how can we new proxied object and set it into a field? about byte-buddy

[raphw]

Currently, you can use the MethodCall instrumentation which is composable. For field access, there is no short cut way to do explicit instrumentations yet. Note that you need to call the super constructor in any constructor. It seems like you use a MethodConstant where you probably want to invoke a constructor. This you can do via a MethodInvocation in combination with a TypeCreation.

[renjie-rm]

I would like to add the following code snippet to a class

public String traceId = ContextUtil.get(DidiContextEnum.TRACE_ID.name);
Can it be done?
@raphw

[raphw]

You would need to define a field via defineField and then set the field via any constructor. You can for example use Advice to add this code to the end or beginning of each constructor.

[renjie-rm]

@raphw
First defineField,
then enhanced before and after the constructor is executed？

[renjie-rm]

I can't define fields and specify initialization code blocks? for example

new AgentBuilder.Default()
.type(named("com.Template"))
.transform((builder, typeDescription, classLoader, module) -> {
DynamicType.Builder.FieldDefinition.Optional.Valuable<?> name = builder.defineField("name", String.class, Modifier.PUBLIC);
// initialization code blocks. name = context.getname();
return name;
})
.installOn(inst);

[raphw]

You can do both, but a non-static field must be initialized in a constructor.

For example, use Advice as follows:

static void exit(@FieldValue(value = "name", readOnly = false) String name) {
  name = ContextUtil.get(DidiContextEnum.TRACE_ID.name);
}

and then apply the visit method with Advice.to(MyAdvice.class).on(isConstructor()).

[renjie-rm]

ByteBuddyAgent.install();
new ByteBuddy()
.redefine(Source.class)
.defineField("name", String.class, Modifier.PUBLIC)
.make()
.load(Source.class.getClassLoader());

Exception in thread "main" java.lang.IllegalStateException: Class already loaded: class com.example.a.Source
at net.bytebuddy.dynamic.loading.ByteArrayClassLoader.load(ByteArrayClassLoader.java:307)
at net.bytebuddy.dynamic.loading.ClassLoadingStrategy$Default$WrappingDispatcher.load(ClassLoadingStrategy.java:357)
at net.bytebuddy.dynamic.loading.ClassLoadingStrategy$Default.load(ClassLoadingStrategy.java:143)
at net.bytebuddy.dynamic.TypeResolutionStrategy$Passive.initialize(TypeResolutionStrategy.java:100)
at net.bytebuddy.dynamic.DynamicType$Default$Unloaded.load(DynamicType.java:5686)
at net.bytebuddy.dynamic.DynamicType$Default$Unloaded.load(DynamicType.java:5675)
at com.example.a.Main.main(Main.java:19)

@raphw

[raphw]

Do you want to add the field or create a new class? If you are doing the latter, you should rename the class. For the former, look into Java agents.

[renjie-rm]

I thought this would add fields to the original class

[renjie-rm]

Instrumentation install = ByteBuddyAgent.install();
new AgentBuilder.Default()
.type(ElementMatchers.is(Source.class))
.transform((builder, typeDescription, classLoader, javaModule) ->
builder.defineField("name", String.class, Modifier.PUBLIC))
.installOn(install);

Instrumentation install = ByteBuddyAgent.install();
new AgentBuilder.Default()
.type(ElementMatchers.named("com.example.a.Source"))
.transform((builder, typeDescription, classLoader, javaModule) ->
builder.defineField("name", String.class, Modifier.PUBLIC))
.installOn(install);

Don't these two codes do the same thing?@raphw

[raphw]

Once you load a class via ElementMatchers.is(Source.class), its layout is set and the JVM does no longer allow adding fields to it.

You would need to add a matcher that does not load the class, for example via named("..."). Additionally, you'd need to run this code before you load the class in other code.

[renjie-rm]

You can do both, but a non-static field must be initialized in a constructor.

For example, use Advice as follows:

static void exit(@FieldValue(value = "name", readOnly = false) String name) {
  name = ContextUtil.get(DidiContextEnum.TRACE_ID.name);
}

and then apply the visit method with Advice.to(MyAdvice.class).on(isConstructor()).

       new AgentBuilder.Default()
           .type(ElementMatchers.named("com.example.a.Source"))
           .transform((builder, typeDescription, classLoader, javaModule) ->
               builder.defineField("name", String.class, Modifier.PUBLIC)
                   .visit(Advice.to(MyAdvice.class).on(MethodDescription::isConstructor)))
           .installOn(install);

@raphw There is something wrong with this code

[raphw]

Have you imported the correct @FieldValue annotation? It needs to be the Advice related one. You can register an AgentBuilder.Listener.StreamWriting.toSystemOut() in the agent builder. Otherwise, you will not see any error messages.

[renjie-rm]

public class MyAdvice {

static void exit(@Advice.FieldValue(value = "name", readOnly = false) String name) {
    name = "andre";
}

}

[renjie-rm]

@GetMapping
public Source test() {
return new Source();
}

before I add this code. Visit (Advice to (MyAdvice. Class.) on (MethodDescription: : isConstructor))
curl localhost. return {"id":null,"name":null}

add this code Visit (Advice to (MyAdvice. Class.) on (MethodDescription: : isConstructor))
curl localhost. return {"id":null}
@raphw

[raphw]

What does your error log say? You can add it as I suggested.

[renjie-rm]

I didn't add @advice. OnMethodExit to the exit method,
java.lang.IllegalArgumentException: No advice defined by class
I can only initialize the name variable after the constructor, right？
@raphw

[raphw]

You can write to fields in an enter advice if that's what you mean.

[renjie-rm]

@Advice.OnMethodEnter
    static void exit( @Advice.FieldValue(value = "name", readOnly = false) String s) {
        s = "andre";
    }

java.lang.IllegalStateException: Cannot access non-static field before calling constructor: public com.example.a.Source()

[raphw]

You are right. It's technically legal, but since reading is not allowed, it's fully forbidden. You would need to create your own offset mapping which does not have this restriction.

[renjie-rm]

thank you!@raphw