Tools and libraries Creating a function that returns logs Using AWS Lambda Powertools for Java and AWS SAM for structured logging Using AWS Lambda Powertools for Java and the AWS CDK for structured logging Using the Lambda console Using the CloudWatch console Using the AWS Command Line Interface (AWS CLI)Deleting logs Advanced logging with Log4j 2 and SLF4J Sample logging code

AWS Lambda function logging in Java

AWS Lambda automatically monitors Lambda functions on your behalf and sends logs to Amazon CloudWatch. Your Lambda function comes with a CloudWatch Logs log group and a log stream for each instance of your function. The Lambda runtime environment sends details about each invocation to the log stream, and relays logs and other output from your function's code. For more information, see Accessing Amazon CloudWatch logs for AWS Lambda.

To output logs from your function code, you can use methods on java.lang.System, or any logging module that writes to stdout or stderr. For simple use cases, this approach might be sufficient.

This page describes how to produce log output from your Lambda function's code, or access logs using the AWS Command Line Interface, the Lambda console, the CloudWatch console, or Infrastructure as code tools such as the AWS Serverless Application Model(AWS SAM).

Sections

Tools and libraries
Creating a function that returns logs
Using AWS Lambda Powertools for Java and AWS SAM for structured logging
Using AWS Lambda Powertools for Java and the AWS CDK for structured logging
Using the Lambda console
Using the CloudWatch console
Using the AWS Command Line Interface (AWS CLI)
Deleting logs
Advanced logging with Log4j 2 and SLF4J
Sample logging code

Tools and libraries

AWS Lambda Powertools for Java is a developer toolkit to implement Serverless best practices and increase developer velocity. The Logging utility provides a Lambda optimized logger which includes additional information about function context across all your functions with output structured as JSON. Use this utility to do the following:

Capture key fields from the Lambda context, cold start and structures logging output as JSON
Log Lambda invocation events when instructed (disabled by default)
Print all the logs only for a percentage of invocations via log sampling (disabled by default)
Append additional keys to structured log at any point in time
Use a custom log formatter (Bring Your Own Formatter) to output logs in a structure compatible with your organization’s Logging RFC

Creating a function that returns logs

To output logs from your function code, you can use methods on java.lang.System, or any logging module that writes to stdout or stderr. The aws-lambda-java-core library provides a logger class named LambdaLogger that you can access from the context object. The logger class supports multiline logs.

The following example uses the LambdaLogger logger provided by the context object.

Example Handler.java


// Handler value: example.Handler
public class Handler implements RequestHandler<Object, String>{
  Gson gson = new GsonBuilder().setPrettyPrinting().create();
  @Override
  public String handleRequest(Object event, Context context)
  {
    LambdaLogger logger = context.getLogger();
    String response = new String("SUCCESS");
    // log execution details
    logger.log("ENVIRONMENT VARIABLES: " + gson.toJson(System.getenv()));
    logger.log("CONTEXT: " + gson.toJson(context));
    // process event
    logger.log("EVENT: " + gson.toJson(event));
    return response;
  }
}

Example log format


START RequestId: 6bc28136-xmpl-4365-b021-0ce6b2e64ab0 Version: $LATEST
ENVIRONMENT VARIABLES: 
{
    "_HANDLER": "example.Handler",
    "AWS_EXECUTION_ENV": "AWS_Lambda_java8",
    "AWS_LAMBDA_FUNCTION_MEMORY_SIZE": "512",
    ...
}
CONTEXT: 
{
    "memoryLimit": 512,
    "awsRequestId": "6bc28136-xmpl-4365-b021-0ce6b2e64ab0",
    "functionName": "java-console",
    ...
}
EVENT:
{
  "records": [
    {
      "messageId": "19dd0b57-xmpl-4ac1-bd88-01bbb068cb78",
      "receiptHandle": "MessageReceiptHandle",
      "body": "Hello from SQS!",
       ...
    }
  ]
}
END RequestId: 6bc28136-xmpl-4365-b021-0ce6b2e64ab0
REPORT RequestId: 6bc28136-xmpl-4365-b021-0ce6b2e64ab0	Duration: 198.50 ms	Billed Duration: 200 ms	Memory Size: 512 MB	Max Memory Used: 90 MB	Init Duration: 524.75 ms

The Java runtime logs the START, END, and REPORT lines for each invocation. The report line provides the following details:

Report Log

RequestId – The unique request ID for the invocation.
Duration – The amount of time that your function's handler method spent processing the event.
Billed Duration – The amount of time billed for the invocation.
Memory Size – The amount of memory allocated to the function.
Max Memory Used – The amount of memory used by the function.
Init Duration – For the first request served, the amount of time it took the runtime to load the function and run code outside of the handler method.
XRAY TraceId – For traced requests, the AWS X-Ray trace ID.
SegmentId – For traced requests, the X-Ray segment ID.
Sampled – For traced requests, the sampling result.

Using AWS Lambda Powertools for Java and AWS SAM for structured logging

Follow the steps below to download, build, and deploy a sample Hello World Java application with integrated AWS Lambda Powertools for Java modules using the AWS SAM. This application implements a basic API backend and uses Powertools for emitting logs, metrics, and traces. It consists of an Amazon API Gateway endpoint and a Lambda function. When you send a GET request to the API Gateway endpoint, the Lambda function invokes, sends logs and metrics using Embedded Metric Format to CloudWatch, and sends traces to AWS X-Ray. The function returns a hello world message.

