JavaClassAtRunTime

This is a sample project of how to generate dynamically a class at runtime, load it, and instantiate it.

Example problem

Suppose you have a Map<String, Object> read from somewhere and you want to turn it into JSON. There are many ways to do that, but simply using jackson and doing ObjectMapper::writeValueAsString will not work.

The code in this repository will generate a Java class at runtime that can be serialized as expected. For example, given the following:

Map<String, Object> record = Map.of("name", "Andrea", "age", 37);

a class similar to this will be generated:

class SomeInternalName {
    private final String name;
    private final Int age;
    
    SomeInternalName(Map<String, Object> record) {
        this.name = record.get("name");
        this.age = record.get("age");
    }
    
    // TODO: getters are not yet implemented
    public String getName() { 
        return name;
    }
    
    public Int getAge() {
        return age;
    }
}

This is all done via bytecode manipulation, using the asm library.

It sounds like a super strange thing to do... but believe me, I had once to do something like this in production! 😅 I will write up about it on my blog in the future!

Implementation

Implementation is a work in progress, there are quite some TODO yet.

andreabergia/JavaClassAtRunTime

JavaClassAtRunTime

Example problem

Implementation