Decoding base64 encoded file back to original format using Clojure

Question

How does one convert a file that has been base64 encoded back to its original format and write it to disk? For instance I have a pdf file which has been mime64 encoded. The file starts with:

data:application/pdf;base64,JVBER

I would like to write this out to disk in the proper format. I have tried several libraries (e.g. ring.util.codec) that decode the string into a byte-array, but if I write the resulting byte-array out to a file (using spit) the file appears corrupted.

UPDATE:

The PHP function base64_decode appears to be doing what I am looking for, as it returns a string. What is the equivalent in Java?

Answer 1

In Clojure, there is data.codec (formerly in clojure-contrib).

Using Java interoperability :

• with Java8, see java.util.base64
• with Java6, see javax.xml.bind.DatatypeConverter/parseBase64Binary and javax.xml.bind.DatatypeConverter/printBase64Binary
• with Google Guava, see com.google.common.io.BaseEncoding
• with the Apache Commons Library 1.1.0, see org.apache.commons.codec.binary.Base64

So those are the helper functions I used for images when using data.codec :

(require '[clojure.data.codec.base64 :as b64-codec])

(defn write-img! [id b64]
  (clojure.java.io/copy
   (decode-str (chop-header b64))
   (java.io.File. (str "/Users/nha/tmp/" id "." (b64-ext b64)))))

(defn decode-str [s]
  (b64-codec/decode (.getBytes s)))

(defn in?
  "true if the seq coll contains the element el"
  [coll el]
  (some #(= el %) coll))

(defn b64-ext [s]
  (if-let [ext (second (first (re-seq #"data:image/(.*);base64.*" s)))]
    (if (in? ["png" "jpeg"] ext)
      ext
      (throw (Exception. (str "Unsupported extension found for image " ext))))
    (throw (Exception. (str "No extension found for image " s)))))

(defn chop-header [s]
  (nth (first (re-seq #"(data:image/.*;base64,)(.*)" s)) 2))