Prerequisites

To complete the steps in this section, you must have the following:

Java 11
AWS CLI version 2
AWS SAM CLI version 1.75 or later. If you have an older version of the AWS SAM CLI, see Upgrading the AWS SAM CLI.

Deploy a sample AWS SAM application

Initialize the application using the Hello World Java template.


sam init --app-template hello-world-powertools-java --name sam-app --package-type Zip --runtime java11 --no-tracing

Build the app.
```
cd sam-app && sam build
```
Deploy the app.
```
sam deploy --guided
```
Follow the on-screen prompts. To accept the default options provided in the interactive experience, press Enter.

Note
For HelloWorldFunction may not have authorization defined, Is this okay?, make sure to enter y.

Get the URL of the deployed application:


aws cloudformation describe-stacks --stack-name sam-app --query 'Stacks[0].Outputs[?OutputKey==`HelloWorldApi`].OutputValue' --output text

Invoke the API endpoint:
```
curl -X GET <URL_FROM_PREVIOUS_STEP>
```
If successful, you'll see this response:
```
{"message":"hello world"}
```

To get the logs for the function, run sam logs. For more information, see Working with logs in the AWS Serverless Application Model Developer Guide.


sam logs --stack-name sam-app

The log output looks like this:


2023/02/03/[$LATEST]851411a899b545eea2cffeba4cfbec81 2023-02-03T09:24:34.095000 INIT_START Runtime Version: java:11.v15    Runtime Version ARN: arn:aws:lambda:eu-central-1::runtime:0a25e3e7a1cc9ce404bc435eeb2ad358d8fa64338e618d0c224fe509403583ca
2023/02/03/[$LATEST]851411a899b545eea2cffeba4cfbec81 2023-02-03T09:24:34.114000 Picked up JAVA_TOOL_OPTIONS: -XX:+TieredCompilation -XX:TieredStopAtLevel=1
2023/02/03/[$LATEST]851411a899b545eea2cffeba4cfbec81 2023-02-03T09:24:34.793000 Transforming org/apache/logging/log4j/core/lookup/JndiLookup (lambdainternal.CustomerClassLoader@1a6c5a9e)
2023/02/03/[$LATEST]851411a899b545eea2cffeba4cfbec81 2023-02-03T09:24:35.252000 START RequestId: 7fcf1548-d2d4-41cd-a9a8-6ae47c51f765 Version: $LATEST
2023/02/03/[$LATEST]851411a899b545eea2cffeba4cfbec81 2023-02-03T09:24:36.531000 {
  "_aws": {
    "Timestamp": 1675416276051,
    "CloudWatchMetrics": [
      {
        "Namespace": "sam-app-powerools-java",
        "Metrics": [
          {
            "Name": "ColdStart",
            "Unit": "Count"
          }
        ],
        "Dimensions": [
          [
            "Service",
            "FunctionName"
          ]
        ]
      }
    ]
  },
  "function_request_id": "7fcf1548-d2d4-41cd-a9a8-6ae47c51f765",
  "traceId": "Root=1-63dcd2d1-25f90b9d1c753a783547f4dd;Parent=e29684c1be352ce4;Sampled=1",
  "FunctionName": "sam-app-HelloWorldFunction-y9Iu1FLJJBGD",
  "functionVersion": "$LATEST",
  "ColdStart": 1.0,
  "Service": "service_undefined",
  "logStreamId": "2023/02/03/[$LATEST]851411a899b545eea2cffeba4cfbec81",
  "executionEnvironment": "AWS_Lambda_java11"
}
2023/02/03/[$LATEST]851411a899b545eea2cffeba4cfbec81 2023-02-03T09:24:36.974000 Feb 03, 2023 9:24:36 AM com.amazonaws.xray.AWSXRayRecorder <init>
2023/02/03/[$LATEST]851411a899b545eea2cffeba4cfbec81 2023-02-03T09:24:36.993000 Feb 03, 2023 9:24:36 AM com.amazonaws.xray.config.DaemonConfiguration <init>
2023/02/03/[$LATEST]851411a899b545eea2cffeba4cfbec81 2023-02-03T09:24:36.993000 INFO: Environment variable AWS_XRAY_DAEMON_ADDRESS is set. Emitting to daemon on address XXXX.XXXX.XXXX.XXXX:2000.
2023/02/03/[$LATEST]851411a899b545eea2cffeba4cfbec81 2023-02-03T09:24:37.331000 09:24:37.294 [main] INFO  helloworld.App - {"version":null,"resource":"/hello","path":"/hello/","httpMethod":"GET","headers":{"Accept":"*/*","CloudFront-Forwarded-Proto":"https","CloudFront-Is-Desktop-Viewer":"true","CloudFront-Is-Mobile-Viewer":"false","CloudFront-Is-SmartTV-Viewer":"false","CloudFront-Is-Tablet-Viewer":"false","CloudFront-Viewer-ASN":"16509","CloudFront-Viewer-Country":"IE","Host":"XXXX.execute-api.eu-central-1.amazonaws.com","User-Agent":"curl/7.86.0","Via":"2.0 f0300a9921a99446a44423d996042050.cloudfront.net (CloudFront)","X-Amz-Cf-Id":"t9W5ByT11HaY33NM8YioKECn_4eMpNsOMPfEVRczD7T1RdhbtiwV1Q==","X-Amzn-Trace-Id":"Root=1-63dcd2d1-25f90b9d1c753a783547f4dd","X-Forwarded-For":"XX.XXX.XXX.XX, XX.XXX.XXX.XX","X-Forwarded-Port":"443","X-Forwarded-Proto":"https"},"multiValueHeaders":{"Accept":["*/*"],"CloudFront-Forwarded-Proto":["https"],"CloudFront-Is-Desktop-Viewer":["true"],"CloudFront-Is-Mobile-Viewer":["false"],"CloudFront-Is-SmartTV-Viewer":["false"],"CloudFront-Is-Tablet-Viewer":["false"],"CloudFront-Viewer-ASN":["16509"],"CloudFront-Viewer-Country":["IE"],"Host":["XXXX.execute-api.eu-central-1.amazonaws.com"],"User-Agent":["curl/7.86.0"],"Via":["2.0 f0300a9921a99446a44423d996042050.cloudfront.net (CloudFront)"],"X-Amz-Cf-Id":["t9W5ByT11HaY33NM8YioKECn_4eMpNsOMPfEVRczD7T1RdhbtiwV1Q=="],"X-Amzn-Trace-Id":["Root=1-63dcd2d1-25f90b9d1c753a783547f4dd"],"X-Forwarded-For":["XXX, XXX"],"X-Forwarded-Port":["443"],"X-Forwarded-Proto":["https"]},"queryStringParameters":null,"multiValueQueryStringParameters":null,"pathParameters":null,"stageVariables":null,"requestContext":{"accountId":"XXX","stage":"Prod","resourceId":"at73a1","requestId":"ba09ecd2-acf3-40f6-89af-fad32df67597","operationName":null,"identity":{"cognitoIdentityPoolId":null,"accountId":null,"cognitoIdentityId":null,"caller":null,"apiKey":null,"principalOrgId":null,"sourceIp":"54.240.197.236","cognitoAuthenticationType":null,"cognitoAuthenticationProvider":null,"userArn":null,"userAgent":"curl/7.86.0","user":null,"accessKey":null},"resourcePath":"/hello","httpMethod":"GET","apiId":"XXX","path":"/Prod/hello/","authorizer":null},"body":null,"isBase64Encoded":false}
2023/02/03/[$LATEST]851411a899b545eea2cffeba4cfbec81 2023-02-03T09:24:37.351000 09:24:37.351 [main] INFO  helloworld.App - Retrieving https://checkip.amazonaws.com
2023/02/03/[$LATEST]851411a899b545eea2cffeba4cfbec81 2023-02-03T09:24:39.313000 {
  "function_request_id": "7fcf1548-d2d4-41cd-a9a8-6ae47c51f765",
  "traceId": "Root=1-63dcd2d1-25f90b9d1c753a783547f4dd;Parent=e29684c1be352ce4;Sampled=1",
  "xray_trace_id": "1-63dcd2d1-25f90b9d1c753a783547f4dd",
  "functionVersion": "$LATEST",
  "Service": "service_undefined",
  "logStreamId": "2023/02/03/[$LATEST]851411a899b545eea2cffeba4cfbec81",
  "executionEnvironment": "AWS_Lambda_java11"
}
2023/02/03/[$LATEST]851411a899b545eea2cffeba4cfbec81 2023-02-03T09:24:39.371000 END RequestId: 7fcf1548-d2d4-41cd-a9a8-6ae47c51f765
2023/02/03/[$LATEST]851411a899b545eea2cffeba4cfbec81 2023-02-03T09:24:39.371000 REPORT RequestId: 7fcf1548-d2d4-41cd-a9a8-6ae47c51f765    Duration: 4118.98 ms    Billed Duration: 4119 ms    Memory Size: 512 MB    Max Memory Used: 152 MB    Init Duration: 1155.47 ms    
XRAY TraceId: 1-63dcd2d1-25f90b9d1c753a783547f4dd    SegmentId: 3a028fee19b895cb    Sampled: true

