The code of my program to be attached is as below.
public class Foo {
}
public class TestEntry {
public TestEntry() {
}
public static void main(String[] args) throws Exception {
try
{
while(true)
{
System.out.println(new Foo().toString());
Thread.sleep(1000);
}
}
catch(Exception e)
{}
}
}
What I attempt to do is to make Foo.toString() returns 'test' by using the following agent.
public class InjectionAgent {
public InjectionAgent() {
}
public static void agentmain(String args, Instrumentation inst) throws Exception
{
System.out.println("agentmain Args:" + args);
new AgentBuilder.Default()
.type(ElementMatchers.named("Foo"))
.transform(new AgentBuilder.Transformer() {
@Override
public Builder<?> transform(Builder<?> arg0, TypeDescription arg1,
ClassLoader arg2, JavaModule arg3) {
return arg0.method(ElementMatchers.named("toString"))
.intercept(FixedValue.value("test"));
}
}).installOn(inst);
}
public static void premain(String args, Instrumentation inst) throws Exception
{
System.out.println("premain Args:" + args);
new AgentBuilder.Default()
.type(ElementMatchers.named("Foo"))
.transform(new AgentBuilder.Transformer() {
@Override
public Builder<?> transform(Builder<?> arg0, TypeDescription arg1,
ClassLoader arg2, JavaModule arg3) {
return arg0.method(ElementMatchers.named("toString"))
.intercept(FixedValue.value("test"));
}
}).installOn(inst);
}
}
I notice that, it was successful when I using -javaagent way, whereas attach way failed, here is code for attach.
public class Injection {
public Injection() {
}
public static void main(String[] args) throws AttachNotSupportedException, IOException, AgentLoadException, AgentInitializationException, InterruptedException {
VirtualMachine vm = null;
String agentjarpath = args[0];
vm = VirtualMachine.attach(args[1]);
vm.loadAgent(agentjarpath, "This is Args to the Agent.");
vm.detach();
}
}
I tried to add AgentBuilder.Listener.StreamWriting.toSystemOut() to the agent, after attaching, the output of TestEntry shows
[Byte Buddy] DISCOVERY Foo [sun.misc.Launcher$AppClassLoader@33909752, null, loaded=true]
[Byte Buddy] TRANSFORM Foo [sun.misc.Launcher$AppClassLoader@33909752, null, loaded=true]
[Byte Buddy] COMPLETE Foo [sun.misc.Launcher$AppClassLoader@33909752, null, loaded=true]
Foo@7f31245a
Foo@6d6f6e28
Foo@135fbaa4
Foo@45ee12a7
Foo@330bedb4
==================================Update===================================== I defined a public method 'Bar' in Foo like this
public class Foo {
public String Bar()
{
return "Bar";
}
}
and then I was trying to make Foo.Bar() returns "modified" in the following way:
public static void agentmain(String args, Instrumentation inst) throws Exception
{
System.out.println("agentmain Args:" + args);
premain(args, inst);
new AgentBuilder.Default()
.with(RedefinitionStrategy.RETRANSFORMATION)
.disableClassFormatChanges()
.with(AgentBuilder.Listener.StreamWriting.toSystemOut())
.type(ElementMatchers.named("Foo"))
.transform(new AgentBuilder.Transformer() {
@Override
public Builder<?> transform(Builder<?> arg0, TypeDescription arg1,
ClassLoader arg2, JavaModule arg3) {
return arg0.visit(Advice.to(InjectionTemplate.class).on(ElementMatchers.named("Bar")));
}
})
.installOn(inst);
}
static class InjectionTemplate {
@Advice.OnMethodExit
static void exit(@Advice.Return String self) {
System.out.println(self.toString() + " " + self.getClass().toString());
self = new String("modified");
}
}
but I got this error:
java.lang.IllegalStateException: Cannot write to read-only parameter class java.lang.String at 1
any suggestions?
