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AWS Lambda
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Logging (Java)

Your Lambda function can contain logging statements. AWS Lambda writes these logs to CloudWatch. We recommend you use one of the following to write logs.

Custom Appender for Log4j™ 2

AWS Lambda recommends Log4j 2 to provide a custom appender. You can use the custom Log4j (see Apache log4j) appender provided by Lambda for logging from your lambda functions. Every call to Log4j methods, such as log.debug() or log.error(), will result in a CloudWatch Logs event. The custom appender is called LambdaAppender and must be used in the log4j2.xml file. You must include the aws-lambda-java-log4j2 artifact (artifactId:aws-lambda-java-log4j2) in the deployment package (.jar file). For an example, see Example 1: Writing Logs Using Log4J v2.8 .

LambdaLogger.log()

Each call to LambdaLogger.log() results in a CloudWatch Logs event, provided the event size is within the allowed limits. For information about CloudWatch Logs limits, see CloudWatch Logs Limits in the Amazon CloudWatch User Guide. For an example, see Example 2: Writing Logs Using LambdaLogger (Java).

In addition, you can also use the following statements in your Lambda function code to generate log entries:

  • System.out()

  • System.err()

However, note that AWS Lambda treats each line returned by System.out and System.err as a separate event. This works well when each output line corresponds to a single log entry. When a log entry has multiple lines of output, AWS Lambda attempts to parse them using line breaks to identify separate events. For example, the following logs the two words ("Hello" and "world") as two separate events:

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System.out.println("Hello \n world");

How to Find Logs

You can find the logs that your Lambda function writes, as follows:

  • Find logs in CloudWatch Logs. The context object (in the aws-lambda-java-core library) provides the getLogStreamName() and the getLogGroupName() methods. Using these methods, you can find the specific log stream where logs are written.

  • If you invoke a Lambda function via the console, the invocation type is always RequestResponse (that is, synchronous execution) and the console displays the logs that the Lambda function writes using the LambdaLogger object. AWS Lambda also returns logs from System.out and System.err methods.

  • If you invoke a Lambda function programmatically, you can add the LogType parameter to retrieve the last 4 KB of log data that is written to CloudWatch Logs. For more information, see Invoke. AWS Lambda returns this log information in the x-amz-log-results header in the response. If you use the AWS Command Line Interface to invoke the function, you can specify the --log-type parameter with value Tail.

Logging Examples (Java)

This section provides examples of using Custom Appender for Log4j and the LambdaLogger objects for logging information.

Example 1: Writing Logs Using Log4J v2.8

  • The following shows how to build your artifact with Maven to correctly include the Log4j v2.8 plugins:

    • For Maven pom.xml:

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      <dependencies> ... <dependency> <groupId>com.amazonaws</groupId> <artifactId>aws-lambda-java-log4j2</artifactId> <version>1.0.0</version> </dependency> <dependency> <groupId>org.apache.logging.log4j</groupId> <artifactId>log4j-core</artifactId> <version>2.8.2</version> </dependency> <dependency> <groupId>org.apache.logging.log4j</groupId> <artifactId>log4j-api</artifactId> <version>2.8.2</version> </dependency> .... </dependencies>
    • If using the Maven shade plugin, set the plugin configuration as follows:

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      <plugins> ... <plugin> <groupId>org.apache.maven.plugins</groupId> <artifactId>maven-shade-plugin</artifactId> <version>2.4.3</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>
    • The following Java code example shows how to use Log4j with Lambda:

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      package example; import com.amazonaws.services.lambda.runtime.Context; 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"); // 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()); } }
    • The example preceding uses the following log4j2.xml file to load properties

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      <?xml version="1.0" encoding="UTF-8"?> <Configuration packages="com.amazonaws.services.lambda.runtime.log4j2.LambdaAppender"> <Appenders> <Lambda name="Lambda"> <PatternLayout> <pattern>%d{yyyy-MM-dd HH:mm:ss} %X{AWSRequestId} %-5p %c{1}:%L - %m%n</pattern> </PatternLayout> </Lambda> </Appenders> <Loggers> <Root level="debug"> <AppenderRef ref="Lambda" /> </Root> </Loggers> </Configuration>

Example 2: Writing Logs Using LambdaLogger (Java)

The following Java code example writes logs using both the System methods and the LambdaLogger object to illustrate how they differ when AWS Lambda logs information to CloudWatch.

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package example; import com.amazonaws.services.lambda.runtime.Context; import com.amazonaws.services.lambda.runtime.LambdaLogger; public class Hello { 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 continuation of system.err"); LambdaLogger logger = context.getLogger(); // Write log to CloudWatch using LambdaLogger. logger.log("log data from LambdaLogger \n this is continuation of logger.log"); // 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()); } }

The following is sample of log entries in CloudWatch Logs.

Note:

  • AWS Lambda parses the log string in each of the System.out.println() and System.err.println() statements logs as two separate events (note the two down arrows in the screenshot) because of the line break.

  • The LambdaLogger.log() produce one CloudWatch event.

You can do the following to test the code:

  • Using the code, create a deployment package.

  • Upload the deployment package to AWS Lambda to create your Lambda function.

  • To test your Lambda function use a string ("this is a test") as sample event. The handler code receives the sample event but does nothing with it. It only shows how to write logs.

Follow the instructions provided in the Getting Started. For more information, see Step 2.3: (Optional) Create a Lambda Function Authored in Java. Note the following differences:

  • When you create a deployment package, don't forget the aws-lambda-java-core library dependency.

  • When you create the Lambda function, specify example.Hello::myHandler (package.class::method) as the handler value.