Read username from log file using java

Question

I want to read username who logged in successfully through SSO and non-sso from log file. I think regular expression would help to achieve, but i need some directions to construct regex.

Input line to read from log:

    12:59:39.924 [ajp-nio-0.0.0.0-8009-exec-87] INFO  AuthenticationStrategy - Authentication for username [user1] & realm [SSORealm] successful.
    13:21:38.017 [ajp-nio-0.0.0.0-8009-exec-87] INFO  AuthenticationStrategy - Authentication for username [user2] & realm [Realm] successful.
    13:19:30.419 [ajp-nio-0.0.0.0-8009-exec-87] INFO  AuthenticationStrategy - Authentication for username [user1] & realm [LdapRealm] failed.

Note: I want to read username who successfully logged in because there will some authentication failure as well and don't want to read those names.

Expected output:

user1
user2

Sample Code which tried to read log file:

public class Log {
public static void main(String as[])
{
    try{
           FileInputStream fstream = new FileInputStream("C:\\Users\\test.log");
           BufferedReader br = new BufferedReader(new InputStreamReader(fstream));
           String strLine;
           while ((strLine = br.readLine()) != null)   {
              System.out.println (strLine);
           }
           fstream.close();
        } catch (Exception e) {
             System.err.println("Error: " + e.getMessage());
        }
}
}

Appreciate your help

Answer 1

You can use a regular expression in order to get the user name of each line:

String input = "12:59:39.924 INFO  AuthenticationStrategy - Authentication for username [user1] & realm [SSORealm] successful.";
Matcher matcher = Pattern.compile(" (\\[.*?\\])").matcher(input);
if (matcher.find(1)) {
    String userName = matcher.group(1);
    System.out.println(userName);
}

Answer 2

This might be an option to look at too:

(?m)(?<=username\\s\\[)[^\\r\\n\\]]+

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If you wish to exclude the failed ones, we can add a negative lookahead, such as with:

(?m)(?!.*\\bfailed)(?<=username\\s\\[)[^\\r\\n\\]]+

RegEx Demo

Test

import java.util.regex.Matcher;
import java.util.regex.Pattern;


public class RegularExpression{

    public static void main(String[] args){

        final String regex = "(?m)(?!.*\\bfailed)(?<=username\\s\\[)[^\\r\\n\\]]+";
        final String string = "    12:59:39.924 [ajp-nio-0.0.0.0-8009-exec-87] INFO  AuthenticationStrategy - Authentication for username [user1] & realm [SSORealm] successful.\n"
             + "    13:21:38.017 [ajp-nio-0.0.0.0-8009-exec-87] INFO  AuthenticationStrategy - Authentication for username [user2] & realm [Realm] successful.\n"
             + "    13:19:30.419 [ajp-nio-0.0.0.0-8009-exec-87] INFO  AuthenticationStrategy - Authentication for username [user1] & realm [LdapRealm] failed.\n"
             + "N";

        final Pattern pattern = Pattern.compile(regex);
        final Matcher matcher = pattern.matcher(string);

        while (matcher.find()) {
            System.out.println("Full match: " + matcher.group(0));
        }
    }
}

Output

Full match: user1
Full match: user2

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If you wish to simplify/update/explore the expression, it's been explained on the top right panel of regex101.com . You can watch the matching steps or modify them in this debugger link , if you'd be interested. The debugger demonstrates that how a RegEx engine might step by step consume some sample input strings and would perform the matching process.

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RegEx Circuit

jex.im visualizes regular expressions: