In this chapter, let us understand in detail how to create a simple AWS Lambda function in Java in detail.

Creating JAR file in Eclipse

Before proceeding to work on creating a lambda function in AWS, we need AWS toolkit support for Eclipse. For any guidance on installation of the same, you can refer to the Environment Setup chapter in this tutorial.

Once you are done with installation, follow the steps given here −

Step 1

Open Eclipse IDE and create a new project with AWS Lambda Java Project. Observe the screenshot given below for better understanding −

Step 2

Once you select Next, it will redirect you the screen shown below −

Step 3

Now, a default code is created for Input Type Custom. Once you click Finish button the project gets created as shown below −

Step 4

Now, right click your project and export it. Select Java / JAR file from the Export wizard and click Next.

Step 5

Now, if you click Next, you will be prompted save the file in the destination folder which will be asked when you click on next.

Once the file is saved, go back to AWS Console and create the AWS Lambda function for Java.

Step 6

Now, upload the .jar file that we created using the Upload button as shown in the screenshot given below −

Handler Details for Java

Handler is package name and class name. Look at the following example to understand handler in detail −

Example

package com.amazonaws.lambda.demo;

import com.amazonaws.services.lambda.runtime.Context;
import com.amazonaws.services.lambda.runtime.RequestHandler;
public class LambdaFunctionHandler implements RequestHandler

Observe that from the above code, the handler will be com.amazonaws.lambda.demo.LambdaFunctionHandler

Now, let us test the changes and see the output −

Context Object in Java

Interaction with AWS Lambda execution is done using the context. It provides following methods to be used inside Java −

Now, let us update the code given above and observe the output for some of the methods listed above. Observe the Example code given below for a better understanding −

package com.amazonaws.lambda.demo;
import com.amazonaws.services.lambda.runtime.Context;
import com.amazonaws.services.lambda.runtime.RequestHandler;
public class LambdaFunctionHandler implements RequestHandler<Object, String> {
   @Override
   public String handleRequest(Object input, Context context) {
      context.getLogger().log("Input: " + input);
      System.out.println("AWS Lambda function name: " + context.getFunctionName());
      System.out.println("Memory Allocated: " + context.getMemoryLimitInMB());
      System.out.println("Time remaining in milliseconds: " + context.getRemainingTimeInMillis());
      System.out.println("Cloudwatch group name " + context.getLogGroupName());
      System.out.println("AWS Lambda Request Id " + context.getAwsRequestId());
      
      // TODO: implement your handler
      return "Hello from Lambda!";
   }
}

Once you run the code given above, you can find the output as given below −

Logs for context

You can observe the following output when you are viewing your log output −

The memory allocated for the Lambda function is 512MB. The time allocated is 25 seconds. The time remaining as displayed above is 24961, which is in milliseconds. So 25000 – 24961 which equals to 39 milliseconds is used for the execution of the Lambda function. Note that Cloudwatch group name and request id are also displayed as shown above.

Note that we have used the following command to print logs in Java −

System.out.println (“log message”)

The same is available in CloudWatch. For this, go to AWS services, select CloudWatchservices and click Logs.

Now, if you select the Lambda function, it will display the logs date wise as shown below −

Logging in Java

You can also use Lambdalogger in Java to log the data. Observe the following example that shows the same −

Example

package com.amazonaws.lambda.demo;
import com.amazonaws.services.lambda.runtime.Context;
import com.amazonaws.services.lambda.runtime.RequestHandler;
import com.amazonaws.services.lambda.runtime.LambdaLogger;
public class LambdaFunctionHandler implements RequestHandler<Object, String> {
   @Override
   public String handleRequest(Object input, Context context) {
      LambdaLogger logger = context.getLogger();
      logger.log("Input: " + input);
      logger.log("AWS Lambda function name: " + context.getFunctionName()+"n");
      logger.log("Memory Allocated: " + context.getMemoryLimitInMB()+"n");
      logger.log("Time remaining in milliseconds: " + context.getRemainingTimeInMillis()+"n");
      logger.log("Cloudwatch group name " + context.getLogGroupName()+"n");
      logger.log("AWS Lambda Request Id " + context.getAwsRequestId()+"n");
      
      // TODO: implement your handler
      return "Hello from Lambda!";
   }
}

The code shown above will give you the following output −

The output in CloudWatch will be as shown below −

Error handling in Java for Lambda Function

This section will explain how to handle errors in Java for Lambda function. Observe the following code that shows the same −

package com.amazonaws.lambda.errorhandling;
import com.amazonaws.services.lambda.runtime.Context;
import com.amazonaws.services.lambda.runtime.RequestHandler;
public class LambdaFunctionHandler implements RequestHandler<Object, String> {
   @Override
   public String handleRequest(Object input, Context context) {
      throw new RuntimeException("Error from aws lambda");
   } 
}

Note that the error details are displayed in json format with errorMessage Error from AWS Lambda. Also, the ErrorType and stackTrace gives more details about the error.

The output and the corresponding log output of the code given above will be as shown in the following screenshots given below −