How to tell if JRE or JDK is installed

Setting up and verifying your Java environment

Configuring your environment

Initially, double-check that your Java path is correctly established in the system environment variables. This path helps your system know where to look for executable files like java and javac.

Checking PATH and JAVA_HOME

For troubleshooting, check what JAVA_HOME is set to using echo $JAVA_HOME on Unix systems or echo %JAVA_HOME% on Windows. Likewise, check if the Java bin directory is included in your system’s PATH via echo $PATH or echo %PATH%. If not properly set, these could lead to errors despite the actual Java installation.

echo $JAVA_HOME  // "home" of Java, or where it showers and sleeps
echo $PATH  // reveals where Java has "permission" to loiter

Finding Java on Unix-based systems

On Unix systems (like Linux or macOS), the which java or command -v java commands reveal the location of the java binary on your system’s execution path. The returned path specifies where java resides—if it's on the PATH.

which java
command -v java

To examine the entire system for Java installations, including those not on the PATH, use find / -name "java". This command may take a while to complete.

find / -name "java"  // "where in the world is Java San Diego?"

Determining Java presence for Mac users

For Mac enthusiasts, the /Library/Java/JavaVirtualMachines/ directory stores your JDK installations. If installed, you'll spot a directory for each version. Ensure to set JAVA_HOME to the path of the version you plan on using.

Checking for JDK, not just JRE

Be mindful that the presence of java in your Unix-based PATH does not guarantee a full JDK. Look specifically for the javac executable. The existence of javac promises JDK, and not just JRE.

javac -version  // "javac, do you exist?"

Verifying JDK on Windows

For Windows users, navigate to the bin directory (generally found in C:\Program Files\Java\jdk<version>\bin). Execute javac -version here to confirm JDK presence.

cd /d C:\Program Files\Java\jdk<version>\bin // "show me the way to go home (to the bin)"
javac -version // "javac, are you in there?"

A warning to Linux users

A note to Linux comrades: many Linux distributions' Java executables are part of the OpenJDK JRE. So, appearances can be deceiving, and do not take openjdk in your which java or command -v java output as a JDK confirmation.

Anticipating and tackling potential issues

Different versions of JDK and JRE

Be aware that your java -version and javac -version should report the same version number. Mismatches could lead to unexpected compilation/execution issues.

Missing JRE?

If you're using JDK version 11 or newer, don't be alarmed if there isn't a separate JRE. The JDK now includes all the capabilities required for JRE.

Check your version

For development, using the JDK is generally necessary, but ensure you have the right version for your projects. Java continues to evolve, and newer versions may introduce breakages in backward compatibility, or you may require features not present in older versions.

Setting environment variables in macOS and Linux

On macOS and Linux, experiencing an error when setting JAVA_HOME or updating to a new JDK usually means you need to do a little extra work. Open up your shell's profile script and modify the JAVA_HOME variable to the correct paths, then use source ~/.bash_profile (or equivalent) to apply your changes. This tweak is the range of motion exercise for your system's Java muscles.