YamlPropertiesFactoryBean incorrect flatten nested map to properties

[remeio]

To fix issue #27020.

For the method org.springframework.beans.factory.config.YamlProcessor#getFlattenedMap before this PR:

• When the key is start with [, will return path + key as map key.
• Otherwise, will return path + '.' + key as map key.

if (key.startsWith("[")) {
	key = path + key;
}
else {
	key = path + '.' + key;
}

For the method org.springframework.beans.factory.config.YamlProcessor#getFlattenedMap after this PR:

• When the key is start with [ or end with ], will return path + '[' + key + ']' as map key.
• Otherwise, will return path + '.' + key as map key.
• And when the key is indexed key at Collection like [0], will return path + key as map key.

For the method org.springframework.beans.factory.config.YamlProcessor#asMap after this PR:
Change result.put("[" + key.toString() + "]", value); to result.put(key.toString(), value); on asMap.

And the test case you can see:

root:
  webservices:
    "[domain.test:8080]":
      - username: foo
        password: bar
# Before this PR:
# root.webservices[domain.test:8080][0].username=foo
# root.webservices[domain.test:8080][0].password=bar
# After this PR:
# root.webservices[[domain.test:8080]][0].username=foo
# root.webservices[[domain.test:8080]][0].password=bar

indexed:
  "[0]": foo
  "[1]": bar
indexed2:
  - "[a]": foo
    "[b]": bar
# Before this PR:
# indexed[0]=foo
# indexed[1]=bar
# indexed2[0][a]=foo
# indexed2[0][b]=bar
# After this PR:
# indexed[[0]]=foo
# indexed[[1]]=bar
# indexed2[0][[a]]=foo
# indexed2[0][[b]]=bar

only-left-bracket:
  "[/key1/": foo
only-right-bracket:
  "/key1/]": foo
special-bracket:
  "][/key1/][": foo
# Before this PR:
# only-left-bracket[/key1/=foo
# only-right-bracket./key1/]=foo
# special-bracket.][/key1/][=foo
# After this PR:
# only-left-bracket[[/key1/]=foo
# only-right-bracket[/key1/]]=foo
# special-bracket.][/key1/][=foo


number-key:
  1: foo
# Before this PR:
# number-key[1]=bar
# After this PR:
# number-key.1=foo

By the way, org.springframework.beans.factory.config.YamlMapFactoryBean#getObject result is:

# Before this PR & After this PR:
# root={webservices={[domain.test:8080]=[{username=foo, password=bar}]}}
# indexed={[0]=foo, [1]=bar}
# indexed2=[{[a]=foo, [b]=bar}]
# only-left-bracket={[/key1/=foo}
# only-right-bracket={/key1/]=foo}
# special-bracket={][/key1/][=foo}

# Before this PR:
# number-key={[1]=foo}
# After this PR:
# number-key={1=foo}

[remeio]

This PR just is a try, I submit the test case for []. It's important to explain what result is correct for this test case. Maybe you can fill it with:

root:
  webservices:
    "[domain.test:8080]":
      - username: foo
        password: bar
# properties should be?
# map should be?

indexed:
  "[0]": foo
  "[1]": bar
indexed2:
  - "[a]": foo
    "[b]": bar
# properties should be?
# map should be?

only-left-bracket:
  "[/key1/": foo
only-right-bracket:
  "/key1/]": foo
special-bracket:
  "][/key1/][": foo
# properties should be?
# map should be?

number-key:
  1: foo
# properties should be?
# map should be?

By the way, If We want to use @ConfigurationProperties on Spring Boot, we need to config yaml file with following the document Binding Maps. We can see:

When binding to Map properties you may need to use a special bracket notation so that the original key value is preserved.