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Java

• Upgrade from version 8 to version 9 was not backward compatible and resulted in a lot of broken packages hence you will see a lot of companies still using Java 8.

• Java 8 version number is 1.8. This changed with version 9.

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What is a JDK?

• JDK is a set of development tools which will help us develop, build and run the java application.
• These tools are present in the bin directory of the jdk
  jdb is used for debugging java applications and javac is used for compiling java code.

• JRE (Java Runtime Environment) is used to run the Java applications on any system.

  • JRE and the java command line utility are the only components required to run already built Java applications on a system.

• Before version 9 of Java JDK and JRE were shipped as separate components.

  • So if you were not developing a Java application and just wanted to run them then you could have only downloaded the JRE.
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• After version 9 JRE is packed into JDK.
• JVM
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Building & Packaging

• JAR stands for Java Archive it helps compress and combine multiple Java class files and its dependencies into a single distributable package.
  • jar cf JarFileName.jar file1 file2 ....
• Incase of a web application we have static files like html and css. They are packaged in a WAR (Web Archive) file.

• When jar command is run it automatically generates a manifest file within the package at path META-INF/MANIFEST.MF

  • This file contains information about the files packaged in jar file and any other meta data about the application
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  • One important information present in the meta data is the entry point to the application.

• Once jar is built it can be run on any platform using java -jar JarFileName.jar

• Generating documentation for the code: javadoc -d doc MyClass.java

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• When working with large software it can get really cumbersome to package and generate documentation for the project.

  • This is where build tools like Maven, ANT and gradle are used.

• ANT example:

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  • If we want to run specific sections of ANT then we can use ant compile jar

NPM (JavaScript)

• When you install a npm package they are installed inside the node_modules folder in the current directory.
• A directory by the name of the package is created it contains different files like README.md, LICENSE, package.json and the lib folder contains the code.
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• The package.json inside the package is the file which contains meta data for the package like its name, version, author, its git repository.
  • This file is useful when you are troubleshooting issues related to dependency.

When we import a package node first looks for the package in the local node_modules directory if the package is not found then it looks in other locations.

We can find the look up order by priority using the following command.

• We can install the package globally using: npm -g install <package-name>
• There are two types of modules:
  • BuiltIn: These are installed automatically when nodejs is installed for the first time.
  • External

Python

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• Python versions:
  • Python2: 2000 - 2010
  • Python3: 2008 - Till Date.
• When we install Python PIP gets installed automatically.
• If we have multiple python versions then we will have different pips.
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So if you don't know to which python pip is pointing just use pip -V

• When Python is installed it creates the following package structure.
  • A directory is created for each version of Python under the /usr/lib directory.
• When we install a python package using pip it places it in the site-packages folder in the appropriate python folder.
  • This may not be completely true as they can also be installed in different locations.
• Packages can also be 32 bit or 64 bit so a separate path is available.
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Finding the location of a particular package

pip show <package-name>

• When importing a package python looks at a list of paths to find and import the package.

  • We can list the paths using print(sys.path)
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• To install a lot of packages we use requirements.txt

  • pip install -r requirements.txt
• It is a best practice to specify the version of packages in requirements.txt file.
  • If no version is specified then python will install the latest version of the package.
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• Upgrading a package: pip install <package-name> --upgrade
• Uninstalling a package: pip uninstall <package-name>

• Other package managers:

  • Eggs don't need to be unpacked but they are obsolete
  • Wheels are similar to eggs but they need to be unpacked.

• To install a wheel package we do pip install package.whl

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Last updated: 2022-09-16