Optimize ClassNameReader.getClassName via direct ASM API

[cookie-meringue]

Change

ClassNameReader.getClassName now calls ClassReader.getClassName() directly instead of delegating to the visitor-based getClassInfo(r)[0].

Before

public static String getClassName(ClassReader r) {
    return getClassInfo(r)[0];
}

After

// SPRING PATCH BEGIN
public static String getClassName(ClassReader r) {
    return r.getClassName().replace('/', '.');
}
// SPRING PATCH END

Motivation

ClassNameReader.getClassName is a small utility that extracts the FQCN from class bytecode (e.g. byte[] → "com.example.MyService").
It sits on the hot path of every CGLIB proxy class definition.

The previous implementation did all of the following:

• Allocated an ArrayList and a ClassVisitor.
• Walked the class header, UTF-8 decoding this_class, super_class, and every interface name.
• Called replace('/', '.') and array.add() on each name.
• Threw EARLY_EXIT and caught it to break out of the visitor.
• Built the result array with toArray(new String[0]).

After all that work, the caller used only [0] of the result — super_class and the interface names were thrown away.

public static String getClassName(ClassReader r) {
    return getClassInfo(r)[0];
}

Impact

Per call, the new path:

• Cuts object allocations from ~7 down to 2.
• Skips UTF-8 decoding of super_class and every interface name.
• Removes the visitor dispatch and the throw / catch round-trip.

JMH measurements

ClassNameReaderBenchmark (5 forks × 5 warmup × 5 measurement)

Without additional interfaces — proxy = extends ProxyTarget implements Factory

Path	Score	Memory Allocation
getClassInfo(r)[0]	216.9 ns/op	816 B/op
r.getClassName().replace('/', '.')	22.0 ns/op	432 B/op
Improvement	9.9× faster	1.9× less (-47%)

With 3 additional interfaces — Cloneable, Comparable, Runnable

Path	Score	Memory Allocation
getClassInfo(r)[0]	314.5 ns/op	1024 B/op
r.getClassName().replace('/', '.')	21.7 ns/op	432 B/op
Improvement	14.5× faster	2.4× less (-58%)

How to run

./gradlew :spring-core:jmhJar
java -jar spring-core/build/libs/spring-core-<version>-jmh.jar \
    "ClassNameReaderBenchmark" -prof gc

On the current main branch, <version> is 7.1.0-SNAPSHOT.

The new path's time and allocation stay constant regardless of how many interfaces the proxy declares. The legacy path, by contrast, decodes every interface name from the class header and runs the extra replace / add work — so the gap widens as the proxy implements more interfaces.