October 6, 2017

Understanding Java for Clojurists, side-by-side

I've been writing a lot of Clojure for almost two years now, but I never actually learned Java until recently, and primarily for the purpose of being able to use Java libraries in Clojure.

Until actually learning Java, I found interop (calling Java from Clojure code) the most difficult part of Clojure, mainly because Java has a lot of mental complexity that you need to take on—IO, for example, tends to involve all kinds of BufferedThis and StreamingThat and casting across like 5 different types before you can do anything, and most tutorials on Clojure-Java interop assume you know that stuff, and also assume you know, e.g., what a static method is, and that a "class" is the name for both the basic organization of java source code and the name for the compiled output files. Stuff like that.

Accordingly, the existing tutorials seem to be mostly limited to telling you how to instantiate Java objects and call methods on them, they leave off the more basic how-the-JVM-thinks kind of stuff. I won't cover much of the stuff the usual tutorials do, but here's a brief & helpful example of a tutorial like that.

Instead, here's a quick example/mini-tutorial of working Clojure-Java interop that tries to assume pretty close to zero knowledge of Java, and only a basic working knowledge of Clojure.

The task will be to generate a thumbnail of an image from the command line. At the end, we want to be able to type "java -jar ourBigImage.jpg targetSmallImage.jpg" and get a thumbnail out the other end.

First, we'll do it in Java, and then we'll do it in Clojure. All the code is housed on github.

Making a Thumbnail in Java

We'll use the imgscalr library, which, although it hasn't been updated in a while, seems to be widely recommended. (IMPORTANT WARNING: don't click the external links on that github: last I checked the domains seem to have been hijacked by what looks suspiciously like malware sites.) We'll also use the Gradle build tool, because Maven uses XML and XML is agony.

The Build File

First, install Gradle from the instructions at the link above. Then we'll need a build.gradle file to set up the process. Here's my build.gradle:

plugins {
id 'com.github.johnrengelman.shadow' version '2.0.1'
id 'java'

jar {
    manifest {
        attributes 'Main-Class': 'thumbnail.Thumbnail'

dependencies {
             compile 'org.imgscalr:imgscalr-lib:4.2'    

repositories {

The first line is a plugin that allows us to create an uberjar — one big jar file (a jar is essentially a tar.gz of compiled java files—"classes" + resources, with a funny extension) with our code plus library code. Even though this is built into Leiningen for Clojureland, it isn't the default for Javaland. For whatever reason, the default behavior in Javaland is to create a jar without library code, requiring you to provide the libraries some other way.

Because we'll be using a library, and we don't want to mess around with putting that library on an explicit classpath, an uberjar will be much more convenient. Honestly, I haven't quite figured out how to use a library jar that isn't bundled into an uberjar yet. But with this plugin we can just use gradle shadowjar to build and everything will just work.

The second line is the plugin to build Java projects. It turns out that Gradle can be used to compile lots of things, though I think it's mostly used for Java and Groovy (I think that this config file is written in Groovy, though I'm not sure)... and maybe Kotlin?

The jar/manifest block will tell Java where to find our main class, which is the entry point to the application. Java applications are organized into packages composed of classes, so this says "go to the thumbnail package and the Thumbnail class, and you'll find the main method."

The rest of that file should be pretty self-explanatory.

The Code

Our only other file is src/main/java/Thumbnail.java, which is as follows:

package thumbnail;

import java.awt.image.BufferedImage;
import javax.imageio.ImageIO;
import java.io.File;
import java.io.IOException;
import org.imgscalr.Scalr;

public class Thumbnail {

    public static BufferedImage readImageFile(String filename) {
        BufferedImage image = null;
        try {
            image = ImageIO.read(new File(filename));
        catch (IOException e) {
        return image;

    public static BufferedImage scale(BufferedImage inImage){
        BufferedImage outImage = Scalr.resize(inImage, 200);
        return outImage;

    public static void main(String[] args) {
        String inFile = args[0];
        String outFile = args[1];
        try {
            BufferedImage inImage = readImageFile(inFile);
            BufferedImage outImage = scale(inImage);
            ImageIO.write(outImage, "jpg", new File(outFile));
        catch (IOException e){

Let's break this down.

First we have our package declaration, which just helps the compiler organize the code.

Then we have a bunch of imports. We're importing the built-in types (classes) BufferedImage and File which we need to use to read in and write out our data. Java is statically typed, and, like I said, IO tends to involve lots of casts.

We also have to import the exception that we're going to have to handle, because Java makes you explicitly throw or handle almost all exceptions.

Finally, we have to import the library that we're using. It provides one class that we need, Scalr, and we'll just be using one static method from that class, Scalr.resize().

FYI, a static method is a method that you can call like a normal function, that's attached to the name of a class. The alternative is an instance method, which is a method that is attached to an instance of a class. For example, if we have a String object named stringeyMcStringFace then to get the number of characters we'd call the instance method stringeyMcStringFace.length(). We can't call it as a static method like String.length(stringeyMcStringFace) — that would fail to compile.

After the imports, we create our class Thumbnail. This isn't actually going to do any object-oriented stuff (maybe I'll write a part 2 of this tutorial for that). Instead, we're just going to use the class as, essentially, a namespace. That means instead of instantiating it (making objects out of our class that combine state and functionality), we're just going to hang a few static methods on it.

