Fallback to in-memory db files

CHANGELOG

@@ -1,3 +1,6 @@
+unreleased
+  - Fall back to an in-memory database when a path can not be pread (such as a path inside a JAR on JRuby)
+
 02-09-2026
   - Version 2.0.0
   - Require Ruby 3.3 or newer

lib/mini_mime.rb

@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 require "mini_mime/version"
+require "stringio"
 
 module MiniMime
   def self.lookup_by_filename(filename)
@@ -126,6 +127,10 @@ def initialize(filename)
           @file = ::File.open(filename, 'rb')
         end
 
+        def size
+          @file.size
+        end
+
         def readline(*args)
           @file.readline(*args)
         end
@@ -143,13 +148,36 @@ def close
       end
     end
 
+    # Holds the whole database in memory
+    class MemoryFile
+      def initialize(content)
+        @io = StringIO.new(content.b.freeze)
+      end
+
+      def size
+        @io.size
+      end
+
+      def readline(*args)
+        @io.readline(*args)
+      end
+
+      def pread(size, offset)
+        @io.pread(size, offset)
+      end
+
+      def close
+        @io.close
+      end
+    end
+
     class RandomAccessDb
       MAX_CACHED = 100
 
       def initialize(path, sort_order)
         @path = path
-        @file = PReadFile.new(@path)
-        @file_length = File.size(@path)
+        @file = open_file_path(path)
+        @file_length = @file.size
         if @file_length.zero?
           @file.close
           raise ArgumentError, "MIME database is empty: #{@path}"
@@ -216,6 +244,16 @@ def resolve(row)
 
         Info.new(buffer)
       end
+
+      private
+
+      def open_file_path(source)
+        return PReadFile.new(source) if File.ftype(source) == "file"
+
+        # NOTE: only a real file has a descriptor to pread, JRuby hands out a File for a path
+        # inside a JAR archive ("uri:classloader:/...") but then reads from the stream position
+        MemoryFile.new(File.binread(source))
+      end
     end
 
     def initialize

test/database_test.rb

@@ -44,6 +44,22 @@ def test_random_access_database_search_boundaries
     end
   end
 
+  def test_falls_back_to_memory_for_a_path_that_is_not_a_file
+    with_ftype("unknown") do
+      with_database("a text/a 7bit\nz text/z 7bit\n") do |database|
+        assert_instance_of MiniMime::Db::MemoryFile, database_file(database)
+        assert_equal "text/a", database.lookup("a").content_type
+        assert_equal "text/z", database.lookup("z").content_type
+      end
+    end
+  end
+
+  def test_uses_the_file_backend_for_a_plain_path
+    with_database("a text/a 7bit\n") do |database|
+      assert_instance_of MiniMime::Db::PReadFile, database_file(database)
+    end
+  end
+
   def test_rejects_an_empty_database
     assert_invalid_database("")
   end
@@ -108,6 +124,19 @@ def test_rebuild_uses_the_selected_types_preferred_extension
 
   private
 
+  # JRuby reports "unknown" for a path inside an archive, a real file is always "file"
+  def with_ftype(ftype)
+    original = File.singleton_class.instance_method(:ftype)
+    File.singleton_class.define_method(:ftype) { |*| ftype }
+    yield
+  ensure
+    File.singleton_class.define_method(:ftype, original)
+  end
+
+  def database_file(database)
+    database.instance_variable_get(:@file)
+  end
+
   def assert_invalid_database(contents)
     Tempfile.create do |file|
       file.binmode