JVM cannot find Agent_OnLoad in agent lib

Question

I am coding a Java Agent library. However, the JVM cannot find Agent_OnLoad function even if I have explicitly exported it in my C++ code.

JNIEXPORT jint JNICALL
    Agent_OnLoad(JavaVM* vm, char* options, void* reserved)
{
    return JNI_OK;
}

It then says Could not find Agent_OnLoad function in the agent library: ./FuncTest.dll. The start-up command line is java -agentpath:./FuncTest.dll -jar .\helloworld.jar

Using Visual Studio 2022, Corretto 17.0.8

Did you check your actual DLL to make sure the export is present and decorated correctly? You may need to use a .DEF file to name the export properly. It may be getting exported like _Agent_OnLoad or _Agent_OnLoad@12 instead of just plain Agent_OnLoad — Remy Lebeau
– Remy Lebeau, Commented Jul 13, 2024 at 4:58
Thanks @RemyLebeau, I looked up the DLL's functions using IDA and found out that the name was mangled. — Rabbit0w0
– Rabbit0w0, Commented Jul 13, 2024 at 5:08

Rabbit0w0 · Accepted Answer · 2024-07-13 05:13:07Z

1

Name decoration is applied to the exported function name.

By editing it to

extern "C" {
    JNIEXPORT jint JNICALL
        Agent_OnLoad(JavaVM* vm, char* options, void* reserved)
    {
        return JNI_OK;
    }
}

solves the problem.

See How to specify a "clean" name of DLL exports? for a detailed answer.

edited Jul 13, 2024 at 5:13

answered Jul 13, 2024 at 5:08

Rabbit0w0

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2 Comments

Remy Lebeau Over a year ago

That is literally the same code you posted originally. Why did you edit your question to change the code just to post the original code in this answer?

Rabbit0w0 Over a year ago

@RemyLebeau The original code is simplified from my project. When simplifying the code I modified it's structure wrongly.

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