This is a public API endpoint that is accessible over the internet. We recommend that you delete the endpoint after testing.
```
sam delete
```

Managing log retention

Log groups aren't deleted automatically when you delete a function. To avoid storing logs indefinitely, delete the log group, or configure a retention period after which CloudWatch automatically deletes the logs. To set up log retention, add the following to your AWS SAM template:


Resources:
  HelloWorldFunction:
    Type: AWS::Serverless::Function
    Properties:
      # Omitting other properties

  LogGroup:
    Type: AWS::Logs::LogGroup
    Properties:
      LogGroupName: !Sub "/aws/lambda/${HelloWorldFunction}"
      RetentionInDays: 7

Using AWS Lambda Powertools for Java and the AWS CDK for structured logging

Follow the steps below to download, build, and deploy a sample Hello World Java application with integrated AWS Lambda Powertools for Java modules using the AWS CDK. This application implements a basic API backend and uses Powertools for emitting logs, metrics, and traces. It consists of an Amazon API Gateway endpoint and a Lambda function. When you send a GET request to the API Gateway endpoint, the Lambda function invokes, sends logs and metrics using Embedded Metric Format to CloudWatch, and sends traces to AWS X-Ray. The function returns a hello world message.

Prerequisites

To complete the steps in this section, you must have the following:

Java 11
AWS CLI version 2
AWS CDK version 2
AWS SAM CLI version 1.75 or later. If you have an older version of the AWS SAM CLI, see Upgrading the AWS SAM CLI.

