How to create protobuf instances using java reflection?

Question

Normally you create an protobuf class instance like this:

Bar.Builder bld = Bar.newBuilder();
bld.setXYZ(...

I have a usecase that uses the Java reflection to instantiate a protobuf class:

Class clsBar = Class.forName("com.xyz.Foo$Bar");
Object instance = clsBar.newInstance(); // error here!
Method mth = clsBar.getMethod(...);

The above code works fine with normal Java classes. But for a generated protobuf class "com.xyz.Foo$Bar", It gives me an NoSuchMethodException, as there isn't a default public constructor there.

Any suggestions on how to use Java refection to create protobuf instances? The question is for someone who's really good at protobuf internals. Thank!

Answer 1

I think you should go the full way: through the Builder class:

//get Bar class
Class barClass = Class.forName("com.xyz.Foo$Bar");

//instantiate Builder through newBuilder method
Method newBuilderMethod = barClass.getMethod("newBuilder");
Bar.Builder builder = (Bar.Builder) newBuilderMethod.invoke(null);

// ... set properties  -- can be through reflection if necessary

//build:
Bar bar = builder.build();

Though I don't exactly see how reflection is of any use in this case, that would probably require a deeper understanding of the exact problem you are trying to solve.