I just spent 2 hours setting this variable. The other answers did not work properly for me. I'm using macOS Catalina 10.15.4.

First, find your actual Java SDK Home directory:

/usr/libexec/java_home

Manually navigate there to make sure you don't have any mistakes due to incorrect versions, etc. For me, this was:

/Library/Java/JavaVirtualMachines/jdk-13.0.2.jdk/Contents/Home

Next, edit your terminal's profile. If you're using zsh, this will be:

vim ~/.zshrc

If you're not using zsh, this will be:

vim ~/.bash_profile

Inside, add the following new line anywhere in the file:

export JAVA_HOME=/Library/Java/JavaVirtualMachines/jdk-13.0.2.jdk/Contents/Home

Restart your terminal app (or source ~/.bash_profile), and it should work properly.

Also, it`s interesting to set your PATH to reflect the JDK. After adding JAVA_HOME (which can be done with the example cited by 'mipadi'):

export JAVA_HOME=$(/usr/libexec/java_home)

Add also in ~/.profile:

export PATH=${JAVA_HOME}/bin:$PATH

P.S.: For OSX, I generally use .profile in the HOME dir instead of .bashrc

Adding the below answer to help those who are looking for step by step instructions on how to setup Java_Home on a Mac.

1. Determine whether Java is installed by using the command below:

    which java
   

You will see something like this - /usr/bin/java

2. Next Step will be to determine the version of Java Installed by using the command below:

    java -version
   

You will see something like java version "1.8.0_131"

3. Next step will be to get the location where the Java is installed:

    cd /Library/Java
   

4. Under the Java folder, select the folder with the version that was displayed earlier:

    /Library/Java/JavaVirtualMachines/jdk1.8.0_131.jdk/Contents/Home
   

5. Next check if Java_Home is setup to the correct location:

    echo $JAVA_HOME
   

It will result in blank output if it is not already setup. If it returns the location, check if it points to the correct folder.

6. You can add or update the Java_Home using the below commands:

    vi ~/.bash_profile
   

Navigate to the end of the file by pressing "Shift + g". Now press "i" to get to insert mode. Add the below lines in the bash_profile after replacing the path to the java home directory on your mac

    # Setting Java_Home
    export JAVA_HOME=/Library/Java/JavaVirtualMachines/jdk1.8.0_131.jdk/Contents/Home

Save this by pressing Esc followed by ":wq!"

7. Finally open a new terminal window and test the variable is setup correctly:

    echo $JAVA_HOME
   

It should return the Java_Home path setup