Add log4j2 to not miss errors.

frameworks/Java/armeria/pom.xml

@@ -25,6 +25,11 @@
       <artifactId>armeria</artifactId>
       <version>${armeria.version}</version>
     </dependency>
+    <dependency>
+      <groupId>org.apache.logging.log4j</groupId>
+      <artifactId>log4j-slf4j-impl</artifactId>
+      <version>2.12.1</version>
+    </dependency>
     <dependency>
       <groupId>org.postgresql</groupId>
       <artifactId>postgresql</artifactId>

frameworks/Java/armeria/src/main/resources/log4j2.xml

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<Configuration status="WARN">
+  <Appenders>
+    <Console name="Console" target="SYSTEM_OUT">
+      <PatternLayout pattern="%d{HH:mm:ss.SSS} [%t] %-5level %logger{36} - %msg%n"/>
+    </Console>
+  </Appenders>
+  <Loggers>
+    <Root level="info">
+      <AppenderRef ref="Console"/>
+    </Root>
+  </Loggers>
+</Configuration>