java 42 lines · 6 steps

Counting HTTP statuses with Java streams

Stream a log file line by line, pull out HTTP status codes with a regex, and tally them without loading everything into memory.

Explained by highlit

Intermediate streams regex file-io grouping resource-management

1package com.example.logs;
2 
3import java.io.IOException;
4import java.io.UncheckedIOException;
5import java.nio.charset.StandardCharsets;
6import java.nio.file.Files;
7import java.nio.file.Path;
8import java.util.Map;
9import java.util.regex.Matcher;
10import java.util.regex.Pattern;
11import java.util.stream.Collectors;
12import java.util.stream.Stream;
13 
14public final class AccessLogAnalyzer {
15 
16    private static final Pattern STATUS = Pattern.compile("\"\\s(\\d{3})\\s");
17 
18    public Map<Integer, Long> countByStatus(Path logFile) throws IOException {
19        try (Stream<String> lines = Files.lines(logFile, StandardCharsets.UTF_8)) {
20            return lines
21                    .map(STATUS::matcher)
22                    .filter(Matcher::find)
23                    .map(m -> Integer.parseInt(m.group(1)))
24                    .collect(Collectors.groupingBy(
25                            status -> status,
26                            Collectors.counting()));
27        }
28    }
29 
30    public long countServerErrors(Path logFile) {
31        try (Stream<String> lines = Files.lines(logFile, StandardCharsets.UTF_8)) {
32            return lines
33                    .map(STATUS::matcher)
34                    .filter(Matcher::find)
35                    .map(m -> Integer.parseInt(m.group(1)))
36                    .filter(status -> status >= 500)
37                    .count();
38        } catch (IOException e) {
39            throw new UncheckedIOException("Failed to read " + logFile, e);
40        }
41    }
42}

01 / 01

STEP 01

Walkthrough

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Three takeaways

1Files.lines returns a lazy stream that must be closed, so wrapping it in try-with-resources frees the file handle even on a partial read.
2Collectors.groupingBy paired with counting turns a stream of values into a frequency map in one pass.
3Wrapping a checked IOException in UncheckedIOException lets a method stay clean while still surfacing failures.

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