Spring AOP cannot generate proxy for lambda on Java 16+

I’m migrating to Java 17 and found an issue with proxy generation for ‘lambda beans’.

• Spring Boot: 2.6.3
• Maven: 3.8.4
• Java:

openjdk version "17.0.2" 2022-01-18 LTS
OpenJDK Runtime Environment Corretto-17.0.2.8.1 (build 17.0.2+8-LTS)
OpenJDK 64-Bit Server VM Corretto-17.0.2.8.1 (build 17.0.2+8-LTS, mixed mode, sharing)

Spring AOP cannot generate a proxy for beans like:

    @Bean
    public Supplier<String> stringSupplier() {
        return () -> "lambda supplier value";
    }

Errors:

Caused by: org.springframework.aop.framework.AopConfigException: 
Could not generate CGLIB subclass of class com.example.issue.aopissue.service.IssueConfiguration$$Lambda$638/0x0000000800e796e0: 
Common causes of this problem include using a final class or a non-visible class; 
nested exception is org.springframework.cglib.core.CodeGenerationException: java.lang.reflect.InaccessibleObjectException--
>Unable to make protected final java.lang.Class java.lang.ClassLoader.defineClass(java.lang.String,byte[],int,int,java.security.ProtectionDomain) 
throws java.lang.ClassFormatError accessible: module java.base does not "opens java.lang" to unnamed module @5ecddf8f

But if I use class implementation it works fine:

public class TestSupplierService implements Supplier<String> {

    @Override
    public String get() {
        return "class supplier value";
    }
}

I prepared demo project aop-issue to show the issue:

• Application Configuration
• Aspect Configuration
• Test with lambda bean, context doesn’t start
• Test with clas implementation, works as expected

[sbrannen]

I’m guessing that the JDK thinks that the com.example.issue.aopissue.service.IssueConfiguration$$Lambda$638/0x0000000800e796e0 type resides in same package as java.util.function.Supplier and therefore in the java.lang module instead of being local to your module, and I’m wondering if that’s perhaps a bug in the JDK.

For the sake of clarity, after investigating this issue, there does not appear to be an issue with the JDK. Rather:

• Spring AOP is attempting to create a CGLIB-based proxy for the lambda expression.
• You therefore need --add-opens java.base/java.lang=ALL-UNNAMED on Java 16 or higher in order to allow CGLIB to create the class-based proxy for the lambda expression.

Note that --add-opens java.base/java.lang=ALL-UNNAMED is not necessary on Java 9 through Java 15.

For the time being, people can use the --add-opens workaround.

However, the Spring team will investigate a way to avoid the creation of CGLIB-based proxies for lambda expressions. A JDK dynamic proxy should always suffice for a class that can only ever be used via the functional interface it implements. Dynamic proxies should also be used for lambda expressions and method references even if proxyTargetClass has been set to true.

[Ferioney]

@sbrannen checked and it worked as expected. Thank you!