如何使用 Java 1.8 更改/重构方法以将 FileInputStream 用于本地文件以及将 InputStream 用于 URL？

Question

Using Java 1.8, I created a class which obtains a zip file from an external HTTP URL:

eg

https://raw.githubusercontent.com/mlampros/DataSets/master/fastText_data.zip

and converts it into a String based MD5 hash:

6aa2fe666f83953a089a2caa8b13b80e

---

My utility class:

public class HashUtils {
 
    public static String makeHashFromUrl(String fileUrl) {
        try {
            MessageDigest md = MessageDigest.getInstance("MD5");
            InputStream is = new URL(fileUrl).openStream();

            try {
                is = new DigestInputStream(is, md);

                // Up to 8K per read
                byte[] ignoredBuffer = new byte[8 * 1024];

                while (is.read(ignoredBuffer) > 0) { }
            } finally {
                is.close();
            }
            byte[] digest = md.digest();
            StringBuilder sb = new StringBuilder();

            for (int i = 0; i < digest.length; i++) {
                sb.append(Integer.toString((digest[i] & 0xff) + 0x100, 16).substring(1));
            }
            return sb.toString();

        } catch (Exception ex) {
            throw new RuntimeException(ex);
        }
    }
}

---

Whereas this is fine for an external URL containing zip file (or any file with any type of file extension), I need to be be able to use the same code (algorithm) for files that reside on a local filesystem.

eg

Inside $CATALINA_HOME/temp/fastText_data.zip

Would need to use this instead:

InputStream fis =  new FileInputStream(filename);

---

How could I do this using the same method (don't want to violate DRY - Don't Repeat Yourself)?

Of course, creating a brand new method containing the same code but using the InputStream fis = new FileInputStream(filename); instead of InputStream is = new URL(fileUrl).openStream(); would be violating the DRY principle?

What would be a good way to refactor this out? Two public methods with a refactored private method containing the same lines of code?

Answer 1

Make three methods: A private method that expects an InputStream argument which is given to your current logic, and two very short public methods which each call the private method with an InputStream they create.

public static String makeHashFromUrl(String url) {
    try (InputStream stream = new URL(url).openStream()) {
        return makeHashFromStream(stream);
    }
}

public static String makeHashFromFile(File file) {
    try (InputStream stream = new BufferedInputStream(new FileInputStream(file))) {
        return makeHashFromStream(stream);
    }
}

private static String makeHashFromStream(InputStream is) {
    try {
        MessageDigest md = MessageDigest.getInstance("MD5");

        try {
            is = new DigestInputStream(is, md);

            // etc.
}