Overview

With the new release model of Java, now new Java version is released after every 6 months. For learning and experimenting with different JDK versions it is mandatory to install and manage multiple versions. In this article we will learn how easily we can manage multiple JDKs on Mac OS.

Install Location

When an Oracle JDK is installed in Mac OS, it is installed in following directory: /Library/Java/JavaVirtualMachines. I have 3 JDK installed in my machine, so it has 3 separate directory. We can view the content of this directory using ls command.

$ ls -la /Library/Java/JavaVirtualMachines
total 0
drwxr-xr-x  5 root  wheel  160 May  2 12:48 .
drwxr-xr-x  4 root  wheel  128 Jan 23  2020 ..
drwxr-xr-x  3 root  wheel   96 May  2 12:48 jdk-16.0.1.jdk
drwxr-xr-x  3 root  wheel   96 Dec 13  2016 jdk1.7.0_79.jdk
drwxr-xr-x  3 root  wheel   96 Sep  8  2019 jdk1.8.0_221.jdk

Now lets see which version of java binary is selected by default.

$ java -version 
java version "16.0.1" 2021-04-20
Java(TM) SE Runtime Environment (build 16.0.1+9-24)
Java HotSpot(TM) 64-Bit Server VM (build 16.0.1+9-24, mixed mode, sharing)

$ which java
/usr/bin/java

$ readlink $(which java)
/System/Library/Frameworks/JavaVM.framework/Versions/Current/Commands/java

$ readlink /System/Library/Frameworks/JavaVM.framework/Versions/Current/Commands/java
$ ls -la /System/Library/Frameworks/JavaVM.framework/Versions/Current/Commands/java
-rwxr-xr-x  1 root  wheel  38880 May 28  2020 /System/Library/Frameworks/JavaVM.framework/Versions/Current/Commands/java

It looks like java is alias to /System/Library/Frameworks/JavaVM.framework/Versions/Current/Commands/java. But this file is not symbolic link. Now lets see which version it is:

$ /System/Library/Frameworks/JavaVM.framework/Versions/Current/Commands/java -version 
java version "16.0.1" 2021-04-20
Java(TM) SE Runtime Environment (build 16.0.1+9-24)
Java HotSpot(TM) 64-Bit Server VM (build 16.0.1+9-24, mixed mode, sharing)

By this it seems like it is coping Java 16 binary. But MD5 sum of these two binary doesn’t match.

$ md5 /Library/Java/JavaVirtualMachines/jdk-16.0.1.jdk/Contents/Home/bin/java
MD5 (/Library/Java/JavaVirtualMachines/jdk-16.0.1.jdk/Contents/Home/bin/java) = 6260ad30c7ac6f36226589f8c31d01b9

$ md5 /System/Library/Frameworks/JavaVM.framework/Versions/Current/Commands/java
MD5 (/System/Library/Frameworks/JavaVM.framework/Versions/Current/Commands/java) = 41a719da5c04632f527db111d61088d0

Actually binaries within /System/Library/Frameworks/JavaVM.framework/Versions/Current/Commands are stub applications that determine which Java to use. And by default top Java version is selected for use.

Introducing java_home Command

/System/Library/Frameworks/JavaVM.framework/Versions/Current/Commands directory has a binary named java_home which can be used for querying various installed Java version’s home directory. This binary is aliased as /usr/libexec/java_home. As /usr/libexec directory is not included in $PATH by default, so always need to enter full command path.

$ readlink /usr/libexec/java_home
/System/Library/Frameworks/JavaVM.framework/Versions/Current/Commands/java_home

Now lets play with java_home:

# Executing without any args prints top installed Java's Home
$ /usr/libexec/java_home
/Library/Java/JavaVirtualMachines/jdk-16.0.1.jdk/Contents/Home

# -V options shows list of all installed JDK 
$ /usr/libexec/java_home -V
Matching Java Virtual Machines (3):
    16.0.1, x86_64:	"Java SE 16.0.1"	/Library/Java/JavaVirtualMachines/jdk-16.0.1.jdk/Contents/Home
    1.8.0_221, x86_64:	"Java SE 8"	/Library/Java/JavaVirtualMachines/jdk1.8.0_221.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/jdk-16.0.1.jdk/Contents/Home

# Can list a specific Java's home with -v 
$ /usr/libexec/java_home -v 1.8
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home

# -exec is used to run specific version of java
$ /usr/libexec/java_home -v 1.8 -exec java -version
java version "1.8.0_221"
Java(TM) SE Runtime Environment (build 1.8.0_221-b11)
Java HotSpot(TM) 64-Bit Server VM (build 25.221-b11, mixed mode)

Switching Java Version

Interesting thing is all commands in /System/Library/Frameworks/JavaVM.framework/Versions/Current/Commands obeys $JAVA_HOME environment variable. So if we update the JAVA_HOME variable we can use different java version.

# Initial Java Version is 16
$ java -version 
java version "16.0.1" 2021-04-20
Java(TM) SE Runtime Environment (build 16.0.1+9-24)
Java HotSpot(TM) 64-Bit Server VM (build 16.0.1+9-24, mixed mode, sharing)

# Exported Java 1.8 as JAVA_HOME
$ export JAVA_HOME=$(/usr/libexec/java_home -v 1.8)

# Now its Java 1.8
$ java -version 
java version "1.8.0_221"
Java(TM) SE Runtime Environment (build 1.8.0_221-b11)
Java HotSpot(TM) 64-Bit Server VM (build 25.221-b11, mixed mode)

To make it permanent we have write this line in .bash_profile.

$ echo 'export JAVA_HOME=$(/usr/libexec/java_home -v 1.8)' >> ~/.bash_profile

Now we are using system wide Java 8 but we still get jshell command which was introduced in Java 9. But running jshell shows following error:

$ readlink $(which jshell)
/System/Library/Frameworks/JavaVM.framework/Versions/Current/Commands/jshell

$ jshell
Unable to locate an executable at "/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/bin/jshell" (-1)

But we can write us a simple alias like below:

$ alias jshell16='/usr/libexec/java_home -v 16 -exec jshell'
$ jshell16
|  Welcome to JShell -- Version 16.0.1
|  For an introduction type: /help intro

jshell>

We can also write alias to easily switch between JDKs like below:

$ alias setJdk7='unset JAVA_HOME; export JAVA_HOME=$(/usr/libexec/java_home -v 1.7)'
$ setJdk7
$ java -version 
java version "1.7.0_79"
Java(TM) SE Runtime Environment (build 1.7.0_79-b15)
Java HotSpot(TM) 64-Bit Server VM (build 24.79-b02, mixed mode)

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