EDIT: I've described our solution at https://stackoverflow.com/a/60235242/3236516
I have a java object. It is an instance of one of many subclasses that extend an abstract class. I would like to modify one of its methods such that it runs some additional code before calling the original method. My goal is conceptually the same as a pointcut in AspectJ.
It is fine if I create some modified version of the original object rather than mutating the original. It is also fine if the solution involves bytecode manipulation.
Prior Work
I've considered creating a proxy via JavaAssist. The trouble is that ProxyFactory's create method expects that I know the constructor input types in advance. I don't. I can create my object without calling the constructor via Objenesis, but then the resulting proxy object will have null values for any values set by the constructor. This means my resulting object will behave differently from the original whenever a value set by the constructor is directly referenced.
Context
We are using Flink via AWS Kinesis Data Analytics to transform some streaming data. We would like to include some common code at the beginning of all of our StreamOperator's open() methods without having to modify each operator. One use case for this is to ensure a custom metrics agent is running on each instance an operator is running on.
Answer from original asker: We solved the problem by creating a ByteBuddy proxy for StreamExecutionEnvironment that intercepted calls to getStreamGraph and recast (using reflection) each node's jobVertexClass to a class that extended the original class type, but included our custom logic. Because different classes require different parameters, we instantiated the proxy without calling a constructor by using Objenesis. To solve the problem of private fields normally set in the constructor being left null, we used reflection to alter the visibility of all private fields and then copied every field value from the original object to the proxy object.
We did not pursue the agent solution proposed by Rafael Winterhalter because it requires the ability to run the agent setup code on every worker instance, which is analogous to the original problem of wanting to start a metrics agent on each worker machine. Though I did not state this in my original question, the code creating the proxy objects occurs on the Flink job management machine, not the worker machines.