Category: appengine

Many web applications need to generate a SHA1 hash for verifying file integrity. If you use the Apache Commons libraries, the DigestUtils class offers convenience methods for generating hashes.

If you don’t use Apache Commons, the following function will generate SHA1 hashes for you. Simply pass the string to calculate the hash from into the function ( to_be_sha1 ); the function will then return the SHA1 hash. If an error is encountered (such as the passed-in string being null), a RuntimeException will be thrown.

/**
 * Calculates a SHA1 hash from a provided String. If 
 * to_be_sha1 is null, a RuntimeException will be thrown.
 * 
 * @param to_be_sha1 String to calculate a SHA1 hash from.
 * @return A SHA1 hash from the provided String.
 */
public static String generateSHA1(String to_be_sha1) {
    String sha1_sum = "";
    //if the provided String is null, throw an Exception.
    if (to_be_sha1 == null) {
        throw new RuntimeException("There is no String to calculate a SHA1 hash from.");
    }
    try {
        MessageDigest digest = MessageDigest.getInstance("SHA1");
        byte[] array = digest.digest(to_be_sha1.getBytes("UTF-8"));
        StringBuffer collector = new StringBuffer();
        for (int i = 0; i < array.length; i++) {
            collector.append(Integer.toString((array[i] & 0xff) + 0x100, 16).substring(1));
        }
        sha1_sum = collector.toString();
    }//end try
    catch (NoSuchAlgorithmException e) {
        throw new RuntimeException("Could not find a SHA1 instance: " + e.getMessage());
    }
    catch (UnsupportedEncodingException e) {
        throw new RuntimeException("Could not translate UTF-8: " + e.getMessage());
    }
    return sha1_sum;
}//end generateSHA1

Add these imports as well:

import java.io.UnsupportedEncodingException;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;

Here’s a short code example demonstrating how to read in a file within your application. This snippet assumes the file data.json is within your application’s root directory (change as necessary). The contents of the file are read into the variable json – insert your own processing code after the while code block. If the file cannot be found, or if there are other difficulties in reading from the file, an IOException will be thrown.

try {
    InputStream is = context.getResourceAsStream("/data.json");
    BufferedReader reader = new BufferedReader(new InputStreamReader(is));  
    String json = "";
    String line = "";   
    while (line != null) {
        json += line;
        line = reader.readLine();
    }   
    //Do something with the read-in data within the json variable.
}
catch (IOException e) {
    throw new RuntimeException("Unable to read in file.");
}

Web applications frequently need to generate hashes for comparing data, verifying file integrity, generating keys, etc. If your application uses the Apache Commons libraries, you can use the included DigestUtils convenience class to create hashes. If you don’t use the Apache libraries, then you need an alternate way to generate MD5 hashes.

The below code snippet is a simple function that generates a MD5 hash from a given string. It’s simple to use: simply pass the string to be hashed into the function (to_be_md5), and the hash is returned. This function will throw a RuntimeException if an error is encountered (for example, if the passed-in String is null).

/**
 * Calculate a MD5 hash from the provided String. If the 
 * provided String is null, this method will throw a 
 * RuntimeException.
 * 
 * @param to_be_md5 A String to calculate a MD5 hash from.
 * @return A MD5 hash calculated from the provided String.
 * @throws RuntimeException If an error was encountered during calculating.
 */
public static String generateMD5(String to_be_md5) {
    String md5_sum = "";
    //If the provided String is null, then throw an Exception.
    if (to_be_md5 == null) {
        throw new RuntimeException("There is no string to calculate a MD5 hash from.");
    }
    try {
        MessageDigest md = MessageDigest.getInstance("MD5");
        byte[] array = md.digest(to_be_md5.getBytes("UTF-8"));
        StringBuffer collector = new StringBuffer();
        for (int i = 0; i < array.length; ++i) {
            collector.append(Integer.toHexString((array[i] & 0xFF) | 0x100).substring(1, 3));
        }
        md5_sum = collector.toString();
    }//end try
    catch (NoSuchAlgorithmException e) {
        throw new RuntimeException("Could not find a MD5 instance: " + e.getMessage());
    }
    catch (UnsupportedEncodingException e) {
        throw new RuntimeException("Could not translate UTF-8: " + e.getMessage());
    }
    return md5_sum;
}//end generateMD5

Don’t forget to import the following classes:

import java.io.UnsupportedEncodingException;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;