OS.

Docker containers provide isolation from other containers without introducing a virtualisation layer. Thus, they are different and more performant than VMs.

Docker can perform several things that the Java JVM cannot do however, programming in Java and running on the JVM will provide several of the advantages of running in a Docker container.I work on a large Java project and this project is 20 years old. We have been evolving and fixing our application without any tools or compatibility issues for all these years. Also, as a bonus, the application is platform independent. Several components of it can run in both Windows and Linux. Because no effort was made initially to build a multiplatform application, there is one component that cannot run on Linux. But it would be relatively easy to make it work on that platform.It would have been much more difficult to do the same thing using C or C++ and associated toolchain.