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Java-related Tricks and Commands

Creating a JAR file

The basic format of the command for creating a JAR file is:

jar cf jar-file-name input-files

The options and arguments used in this command are:

  • The c option indicates that you want to create a JAR file.
  • The f option indicates that you want the output to go to a file rather than to stdout.
  • jar-file-name is the name that you want the resulting JAR file to have. You can use any filename for a JAR file. By convention, JAR filenames are given a .jar extension, though this is not required.
  • The input-files argument is a space-separated list of one or more files that you want to include in your JAR file. This argument can contain the wildcard symbol. If any of the input files are directories, the contents of those directories are added to the JAR archive recursively.
  • This command will also generate a default manifest file for the JAR archive.
  • The metadata in the JAR file, such as the entry names, comments, and contents of the manifest, must be encoded in UTF8.

Several other options to the cf options of the basic command:

  • v: produces verbose output on stdout whlie the JAR file is being built;
  • 0: indicates that you don't want the JAR file to be compressed;
  • M: indicates that the default manifest file should not be produced;
  • m: used to include manifest information from an existing manifest file;
    • The format for using this option is jar cmf jar-file-name existing-manifest-file input-files
    • The manifest must end with a new line or carriage return. The last line will not be parsed properly if it does not end with a new line or carriage return.
  • C to change directories during execution of the command.

An example:

In directory PropertyGraph/target/classes (under the class/ directory, there are com/ and application.properties):

jar cvmf ../../src/META-INF/MANIFEST.MF test.jar com

Run the JAR application:

java -jar test.jar arguments_to_JAR

Maven

Maven is a build automation tool provided by Apache which does more than dependency management. We can make it as a peer of Ant and Makefile which downloads all of the dependencies required.

Refer to: https://maven.apache.org/guides/getting-started/index.html

On a mvn install, it frames a dependency tree based on the project configuration pom.xml on all the sub-projects under the super pom.xml (the root POM) and downloads/compiles all the needed components in a directory called .m2 under the user's folder. These dependencies will have to be resolved for the project to be built without any errors, and mvn install is one utility that could download most of the dependencies.

Further, there are other utils within Maven like dependency:resolve which can be used separately in any specific cases. The build life cycle of the maven is as below:

  • process-resources
  • compile
  • process-test-resources
  • test-compile
  • test
  • package
  • install
  • deploy

The test phase of this mvn can be ignored by using a flag -DskipTests=true.