Deploy a sample AWS CDK application

Create a project directory for your new application.
```
mkdir hello-world
cd hello-world
```
Initialize the app.
```
cdk init app --language java
```

Create a maven project with the following command:


mkdir app
cd app
mvn archetype:generate -DgroupId=helloworld -DartifactId=Function -DarchetypeArtifactId=maven-archetype-quickstart -DinteractiveMode=false

Open pom.xml in the hello-world\app\Function directory and replace the existing code with the following code that includes dependencies and maven plugins for Powertools.


<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <groupId>helloworld</groupId>
  <artifactId>Function</artifactId>
  <packaging>jar</packaging>
  <version>1.0-SNAPSHOT</version>
  <name>Function</name>
  <url>http://maven.apache.org</url>
<properties>
    <maven.compiler.source>11</maven.compiler.source>
    <maven.compiler.target>11</maven.compiler.target>
    <log4j.version>2.17.2</log4j.version>
</properties>
    <dependencies>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>3.8.1</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>software.amazon.lambda</groupId>
            <artifactId>powertools-tracing</artifactId>
            <version>1.3.0</version>
        </dependency>
        <dependency>
            <groupId>software.amazon.lambda</groupId>
            <artifactId>powertools-metrics</artifactId>
            <version>1.3.0</version>
        </dependency>
        <dependency>
            <groupId>software.amazon.lambda</groupId>
            <artifactId>powertools-logging</artifactId>
            <version>1.3.0</version>
        </dependency>
        <dependency>
            <groupId>com.amazonaws</groupId>
            <artifactId>aws-lambda-java-core</artifactId>
            <version>1.2.2</version>
        </dependency>
        <dependency>
            <groupId>com.amazonaws</groupId>
            <artifactId>aws-lambda-java-events</artifactId>
            <version>3.11.1</version>
        </dependency>
  </dependencies>
<build>
    <plugins>
        <plugin>
            <groupId>org.codehaus.mojo</groupId>
            <artifactId>aspectj-maven-plugin</artifactId>
            <version>1.14.0</version>
            <configuration>
                <source>${maven.compiler.source}</source>
                <target>${maven.compiler.target}</target>
                <complianceLevel>${maven.compiler.target}</complianceLevel>
                <aspectLibraries>
                    <aspectLibrary>
                        <groupId>software.amazon.lambda</groupId>
                        <artifactId>powertools-tracing</artifactId>
                    </aspectLibrary>
                    <aspectLibrary>
                        <groupId>software.amazon.lambda</groupId>
                        <artifactId>powertools-metrics</artifactId>
                    </aspectLibrary>
                    <aspectLibrary>
                        <groupId>software.amazon.lambda</groupId>
                        <artifactId>powertools-logging</artifactId>
                    </aspectLibrary>
                </aspectLibraries>
            </configuration>
            <executions>
                <execution>
                    <goals>
                        <goal>compile</goal>
                    </goals>
                </execution>
            </executions>
        </plugin>
        <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-shade-plugin</artifactId>
                <version>3.4.1</version>
                <executions>
                    <execution>
                        <phase>package</phase>
                        <goals>
                            <goal>shade</goal>
                        </goals>
                        <configuration>
                            <transformers>
                                <transformer
                                        implementation="com.github.edwgiz.maven_shade_plugin.log4j2_cache_transformer.PluginsCacheFileTransformer">
                                </transformer>
                            </transformers>
                            <createDependencyReducedPom>false</createDependencyReducedPom>
                            <finalName>function</finalName>

                        </configuration>
                    </execution>
                </executions>
                <dependencies>
                    <dependency>
                        <groupId>com.github.edwgiz</groupId>
                        <artifactId>maven-shade-plugin.log4j2-cachefile-transformer</artifactId>
                        <version>2.15</version>
                    </dependency>
                </dependencies>
        </plugin>
    </plugins>
</build>
</project>

Create the hello-world\app\src\main\resource directory and create log4j.xml for the log configuration.
```
mkdir -p src/main/resource
cd src/main/resource
touch log4j.xml
```

Open log4j.xml and add the following code.


<?xml version="1.0" encoding="UTF-8"?>
<Configuration>
    <Appenders>
        <Console name="JsonAppender" target="SYSTEM_OUT">
            <JsonTemplateLayout eventTemplateUri="classpath:LambdaJsonLayout.json" />
        </Console>
    </Appenders>
    <Loggers>
        <Logger name="JsonLogger" level="INFO" additivity="false">
            <AppenderRef ref="JsonAppender"/>
        </Logger>
        <Root level="info">
            <AppenderRef ref="JsonAppender"/>
        </Root>
    </Loggers>
</Configuration>

Open App.java from the hello-world\app\Function\src\main\java\helloworld directory and replace the existing code with the following code. This is the code for the Lambda function.


package helloworld;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.URL;
import java.util.HashMap;
import java.util.Map;
import java.util.stream.Collectors;

import com.amazonaws.services.lambda.runtime.Context;
import com.amazonaws.services.lambda.runtime.RequestHandler;
import com.amazonaws.services.lambda.runtime.events.APIGatewayProxyRequestEvent;
import com.amazonaws.services.lambda.runtime.events.APIGatewayProxyResponseEvent;
import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import software.amazon.lambda.powertools.logging.Logging;
import software.amazon.lambda.powertools.metrics.Metrics;
import software.amazon.lambda.powertools.tracing.CaptureMode;
import software.amazon.lambda.powertools.tracing.Tracing;

import static software.amazon.lambda.powertools.tracing.CaptureMode.*;

