Question about ClassNotFoundException

Hi, everyone! I found a java.lang.ClassNotFoundException when i use ElasticApm and my own agent.

I prepared a demo to reproduce it:

Elastic Apm version: 1.34.1 bytebuddy version: 1.12.10 (same with elastic apm)

1. i generate a springboot web demo in start.spring.io and new a DemoController

DemoController:

@RestController
public class DemoController {
    @GetMapping("/hello")
    public String hello(String message) {
        System.out.println("i am hello");
        return message;
    }
}

pom.xml:

......
	<properties>
		<java.version>17</java.version>
	</properties>
	<dependencies>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-web</artifactId>
		</dependency>
	</dependencies>

1. write a simple agent with bytebuddy, enhance DemoController#hello and print : “i am agent”, i load my agent with bootstrap classloader:

premain:

    public static void premain(String args, Instrumentation inst) {
        File jar = new File("my agent path");
        try {
            inst.appendToBootstrapClassLoaderSearch(new JarFile(jar));
            // use bootstrap classloader
            Class.forName("Myagent", true, null)
                    .getMethod("main", Instrumentation.class)
                    .invoke(null, inst);
    }

Myagent:

    public static void main(Instrumentation inst) {
        new AgentBuilder.Default()
                .with(AgentBuilder.LocationStrategy.ForClassLoader.WEAK)
                .with(AgentBuilder.RedefinitionStrategy.RETRANSFORMATION)
                .with(AgentBuilder.LocationStrategy.ForClassLoader.WEAK)
                .ignore(ElementMatchers.<TypeDescription>nameStartsWith("net.bytebuddy.")
                        .or(ElementMatchers.<TypeDescription>nameStartsWith(".asm."))
                        .or(nameStartsWith("java."))//rt.jar bootstrap classloader
                        .or(nameStartsWith("com.sun.")))
                .type(ElementMatchers.<TypeDescription>named("com.example.demo.DemoController"))
                .transform(new AgentBuilder.Transformer() {
                    @Override
                    public DynamicType.Builder<?> transform(DynamicType.Builder<?> builder, TypeDescription typeDescription, ClassLoader classLoader, JavaModule module) {
                        return builder
                                .method(ElementMatchers.named("hello"))
                                .intercept(MethodDelegation.to(new Interceptor()));
                    }
                })
                .installOn(inst);
    }

Interceptor:

    @RuntimeType
    public String interceptor(@SuperCall Callable<?> zuper) throws Exception {
        System.out.println("i am interceptor");
        zuper.call();
        return "";
    }

Here are some experiments I did:

1. jdk version: 17.05, jvm options: -javaagent:Myagent.jar -javaagent:elastic-apm-agent-1.34.1.jar curl localhost:8080/hello will thrown exception:

java.lang.ClassNotFoundException: com.example.demo.DemoController$auxiliary$0QZj0xRL
	at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:641) ~[na:na]
	at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:188) ~[na:na]
	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:520) ~[na:na]
	at com.example.demo.DemoController.hello(DemoController.java) ~[classes/:na]

1. if premain use AppClassloader load Myagent, it works great:

    public static void premain(String args, Instrumentation inst) {
        File jar = new File("my agent path");
        try {
            inst.appendToBootstrapClassLoaderSearch(new JarFile(jar));
            // use bootstrap classloader
            Class.forName("Myagent", true, ClassLoader.getSystemClassLoader())
                    .getMethod("main", Instrumentation.class)
                    .invoke(null, inst);
    }

1. if i use elastic-apm-agent-1.26.0.jar, it work great: jdk version: 17.05, jvm options: -javaagent:Myagent.jar -javaagent:elastic-apm-agent-1.26.0.jar

2. if i use jdk8 and elastic-apm-agent-1.34.1.jar, it work great: jdk version: 1.8.0_211, jvm options: -javaagent:Myagent.jar -javaagent:elastic-apm-agent-1.34.1.jar

i think when i use elastic-apm-agent-1.34.1 in jdk17 with Myagent，bytebuddy can’t inject auxiliary class to app classloader？ Can you help me? thank

[raphw]

The problem will be that this injects the Nexus class into the boot loader. However, the NexusAccessor is explicitly aiming for the system class loader, so this should not be a problem?

[LeePui]

The problem will be that this injects the Nexus class into the boot loader. However, the NexusAccessor is explicitly aiming for the system class loader, so this should not be a problem?

Thank you very much！I think I have probably understood the general logic. Thank you for your guidance. If necessary later, I will provide a reproduction