I have a Lambda function written in Java I and I want it to access S3 (putObject).
I do not want to use or store credentials in my Lambda function in order to access S3. Instead, I would like to use IAM roles.
How can I code an AWS S3 client inside my java code (that would be ran by Lambda) that won't need any credentials and assume that the Lambda has the appropriate Role?
You don't need to store credentials in your lambda functions. All funtions run with a role - the role you set when you created the function. Since the lambda function has a role, you can add or remove permissions from this role as needed, without changing the function itself
Manage Permissions: Using an IAM Role (Execution Role)
Each Lambda function has an IAM role (execution role) associated with it. You specify the IAM role when you create your Lambda function. Permissions you grant to this role determine what AWS Lambda can do when it assumes the role. There are two types of permissions that you grant to the IAM role:
If your Lambda function code accesses other AWS resources, such as to read an object from an S3 bucket or write logs to CloudWatch Logs, you need to grant permissions for relevant Amazon S3 and CloudWatch actions to the role. If the event source is stream-based (Amazon Kinesis Streams and DynamoDB streams), AWS Lambda polls these streams on your behalf. AWS Lambda needs permissions to poll the stream and read new records on the stream so you need to grant the relevant permissions to this role.
http://docs.aws.amazon.com/lambda/latest/dg/intro-permission-model.html