The first method, readImageFile is just a helper method for IO. It declares and instantiates a BufferedImage, and then it tries to read a file with the filename it's passed. Because this can throw an IOException, we have to wrap it in a try/catch. Also note how we have to explicitly instantiate a File object to pass to ImageIO.read(), which returns our BufferedImage. This is Java, it's gonna be verbose.

The second method just wraps the method we got from imgscalr. It's mostly in there for comparision with the equivalent Clojure code in a bit. The second argument to the resize method is the maximum size of our output image, which I've hardcoded in for simplicity.

The last method is our main method. It's the entry point that will be called from the command line, and it always has the same signature and name in every Java program. It's pretty self-explanatory: we read the filenames off the commandline, then we read the filenames from the argument and pass them to the other methods we've defined. Then we write it back out, again using ImageIO.

That's it! If you clone the repository, go to javathumbnail/thumbnail, and compile with gradle shadowjar this will give you a jar file. Then if you go to the root directory of the repo (with the the sample image) and run java -jar javathumbnail/thumbnail/build/libs/thumbnail-all.jar bigkitty.jpg smallkitty-java.jpg we should get a very cute small kitty!

His name is Leonidas. He's the sweetest.

Let's look at the equivalent in Clojure!

Making a Thumbnail in Clojure

Instead of Gradle now we'll use Leiningen. Here's our project.clj:

(defproject thumbnail "0.1.0-SNAPSHOT"
  :dependencies [[org.clojure/clojure "1.8.0"]
                 [org.imgscalr/imgscalr-lib "4.2"]]
  :main ^:skip-aot thumbnail.core
  :target-path "target/%s"
  :profiles {:uberjar {:aot :all}})

Because I'm assuming some basic Clojure knowledge, I won't go into too much detail here, but note that you can fetch a Java library from Maven Central in exactly the same way as you'd fetch a Clojure library from Clojars. Leiningen makes it seamless.

The only other file we need is our src/thumbnail/core.clj:

(ns thumbnail.core
  (:require [clojure.java.io :as io])
  (:import [org.imgscalr Scalr]
           [java.awt.image BufferedImageOp]
           [javax.imageio ImageIO]))

(defn read-image [filename]
  (ImageIO/read (io/file filename)))

(defn scale [image]
  (Scalr/resize image 200 (into-array BufferedImageOp [])))

(defn -main
  [infile outfile]
  (let [image (read-image infile)
        scaled (scale image)]
    (ImageIO/write scaled "jpg" (io/file outfile))))

Again, I'm assuming you already know Clojure, so I'm only going to explain the interop bits.

First, note that in addition to (:require) in our ns declaration we also have (:import). The format of an import is that we give it the Java package identifier and then any classes we want from that package, separated by spaces. So if we wanted to import multiple classes from java.awt.image we could do an import like [java.awt.image BufferedImageOp BufferedImage]. (Incidentally, I'll explain this BufferedImageOp business in a minute).

We'll also want to require the clojure.java.io built-in namespace, which wraps some of that Java verbosity. Note, however, that we do not need to import errors—we're in Clojure now, we don't have to explicitly catch them!—and we don't need to import the types we're using either, Clojure will just let us use reflection to sort them out. (Reflection is the capacity of the JVM to inspect an object and figure out what type it is. I believe, though I'm not sure, that a lot of the reflection here happens on the compilation end: the Clojure compiler figures out that ImageIO/read has to return a BufferedImage, for example, and hence can verify that Scalr/resize is going to get the right type for the first argument.)

Thus, read-image is a much less verbose version of readImageFile from the Java side that does exactly the same thing: io/file returns a Java File which we can then pass straight to the static read() method of the ImageIO class by calling ImageIO/read just as if it were a Clojure function.

Likewise, scale is pretty straightforward: we call the static method from the imported library as before.

There is one little glitch at the end, though. It turns out that if we look at the signature for the resize method, it actually has three parameters. The third is a varargs parameter, which means it takes an array of objects of type BufferedImageOp.

We don't actually need to give that parameter anything for our simple example, and the Java compiler is kind enough that it will let us leave off a varargs parameter entirely if we don't need it. Unfortunately, the Clojure compiler is not so generous (this might be the only case in the entire history of coding where Clojure is more verbose than Java), and makes us explicitly pass these arguments in.

Accordingly, we have to construct an empty array that would have the Java type BufferedImageOp[] (that's Java-ese for an array of BufferedImageOps, where a BufferedImageOp is just... I'm not sure, but it's the thing that the signature of resize demands, so it's what we have to give it) and pass that into resize as the third argument. The into-array function in clojure.core provides that capacity. Arrays are super primitive, and the JVM needs a type even for a zero-length array.

The main function is self-explanatory after all that—it's just a direct translation of method calls to Clojure function calls.

And we're done! lein uberjar from the clojure project root, plus java -jar clojurethumbnail/thumbnail/target/uberjar/thumbnail-0.1.0-SNAPSHOT-standalone.jar bigkitty.jpg smallkitty-clojure.jpg from the will give us the same cute kitty as before!


A real-world example of using imgscalr from Clojure

Another more complicated example of using imgscalr from Clojure

StackOverflow answer explaining the varargs quirk

JIRA issue on Clojure for fixing the varargs quirk, has been open for over 7 years (!!!)

Tags: clojure java