Synthesized annotation toString() doesn't match non-synthesized annotation on Java 9+

[jeff-miller-cfa]

Synthesized annotations that contain arrays and/or strings do not generate the same toString() value as the non-synthesized version.

The non-synthesized annotation's toString() method returns a string which is a code equivalent version of that annotation meaning that you can take the result of the toString(), copy and paste it into code, and have it compile. This is very convenient when writing a code generation library since it allows you to copy annotations.

This does not work with synthesized annotations though as the way the array gets serialized in toString() doesn't match.

Actual Annotation:

@Bean("testValue")

Non-Synthesized Annotation toString():

@org.springframework.context.annotation.Bean(value = {"testValue"})

Synthesized Annotation toString():

@org.springframework.context.annotation.Bean(value = [testValue])

spring-framework/spring-core/src/main/java/org/springframework/core/annotation/SynthesizedMergedAnnotationInvocationHandler.java

Line 202 in a0c97e4

StringBuilder builder = new StringBuilder("[");

[sbrannen]

This is actually by design.

The result of invoking toString() on a synthesized annotation is identical to the result of invoking toString() on the same non-synthesized annotation on Java 8. In fact we used to have tests in place to verify that, but those broke once we started building the framework on Java 9, since Java 9 changed the output format for toString() in annotations.

See:

• Out-of-the-box build compatibility with JDK 9 build 136 [SPR-14645] #19211
• commit faf6e5d

In any case, it does not appear that the JDK format has changed since Java 9, so I'll go ahead and update to the newer format.

[jhoeller]

Ah ok, I wasn't aware of the change between JDK 8 and 9 there. If 5.3.x is currently aligned with the JDK 8 toString format there, we could also simply upgrade to the JDK 9+ format for 6.0.

[sbrannen]

Interestingly enough, it turns out there are actually 2 bugs (🐛) lurking behind the scenes here.

1. Spring's toString() for synthesized annotations in fact did not match the JDK 8 behavior for class attributes. Spring used Class#getName; JDK 8 used Class#toString.
2. JDK 9+ incorrectly invokes Enum#toString for enum attributes; whereas, the commit I am about to push will use Enum#name, since an enum can in fact override toString() (for example, with return name().toLowerCase()).

I'll see if the OpenJDK team is willing to update the toString() implementation for annotations to use Enum#name.

[sbrannen]

I'll see if the OpenJDK team is willing to update the toString() implementation for annotations to use Enum#name.

See: https://bugs.openjdk.java.net/browse/JDK-8281462

[sbrannen]

Reopening to address additional shortcomings in the toString() implementation for synthesized annotations.