Switched capture strategy directly adding spy appender (#3)

Reworked log capture in setting_with_deprecated_alias_spec to use Logger's addAppender and a spy appender implementation.
This leverages the idea to don't reconfigure the Log4J with a custom build configuration, but simply adding an appender that could spy the appended lines to a logger.

logstash-core/spec/logstash/settings/setting_with_deprecated_alias_spec.rb

@@ -17,6 +17,7 @@
 
 require "spec_helper"
 require "logstash/settings"
+java_import org.apache.logging.log4j.core.appender.AbstractAppender
 
 describe LogStash::Setting::SettingWithDeprecatedAlias do
   let(:canonical_setting_name) { "canonical.setting" }
@@ -27,21 +28,38 @@
   let(:settings) { LogStash::Settings.new }
   let(:canonical_setting) { LogStash::Setting::StringSetting.new(canonical_setting_name, default_value, true) }
 
-  log_spy = nil
-  log_ctx = nil
+  class CustomAppender < AbstractAppender
 
-  def log_ctx
-    @log_ctx
-  end
+    attr_reader :events_collector
+
+    def initialize(events)
+      super("CustomCaptorAppender", nil, nil, true, org.apache.logging.log4j.core.config.Property::EMPTY_ARRAY)
+      @events_collector = events
+    end
 
-  def log_spy
-    @log_spy
+    # override the append to catch all the calls and collect the events
+    def append(log_event)
+      @events_collector << log_event.message.formatted_message
+    end
   end
 
+  let(:events) { [] }
+
   before(:each) do
-    # Initialization of appender and logger use to spy, need to be freshly recreated on each test is context shutdown is used.
-    @log_ctx = setup_logger_spy
-    @log_spy = retrieve_logger_spy(@log_ctx)
+    java_import org.apache.logging.log4j.LogManager
+    logger = LogManager.getLogger("org.logstash.settings.DeprecatedAlias")
+    deprecated_logger = LogManager.getLogger("org.logstash.deprecation.settings.DeprecatedAlias")
+
+    @custom_appender = CustomAppender.new(events).tap {|appender| appender.start }
+
+    java_import org.apache.logging.log4j.Level
+    logger.addAppender(@custom_appender)
+    deprecated_logger.addAppender(@custom_appender)
+    # had to set level after appending as it was "error" for some reason
+    logger.setLevel(Level::INFO)
+    deprecated_logger.setLevel(Level::INFO)
+
+    expect(@custom_appender.started?).to be_truthy
 
     allow(LogStash::Settings).to receive(:logger).and_return(double("SettingsLogger").as_null_object)
     allow(LogStash::Settings).to receive(:deprecation_logger).and_return(double("SettingsDeprecationLogger").as_null_object)
@@ -50,9 +68,15 @@ def log_spy
   end
 
   after(:each) do
-    @log_ctx.close
-    @log_spy = nil
-    @log_ctx = nil
+    events.clear
+    java_import org.apache.logging.log4j.LogManager
+    logger = LogManager.getLogger("org.logstash.settings.DeprecatedAlias")
+    deprecated_logger = LogManager.getLogger("org.logstash.deprecation.settings.DeprecatedAlias")
+    # The Logger's AbstractConfiguration contains a cache of Appender, by class name. The cache is updated
+    # iff is absent, so to make subsequent add_appender effective we have cleanup on teardown, else the new
+    # appender instance is simply not used by the logger
+    logger.remove_appender(@custom_appender)
+    deprecated_logger.remove_appender(@custom_appender)
   end
 
   shared_examples '#validate_value success' do
@@ -78,8 +102,7 @@ def log_spy
       it 'does not emit a deprecation warning' do
         expect(LogStash::Settings.deprecation_logger).to_not receive(:deprecated).with(a_string_including(deprecated_setting_name))
         settings.get_setting(deprecated_setting_name).observe_post_process
-        log_spy = retrieve_logger_spy(log_ctx)
-        expect(log_spy.messages).to be_empty
+        expect(events).to be_empty
       end
     end
   end
@@ -97,15 +120,15 @@ def log_spy
     end
 
     it 'logs a deprecation warning' do
-      expect(log_spy.messages[0]).to include(deprecated_setting_name)
+      expect(events[0]).to include(deprecated_setting_name)
     end
 
     include_examples '#validate_value success'
 
     context "#observe_post_process" do
       it 're-emits the deprecation warning' do
         settings.get_setting(deprecated_setting_name).observe_post_process
-        expect(log_spy.messages[0]).to include(deprecated_setting_name)
+        expect(events[0]).to include(deprecated_setting_name)
       end
     end
 
@@ -142,15 +165,15 @@ def log_spy
 
     it 'does not produce a relevant deprecation warning' do
       settings.get_setting(deprecated_setting_name).observe_post_process
-      expect(log_spy.messages).to be_empty
+      expect(events).to be_empty
     end
 
     include_examples '#validate_value success'
 
     context "#observe_post_process" do
       it 'does not emit a deprecation warning' do
         settings.get_setting(deprecated_setting_name).observe_post_process
-        expect(log_spy.messages).to be_empty
+        expect(events).to be_empty
       end
     end
   end
@@ -172,7 +195,8 @@ def log_spy
   context 'Settings#get on deprecated alias' do
     it 'produces a WARN-level message to the logger' do
       settings.get(deprecated_setting_name)
-      expect(log_spy.messages[0]).to include("setting `#{canonical_setting_name}` has been queried by its deprecated alias `#{deprecated_setting_name}`")
+      sleep 0.1
+      expect(events[0]).to include("setting `#{canonical_setting_name}` has been queried by its deprecated alias `#{deprecated_setting_name}`")
     end
   end
 end