How to Use the Command 'cs java' (with examples)

The cs java command is a versatile tool provided by Coursier, a software utility for fetching, updating, and managing Java Virtual Machines (JVMs) within a Java development environment. This command empowers developers to manage multiple JVM versions concurrently, facilitating seamless version switching, installation, and execution of Java Runtime Environments. Whether you’re ensuring compatibility for different projects, optimizing performance, or simply experimenting with various JVM features, cs java provides an efficient and flexible solution.

Use case 1: Display Java Version Using Coursier

Code:

cs java -version

Motivation:

Checking the Java version currently in use is fundamental for developers who need to ensure that their environment matches the project’s requirements. This is particularly useful when maintaining or deploying applications that are sensitive to specific Java versions, ensuring compatibility and mitigating potential runtime issues.

Explanation:

• cs: The prefix for all Coursier commands.

• java: Invokes the Java command functionality within Coursier.

• -version: Requests the command to return the currently active Java version, providing clarity on which JVM is being utilized.

Example Output:

java version "17.0.1" 2021-10-19 LTS
Java(TM) SE Runtime Environment (build 17.0.1+12-LTS-39)
Java HotSpot(TM) 64-Bit Server VM (build 17.0.1+12-LTS-39, mixed mode, sharing)

Use case 2: Call a Specific Java Version with Custom Properties Using Coursier

Code:

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

Motivation:

There are scenarios where applications require specific JVM versions or options for optimal performance or compatibility. This use case allows developers to tailor their application’s runtime environment precisely, using a specified JVM and adjusting the Java Virtual Machine options to align with the application’s needs.

Explanation:

• --jvm jvm_name:jvm_version: Specifies the exact JVM to be used, where jvm_name is the identifier of the JVM provider and jvm_version is the desired version number.

• -Xmx32m: A JVM option that sets the maximum heap size to 32 megabytes, crucial for controlling memory usage.

• -Xanother_jvm_opt: Placeholder for any additional JVM options the developer may wish to utilize.

• -jar path/to/jar_name.jar: Indicates the specific Java Archive (JAR) file to execute, crucial for running Java applications packaged in this format.

Example Output:

This is a sample output from a Java application executed using JVM x.y.z by Coursier

Use case 3: List All Available JVMs in the Coursier Default Index

Code:

cs java --available

Motivation:

Understanding the available JVMs allows developers to evaluate and select the most appropriate options for their projects. This can be pertinent when adopting new language features introduced in newer Java versions or when troubleshooting issues linked to JVM differences.

Explanation:

• --available: Instructs Coursier to retrieve and display a list of all JVM versions available from the default index, effectively aiding in decision-making regarding JVM adoption.

Example Output:

adopt:1.8.0-302
graalvm:21.2.0
openjdk:16

Use case 4: List All Installed JVMs in the System with Their Locations

Code:

cs java --installed

Motivation:

Knowing which JVMs are installed and their respective locations is critical for maintaining a clean and organized development environment. It allows developers to verify and manage JVM installations, reducing clutter and ensuring that only desired versions are in use.

Explanation:

• --installed: This flag requests the command to list all JVMs currently installed on the system, along with their installation paths.

Example Output:

adopt:1.8.0-292 (/Users/username/.coursier/jvm/adopt-1.8.0-292)
graalvm:21.1.0 (/Users/username/.coursier/jvm/graalvm-21.1.0)

Use case 5: Set a Specific JVM as a One-off Default for the Shell Instance

Code:

cs java --jvm jvm_name:jvm_version --env

Motivation:

In scenarios where a temporary switch of the JVM environment is necessary—for testing or compatibility checks in an ongoing shell session—this command offers a precise method. It helps in swiftly adapting the development environment without making lasting changes.

Explanation:

• --jvm jvm_name:jvm_version: Specifies the desired JVM version to be temporarily set as default.

• --env: Applies the JVM setting only to the current shell session, allowing for transient configuration adjustments.

Example Output:

export JAVA_HOME=/Users/username/.coursier/jvm/openjdk-11.0.11
export PATH=/Users/username/.coursier/jvm/openjdk-11.0.11/bin:$PATH

Use case 6: Revert Changes for the Default JVM Settings

Code:

eval "$(cs java --disable)"

Motivation:

After experimenting with or testing different JVM configurations, reverting to the default settings ensures continuity and stability in the development environment. This command is beneficial for quickly regaining your previous JVM setup post-testing.

Explanation:

• --disable: Signals Coursier to restore the JVM settings to their original defaults before any modifications were made.

• eval "$(…)": This shell construct runs the output of the command, executing the necessary commands to revert the environment settings.

Example Output:

# No output; the JVM setting reverts silently and seamlessly.

Use case 7: Set a Specific JVM as Default for the Whole System

Code:

cs java --jvm jvm_name:jvm_version --setup

Motivation:

Consistency across multiple projects or teams often requires setting a particular JVM as the system-wide default. Whether adhering to company standards or ensuring compatibility with all development and production environments, this command is key to achieving such uniformity.

Explanation:

• --jvm jvm_name:jvm_version: Selects the specific JVM version to be established as the global default.

• --setup: Applies the default JVM setting to the entire system, cementing the change beyond the scope of individual shell sessions.

Example Output:

Setting /usr/local/bin/java to /Users/username/.coursier/jvm/graalvm-21.2.0/bin/java

Conclusion

Coursier’s cs java command offers a powerful suite of tools tailored for managing JVM environments efficiently. By supporting operations such as listing, installing, and configuring multiple JVM versions, developers gain flexibility and control over their project requirements and development settings. Whether transient or permanent, these functionalities streamline Java development and ensure a consistent and compatible environment across applications.