Working with Zip ​

ZipInputStream is a type of FilterInputStream that can directly read the contents of a zip file:

┌───────────────────┐
│    InputStream    │
└───────────────────┘
          ▲
          │
┌───────────────────┐
│ FilterInputStream │
└───────────────────┘
          ▲
          │
┌───────────────────┐
│InflaterInputStream│
└───────────────────┘
          ▲
          │
┌───────────────────┐
│  ZipInputStream   │
└───────────────────┘
          ▲
          │
┌───────────────────┐
│  JarInputStream   │
└───────────────────┘

JarInputStream is derived from ZipInputStream and adds the main functionality of directly reading the MANIFEST.MF file inside a jar file. Essentially, a jar file is a zip file with additional fixed descriptor files.

Reading a Zip File ​

Let’s look at the basic usage of ZipInputStream. Typically, you create a ZipInputStream by passing a FileInputStream as the data source, then loop through getNextEntry() until it returns null, indicating the end of the zip stream.

A ZipEntry represents a compressed file or directory. If it’s a file, you continuously read it using the read() method until it returns -1:

try (ZipInputStream zip = new ZipInputStream(new FileInputStream(...))) {
    ZipEntry entry = null;
    while ((entry = zip.getNextEntry()) != null) {
        String name = entry.getName();
        if (!entry.isDirectory()) {
            int n;
            while ((n = zip.read()) != -1) {
                ...
            }
        }
    }
}

Writing to a Zip File ​

ZipOutputStream is a type of FilterOutputStream that can directly write content to a zip file. You typically create a ZipOutputStream by wrapping a FileOutputStream. Before writing each file, you call putNextEntry(), then write the byte[] data using write(), and call closeEntry() to finish packaging that file.

try (ZipOutputStream zip = new ZipOutputStream(new FileOutputStream(...))) {
    File[] files = ...
    for (File file : files) {
        zip.putNextEntry(new ZipEntry(file.getName()));
        zip.write(Files.readAllBytes(file.toPath()));
        zip.closeEntry();
    }
}

The code above does not consider the directory structure. To implement a hierarchical structure, the name passed to new ZipEntry(name) should use a relative path.

Summary ​

ZipInputStream can read zip format streams, while ZipOutputStream can write multiple data entries into a zip file. When combined with FileInputStream and FileOutputStream, you can effectively read and write zip files.