resources/src/main/java/org/robolectric/res/Fs.java
private static FileSystem getJarFs(Path jarFile) throws IOException {
    Path key = jarFile.toAbsolutePath();

    synchronized (ZIP_FILESYSTEMS) {
      FsWrapper fs = ZIP_FILESYSTEMS.get(key);
      if (fs == null) {
        fs = new FsWrapper(FileSystems.newFileSystem(key, null), key);
        fs.incrRefCount();

        ZIP_FILESYSTEMS.put(key, fs);
      } else {
        fs.incrRefCount();
      }

      return fs;
    }
  }
private static FileSystem getJarFs(Path jarFile) throws IOException {
    Path key = jarFile.toAbsolutePath();

    synchronized (ZIP_FILESYSTEMS) {
      FsWrapper fs = ZIP_FILESYSTEMS.get(key);
      if (fs == null) {
        fs = new FsWrapper(FileSystems.newFileSystem(key, null), key);
        fs.incrRefCount();

        ZIP_FILESYSTEMS.put(key, fs);
      } else {
        fs.incrRefCount();
      }

      return fs;
    }
  }
Returns a reference-counted Jar FileSystem, possibly one that was previously returned.