/**
 * Handler for requests to Lambda function.
 */
public class App implements RequestHandler<APIGatewayProxyRequestEvent, APIGatewayProxyResponseEvent> {
    Logger log = LogManager.getLogger(App.class);


    @Logging(logEvent = true)
    @Tracing(captureMode = DISABLED)
    @Metrics(captureColdStart = true)
    public APIGatewayProxyResponseEvent handleRequest(final APIGatewayProxyRequestEvent input, final Context context) {
        Map<String, String> headers = new HashMap<>();
        headers.put("Content-Type", "application/json");
        headers.put("X-Custom-Header", "application/json");

        APIGatewayProxyResponseEvent response = new APIGatewayProxyResponseEvent()
                .withHeaders(headers);
        try {
            final String pageContents = this.getPageContents("https://checkip.amazonaws.com");
            String output = String.format("{ \"message\": \"hello world\", \"location\": \"%s\" }", pageContents);

            return response
                    .withStatusCode(200)
                    .withBody(output);
        } catch (IOException e) {
            return response
                    .withBody("{}")
                    .withStatusCode(500);
        }
    }
    @Tracing(namespace = "getPageContents")
    private String getPageContents(String address) throws IOException {
        log.info("Retrieving {}", address);
        URL url = new URL(address);
        try (BufferedReader br = new BufferedReader(new InputStreamReader(url.openStream()))) {
            return br.lines().collect(Collectors.joining(System.lineSeparator()));
        }
    }
}

Open HelloWorldStack.java from the hello-world\src\main\java\com\myorg directory and replace the existing code with the following code. This code uses Lambda Constructorand the ApiGatewayv2 Constructor to create a REST API and a Lambda function.


package com.myorg;

import software.amazon.awscdk.*;
import software.amazon.awscdk.services.apigatewayv2.alpha.*;
import software.amazon.awscdk.services.apigatewayv2.integrations.alpha.HttpLambdaIntegration;
import software.amazon.awscdk.services.apigatewayv2.integrations.alpha.HttpLambdaIntegrationProps;
import software.amazon.awscdk.services.lambda.Code;
import software.amazon.awscdk.services.lambda.Function;
import software.amazon.awscdk.services.lambda.FunctionProps;
import software.amazon.awscdk.services.lambda.Runtime;
import software.amazon.awscdk.services.logs.RetentionDays;
import software.amazon.awscdk.services.s3.assets.AssetOptions;
import software.constructs.Construct;

import java.util.Arrays;
import java.util.List;

import static java.util.Collections.singletonList;
import static software.amazon.awscdk.BundlingOutput.ARCHIVED;

public class HelloWorldStack extends Stack {
    public HelloWorldStack(final Construct scope, final String id) {
        this(scope, id, null);
    }

    public HelloWorldStack(final Construct scope, final String id, final StackProps props) {
        super(scope, id, props);

        List<String> functionPackagingInstructions = Arrays.asList(
                "/bin/sh",
                "-c",
                "cd Function " +
                        "&& mvn clean install " +
                        "&& cp /asset-input/Function/target/function.jar /asset-output/"
        );
        BundlingOptions.Builder builderOptions = BundlingOptions.builder()
                .command(functionPackagingInstructions)
                .image(Runtime.JAVA_11.getBundlingImage())
                .volumes(singletonList(
                        // Mount local .m2 repo to avoid download all the dependencies again inside the container
                        DockerVolume.builder()
                                .hostPath(System.getProperty("user.home") + "/.m2/")
                                .containerPath("/root/.m2/")
                                .build()
                ))
                .user("root")
                .outputType(ARCHIVED);

        Function function = new Function(this, "Function", FunctionProps.builder()
                .runtime(Runtime.JAVA_11)
                .code(Code.fromAsset("app", AssetOptions.builder()
                        .bundling(builderOptions
                                .command(functionPackagingInstructions)
                                .build())
                        .build()))
                .handler("helloworld.App::handleRequest")
                .memorySize(1024)
                .timeout(Duration.seconds(10))
                .logRetention(RetentionDays.ONE_WEEK)
                .build());

        HttpApi httpApi = new HttpApi(this, "sample-api", HttpApiProps.builder()
                .apiName("sample-api")
                .build());

        httpApi.addRoutes(AddRoutesOptions.builder()
                .path("/")
                .methods(singletonList(HttpMethod.GET))
                .integration(new HttpLambdaIntegration("function", function, HttpLambdaIntegrationProps.builder()
                        .payloadFormatVersion(PayloadFormatVersion.VERSION_2_0)
                        .build()))
                .build());

        new CfnOutput(this, "HttpApi", CfnOutputProps.builder()
                .description("Url for Http Api")
                .value(httpApi.getApiEndpoint())
                .build());
    }
}

Open pom.xml from the hello-world directory and replace the existing code with the following code.


