How Can I List Files Inside a JAR File Using Java?

Listing Files Within a JAR with Java

One common task when working with JAR archives is listing the files they contain. This can be useful for various purposes, such as extracting specific files or creating an inventory of the archive's contents.

Using the Java API

Java provides an extensive set of classes for handling ZIP files, including JAR archives. The ZipInputStream class allows you to iterate over the entries in a JAR file and access their metadata.

To list the files within a JAR, follow these steps:

1. Obtain the CodeSource object for the class that is running within the JAR. This can be done using the getProtectionDomain() and getCodeSource() methods of the Class object.
2. Convert the CodeSource to a URL object using the getLocation() method.
3. Create a ZipInputStream object from the URL.
4. Iterate over the entries in the JAR by calling the getNextEntry() method of the ZipInputStream.
5. For each entry, check if the name matches the desired pattern or criterion.

Example Code

The following code shows an example of listing all the files within a JAR:

CodeSource src = MyClass.class.getProtectionDomain().getCodeSource();
if (src != null) {
  URL jar = src.getLocation();
  ZipInputStream zip = new ZipInputStream(jar.openStream());
  while (true) {
    ZipEntry e = zip.getNextEntry();
    if (e == null) {
      break;
    }
    System.out.println(e.getName());
  }
} else {
  System.out.println("JAR not found");
}

Java 7 Enhancements

In Java 7, a new feature was introduced that simplifies the process of working with ZIP files. The FileSystem class can be used to mount a ZIP file as a read-only file system. This allows you to use the standard Java I/O libraries to navigate and list the contents of the JAR file.

The following code shows an example of using the FileSystem class to list the files within a JAR:

Path jarPath = Paths.get("/path/to/my.jar");
FileSystem fs = FileSystems.newFileSystem(jarPath, null);
Path root = fs.getPath("/");
Files.walk(root)
    .filter(path -> path.toString().startsWith("path/to/your/dir/"))
    .forEach(System.out::println);

This code uses the Files class, which provides a high-level API for working with files and directories, to walk through the root directory of the JAR file and filter out any files that do not match the desired criterion.

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