Add compile-time null safety with @NullMarked and NullAway

[jbl428]

Describe the feature you request

1. Adding package-level null safety configuration - Add package-info.java files with @NullMarked annotation from JSpecify to establish non-null as the default for all elements in each package
2. Integrating NullAway static analysis - Add NullAway as a compile-time checker to automatically detect potential null pointer exceptions and enforce proper null handling throughout the codebase

ref:

• https://docs.spring.io/spring-framework/reference/7.0/core/null-safety.html
• Add JSpecify nullability annotations spring-projects/spring-boot#46587

(Optional): Suggest A Solution

Proposed implementation approach:

• Add package-info.java files to all existing packages with @NullMarked annotation
• NullAway integration

Example package-info.java:

@NullMarked
package com.example.project.core;

import org.jspecify.annotations.NullMarked;

If the feature request is approved, would you be willing to submit a PR?

Yes

[seongahjo]

@jbl428
I think it's a great feature!

PR is welcome.

[jbl428]

@seongahjo
I looked into the NullAway documentation and found that it requires a minimum ErrorProne version of 2.14.0. After testing, it appears that at least JDK 11 is needed for it to work properly.

Since this project uses JDK 8, we would need to find alternative tools that support JDK 8 instead of NullAway for null safety checking.

[jbl428]

Looking at the JSpecify nullness checker support list, I found that the Checker Framework could be a good alternative. It supports @Nullable and @NonNull annotations, and since NonNull is the default, we wouldn't need to create package-info.java files in every package with Java files, which makes it more convenient to use.