<?xml version="1.0" encoding="UTF-8"?>
<project xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"
         xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.myorg</groupId>
    <artifactId>hello-world</artifactId>
    <version>0.1</version>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <cdk.version>2.70.0</cdk.version>
        <constructs.version>[10.0.0,11.0.0)</constructs.version>
        <junit.version>5.7.1</junit.version>
    </properties>

    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.8.1</version>
                <configuration>
                    <source>1.8</source>
                    <target>1.8</target>
                </configuration>
            </plugin>

            <plugin>
                <groupId>org.codehaus.mojo</groupId>
                <artifactId>exec-maven-plugin</artifactId>
                <version>3.0.0</version>
                <configuration>
                    <mainClass>com.myorg.HelloWorldApp</mainClass>
                </configuration>
            </plugin>
        </plugins>
    </build>

    <dependencies>
        <!-- AWS Cloud Development Kit -->
        <dependency>
            <groupId>software.amazon.awscdk</groupId>
            <artifactId>aws-cdk-lib</artifactId>
            <version>${cdk.version}</version>
        </dependency>
        <dependency>
            <groupId>software.constructs</groupId>
            <artifactId>constructs</artifactId>
            <version>${constructs.version}</version>
        </dependency>
        <dependency>
          <groupId>org.junit.jupiter</groupId>
          <artifactId>junit-jupiter</artifactId>
          <version>${junit.version}</version>
          <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>software.amazon.awscdk</groupId>
            <artifactId>apigatewayv2-alpha</artifactId>
            <version>${cdk.version}-alpha.0</version>
        </dependency>
        <dependency>
            <groupId>software.amazon.awscdk</groupId>
            <artifactId>apigatewayv2-integrations-alpha</artifactId>
            <version>${cdk.version}-alpha.0</version>
        </dependency>
    </dependencies>
</project>

Make sure you’re in the hello-world directory and deploy your application.
```
cdk deploy
```

Get the URL of the deployed application:


aws cloudformation describe-stacks --stack-name HelloWorldStack --query 'Stacks[0].Outputs[?OutputKey==`HttpApi`].OutputValue' --output text

Invoke the API endpoint:
```
curl -X GET <URL_FROM_PREVIOUS_STEP>
```
If successful, you'll see this response:
```
{"message":"hello world"}
```

To get the logs for the function, run sam logs. For more information, see Working with logs in the AWS Serverless Application Model Developer Guide.


sam logs --stack-name HelloWorldStack

The log output looks like this:


2023/02/03/[$LATEST]851411a899b545eea2cffeba4cfbec81 2023-02-03T09:24:34.095000 INIT_START Runtime Version: java:11.v15    Runtime Version ARN: arn:aws:lambda:eu-central-1::runtime:0a25e3e7a1cc9ce404bc435eeb2ad358d8fa64338e618d0c224fe509403583ca
2023/02/03/[$LATEST]851411a899b545eea2cffeba4cfbec81 2023-02-03T09:24:34.114000 Picked up JAVA_TOOL_OPTIONS: -XX:+TieredCompilation -XX:TieredStopAtLevel=1
2023/02/03/[$LATEST]851411a899b545eea2cffeba4cfbec81 2023-02-03T09:24:34.793000 Transforming org/apache/logging/log4j/core/lookup/JndiLookup (lambdainternal.CustomerClassLoader@1a6c5a9e)
2023/02/03/[$LATEST]851411a899b545eea2cffeba4cfbec81 2023-02-03T09:24:35.252000 START RequestId: 7fcf1548-d2d4-41cd-a9a8-6ae47c51f765 Version: $LATEST
2023/02/03/[$LATEST]851411a899b545eea2cffeba4cfbec81 2023-02-03T09:24:36.531000 {
  "_aws": {
    "Timestamp": 1675416276051,
    "CloudWatchMetrics": [
      {
        "Namespace": "sam-app-powerools-java",
        "Metrics": [
          {
            "Name": "ColdStart",
            "Unit": "Count"
          }
        ],
        "Dimensions": [
          [
            "Service",
            "FunctionName"
          ]
        ]
      }
    ]
  },
  "function_request_id": "7fcf1548-d2d4-41cd-a9a8-6ae47c51f765",
  "traceId": "Root=1-63dcd2d1-25f90b9d1c753a783547f4dd;Parent=e29684c1be352ce4;Sampled=1",
  "FunctionName": "sam-app-HelloWorldFunction-y9Iu1FLJJBGD",
  "functionVersion": "$LATEST",
  "ColdStart": 1.0,
  "Service": "service_undefined",
  "logStreamId": "2023/02/03/[$LATEST]851411a899b545eea2cffeba4cfbec81",
  "executionEnvironment": "AWS_Lambda_java11"
}

This is a public API endpoint that is accessible over the internet. We recommend that you delete the endpoint after testing.
```
cdk destroy
```

Using the Lambda console

You can use the Lambda console to view log output after you invoke a Lambda function. For more information, see Accessing Amazon CloudWatch logs for AWS Lambda.

Using the CloudWatch console

You can use the Amazon CloudWatch console to view logs for all Lambda function invocations.

To view logs on the CloudWatch console

Open the Log groups page on the CloudWatch console.
Choose the log group for your function (/aws/lambda/your-function-name).
Choose a log stream.

Each log stream corresponds to an instance of your function. A log stream appears when you update your Lambda function, and when additional instances are created to handle multiple concurrent invocations. To find logs for a specific invocation, we recommend instrumenting your function with AWS X-Ray. X-Ray records details about the request and the log stream in the trace.

To use a sample application that correlates logs and traces with X-Ray, see Error processor sample application for AWS Lambda.

Using the AWS Command Line Interface (AWS CLI)

The AWS CLI is an open-source tool that enables you to interact with AWS services using commands in your command line shell. To complete the steps in this section, you must have the following:

You can use the AWS CLI to retrieve logs for an invocation using the --log-type command option. The response contains a LogResult field that contains up to 4 KB of base64-encoded logs from the invocation.

