{"id":3659150,"date":"2024-02-12T08:00:54","date_gmt":"2024-02-12T13:00:54","guid":{"rendered":"https:\/\/spin.atomicobject.com\/?p=3659150"},"modified":"2024-02-09T17:49:06","modified_gmt":"2024-02-09T22:49:06","slug":"java-stream-api","status":"publish","type":"post","link":"https:\/\/spin.atomicobject.com\/java-stream-api\/","title":{"rendered":"Zip Archives with the Java Stream API"},"content":{"rendered":"

I’ve been working on a Java project that extracts files from Zip archives in several places. We’re making heavy use of the Java Stream API<\/a> in this codebase, and I wanted to deal with the contents of the Zip archives in the same way. (That is, mapping the entries, filtering the entries, processing entries in parallel, etc.)<\/p>\n

The code I’m working with is provided with an InputStream<\/code> that provides the bytes that make up the Zip archive. That means I don’t know if the contents are coming from the local file or from somewhere else (S3 bucket, database, etc.). I’m also only concerned with pulling out files and can ignore entries that represent a directory.<\/p>\n

ExtractedFile record<\/h2>\n

To provide full separation from all the Java Zip APIs, I want the resulting Stream to yield records similar to the ZipEntry<\/a> class. However, they should only contain the filename and the contents of the file (the bytes).<\/p>\n

\n  public record ExtractedFile(String filename, byte[] bytes) {}\n<\/code><\/pre>\n
Using ZipFile<\/h2>\n
On solution would be to save the contents of the InputStream<\/code> to a temp file, use the built-in ZipFile<\/a> class to read\/stream the entries, and then delete the temp file when we’re done. This Listing a ZIP file contents with Stream API in Java 8<\/a> post shows a simple example of how the .stream()<\/code> method on ZipFile<\/code> can be used.<\/p>\n
Building on that, here’s a full implementation that filters out directories, reads each entry into a byte array, and yields an ExtractedFile<\/code> record for each entry:<\/p>\n
\n  public Stream extractFiles(InputStream archiveStream) throws IOException {\n\n    Path tempFile = Files.createTempFile(\"temp\", \".zip\");\n    try (archiveStream) {\n      Files.copy(archiveStream, tempFile, java.nio.file.StandardCopyOption.REPLACE_EXISTING);\n    }\n\n    var zipFile = new ZipFile(tempFile.toFile());\n\n    return zipFile\n      .stream()\n      .filter(entry -> !entry.isDirectory())\n      .map(entry -> {\n        try (var zipEntryInputStream = zipFile.getInputStream(entry)) {\n          return new ExtractedFile(\n            entry.getName(),\n            zipEntryInputStream.readAllBytes());\n        } catch (IOException e) {\n          throw new RuntimeException(\"Failed to extract entry\", e);\n        }\n      })\n      .onClose(() -> {\n        try {\n          zipFile.close();\n          Files.delete(tempFile);\n        } catch (IOException e) {\n          logger.warn(\"Error closing zip archive\", e);\n        }\n      });\n  }\n<\/code><\/pre>\n
And an example usage might look like:<\/p>\n
\n  \/\/ Use a try-with-resources to ensure the stream gets closed\n  try (var entryStream = ZipUtils.extractFiles(archiveStream)) {\n    entryStream\n        .filter(entry -> entry.filename().endsWith(\".txt\"))\n        .parallel()\n        .forEach(entry -> {\n          \/\/ do something with the text file\n        });\n  }\n<\/code><\/pre>\n
Things to note about this implementation:<\/p>\n