How to Check Java Version

By 

Dejan Panovski
•

Updated on

•

5 min read

Check Java Version

Before installing an application that runs on Java, such as Tomcat, Jenkins, or Maven, you need to know whether Java is installed on your system and which version it is. Many applications require a specific minimum version and refuse to start with older releases.

This guide explains how to check the installed Java version from the Linux command line, verify the JDK compiler, and find out which installation is the default when multiple versions are present.

Checking the Java Runtime Version

To find out which Java version is installed on your system, run the java -version command:

Terminal
java -version

The command prints the default Java runtime version:

output
openjdk version "21.0.11" 2026-04-21
OpenJDK Runtime Environment (build 21.0.11+10-Ubuntu-1ubuntu124.04.2)
OpenJDK 64-Bit Server VM (build 21.0.11+10-Ubuntu-1ubuntu124.04.2, mixed mode, sharing)

In this example, Java version 21.0.11 is installed. The version on your system may differ depending on which package you installed.

Java 9 and newer also accept java --version (with two dashes), which prints the same information to standard output instead of standard error. This distinction matters when you pipe the output to another command.

If you get “java: command not found”, it means that Java is not installed on the system. To install it, use one of the following guides depending on your Linux distribution:

Understanding Java Version Numbers

Since JDK 10, Java version numbers follow the scheme defined in JEP 322:

txt
FEATURE.INTERIM.UPDATE.PATCH

For example, in Java 21.0.11, 21 is the feature release, 0 is the interim release, and 11 is the update release. The patch component is omitted because its value is zero.

  • FEATURE - Increases every six months with each feature release that brings new language and platform features.
  • INTERIM - Reserved for releases between feature releases. In practice, it is always 0.
  • UPDATE - Increases with each quarterly update that delivers bug and security fixes.
  • PATCH - Increases only for emergency releases that fix critical issues. It is omitted when its value is zero.

Oracle publishes a new feature release every six months. Long-term support (LTS) releases, which receive updates for several years, come out every two years. Java 8, 11, 17, 21, and 25 are LTS releases.

Checking the Java Compiler Version

The java command shows the runtime (JRE) version. If you need to compile Java code, you also need the Java Development Kit (JDK). To check whether the JDK is installed and which version it provides, run:

Terminal
javac -version

The output shows the compiler version:

output
javac 21.0.11

If the command is not found, the JDK may not be installed, or the directory containing javac may not be in your PATH. Install the JDK package to get the compiler. On Ubuntu and Debian, the package is named default-jdk or a version-specific package such as openjdk-21-jdk.

Listing Installed Java Versions

A system can have several Java versions installed side by side. To list all of them on Ubuntu, Debian, and derivatives, run:

Terminal
update-alternatives --list java

The command prints the full path of every registered Java executable:

output
/usr/lib/jvm/java-17-openjdk-amd64/bin/java
/usr/lib/jvm/java-21-openjdk-amd64/bin/java

In this example, two versions are installed: Java 17 and Java 21. You can also inspect the /usr/lib/jvm directory, where Linux distributions place Java installations. On Fedora, RHEL, and derivatives, use alternatives --display java instead.

Checking the JAVA_HOME Variable

Many Java applications and build tools such as Maven, Gradle, and Tomcat rely on the JAVA_HOME environment variable to locate the Java installation. To check whether it is set, run:

Terminal
echo $JAVA_HOME

If the variable is set, the output shows the installation path:

output
/usr/lib/jvm/java-21-openjdk-amd64

An empty output means JAVA_HOME is not configured. You can set it by adding the following line to your ~/.bashrc or ~/.profile file:

sh
export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64

Replace the path with the actual location of your Java installation. Use one of the paths from the previous section, without the /bin/java suffix.

Managing Multiple Java Versions

Your system may have more than one Java version installed at the same time. To check whether that is the case and to switch between them, run:

Terminal
sudo update-alternatives --config java

If you have only one Java installation, the output looks like this:

output
There is only one alternative in link group java (providing /usr/bin/java): /usr/lib/jvm/java-21-openjdk-amd64/bin/java
Nothing to configure.

If multiple versions are installed, the command displays a menu where you can select which version will be the default:

output
There are 3 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                            Priority   Status
------------------------------------------------------------
* 0            /usr/lib/jvm/java-21-openjdk-amd64/bin/java      2111      auto mode
  1            /usr/lib/jvm/java-17-openjdk-amd64/bin/java      1711      manual mode
  2            /usr/lib/jvm/java-21-openjdk-amd64/bin/java      2111      manual mode

Press <enter> to keep the current choice[*], or type selection number:

Enter the number from the Selection column and press Enter to change the default. This command is available on Debian-based distributions (Ubuntu, Debian, Linux Mint). On Fedora and RHEL-based systems, use alternatives --config java instead.

FAQ

Does java -version show the JDK or the JRE?
It shows whichever runtime is set as the default. Both the JRE-only package and the full JDK include the java command. To confirm that the JDK is installed, check for javac as described above.

How do I check the Java version inside a shell script?
Parse the output of java -version, which prints to standard error:

Terminal
java -version 2>&1 | head -n 1 | awk -F '"' '{print $2}'

This returns a string such as 21.0.11. Do not compare Java versions as ordinary strings because values such as 9 and 17 sort incorrectly. Use a version-aware comparison such as sort -V when your script needs to enforce a minimum version.

Conclusion

Use java -version to check the installed runtime and javac -version to verify the JDK. If you need to manage multiple versions, our update-alternatives guide explains how to inspect, switch, and automate the default selection.

Tags

java

Linuxize Weekly Newsletter

A quick weekly roundup of new tutorials, news, and tips.

About the authors

Dejan Panovski

Dejan Panovski

Dejan Panovski is the founder of Linuxize, an RHCSA-certified Linux system administrator and DevOps engineer based in Skopje, Macedonia. Author of 800+ Linux tutorials with 20+ years of experience turning complex Linux tasks into clear, reliable guides.

View author page