Example retrieve a log ID

The following example shows how to retrieve a log ID from the LogResult field for a function named my-function.


aws lambda invoke --function-name my-function out --log-type Tail

You should see the following output:


{
    "StatusCode": 200,
    "LogResult": "U1RBUlQgUmVxdWVzdElkOiA4N2QwNDRiOC1mMTU0LTExZTgtOGNkYS0yOTc0YzVlNGZiMjEgVmVyc2lvb...",
    "ExecutedVersion": "$LATEST"
}

Example decode the logs

In the same command prompt, use the base64 utility to decode the logs. The following example shows how to retrieve base64-encoded logs for my-function.


aws lambda invoke --function-name my-function out --log-type Tail \
--query 'LogResult' --output text |  base64 -d

You should see the following output:


START RequestId: 57f231fb-1730-4395-85cb-4f71bd2b87b8 Version: $LATEST
"AWS_SESSION_TOKEN": "AgoJb3JpZ2luX2VjELj...", "_X_AMZN_TRACE_ID": "Root=1-5d02e5ca-f5792818b6fe8368e5b51d50;Parent=191db58857df8395;Sampled=0"",ask/lib:/opt/lib",
END RequestId: 57f231fb-1730-4395-85cb-4f71bd2b87b8
REPORT RequestId: 57f231fb-1730-4395-85cb-4f71bd2b87b8  Duration: 79.67 ms      Billed Duration: 80 ms         Memory Size: 128 MB     Max Memory Used: 73 MB

The base64 utility is available on Linux, macOS, and Ubuntu on Windows. macOS users may need to use base64 -D.

Example get-logs.sh script

In the same command prompt, use the following script to download the last five log events. The script uses sed to remove quotes from the output file, and sleeps for 15 seconds to allow time for the logs to become available. The output includes the response from Lambda and the output from the get-log-events command.

Copy the contents of the following code sample and save in your Lambda project directory as get-logs.sh.

The cli-binary-format option is required if you're using AWS CLI version 2. To make this the default setting, run aws configure set cli-binary-format raw-in-base64-out. For more information, see AWS CLI supported global command line options.


#!/bin/bash
aws lambda invoke --function-name my-function --cli-binary-format raw-in-base64-out --payload '{"key": "value"}' out
sed -i'' -e 's/"//g' out
sleep 15
aws logs get-log-events --log-group-name /aws/lambda/my-function --log-stream-name stream1 --limit 5

Example macOS and Linux (only)

In the same command prompt, macOS and Linux users may need to run the following command to ensure the script is executable.


chmod -R 755 get-logs.sh

Example retrieve the last five log events

In the same command prompt, run the following script to get the last five log events.


./get-logs.sh

You should see the following output:


{
    "StatusCode": 200,
    "ExecutedVersion": "$LATEST"
}
{
    "events": [
        {
            "timestamp": 1559763003171,
            "message": "START RequestId: 4ce9340a-b765-490f-ad8a-02ab3415e2bf Version: $LATEST\n",
            "ingestionTime": 1559763003309
        },
        {
            "timestamp": 1559763003173,
            "message": "2019-06-05T19:30:03.173Z\t4ce9340a-b765-490f-ad8a-02ab3415e2bf\tINFO\tENVIRONMENT VARIABLES\r{\r  \"AWS_LAMBDA_FUNCTION_VERSION\": \"$LATEST\",\r ...",
            "ingestionTime": 1559763018353
        },
        {
            "timestamp": 1559763003173,
            "message": "2019-06-05T19:30:03.173Z\t4ce9340a-b765-490f-ad8a-02ab3415e2bf\tINFO\tEVENT\r{\r  \"key\": \"value\"\r}\n",
            "ingestionTime": 1559763018353
        },
        {
            "timestamp": 1559763003218,
            "message": "END RequestId: 4ce9340a-b765-490f-ad8a-02ab3415e2bf\n",
            "ingestionTime": 1559763018353
        },
        {
            "timestamp": 1559763003218,
            "message": "REPORT RequestId: 4ce9340a-b765-490f-ad8a-02ab3415e2bf\tDuration: 26.73 ms\tBilled Duration: 27 ms \tMemory Size: 128 MB\tMax Memory Used: 75 MB\t\n",
            "ingestionTime": 1559763018353
        }
    ],
    "nextForwardToken": "f/34783877304859518393868359594929986069206639495374241795",
    "nextBackwardToken": "b/34783877303811383369537420289090800615709599058929582080"
}

Deleting logs

Log groups aren't deleted automatically when you delete a function. To avoid storing logs indefinitely, delete the log group, or configure a retention period after which logs are deleted automatically.

Advanced logging with Log4j 2 and SLF4J

Note

AWS Lambda does not include Log4j2 in its managed runtimes or base container images. These are therefore not affected by the issues described in CVE-2021-44228, CVE-2021-45046, and CVE-2021-45105.

For cases where a customer function includes an impacted Log4j2 version, we have applied a change to the Lambda Java managed runtimes and base container images that helps to mitigate the issues in CVE-2021-44228, CVE-2021-45046, and CVE-2021-45105. As a result of this change, customers using Log4J2 may see an additional log entry, similar to "Transforming org/apache/logging/log4j/core/lookup/JndiLookup (java.net.URLClassLoader@...)". Any log strings that reference the jndi mapper in the Log4J2 output will be replaced with "Patched JndiLookup::lookup()".

