1. echo $SHELL can show what shell you are using.

$ echo $SHELL
/bin/bash

2. What is /usr/libexec/java_home

2.1 On Mac OS X 10.5 or later, we can use /usr/libexec/java_home to return the location of the default JDK.

$ /usr/libexec/java_home 
/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home

2.2 Also, find all installed JDKs.

$ /usr/libexec/java_home -V
Matching Java Virtual Machines (2):
    1.8.0_91, x86_64:   "Java SE 8" /Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home
    1.7.0_79, x86_64:   "Java SE 7" /Library/Java/JavaVirtualMachines/jdk1.7.0_79.jdk/Contents/Home

/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home

2.3 Also, run a specified JDK command.

$ /usr/libexec/java_home -v1.8
/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home

3. Set JAVA_HOME

3.1 Open the ~/.bash_profile

vi ~/.bash_profile

3.2 add the following content

export JAVA_HOME=/System/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home

$ source ~/.bash_profile
$ echo $JAVA_HOME
/System/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home

4. Switch between different JDK versions

4.1 if you want to switch between different JDK, you can see this article.

How to Set $JAVA_HOME environment variable on macOS