Java Runtime now incorrectly reports Cannot read field "stderr" because "consoleOutput" is null

[TimothyJones]

Describe the bug

In the java version of jsii 1.109.0, the runner now reports:

Unexpected error in background thread "software.amazon.jsii.JsiiRuntime.ErrorStreamSink": java.lang.NullPointerException: Cannot read field "stderr" because "consoleOutput" is null

on every run.

The way the jsii Java runner works is that there's a thread that reads standard error for json objects. It reads each line, tries to parse them to json, and if it fails with an exception, it will print the line instead.

There's a small mistake in the runtime, where the buffer is flushed on EOF, even if it is empty. This would have failed the json parsing, and printed a newline to standard error (probably no one noticed this - I certainly didn't).

Unfortunately, Jackson-Databind 2.18.3 contains a change, where json nodes that aren't present will now return null instead of throwing an exception as they did previously. This changes the behaviour for empty string.

Regression Issue

• Select this option if this issue appears to be a regression.

Expected Behavior

No errors to be printed on a successful run

Current Behavior

The following error is printed to standard out

Unexpected error in background thread "software.amazon.jsii.JsiiRuntime.ErrorStreamSink": java.lang.NullPointerException: Cannot read field "stderr" because "consoleOutput" is null

Reproduction Steps

Run any java code that calls jsii with v1.109.0

Possible Solution

The fix is twofold:

1. Don't flush an empty buffer on EOF (ie, this line)
2. Handle a null return from objectMapper.treeToValue here

Additional Information/Context

Happy to produce a PR if you like

SDK version used

1.109.0

Environment details (OS name and version, etc.)

openjdk 23.0.1, node v22.9.0, MacOS 14.4.1

[TimothyJones]

Pull request #4802 raised to address this