Independent of this change, we strongly encourage all customers whose functions include Log4j2 to update to the latest version. Specifically, customers using the aws-lambda-java-log4j2 library in their functions should update to version 1.5.0 (or later), and redeploy their functions. This version updates the underlying Log4j2 utility dependencies to version 2.17.0 (or later). The updated aws-lambda-java-log4j2 binary is available at the Maven repository and its source code is available in Github.

To customize log output, support logging during unit tests, and log AWS SDK calls, use Apache Log4j 2 with SLF4J. Log4j is a logging library for Java programs that enables you to configure log levels and use appender libraries. SLF4J is a facade library that lets you change which library you use without changing your function code.

To add the request ID to your function's logs, use the appender in the aws-lambda-java-log4j2 library. The following example shows a Log4j 2 configuration file that adds a timestamp and request ID to all logs.

Example src/main/resources/log4j2.xml – Appender configuration


<Configuration status="WARN">
  <Appenders>
    <Lambda name="Lambda">
      <PatternLayout>
          <pattern>%d{yyyy-MM-dd HH:mm:ss} %X{AWSRequestId} %-5p %c{1} - %m%n</pattern>
      </PatternLayout>
    </Lambda>
  </Appenders>
  <Loggers>
    <Root level="INFO">
      <AppenderRef ref="Lambda"/>
    </Root>
    <Logger name="software.amazon.awssdk" level="WARN" />
    <Logger name="software.amazon.awssdk.request" level="DEBUG" />
  </Loggers>
</Configuration>

With this configuration, each line is prepended with the date, time, request ID, log level, and class name.

Example log format with appender


START RequestId: 6bc28136-xmpl-4365-b021-0ce6b2e64ab0 Version: $LATEST
2020-03-18 08:52:43 6bc28136-xmpl-4365-b021-0ce6b2e64ab0 INFO  Handler - ENVIRONMENT VARIABLES:
{
    "_HANDLER": "example.Handler",
    "AWS_EXECUTION_ENV": "AWS_Lambda_java8",
    "AWS_LAMBDA_FUNCTION_MEMORY_SIZE": "512",
    ...
}
2020-03-18 08:52:43 6bc28136-xmpl-4365-b021-0ce6b2e64ab0 INFO  Handler - CONTEXT:
{
    "memoryLimit": 512,
    "awsRequestId": "6bc28136-xmpl-4365-b021-0ce6b2e64ab0",
    "functionName": "java-console",
    ...
}

SLF4J is a facade library for logging in Java code. In your function code, you use the SLF4J logger factory to retrieve a logger with methods for log levels like info() and warn(). In your build configuration, you include the logging library and SLF4J adapter in the classpath. By changing the libraries in the build configuration, you can change the logger type without changing your function code. SLF4J is required to capture logs from the SDK for Java.

In the following example, the handler class uses SLF4J to retrieve a logger.

Example src/main/java/example/HandlerS3.java – Logging with SLF4J


import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

// Handler value: example.Handler
public class HandlerS3 implements RequestHandler<S3Event, String>{
    private static final Logger logger = LoggerFactory.getLogger(HandlerS3.class);
    Gson gson = new GsonBuilder().setPrettyPrinting().create();
    @Override
    public String handleRequest(S3Event event, Context context)
    {
        ...
        logger.info("RECORD: " + record);
        logger.info("SOURCE BUCKET: " + srcBucket);
        logger.info("SOURCE KEY: " + srcKey);
        ...
    }
}

The build configuration takes runtime dependencies on the Lambda appender and SLF4J adapter, and implementation dependencies on Log4J 2.

Example build.gradle – Logging dependencies


dependencies {
    ...
    implementation 'org.apache.logging.log4j:log4j-api:[2.17.1,)'
    implementation 'org.apache.logging.log4j:log4j-core:[2.17.1,)'
    implementation 'org.apache.logging.log4j:log4j-slf4j18-impl:[2.17.1,)'
    ...
}

When you run your code locally for tests, the context object with the Lambda logger is not available, and there's no request ID for the Lambda appender to use. For example test configurations, see the sample applications in the next section.

Sample logging code

The GitHub repository for this guide includes sample applications that demonstrate the use of various logging configurations. Each sample application includes scripts for easy deployment and cleanup, an AWS SAM template, and supporting resources.

Sample Lambda applications in Java

java-basic – A collection of minimal Java functions with unit tests and variable logging configuration.
java-events – A collection of Java functions that contain skeleton code for how to handle events from various services such as Amazon API Gateway, Amazon SQS, and Amazon Kinesis. These functions use the latest version of the aws-lambda-java-events library (3.0.0 and newer). These examples do not require the AWS SDK as a dependency.
s3-java – A Java function that processes notification events from Amazon S3 and uses the Java Class Library (JCL) to create thumbnails from uploaded image files.
Use API Gateway to invoke a Lambda function – A Java function that scans a Amazon DynamoDB table that contains employee information. It then uses Amazon Simple Notification Service to send a text message to employees celebrating their work anniversaries. This example uses API Gateway to invoke the function.

The java-basic sample application shows a minimal logging configuration that supports logging tests. The handler code uses the LambdaLogger logger provided by the context object. For tests, the application uses a custom TestLogger class that implements the LambdaLogger interface with a Log4j 2 logger. It uses SLF4J as a facade for compatibility with the AWS SDK. Logging libraries are excluded from build output to keep the deployment package small.

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