Faster Clojure REPL Startup With Java 9 Snapshot

After upgrading to Java 8, Clojure development seemed faster due to quicker REPL startup times. So when I saw a snapshot of Java 9 had been released I was hopeful that startup performance would be even faster.

As Clojure runs on the Java Virtual machine (JVM), each time you start a REPL then you wait for a new JVM to start. Other than this REPL startup, Clojure feels faster than developing with Java directly.

Here is how I set up Java 9 Snapshot on my Linux laptop (Ubuntu 14.10), it should be the same for any decent operating system.

Installing Java 9 Snapshot

I could have built Java 9 from source and made a .deb file of it for a nice install, however the manual install is a lot quicker.

Download the Java 9 snapshot from the OpenJDK9 website.

I extracted the .tar.gz file into the directory ~/apps/openjdk and created a symbolic link called current that pointed to the extracted directory

tar zvxf ~/Downloads/jdk-9-ea-bin-b44-linux-x64-23_dec_2014.tar.gz ~/apps/openjdk
cd ~/apps/openjdk
ln -s jdk-9-ea-bin-b44-linux-x64-23_dec_2014 current

Add Java 9 to the system PATH

I currently have Java 8 installed and its picked up by the alternatives system in Ubuntu, which has java in the /usr/bin path. So to run Java 9 without removing Java 8 or creating an Ubuntu package, I can simply add Java 9 executable to the start of the system path so it is picked up first.

To make the manual adding of Java to the path more robust, I use the environment vairable JAVA_HOME and set that to the location pointed to by the current symbolic link. If I want to try a new version of Java I can simply change the symbolic link.

Add the environment variable to your shell resource configuration, eg ~/.bashrc or ~/.zshrc as follows

### Java9 - from
export JAVA_HOME=/home/jr0cket/apps/openjdk/current
export PATH=$JAVA_HOME/bin:$PATH

Now when ever I open a new command line terminal I can run Java 9 as the default Java. I could also use source ~/.bashrc or source ~/.zshrc command to update the path in the current command line terminal.

Testing Java 9

To test I have successfulling installed Java 9 I run the following commands:

java -version
javac -version

To test the speed performance of Java 9 over Java 8 I used Light Table, a modern and easy to use development environment for Clojure. For my performance test I opened a small project in Light Table and opened its main Clojure file. I then started an Instarepl in Light Table for the current file.

Using Java 8 the Instarepl took 17 seconds to start up. Using Java 9 the Instarepl took 14 seconds to start up.

The time taken for the REPL to start included checking for dependencies each time I ran it. In each test the dependencies were all ready present so time difference is not due to downloading libraries. There are many more tests I could run, but the biggest difference for me is in REPL startup time.

So in this basic test there is a visible improvement in REPL startup time with Java 9. I hope that this startup time can be further reduced as Java 9 develops and the componentisation of Java via Project Jigsaw helps make Java smaller and quicker to start.

Thank you.

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