A traditional virtual machine represents the hardware-level virtualization. In essence, it's a complete, virtualized physical machine with BIOS and an operating system installed. It runs on top of the host operating system. Your Java application runs in the virtualized environment as it would normally do on your own machine. There are a lot of advantages from using virtual machines for your applications. Each virtual machine can have a totally different operating system; those can be different Linux flavors, Solaris, or Windows, for example. Virtual machines are also very secure by definition; they are totally isolated, complete operating systems in a box.

However, nothing comes without a price. Virtual machines contain all the features that an operating system needs to have to be operational: core system libraries, device drivers, and so on. Sometimes they can be resource hungry and heavyweight. Virtual machines require full installation, which sometimes can be cumbersome and not so easy to set up. Last, but not least, you will need more compute power and resources to execute your application in the virtual machine the hypervisor needs to first import the virtual machine and then power it up and this takes time. However, I believe, when it comes to running Java applications, having the complete virtualized environment is not something that we would want very often. Docker comes to the rescue with the concept of containerization. Java applications (but of course, it's not limited to Java) run on Docker in an isolated environment called a container. A container is not a virtual machine in the popular sense. It behaves as a kind of operating system virtualization, but there's no emulation at all. The main difference is that while each traditional virtual machine image runs on an independent guest operating system, the Docker containers run within the same kernel running on the host machine. A container is self-sufficient and isolated not only from the underlying OS, but from other containers as well. It has its own separated filesystem and environment variables. Naturally, containers can communicate with each other (as an application and a database container for example) and also can share the files on disk. Here comes the main difference when comparing to traditional virtualization because the containers run within the same kernel they utilize fewer system resources. All the operating system core software is removed from the Docker image. The base container can be, and usually is, very lightweight. There is no overhead related to a classic virtualization hypervisor and a guest operating system. This way you can achieve almost bare metal, core performance for your Java applications. Also, the startup time of a containerized Java application is usually very low due to the minimal overhead of the container. You can also roll-out hundreds of application containers in seconds to reduce the time needed for provisioning your software. We will do this using Kubernetes in one of the coming chapters. Although Docker is quite different from the traditional virtualization engines. Be aware that containers cannot substitute virtual machines for all use cases; a thoughtful evaluation is still required to determine what is best for your application. Both solutions have their advantages. On the one hand, we have the fully isolated secure virtual machine with average performance. On the other hand, we have the containers that are missing some of the key features, but are equipped with high performance that can be provisioned very fast. Let's see what other benefits you will get when using Docker containerization.

As we have said before, the major visible benefit of using Docker will be very fast performance and short provisioning time. You can create or destroy containers quickly and easily. Containers share resources such as the operating system's kernel and the needed libraries efficiently with other Docker containers. Because of that, multiple versions of an application running in containers will be very lightweight. The result is faster deployment, easier migration, and startup times.

Docker can be especially useful when deploying Java microservices. We will get back to microservices in detail in one of the coming chapters. A microservices application is composed of a series of discrete services, communicating with others via an API. Microservices break an app into a large number of small processes. They are the opposite of the monolithic applications, which run all operations as a single process or a set of large processes.

Using Docker containers enables you to deploy ready-to-run software, which is portable and extremely easy to distribute. Your containerized application simply runs within its container; there's no need for installation. The lack of an installation process has a huge advantage; it eliminates problems such as software and library conflicts or even driver compatibility issues. Docker containers are portable; they can be run from anywhere: your local machine, a remote server, and private or public cloud. All major cloud computing providers, such as Amazon Web Services (AWS) and Google's compute platform support Docker now. A container running on, let's say, an Amazon EC2 instance, can easily be transferred to some other environment, achieving exactly the same consistency and functionality. The additional level of abstraction Docker provides on the top of your infrastructure layer is an indispensable feature. Developers can create the software without worrying about the platform it will later be run on. Docker has the same promise as Java; write once, run anywhere; except instead of code, you configure your server exactly the way you want it (by picking the operating system, tuning the configuration files, installing dependencies) and you can be certain that your server template will run exactly the same on any host that runs Docker.

Because of Docker's reproducible build environment, it's particularly well suited for testing, especially in your continuous integration or continuous delivery flow. You can quickly boot up identical environments to run the tests. And because the container images are all identical each time, you can distribute the workload and run tests in parallel without a problem. Developers can run the same image on their machine that will be run in production later, which again has a huge advantage in testing.

The use of Docker containers speeds up continuous integration. There are no more endless build-test-deploy cycles; Docker containers ensure that applications run identically in development, test, and production environments. The code grows over time and becomes more and more troublesome. That's why the idea of an immutable infrastructure becomes more and more popular nowadays and the concept of containerization has become so popular. By putting your Java applications into containers, you can simplify the process of deployment and scaling. By having a lightweight Docker host that needs almost no configuration management, you manage your applications simply by deploying and redeploying containers to the host. And again, because the containers are very lightweight, it takes only seconds.

We have been talking a lot about images and containers, without getting much into the details. Let's do it now and see what Docker images and containers are.