How Can I Access and Retrieve Files Inside a JAR File in Java?

Navigating and Retrieving Files Within a JAR File

When working with JAR files, it becomes necessary to access the internal files for various purposes. This question arises when you need to enumerate files within a JAR file, specifically to load images and dynamically access them.

The solution involves utilizing a combination of methods. First, determine the JAR file where your main class resides. You can retrieve this information using java.util.Zip.

Once you know the JAR file name, create a CodeSource object and obtain the URL of the JAR file. Next, use ZipInputStream to access the contents of the JAR file. Continually retrieve ZipEntry objects from the stream and examine each entry's name. If the name matches the desired directory path, you can perform any necessary operations on that entry.

Here's an example code snippet to list images within the JAR file:

CodeSource src = MyClass.class.getProtectionDomain().getCodeSource();
if (src != null) {
  URL jar = src.getLocation();
  ZipInputStream zip = new ZipInputStream(jar.openStream());
  while (true) {
    ZipEntry entry = zip.getNextEntry();
    if (entry == null) break;
    if (entry.getName().startsWith("images/")) {
      // Process the image entry
    }
  }
}

Alternatively, you can use Java 7's FileSystem and NIO's file walking mechanisms to traverse the JAR file's contents. This simplifies the process of searching and filtering for files within the JAR file.

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