{"id":29874,"date":"2015-12-21T11:00:46","date_gmt":"2015-12-21T09:00:46","guid":{"rendered":"http:\/\/examples.javacodegeeks.com\/?p=29874"},"modified":"2019-03-29T14:18:00","modified_gmt":"2019-03-29T12:18:00","slug":"introduction-docker-java-developers","status":"publish","type":"post","link":"https:\/\/examples.javacodegeeks.com\/devops\/docker\/introduction-docker-java-developers\/","title":{"rendered":"Introduction to Docker for Java Developers – Getting started with Docker"},"content":{"rendered":"

1. Introduction<\/h2>\n

According the official definition of Docker, it is an open platform for building, shipping and running distributed applications. It gives programmers, development teams and operations engineers the common toolbox they need to take advantage of the distributed and networked nature of modern applications. In other words, Docker is a tool to avoid the usual headaches of conflicts, dependencies and inconsistent environments, which is an important problem for distributed applications, where we need to install or upgrade several nodes with the same configuration.<\/p>\n

Docker is a container manager, which means that is able to create and execute containers that represent specific runtime environments for your software. In contrast with virtual machines like VirualBox, Docker uses resource isolation features of the Linux kernel to allow independent “containers” to run within a single Linux instance, avoiding the overhead of starting and maintaining virtual machines. A computer with docker can run multiple containers at the same time.<\/p>\n

Docker has a public repository of runtime environments(i.e. Docker images), which is called Docker Hub<\/a>. In this repository allows Docker download and start an specific runtime environments for an specific software (e.g. MongoDB or MySQL) without any installation procedure.\n<\/p>\n

2. Docker Setup<\/h2>\n

The installation instructions for Docker depends on your operative system and are widely explained (here<\/a> for mac, here<\/a> for windows and here<\/a> for ubuntu). Once, you have followed the installation instructions, you should be able to run the following command.<\/p>\n

Docker installation test<\/em><\/span><\/p>\n

$ docker run ubuntu:14.04 \/bin\/echo 'It works!'\nIt works!\n<\/pre>\n
3. Java Hello World for Docker<\/h2>\n
In this example, we are going to run the typical Java Hello World example in docker. We shall use Maven<\/em> to setup a new project for Hello word example. The contents of our maven project descriptor should be as follows:<\/p>\n
pom.xml<\/em><\/span><\/p>\n
<project xmlns=\"http:\/\/maven.apache.org\/POM\/4.0.0\" xmlns:xsi=\"http:\/\/www.w3.org\/2001\/XMLSchema-instance\"\nxsi:schemaLocation=\"http:\/\/maven.apache.org\/POM\/4.0.0 http:\/\/maven.apache.org\/xsd\/maven-4.0.0.xsd\">\n  <modelVersion>4.0.0<\/modelVersion>\n  <groupId>com.javacodegeeks<\/groupId>\n  <artifactId>helloworld<\/artifactId>\n  <version>1.0<\/version>\n  <\/project>\n<\/pre>\n
Now, add a Java class into the source folder (src\/main\/java<\/b> directory) with the following code.
 
<\/div> <\/div><\/p>\n
HelloWorld.java<\/em><\/span><\/p>\n
public class HelloWorld { \n  public static void main(String[] args) throws Exception{ \n    System.out.println(\"Hello World\"); \n  } \n}\n<\/pre>\n
After that, create a file called Dockerfile<\/em> into the root directory of the project and copy the following contents.<\/p>\n
Dockerfile<\/em><\/span><\/p>\n
FROM ubuntu:14.04\nMAINTAINER javacodegeeks\n\nRUN apt-get update && apt-get install -y python-software-properties software-properties-common\nRUN add-apt-repository ppa:webupd8team\/java\n\nRUN echo \"oracle-java8-installer shared\/accepted-oracle-license-v1-1 boolean true\" | debconf-set-selections\n\nRUN apt-get update && apt-get install -y oracle-java8-installer maven\n\nADD . \/usr\/local\/helloworld\nRUN cd \/usr\/local\/helloworld && mvn install\nCMD [\"java\", \"-cp\", \"\/usr\/local\/helloworld\/target\/helloworld-1.0.jar\", \"HelloWorld\"]\n<\/pre>\n
This file contains the set of commands that docker executes to create an execution environment for our application. An execution environment is called Docker image<\/em>. The last line, which starts with CMD<\/code>, is the command that launches our application. A single machine can have different simultaneous executions of the same docker image. Each of these executions is called Docker container<\/em>.<\/p>\n
Now, let’s understand the Dockerfile commands of our example:<\/p>\n
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