jenv

(This document is for macOS, ChatGPT is the co-author)

jenv (https://github.com/jenv/jenv) is a Java version manager that makes it easy to switch between Java versions.

📌 How does jenv work?

jenv works by changing the JAVA_HOME environment variable in your current shell to point to the Java version you want to use.

It doesn’t uninstall, overwrite, or modify any existing Java installations.

When you add a Java version, jenv simply records its path — it doesn’t copy any files.

Switching versions only updates JAVA_HOME, like this:

export JAVA_HOME=/Library/Java/JavaVirtualMachines/jdk-17.jdk/Contents/Home

jenv doesn’t touch anything in /usr/bin/java or /Library, so it’s safe and easy to manage.

Installing jenv

1. Install the project first

(If you use Homebrew or MacPorts, you can install jenv directly — they’ll also set up your shell configuration automatically)

Source code installation:

git clone https://github.com/jenv/jenv.git ~/.jenv

(This example clones the repo into your home directory, but you can choose a different location if you prefer)

1. Depending on your shell, run the appropriate commands below to add jenv to your PATH and initialize it.

(This example uses the home directory — change the PATH if needed)

Bash user, execute:

echo 'export PATH="$HOME/.jenv/bin:$PATH"' >> ~/.bash_profile
echo 'eval "$(jenv init -)"' >> ~/.bash_profile
source ~/.bash_profile

zsh user, execute:

echo 'export PATH="$HOME/.jenv/bin:$PATH"' >> ~/.zshrc
echo 'eval "$(jenv init -)"' >> ~/.zshrc
source ~/.zshrc

1. Enable the export plugin (to enable automatic switching of JAVA_HOME)

eval "$(jenv init -)"
jenv enable-plugin export

(More on this plugin in the Replenish below)

Then either restart your terminal or run:

exec $SHELL -l

1. Prepare Java

It depends on how you want to install it. If you have it, don’t install it.

1. Add Java to jenv management

Check the version of java you installed:

/usr/libexec/java_home -V

Then add it to jenv

jenv add "$(/usr/libexec/java_home)"

You can confirm the addition is successful with the following command:

jenv versions

1. Set the default Java version

Set the global default version:

jenv global 21.0.2

(You can also not set it and continue to use the system set version)

Set the local version of a project directory:

cd /path/to/project
jenv local 21.0.2

Set the version only in the current shell session:

jenv shell 21.0.2

1. Verify that JAVA_HOME is effective

echo $JAVA_HOME

It should point to something like ~/.jenv/versions/21.0.2

-Extra Tips-

After installation, run:

jenv doctor

You can check if jenv is working properly

If you use fish shell, the configuration method is slightly different. For details, please refer to the official jenv documentation

Replenish

eval

eval is a command in Linux/macOS shell. Its function is to execute the string content in the parameter as a command

For a simple example:

eval "echo hello"

It is equivalent to directly executing:

echo hello

In the sentence you said:

eval "$(jenv init -)"

It means:

First execute the command jenv init - and take out its output (usually a string of shell commands, such as setting environment variables, defining functions, etc.)

Then use eval to execute this string of output commands to complete jenv Initialization configuration

Why do this?

Because jenv init - actually outputs commands to be written into the shell configuration, such as:

export PATH="$HOME/.jenv/bin:$PATH"
export some_other_variable=...
# And define some functions, etc.

With eval "$(jenv init -)", you don’t have to copy and paste manually, the shell will automatically execute all the initialization steps it outputs

Brief summary:

eval: Execute parameters as code

jenv init -: Output the shell commands required to initialize jenv

eval "$(jenv init -)": Execute the jenv initialization command to make jenv effective

If you want to see it more intuitively, try it first:

jenv init -

You will see that it outputs a bunch of shell commands. Executing eval "$(jenv init -)" is to let the shell Run these commands

export

jenv enable-plugin export is used to enable a plugin function of jenv, called export plugin

What does it do?

• By default, jenv only helps you switch Java versions, but does not automatically modify the environment variable JAVA_HOME

• Many programs rely on the environment variable JAVA_HOME to find the correct Java path

What does the export plugin do?

• It automatically sets and updates the JAVA_HOME environment variable for you, so that your terminal and applications can use your currently selected Java version

• After enabling, you switch Java versions (such as using jenv global 17), and JAVA_HOME will automatically change accordingly

For a simple example:

Before enabling the export plugin:

jenv global 17
echo $JAVA_HOME
# It may be empty or an old version path

After enabling:

jenv enable-plugin export
jenv global 17
echo $JAVA_HOME
# Displays a path like /Users/yourname/.jenv/versions/17

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