Using log4j2 with AWS Lambda

1. Pull in log4j2 dependencies

<dependencies>
  ...
  <dependency>
    <groupId>com.amazonaws</groupId>
    <artifactId>aws-lambda-java-log4j2</artifactId>
    <version>1.6.5</version>
  </dependency>
  <dependency>
    <groupId>org.apache.logging.log4j</groupId>
    <artifactId>log4j-core</artifactId>
    <version>2.25.5</version>
  </dependency>
  <dependency>
    <groupId>org.apache.logging.log4j</groupId>
    <artifactId>log4j-api</artifactId>
    <version>2.25.5</version>
  </dependency>
  <dependency>
    <groupId>org.apache.logging.log4j</groupId>
    <artifactId>log4j-layout-template-json</artifactId>
    <version>2.25.5</version>
  </dependency>
  ....
</dependencies>

<plugins>
  ...
  <plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-shade-plugin</artifactId>
    <version>3.6.1</version>
    <executions>
      <execution>
        <phase>package</phase>
        <goals>
          <goal>shade</goal>
        </goals>
        <configuration>
          <transformers>
            <transformer
                    implementation="com.github.edwgiz.mavenShadePlugin.log4j2CacheTransformer.PluginsCacheFileTransformer">
            </transformer>
          </transformers>
        </configuration>
      </execution>
    </executions>
    <dependencies>
      <dependency>
        <groupId>com.github.edwgiz</groupId>
        <artifactId>maven-shade-plugin.log4j2-cachefile-transformer</artifactId>
        <version>2.8.1</version>
      </dependency>
    </dependencies>
  </plugin>
  ...
</plugins>

dependencies{
  ...
    implementation group: 'com.amazonaws', name: 'aws-lambda-java-log4j2', version: '1.6.5'
    implementation group: 'org.apache.logging.log4j', name: 'log4j-core', version: log4jVersion
    implementation group: 'org.apache.logging.log4j', name: 'log4j-api', version: log4jVersion
}

jar {
    enabled = false
}
shadowJar {
    transform(com.github.jengelman.gradle.plugins.shadow.transformers.Log4j2PluginsCacheFileTransformer)
}

build.dependsOn(shadowJar)

2. Configure log4j2 using log4j2.xml file

<?xml version="1.0" encoding="UTF-8"?>
<Configuration>
  <Appenders>
    <Lambda name="Lambda" format="${env:AWS_LAMBDA_LOG_FORMAT:-TEXT}">
      <LambdaTextFormat>
        <PatternLayout>
          <pattern>%d{yyyy-MM-dd HH:mm:ss} %X{AWSRequestId} %-5p %c{1}:%L - %m%n</pattern>
        </PatternLayout>
      </LambdaTextFormat>
      <LambdaJSONFormat>
        <JsonTemplateLayout eventTemplateUri="classpath:LambdaLayout.json" />
      </LambdaJSONFormat>
    </Lambda>
  </Appenders>
  <Loggers>
    <Root level="${env:AWS_LAMBDA_LOG_LEVEL:-INFO}">
      <AppenderRef ref="Lambda" />
    </Root>
  </Loggers>
</Configuration>

3. Example code

package example;

import com.amazonaws.services.lambda.runtime.Context;

import static org.apache.logging.log4j.CloseableThreadContext.put;
import org.apache.logging.log4j.CloseableThreadContext.Instance;
import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;

public class Hello {
    // Initialize the Log4j logger.
    static final Logger logger = LogManager.getLogger(Hello.class);

    public String myHandler(String name, Context context) {
        // System.out: One log statement but with a line break (AWS Lambda writes two events to CloudWatch).
        System.out.println("log data from stdout \n this is continuation of system.out");

       // System.err: One log statement but with a line break (AWS Lambda writes two events to CloudWatch).
        System.err.println("log data from stderr. \n this is a continuation of system.err");

        // Use log4j to log the same thing as above and AWS Lambda will log only one event in CloudWatch.
        logger.debug("log data from log4j debug \n this is continuation of log4j debug");

        logger.error("log data from log4j err. \n this is a continuation of log4j.err");

        // When logging in JSON, you can also add custom fields
        // In java11+ you can use the `try (var ctx = put("name", name)) {}` structure
        Instance ctx = put("name", name);
        logger.info("log line with input name");
        ctx.close();

        // Return will include the log stream name so you can look
        // up the log later.
        return String.format("Hello %s. log stream = %s", name, context.getLogStreamName());
    }
}