Unravel › Java concepts › I/O and files

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At a glance

1. 1Compress: try (GZIPOutputStream out = new GZIPOutputStream(new FileOutputStream("a.gz"))) { out.write(bytes); }Decompress by wrapping a GZIPInputStream around the input.
2. 2ZIP: zip.putNextEntry(new ZipEntry("dir/file.txt")), write bytes, zip.closeEntry()Read with ZipInputStream.getNextEntry() in a loop, or open random entries with ZipFile.
3. 3Close the outermost stream: it writes the trailer (checksum, directory) that makes the file validAn unclosed GZIP stream produces a truncated file.
4. 4Always close in try-with-resources; large data should be copied with transferTo rather than loaded into memory

• Zip slip: an entry named ../../etc/cron.d/x extracted naively overwrites files outside the target folder
• Zip bombs: a tiny archive can expand to terabytes
• Both use the DEFLATE algorithm through java.util.zip.Deflater and Inflater, which call the zlib native library

import java.io.*;import java.nio.file.*;import java.util.zip.*;public class Unpack {  static void extract(Path zip, Path dest) throws IOException {    try (ZipInputStream in = new ZipInputStream(Files.newInputStream(zip))) {      ZipEntry e;      while ((e = in.getNextEntry()) != null) {        Path out = dest.resolve(e.getName()).normalize();        if (!out.startsWith(dest)) throw new IOException("bad entry " + e.getName());        if (!e.isDirectory()) Files.copy(in, out, StandardCopyOption.REPLACE_EXISTING);      }    }  }  static byte[] gzip(byte[] data) throws IOException {    ByteArrayOutputStream bytes = new ByteArrayOutputStream();    try (GZIPOutputStream gz = new GZIPOutputStream(bytes)) { gz.write(data); }    return bytes.toByteArray();  }}

1. line 6 uses ZipInputStream, the core of ZIP and GZIP compression
2. line 7 uses ZipEntry, the core of ZIP and GZIP compression
3. line 17 uses GZIPOutputStream, the core of ZIP and GZIP compression

How Unravel recognises it

When you paste code, Unravel looks for these signs of zip and gzip compression. Spotting them yourself is a good habit when you read unfamiliar code:

• Types: ZipInputStream ZipOutputStream ZipFile GZIPInputStream GZIPOutputStream Deflater Inflater ZipEntry JarFile DeflaterOutputStream InflaterInputStream

Part of a learning path

• I/O, networking and time · previous: Java serialization · next: Sockets, URLs and networking

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