Update README.md

Author: sigpwned

README.md

@@ -1,28 +1,35 @@
-# AWS Lambda Java 17 Custom Runtime (DEPRECATED)
+# AWS Lambda Java 21 Custom Runtime
 
-This project is no longer required, as [AWS Lambda now supports Java 17](https://aws.amazon.com/blogs/compute/java-17-runtime-now-available-on-aws-lambda/). I've left it here because others might find it's approach useful for other experimentation. 
+An AWS Lambda layer to enable Java 21 support.
 
+## Usage
 
-An AWS Lambda layer to enable Java 17 support
+### In AWS Console
 
-## Usage
+- Download [java21layer.zip](https://github.com/aleph0io/lambda-java21-layer/releases/download/v21.0.0-alpha.0/java21layer.zip)
+- Upload to S3 (required to create a layer from a ZIP file greater than 50MB in size)
+- In Lambda console, create a layer using this ZIP file using the `x86_64` architecture and the `Custom runtime on Amazon Linux 2` runtime
+- In Lambda console, create a function with `Provide your own bootstrap on Amazon Linux 2` and architecture `x86_64`
+- In Lambda function console, add the layer you created, and upload your [Java deployment package](https://docs.aws.amazon.com/lambda/latest/dg/java-package.html)
+- Run
+- (Hopefully) celebrate! 🥳
 
 ### with CDK
 
-- Download [java17layer.zip](https://github.com/msailes/lambda-java17-layer/releases/download/v0.0.1-alpha/java17layer.zip)
-- Create a Lambda layer using Code.fromAsset `java17layer.zip`
+- Download [java21layer.zip](https://github.com/aleph0io/lambda-java21-layer/releases/download/v21.0.0-alpha.0/java21layer.zip)
+- Create a Lambda layer using Code.fromAsset `java21layer.zip`
 - Note you might need to adjust the path for your own project structure
 
 ```java 
-LayerVersion java17layer = new LayerVersion(this, "Java17Layer", LayerVersionProps.builder()
-        .layerVersionName("Java17Layer")
-        .description("Java 17")
+LayerVersion java21layer = new LayerVersion(this, "Java21Layer", LayerVersionProps.builder()
+        .layerVersionName("Java21Layer")
+        .description("Java 21")
         .compatibleRuntimes(Arrays.asList(Runtime.PROVIDED_AL2))
-        .code(Code.fromAsset("java17layer.zip"))
+        .code(Code.fromAsset("java21layer.zip"))
         .build());
 ```
 
-- Create a function using the PROVIDED_AL2 runtime.
+- Create a function using the `PROVIDED_AL2` runtime.
 - Add this layer to your function.
 
 ```java
@@ -34,36 +41,42 @@ Function exampleWithLayer = new Function(this, "ExampleWithLayer", FunctionProps
         .code(Code.fromAsset("../software/ExampleFunction/target/example.jar"))
         .memorySize(512)
         .logRetention(RetentionDays.ONE_WEEK)
-        .layers(singletonList(java17layer))
+        .layers(singletonList(java21layer))
         .build());
 ```
 
 ## Layer Details
 
-### Java 17 
+### Java 21
 
 A custom JRE is created to reduce final file size. Lambda has a 250MB unzipped file size limit.
 
 [Dockerfile](Dockerfile) describes how the JRE is built.
 
+## Known Issues
+
+1. The layer fails [CDS](https://docs.oracle.com/javase/8/docs/technotes/guides/vm/class-data-sharing.html) (Class Data Sharing) during initialization. This creates some log traffic, and may adversely affect performance.
+2. It is only (lightly) tested
+3. Only x86_64 is supported
+
 ### JVM Settings
 
 The following JVM settings are added by default.
 
 ```
 --add-opens java.base/java.util=ALL-UNNAMED 
 -XX:+TieredCompilation -XX:TieredStopAtLevel=1 
--Xshare:on
+-Xshare:auto
 ```
 
 Further suggestions welcomed
 
 ### Java Class Path
 
 ```
-aws-lambda-java-runtime-interface-client-1.1.0.jar
-aws-lambda-java-core-1.2.1.jar
-aws-lambda-java-serialization-1.0.0.jar
+aws-lambda-java-core-1.2.3.jar
+aws-lambda-java-serialization-1.1.2.jar
+aws-lambda-java-runtime-interface-client-2.4.1.jar
 $LAMBDA_TASK_ROOT
 $LAMBDA_TASK_ROOT/*
 $LAMBDA_TASK_ROOT/lib/*
@@ -77,6 +90,10 @@ $LAMBDA_TASK_ROOT/lib/*
 
 ### Steps
 
-- Run `build-jre.sh` to build the minimal Java 17 runtime
+- Run `build-jre.sh` to build the minimal Java 21 runtime
 - Run `make-layer.sh` to package the runtime, dependencies and bootstrap as a zip
 
+## Acknowledgements
+
+Much credit to [@msailes](https://github.com/msailes), who created the original repo from which this project is forked!
+