As the Java 8 Lambda libraries are statically compiled into jars, it's not possible to just swap in a mock interface source file, as it is in the other dynamic runtimes, without compiling first.

The Dockerfile here will build using a patched LambdaSandboxJava-1.0.jar, which is checked into git. This jar was built using the jar from the Lambda runtime, /var/runtime/lib/LambdaSandboxJava-1.0.jar, with a single class (lambdainternal/LambdaRuntime.class) updated to use local/mock methods instead of native ones.

The build script to perform this patch/update is at ./lambda-runtime-mock/build.sh