diff --git a/lib/fluent/plugin/buffer/file_chunk.rb b/lib/fluent/plugin/buffer/file_chunk.rb
index 06bbf017c2..7930574a58 100644
--- a/lib/fluent/plugin/buffer/file_chunk.rb
+++ b/lib/fluent/plugin/buffer/file_chunk.rb
@@ -399,7 +399,7 @@ def restore_metadata_with_new_format(chunk)
           end
 
           if chunk.slice(0, 2) == BUFFER_HEADER
-            size = chunk.slice(2, 4).unpack('N').first
+            size = chunk.slice(2, 4).unpack1('N')
             if size
               return Fluent::MessagePackFactory.msgpack_unpacker(symbolize_keys: true).feed(chunk.slice(6, size)).read rescue nil
             end
diff --git a/lib/fluent/plugin/buffer/file_single_chunk.rb b/lib/fluent/plugin/buffer/file_single_chunk.rb
index 7225438af6..4cc9d6c625 100644
--- a/lib/fluent/plugin/buffer/file_single_chunk.rb
+++ b/lib/fluent/plugin/buffer/file_single_chunk.rb
@@ -238,17 +238,16 @@ def file_rename(file, old_path, new_path, callback = nil)
           callback.call(file) if callback
         end
 
-        URI_PARSER = URI::Parser.new
         ESCAPE_REGEXP = /[^-_.a-zA-Z0-9]/n
 
         def encode_key(metadata)
           k = @key ? metadata.variables[@key] : metadata.tag
           k ||= ''
-          URI_PARSER.escape(k, ESCAPE_REGEXP)
+          URI::DEFAULT_PARSER.escape(k, ESCAPE_REGEXP)
         end
 
         def decode_key(key)
-          URI_PARSER.unescape(key)
+          URI::DEFAULT_PARSER.unescape(key)
         end
 
         def create_new_chunk(path, metadata, perm)
diff --git a/lib/fluent/registry.rb b/lib/fluent/registry.rb
index 9e473c27fd..b321f6f748 100644
--- a/lib/fluent/registry.rb
+++ b/lib/fluent/registry.rb
@@ -60,13 +60,13 @@ def search(type)
       # search from additional plugin directories
       if @dir_search_prefix
         path = "#{@dir_search_prefix}#{type}"
-        files = @paths.map { |lp|
+        files = @paths.filter_map { |lp|
           lpath = File.expand_path(File.join(lp, "#{path}.rb"))
           File.exist?(lpath) ? lpath : nil
-        }.compact
+        }
         unless files.empty?
           # prefer newer version
-          require files.sort.last
+          require files.max
           return
         end
       end
@@ -74,17 +74,17 @@ def search(type)
       path = "#{@search_prefix}#{type}"
 
       # prefer LOAD_PATH than gems
-      files = $LOAD_PATH.map { |lp|
+      files = $LOAD_PATH.filter_map { |lp|
         if lp == FLUENT_LIB_PATH
           nil
         else
           lpath = File.expand_path(File.join(lp, "#{path}.rb"))
           File.exist?(lpath) ? lpath : nil
         end
-      }.compact
+      }
       unless files.empty?
         # prefer newer version
-        require files.sort.last
+        require files.max
         return
       end
 
diff --git a/lib/fluent/unique_id.rb b/lib/fluent/unique_id.rb
index 060c2e2290..7dce181418 100644
--- a/lib/fluent/unique_id.rb
+++ b/lib/fluent/unique_id.rb
@@ -23,7 +23,7 @@ def self.generate
     end
 
     def self.hex(unique_id)
-      unique_id.unpack('H*').first
+      unique_id.unpack1('H*')
     end
 
     module Mixin