java 46 lines · 7 steps

Parsing Apache access logs in Java

A named-group regex turns each Common Log Format line into a typed, immutable record.

Explained by highlit

Intermediate regex named-groups parsing optional records

1public final class AccessLogParser {
2 
  private static final Pattern LINE = Pattern.compile(
      "^(?<ip>\\S+) \\S+ \\S+ \\[(?<time>[^\\]]+)\\] "
      + "\"(?<method>[A-Z]+) (?<path>\\S+) (?<protocol>[^\"]+)\" "
      + "(?<status>\\d{3}) (?<size>\\d+|-) "
      + "\"(?<referer>[^\"]*)\" \"(?<agent>[^\"]*)\"$");
8 
  private static final DateTimeFormatter CLF_TIME =
      DateTimeFormatter.ofPattern("dd/MMM/yyyy:HH:mm:ss Z", Locale.ENGLISH);
11 
  public Optional<AccessLogEntry> parse(String line) {
      Matcher m = LINE.matcher(line);
      if (!m.matches()) {
          return Optional.empty();
      }
17 
      String rawSize = m.group("size");
      long size = "-".equals(rawSize) ? 0L : Long.parseLong(rawSize);
20 
      AccessLogEntry entry = new AccessLogEntry(
          m.group("ip"),
          OffsetDateTime.parse(m.group("time"), CLF_TIME),
          m.group("method"),
          m.group("path"),
          m.group("protocol"),
          Integer.parseInt(m.group("status")),
          size,
          m.group("referer"),
          m.group("agent"));
31 
      return Optional.of(entry);
  }
34 
  public record AccessLogEntry(
      String ip,
      OffsetDateTime timestamp,
      String method,
      String path,
      String protocol,
      int status,
      long responseBytes,
      String referer,
      String userAgent) {
  }
46}

01 / 01

STEP 01

Walkthrough

Space play ←→ step click any line

Three takeaways

1Named capture groups make a complex regex readable and self-documenting when you extract fields.
2Returning Optional signals parse failure without exceptions, forcing callers to handle malformed lines.
3Compiling patterns and formatters once as static finals avoids rebuilding them on every call.

Related explainers

ruby

class UserAgentParser
  BROWSERS = [
    [/Edg\/([\d.]+)/, "Edge"],
    [/OPR\/([\d.]+)/, "Opera"],

Parsing user-agent strings in Ruby

regex pattern-matching lookup-tables

Intermediate 8 steps

java

@Component
@Converter
public class EncryptedStringConverter implements AttributeConverter<String, String> {

Transparent column encryption in Spring & JPA

encryption aes-gcm jpa-converter

Advanced 10 steps

javascript

function evaluate(expression) {
  const tokens = tokenize(expression);
  let pos = 0;

Building a recursive descent calculator

parsing recursion operator-precedence

Intermediate 8 steps

java

package com.acme.billing.config;
 
import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
import org.springframework.boot.context.properties.ConfigurationProperties;

Feature-flagged beans with Spring @ConditionalOnProperty

feature-flags conditional-beans strategy-pattern

Intermediate 5 steps

rust

use chrono::{Duration, NaiveDate};
 
#[derive(Debug)]
pub struct DateRange {

Parsing and iterating date ranges in Rust

error-handling iterators parsing

Intermediate 7 steps

java

public static Map<String, String> parseCookieHeader(String header) {
    Map<String, String> cookies = new LinkedHashMap<>();
    if (header == null || header.isBlank()) {
        return cookies;