How to use the command 'cs java' (with examples)

The ‘cs java’ command is used in the coursier command-line interface (CLI) to fetch, install, and run JVMs. It provides a convenient way to manage different Java virtual machine (JVM) installations and switch between them. This article will illustrate different use cases of the ‘cs java’ command to showcase its versatility and functionality.

Use case 1: Call the java version

Code:

cs java -version

Motivation: Sometimes, it is necessary to check the version of Java installed on your system. This information can be useful for troubleshooting purposes or ensuring compatibility with certain software.

Explanation: The ‘-version’ flag is used to get the version of Java installed. By running ‘cs java -version’, the command will return the version of Java installed on your system.

Example output:

java version "11.0.12"
Java(TM) SE Runtime Environment (build 11.0.12+8-LTS-237)
Java HotSpot(TM) 64-Bit Server VM (build 11.0.12+8-LTS-237, mixed mode)

Use case 2: Call a specific java version with custom properties

Code:

cs java --jvm jvm_name:jvm_version -Xmx32m -Xanother_jvm_opt -jar path/to/jar_name.jar

Motivation: In some cases, you may need to run a specific version of Java with custom properties. This could be useful for testing compatibility or running applications that require specific JVM options.

Explanation: The ‘–jvm’ flag is followed by the JVM name and version, separated by a colon. Additional JVM options can be specified using the ‘-X’ flag. Finally, the command is used to execute a JAR file.

Example output: This will vary depending on the specific JAR file being executed and the chosen JVM options.

Use case 3: List all available JVMs

Code:

cs java --available

Motivation: When managing multiple JVM installations, it can be helpful to know which versions are available for installation.

Explanation: The ‘–available’ flag is used to list all the available JVMs in the coursier default index. Running ‘cs java –available’ will display a list of available JVMs with their corresponding names and versions.

Example output:

Available JVMs:
- graalvm-ce-java11:21.3.0
- openjdk:13.0.5+1
- openjdk-jolokia:11.0.10

Use case 4: List all installed JVMs

Code:

cs java --installed

Motivation: Similar to listing available JVMs, being able to list the JVMs already installed on the system can be helpful for managing different installations.

Explanation: The ‘–installed’ flag is used to list all the installed JVMs along with their respective locations. Running ‘cs java –installed’ will display a list of installed JVMs with their corresponding names and locations.

Example output:

Installed JVMs:
- graalvm-ce-java11:21.3.0 (/usr/lib/graalvm-ce-java11)
- openjdk:13.0.5 (/usr/lib/jvm/openjdk-13.0.5)
- openjdk-jolokia:11.0.10 (/opt/openjdk-jolokia-11.0.10)

Use case 5: Set a specific JVM as a temporary “default”

Code:

cs java --jvm jvm_name:jvm_version --env

Motivation: There may be situations where you want to temporarily set a specific JVM as the default for the current shell session. This can be useful when testing code against different JVM versions.

Explanation: The ‘–jvm’ flag is used to specify the JVM to set as the default, followed by the JVM name and version separated by a colon. The ‘–env’ flag ensures that the changes are applied to the current shell session.

Example output: This will not produce any output but will set the specified JVM as the default for the current shell session.

Use case 6: Revert changes for the default JVM settings

Code:

eval "$(cs java --disable)"

Motivation: After setting a specific JVM as the default, you may want to revert back to the original default JVM settings.

Explanation: The ‘cs java –disable’ command retrieves the original default JVM settings and wraps them in an ’eval’ statement. By running ’eval “$(cs java –disable)”’, the changes made for the default JVM settings will be reverted.

Example output: This will not produce any output but will revert the changes made for the default JVM settings.

Use case 7: Set a specific JVM as default for the entire system

Code:

cs java --jvm jvm_name:jvm_version --setup

Motivation: In certain cases, you may want to set a specific JVM version as the default for the entire system. This can be useful when working on projects that require a specific JVM version.

Explanation: The ‘–jvm’ flag is used to specify the JVM to set as the default, followed by the JVM name and version separated by a colon. The ‘–setup’ flag ensures that the specified JVM is set as the default for the entire system.

Example output: This will not produce any output but will set the specified JVM as the default for the entire system.

Conclusion:

The ‘cs java’ command in the coursier CLI provides a powerful set of tools for managing and running JVMs. Whether you need to check the Java version, manage multiple JVM installations, or set a specific JVM as the default, the ‘cs java’ command offers a convenient and efficient solution. By understanding and utilizing the various use cases demonstrated in this article, users can take full advantage of the ‘cs java’ command’s capabilities.