fix java enumerable integration bug

Fixes #3022

lib/logstash/java_integration.rb

@@ -24,6 +24,24 @@ class Hash
   end
 end
 
+
+# this is a temporary fix to solve a bug in JRuby where classes implementing the Map interface, like LinkedHashMap
+# have a bug in the has_key? method that is implemented in the Enumerable module that is somehow mixed in the Map interface.
+# this bug makes has_key? (and all its aliases) return false for a key that has a nil value.
+# Only LinkedHashMap is patched here because patching the Map interface is not working.
+# TODO find proper fix, and submit upstream
+# releavant JRuby files:
+# https://github.com/jruby/jruby/blob/master/core/src/main/ruby/jruby/java/java_ext/java.util.rb
+# https://github.com/jruby/jruby/blob/master/core/src/main/java/org/jruby/java/proxies/MapJavaProxy.java
+class Java::JavaUtil::LinkedHashMap
+  def has_key?(key)
+    self.containsKey(key)
+  end
+  alias_method :include?, :has_key?
+  alias_method :member?, :has_key?
+  alias_method :key?, :has_key?
+end
+
 module java::util::Map
   # have Map objects like LinkedHashMap objects report is_a?(Array) == true
   def is_a?(clazz)

spec/lib/logstash/java_integration_spec.rb

@@ -166,4 +166,63 @@ describe "Java integration" do
       end
     end
   end
+
+  context "Enumerable implementation" do
+    context "Java Map interface should report key with nil value as included" do
+
+      it "should support include? method" do
+        expect(Java::JavaUtil::LinkedHashMap.new({"foo" => nil}).include?("foo")).to be_true
+      end
+
+      it "should support has_key? method" do
+        expect(Java::JavaUtil::LinkedHashMap.new({"foo" => nil}).has_key?("foo")).to be_true
+      end
+
+      it "should support member? method" do
+        expect(Java::JavaUtil::LinkedHashMap.new({"foo" => nil}).member?("foo")).to be_true
+      end
+
+      it "should support key? method" do
+        expect(Java::JavaUtil::LinkedHashMap.new({"foo" => nil}).key?("foo")).to be_true
+      end
+    end
+
+    context "Java Map interface should report key with a value as included" do
+
+      it "should support include? method" do
+        expect(Java::JavaUtil::LinkedHashMap.new({"foo" => 1}).include?("foo")).to be_true
+      end
+
+      it "should support has_key? method" do
+        expect(Java::JavaUtil::LinkedHashMap.new({"foo" => 1}).has_key?("foo")).to be_true
+      end
+
+      it "should support member? method" do
+        expect(Java::JavaUtil::LinkedHashMap.new({"foo" => 1}).member?("foo")).to be_true
+      end
+
+      it "should support key? method" do
+        expect(Java::JavaUtil::LinkedHashMap.new({"foo" => 1}).key?("foo")).to be_true
+      end
+    end
+
+    context "Java Map interface should report non existing key as not included" do
+
+      it "should support include? method" do
+        expect(Java::JavaUtil::LinkedHashMap.new({"foo" => 1}).include?("bar")).to be_false
+      end
+
+      it "should support has_key? method" do
+        expect(Java::JavaUtil::LinkedHashMap.new({"foo" => 1}).has_key?("bar")).to be_false
+      end
+
+      it "should support member? method" do
+        expect(Java::JavaUtil::LinkedHashMap.new({"foo" => 1}).member?("bar")).to be_false
+      end
+
+      it "should support key? method" do
+        expect(Java::JavaUtil::LinkedHashMap.new({"foo" => 1}).key?("bar")).to be_false
+      end
+    end
+  end
 end