AWS Developer Tools Blog

Testing Lambda functions using the AWS Toolkit for Eclipse

In this blog post, I will introduce how to test AWS Lambda functions in Eclipse by using the AWS Toolkit for Eclipse. The AWS Toolkit for Eclipse Lambda Plugin provides a feature in which a JUnit test class is created automatically upon creation of a new AWS Lambda Java project. I will give you step-by-step instructions for creating an AWS Lambda Java project, creating an AWS Lambda function, unit testing the AWS Lambda function, uploading the AWS Lambda function to AWS Lambda, and testing the AWS Lambda function remotely. Currently, AWS Lambda supports five AWS service event sources for Java: S3 Event, SNS Event, Kinesis Event, Cognito Event, and DynamoDB Event. You can also define a custom event. In this post, I will give examples using the S3 Event and a custom event. The other event types can be used in the same way.


  1. Install Eclipse version 3.6 (Helios) or later on your computer.
  2. Follow the instructions on the AWS Toolkit for Eclipse page to Install the AWS Toolkit for Eclipse.
  3. After it is installed, the new AWS Toolkit for Eclipse icon will appare on the toolbar.

Steps for testing a Lambda function

  1. Create an AWS Lambda Java project.

    • Choose the AWS Toolkit for Eclipse or New icon, and then choose AWS Lambda Java Project.
    • When you create a name for the project and package, you should see the corresponding changes in the Preview text area. For Input Type, you can choose from the five AWS service event sources plus the custom event. You can also complete the Class Name, Input Type, and Output Type fileds. The AWS Toolkit for Eclipse will auto-generate the Lambda function class in the src/ folder and the unit test class in the tst/ folder. In this example, we set Project Name as S3EventDemo, Package Name to  com.lambda.demo.s3, and left the other settings at their defaults.
    • The AWS Toolkit for Eclipse will create the following folder structure for the S3 Event.

      The LambdaFunctionHandler class is an implementation of the RequestHandler interface that defines the Lambda function you need to implement. The LambdaFunctionHandlerTest class is where the unit tests reside. The TestContext class is an implementation of the Context interface, which acts as a parameter for the Lambda function. The TestUtils class is a supporting class to parse JSON file. The s3-event.put.json file is the sample S3 event source configuration you can use for testing.
  2. Create an AWS Lambda function.
    You need to implement the Lambda function handleRequest in the LambdaFunctionHandler class. It takes S3Event and Context as parameters, and returns an Object. You can always define a custom output class instead of the default Object class. The following is the sample implementation of the Lambda function, which returns a string of the bucket name from the S3 Event.

    public Object handleRequest(S3Event input, Context context) {
        context.getLogger().log("Input: " + input);
        return input.getRecords().get(0).getS3().getBucket().getName();
  3. Unit-test the AWS Lambda function.
    In the unit test, the S3Event parameter is loaded from the s3-event.put.json file in the tst/ folder and the Context is implemented and instantiated by the customers for testing. The default unit test in the LambdaFunctionHandlerTest class is simply printing the output. You may want to change this to a validation as shown in the following code. From the s3-event.put.json file, the bucket name returned from the Lambda function is expected to be “sourcebucket.

    public void testLambdaFunctionHandler() {
        LambdaFunctionHandler handler = new LambdaFunctionHandler();
        Context ctx = createContext();
        Object output = handler.handleRequest(input, ctx);
        if (output != null) {
        Assert.assertEquals("sourcebucket", output);

    This is the simplest way to write the test case. When you run the unit test, output like that shown in the following screenshot will appear in the console.

  4. Upload and run the AWS Lambda function.You can also test the Lambda function after you upload it to AWS Lambda. To do this, right-click anywhere in the workspace of the project, choose Amazon Web Services, and choose Run function on AWS Lambda…, as shown in the following screenshot.

    You will be asked to select the JSON file as the S3Event input. Choose the default one provided by the AWS Toolkit for Eclipse, as shown in the following screenshot.

    Choose Invoke. You will see output similar to the following screenshot in the console. The function output is the bucket name returned by the Lambda function.

  5. Test the custom event Lambda function.
    The workflow for testing a custom event is very similar to testing the S3 Event. Let’s define a Lambda function that calculates the maximum value from a list of integer values.

    • First, define the custom event input class.
      public class CustomEventInput {
          private List<Integer> values;
          public List<Integer> getValues() {
              return values;
          public void setValues(List<Integer> values) {
              this.values = values;
    • Second, define the custom event output class.
      public class CustomEventOutput {
          private Integer value;
          public CustomEventOutput(int value) {
          public Integer getValue() {
              return value;
          public void setValue(Integer value) {
              this.value = value;
    • Third, implement the Lambda function.
      public CustomEventOutput handleRequest(CustomEventInput input, Context context) {
          context.getLogger().log("Input: " + input);
          int maxValue = Integer.MIN_VALUE;
          for (Integer value : input.getValues()) {
              if (value > maxValue) {
                  maxValue = value;
          return new CustomEventOutput(maxValue);
    • Fourth, prepare a sample JSON file as the CustomEventInput object for testing. AWS Lambda will use JSON format to represent THE object you defined. Here is an example using POJOs for handler input/output.
          "values" : [34, 52, 335, 32]
    • Lastly, upload this Lambda function to AWS Lambda, and test remotely. You should see console output similar to the following. The output is the JSON format of the CustomEventOutput object returned by the Lambda function.

This is how a typical Lambda function is written and tested using the AWS Toolkit for Eclipse. For more advanced use cases, you can use the S3 Event and DynamoDB Event examples provisioned by AWS Lambda.