java 36 lines · 7 steps

Streaming a SHA-256 file checksum in Java

Compute a file's SHA-256 hash by feeding its bytes through a DigestInputStream, then render the result as hex.

Explained by highlit

Intermediate hashing streams resource-management bit-manipulation

1public final class FileChecksum {
2 
  private static final char[] HEX = "0123456789abcdef".toCharArray();
4 
  private FileChecksum() {
  }
7 
  public static String sha256(Path file) throws IOException {
      MessageDigest digest;
      try {
          digest = MessageDigest.getInstance("SHA-256");
      } catch (NoSuchAlgorithmException e) {
          throw new IllegalStateException("SHA-256 not available", e);
      }
15 
      try (InputStream in = Files.newInputStream(file);
           DigestInputStream dis = new DigestInputStream(in, digest)) {
          byte[] buffer = new byte[8192];
          while (dis.read(buffer) != -1) {
              // draining the stream feeds the digest
          }
      }
23 
      return toHex(digest.digest());
  }
26 
  private static String toHex(byte[] bytes) {
      char[] out = new char[bytes.length * 2];
      for (int i = 0; i < bytes.length; i++) {
          int b = bytes[i] & 0xff;
          out[i * 2] = HEX[b >>> 4];
          out[i * 2 + 1] = HEX[b & 0x0f];
      }
      return new String(out);
  }
36}

01 / 01

STEP 01

Walkthrough

Space play ←→ step click any line

Three takeaways

1DigestInputStream lets you hash data as you read it, avoiding loading the whole file into memory.
2Try-with-resources guarantees the stream closes even when reading throws.
3Bytes must be masked with 0xff before hex conversion because Java bytes are signed.

Related explainers

java

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

Transparent column encryption in Spring & JPA

encryption aes-gcm jpa-converter

Advanced 10 steps

rust

use axum::{
    extract::{Path, State},
    response::sse::{Event, KeepAlive, Sse},
};

Streaming import progress with SSE in Axum

server-sent-events streams watch-channel

Advanced 7 steps

ruby

class LogAggregator
  BUCKET_FORMAT = "%Y-%m-%dT%H:%M"
 
  def initialize(entries)

Bucketing log entries by the minute in Ruby

aggregation hashing enumerable

Intermediate 5 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

java

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

Parsing an HTTP Cookie header in Java

string-parsing http url-decoding

Intermediate 6 steps

ruby

class ApplicationController < ActionController::Base
  EXPERIMENTS = {
    checkout_button_color: %w[control blue green],
    onboarding_flow: %w[control streamlined]