How to check the installed Java version on Linux

A Linux host can have several Java runtimes installed, and the version that matters is the one reached when the shell runs java. Checking only the package list can miss a manually installed runtime, a changed PATH entry, or an alternatives selection that points applications at a different executable.

The Java launcher prints the product version, runtime build, and virtual machine line. The traditional java -version form writes that output to the error stream, while java --version writes to the output stream and is easier to capture in scripts.

Command validation used an Ubuntu 26.04 container with the distribution default-jre-headless package, which installed an OpenJDK runtime. Use the path check first, then read the version output, because the same host may have package-managed, manually unpacked, and project-local runtimes available at the same time.

Steps to check the installed Java version on Linux:

Confirm that the shell can find java.
```
$ command -v java
/usr/bin/java
```
If the command prints nothing or the shell reports java: command not found, install a runtime or fix the shell PATH before checking a version.

Related: How to install Java Runtime Environment (JRE) on Ubuntu
Related: How to install JDK on Ubuntu
Resolve the executable behind the shell path.
```
$ readlink -f /usr/bin/java
/usr/lib/jvm/java-25-openjdk-arm64/bin/java
```
Use the path printed by command -v java if it is not /usr/bin/java. A path under /usr/lib/jvm/ usually belongs to a package-managed runtime, while a path under /opt/, /usr/local/, or a project directory often points to a manually installed runtime.
Print the active Java runtime version.
```
$ java -version
openjdk version "25.0.3" 2026-04-21
OpenJDK Runtime Environment (build 25.0.3+9-2-26.04.2-Ubuntu)
OpenJDK 64-Bit Server VM (build 25.0.3+9-2-26.04.2-Ubuntu, mixed mode, sharing)
```
The first line gives the implementation family and version number. The runtime and virtual machine lines show the build and VM variant used by the active launcher.
Use the stdout form when a script needs to capture the version text.
```
$ java --version
openjdk 25.0.3 2026-04-21
OpenJDK Runtime Environment (build 25.0.3+9-2-26.04.2-Ubuntu)
OpenJDK 64-Bit Server VM (build 25.0.3+9-2-26.04.2-Ubuntu, mixed mode, sharing)
```
Current Java launchers print --version output to stdout and exit. java -version remains the common terminal check and prints to stderr, so visible version output is not a failure by itself.
Identify the Debian or Ubuntu package that owns the active binary when the runtime came from packages.
```
$ dpkg -S /usr/lib/jvm/java-25-openjdk-arm64/bin/java
openjdk-25-jre-headless:arm64: /usr/lib/jvm/java-25-openjdk-arm64/bin/java
```
Use the resolved executable path from the earlier step. On RPM-based Linux distributions, query the same path with the native package database when the runtime was installed from packages.
Decide whether the active runtime matches the application requirement.

If the required version is installed but not selected, switch the default java alternative or set the application-specific environment, such as JAVA_HOME, instead of uninstalling other runtimes.

Related: How to set the default Java version on Ubuntu
Related: How to set JAVA_HOME on Linux