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 
3 private static final char[] HEX = "0123456789abcdef".toCharArray();
4 
5 private FileChecksum() {
6 }
7 
8 public static String sha256(Path file) throws IOException {
9 MessageDigest digest;
10 try {
11 digest = MessageDigest.getInstance("SHA-256");
12 } catch (NoSuchAlgorithmException e) {
13 throw new IllegalStateException("SHA-256 not available", e);
14 }
15 
16 try (InputStream in = Files.newInputStream(file);
17 DigestInputStream dis = new DigestInputStream(in, digest)) {
18 byte[] buffer = new byte[8192];
19 while (dis.read(buffer) != -1) {
20 // draining the stream feeds the digest
21 }
22 }
23 
24 return toHex(digest.digest());
25 }
26 
27 private static String toHex(byte[] bytes) {
28 char[] out = new char[bytes.length * 2];
29 for (int i = 0; i < bytes.length; i++) {
30 int b = bytes[i] & 0xff;
31 out[i * 2] = HEX[b >>> 4];
32 out[i * 2 + 1] = HEX[b & 0x0f];
33 }
34 return new String(out);
35 }
36}
01 / 01
STEP 01

Walkthrough

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

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