BAEL-1960: Custom appender for log4j (#4731)

* BAEL-1960: Custom appender for log4j

* Changes as per suggestion to BAEL-1960

* Changes as [er review for BAEL-1960

* Changes for formatting as per suggestion.

* BAEL-1960. Copied pom.xml from master and pasted my changes against it.

* Chnages for spaces instead of tabs.

* Changes for spaces instead of tabs.

logging-modules/log4j2/pom.xml

@@ -18,6 +18,13 @@
             <artifactId>log4j-core</artifactId>
             <version>${log4j-core.version}</version>
         </dependency>
+        
+        <!-- This is the needed API component. -->
+        <dependency>
+          <groupId>org.apache.logging.log4j</groupId>
+          <artifactId>log4j-api</artifactId>
+          <version>${log4j-core.version}</version>
+        </dependency>
 
         <!-- This is used by JSONLayout. -->
         <dependency>
@@ -114,4 +121,4 @@
         <maven.build.timestamp.format>yyyyMMddHHmmss</maven.build.timestamp.format>
     </properties>
 
-</project>
+</project>

logging-modules/log4j2/src/main/java/com/baeldung/logging/log4j2/appender/MapAppender.java

@@ -0,0 +1,54 @@
+/**
+ * 
+ */
+package com.baeldung.logging.log4j2.appender;
+
+import java.time.Instant;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ConcurrentMap;
+
+import org.apache.logging.log4j.Level;
+import org.apache.logging.log4j.core.Appender;
+import org.apache.logging.log4j.core.Core;
+import org.apache.logging.log4j.core.Filter;
+import org.apache.logging.log4j.core.LogEvent;
+import org.apache.logging.log4j.core.appender.AbstractAppender;
+import org.apache.logging.log4j.core.config.plugins.Plugin;
+import org.apache.logging.log4j.core.config.plugins.PluginAttribute;
+import org.apache.logging.log4j.core.config.plugins.PluginElement;
+import org.apache.logging.log4j.core.config.plugins.PluginFactory;
+
+@Plugin(name = "MapAppender", category = Core.CATEGORY_NAME, elementType = Appender.ELEMENT_TYPE)
+public class MapAppender extends AbstractAppender {
+
+    private ConcurrentMap<String, LogEvent> eventMap = new ConcurrentHashMap<>();
+
+    protected MapAppender(String name, Filter filter) {
+        super(name, filter, null);
+    }
+
+    @PluginFactory
+    public static MapAppender createAppender(@PluginAttribute("name") String name, @PluginElement("Filter") final Filter filter) {
+        return new MapAppender(name, filter);
+    }
+
+    @Override
+    public void append(LogEvent event) {
+        if (event.getLevel()
+            .isLessSpecificThan(Level.WARN)) {
+            error("Unable to log less than WARN level.");
+            return;
+        }
+        eventMap.put(Instant.now()
+            .toString(), event);
+    }
+
+    public ConcurrentMap<String, LogEvent> getEventMap() {
+        return eventMap;
+    }
+
+    public void setEventMap(ConcurrentMap<String, LogEvent> eventMap) {
+        this.eventMap = eventMap;
+    }
+
+}

logging-modules/log4j2/src/test/java/com/baeldung/logging/log4j2/appender/MapAppenderIntegrationTest.java

@@ -0,0 +1,35 @@
+package com.baeldung.logging.log4j2.appender;
+
+import static org.junit.Assert.assertEquals;
+
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
+import org.apache.logging.log4j.core.LoggerContext;
+import org.apache.logging.log4j.core.config.Configuration;
+import org.junit.Before;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+@RunWith(JUnit4.class)
+public class MapAppenderIntegrationTest {
+
+    private Logger logger;
+
+    @Before
+    public void setup() {
+        logger = LogManager.getLogger(MapAppenderIntegrationTest.class);
+    }
+
+    @Test
+    public void whenLoggerEmitsLoggingEvent_thenAppenderReceivesEvent() throws Exception {
+        logger.info("Test from {}", this.getClass()
+            .getSimpleName());
+        LoggerContext context = LoggerContext.getContext(false);
+        Configuration config = context.getConfiguration();
+        MapAppender appender = config.getAppender("MapAppender");
+        assertEquals(appender.getEventMap()
+            .size(), 1);
+    }
+
+}

logging-modules/log4j2/src/test/resources/log4j2.xml

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<Configuration xmlns:xi="http://www.w3.org/2001/XInclude"
-    status="WARN">
+<Configuration xmlns:xi="http://www.w3.org/2001/XInclude" 
+    packages="com.baeldung" status="WARN">
     <Appenders>
         <xi:include
             href="log4j2-includes/console-appender_pattern-layout_colored.xml" />
@@ -50,6 +50,7 @@
                     size="17 kB" />
             </Policies>
         </RollingFile>
+        <MapAppender name="MapAppender"/>
     </Appenders>
     <Loggers>
         <Logger name="CONSOLE_PATTERN_APPENDER_MARKER" level="TRACE"
@@ -82,6 +83,7 @@
         </Logger>
         <Root level="DEBUG">
             <AppenderRef ref="ConsoleAppender" />
+            <AppenderRef ref="MapAppender" />
         </Root>
     </Loggers>
 </Configuration>