Add support for list value resolution from YAML block-style in Environment.getProperty()

[xyraclius]

Overview

This PR enables Environment.getProperty() to retrieve list values defined using YAML block-style notation, aligning its behavior with the already-supported inline style.

Previously, this method supported only inline list syntax (key: i,like,fish) but failed when the YAML used block-style notation:

key:
  - i
  - like
  - fish

As a result, calling:

List<String> values = environment.getProperty("key", (Class<List<String>>) List.class);

would return null for block-style YAML. The issue is tracked in GitHub issue #35179.

Spring’s Environment.getProperty() currently supports retrieving List<String> values only when defined as a comma-separated string. YAML block-style list definitions, which are semantically equivalent, are ignored.

✅ YAML Supported (Inline):

first:
  second: i,like,fish

This works as expected:

List<String> values = environment.getProperty("first.second", List.class);

❌ YAML Not Supported (Block-style):

first:
  second:
    - i
    - like
    - fish

This returns null for the same code:

List<String> values = environment.getProperty("first.second", List.class); 

The Solution

This PR enhances property resolution logic to support indexed keys such as:

first.second[0]=i
first.second[1]=love
first.second[2]=spring

This format corresponds to block-style YAML and is now properly aggregated into a List when calling:

environment.getProperty("first.second", List.class);

Internally, PropertySource#getProperty() is updated to collect indexed entries into a list when a direct match is not found.

Test Coverage

This fix is verified by the following test cases:

@Test
void returnsListOfStringsFromIndexedKeys() { ... }

@Test
void returnsListOfMixedTypesFromIndexedKeys() { ... }

@Test
void returnsListOfIntegersFromIndexedKeys() { ... }

@Test
void returnsNullWhenNoDirectMatchAndNoIndexedKeys() { ... }

These tests ensure list values can be resolved from indexed keys when using Environment.getProperty().

Related Issue

• Fixes: #35179

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• [x] I have read the Spring Framework contribution guidelines
• [x] The fix is verified by a test cases
• [x] I included a Signed-off-by trailer in the commit
• [x] I avoided unrelated formatting changes
• [x] I added myself as the author where applicable