Premain

package org.example;

import net.bytebuddy.agent.builder.AgentBuilder;
import net.bytebuddy.asm.Advice;
import net.bytebuddy.description.type.TypeDescription;
import net.bytebuddy.dynamic.DynamicType;
import net.bytebuddy.matcher.ElementMatchers;
import net.bytebuddy.utility.JavaModule;

import java.io.IOException;
import java.lang.instrument.Instrumentation;
import java.net.URISyntaxException;
import java.util.jar.JarFile;

public class Main {
    public static void premain(String arg, Instrumentation inst) {
        System.out.println("FileOutputStream intercepted start!");
        try {
            inst.appendToBootstrapClassLoaderSearch(new JarFile(AgentJarLocator.getAgentJarFile()));
            mode2(inst);
        } catch (IOException e) {
            throw new RuntimeException(e);
        } catch (URISyntaxException e) {
            throw new RuntimeException(e);
        }
        System.out.println("FileOutputStream intercepted end!");
    }
    private static void mode2(Instrumentation instrumentation) throws IOException {
        new AgentBuilder.Default()
                .disableClassFormatChanges()
                .with(AgentBuilder.RedefinitionStrategy.RETRANSFORMATION)
                .with(new AgentBuilder.Listener() {
                    @Override
                    public void onDiscovery(String typeName, ClassLoader classLoader, JavaModule module, boolean loaded) {
                       // System.out.println("Discovered Type: " + typeName);
                    }

                    @Override
                    public void onTransformation(TypeDescription typeDescription, ClassLoader classLoader, JavaModule module, boolean loaded, DynamicType dynamicType) {
                        System.out.println("Transformed: " + typeDescription);
                    }

                    @Override
                    public void onIgnored(TypeDescription typeDescription, ClassLoader classLoader, JavaModule module, boolean loaded) {}

                    @Override
                    public void onError(String typeName, ClassLoader classLoader, JavaModule module, boolean loaded, Throwable throwable) {
                        throwable.printStackTrace();
                    }

                    @Override
                    public void onComplete(String typeName, ClassLoader classLoader, JavaModule module, boolean loaded) {}
                })
                .type(ElementMatchers.named("java.io.FileOutputStream"))
                .transform((builder, typeDescription, classLoader, module,v) ->
                        builder.visit(Advice.to(FileOutputStreamConstructorAdvice.class)
                                .on(ElementMatchers.isConstructor())))
                .installOn(instrumentation);
    
    }
}

AgentJarLocator

package org.example;

import java.io.File;
import java.net.URISyntaxException;
import java.net.URL;
import java.security.CodeSource;
import java.security.ProtectionDomain;
public class AgentJarLocator {
    static File getAgentJarFile() throws URISyntaxException {
        ProtectionDomain protectionDomain = Main.class.getProtectionDomain();
        CodeSource codeSource = protectionDomain.getCodeSource();
        if (codeSource == null) {
            throw new IllegalStateException(String.format("Unable to get agent location, protection domain = %s", protectionDomain));
        }
        URL location = codeSource.getLocation();
        if (location == null) {
            throw new IllegalStateException(String.format("Unable to get agent location, code source = %s", codeSource));
        }
        final File agentJar = new File(location.toURI());
        if (!agentJar.getName().endsWith(".jar")) {
            throw new IllegalStateException("Agent is not a jar file: " + agentJar);
        }
        return agentJar.getAbsoluteFile();
    }
}

Advice

package org.example;

import net.bytebuddy.asm.Advice;

public class FileOutputStreamConstructorAdvice {
    @Advice.OnMethodEnter
    public static void onEnter(@Advice.AllArguments Object[] args) {
        System.out.println("FileOutputStream constructor called with arguments: " + java.util.Arrays.toString(args));
    }
}

build.gradle

plugins {
    id 'java'
}

archivesBaseName = 'my-agent'
group = 'org.example'
version = '1.0'

repositories {
    mavenCentral()
}

dependencies {
    testImplementation platform('org.junit:junit-bom:5.9.1')
    testImplementation 'org.junit.jupiter:junit-jupiter'

    implementation group: 'net.bytebuddy', name: 'byte-buddy', version: '1.14.9'
    implementation group: 'net.bytebuddy', name: 'byte-buddy-agent', version: '1.14.9'
}

test {
    useJUnitPlatform()
}
task fatJar(type: Jar) {
    duplicatesStrategy = DuplicatesStrategy.EXCLUDE
    manifest {
        attributes (
                "Class-path": configurations.compileClasspath.collect { it.getName() }.join(' '),
                "Premain-Class": "org.example.Main",
                "Agent-Class": "org.example.Main",
                "Can-Retransform-Classes": "true",
                "Can-Redefine-Classes": "true",
                "Implementation-Version": rootProject.version,
        )
    }
    from { configurations.runtimeClasspath.collect { it.isDirectory() ? it : zipTree(it) } }
    with jar
}

VMOption

--add-opens java.base/java.io=ALL-UNNAMED -javaagent:"E:DevelopmentProjectsmy-agent-1.0.jar"

Getting below exception

Exception in thread "main" java.lang.reflect.InvocationTargetException
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.base/java.lang.reflect.Method.invoke(Method.java:566)
        at java.instrument/sun.instrument.InstrumentationImpl.loadClassAndStartAgent(InstrumentationImpl.java:513)
        at java.instrument/sun.instrument.InstrumentationImpl.loadClassAndCallPremain(InstrumentationImpl.java:525)
    Caused by: java.lang.IllegalAccessError: failed to access class org.example.Main$1 from class org.example.Main (org.example.Main$1 is in unnamed module of loader 'bootstrap'; org.example.Main is in unnamed module of loader 'app')
        at org.example.Main.mode2(Main.java:40)
        at org.example.Main.premain(Main.java:29)
        ... 6 more
    *** java.lang.instrument ASSERTION FAILED ***: "result" with message agent load/premain call failed at ./src/java.instrument/share/native/libinstrument/JPLISAgent.c line: 422