Introduce support for escaping the escape character for property placeholders

[bitb4ker]

Up until spring-boot 3.3.8, an escaped backslash followed by a placeholder like:

prop1=value1  
prop2=value2\\${prop1}  

Resulted in prop2 resolving to:

value2\value1

Starting with spring-boot 3.4.0, the result is:

value2value1

Doubling the backslashes like this:

prop1=value1  
prop2=value2\\\\${prop1}  

Results in:

value2${prop1}

This is due to the escaping logic introduced in org.springframework.util.PlaceholderParser which does two passes of PlaceholderParser.SimplePlaceholderPart.resolveRecursively(PartResolutionContext, String), each calling PlaceholderParser.parse(String, boolean) in which the escaping logic is implemented.

This last method does not handle escaping the escape character, causing the issue.

[snicoll]

The parser in 6.2 has been rewritten to fix, amongst other things, #9628. Unfortunately, this means that if the placeholder is escaped, it's rendered as is (i.e. not evaluated).

I wrote a test:

@Test
void gh34315() {
    Properties properties = new Properties();
    properties.setProperty("prop1", "value1");
    properties.setProperty("prop2", "value2\\${prop1}");
    PlaceholderParser parser = new PlaceholderParser("${", "}", ":", '\\', true);
    assertThat(parser.replacePlaceholders("${prop2}", properties::getProperty)).isEqualTo("value2${prop1}");
}

and it passes. I've also added those two properties in an empty Spring 6.2 app and it resolved the same way. Can you share a sample that actually fails the way you've described?

[bitb4ker]

props-spring-3.3.zip

props-spring-3.4.zip

To run:

gradlew bootRun

The 3.3 one shows the expected behavior, the 3.4 shows what I described as the broken behavior in both '\' and '\\' in the original post.

[snicoll]

I got it now. The problem is PropertySourcesPlaceholderConfigurer getting in the way of the placeholder resolution. It does its own round, removes the backslash to render the placeholder as is. As a result, the parser continues its work and sees a new placeholder to resolve.

[snicoll]

The issue as reported can't be fixed without re-introducing the bug that we fixed. The side effect that was found as part of the report is going to be handled by #34326.

For that case above, you'll need to restructure your configuration to move the backslash elsewhere to avoid the escaping, something like:

prop1=\\value1
prop2=value2${prop1}

[bitb4ker]

Couldn't we introduce a way to escape the escape character?
This is essentially the root of the issue I raised.
Why does it break previous fixes?