Android runtime code injection / method hooking with Java Reflection

I am currently researching on runtime injection techniques for Android. Is there any way to achieve a similar result as the Xposed Framework with Java Reflections? As far as I have seen, there is just the possibility to call methods of another apps or to read its fields using reflections. However, my goal is to hook into method calls to inject custom code. Is this anyhow possible with Java reflection? For example to load my own classes/dex files into other apps that change a certain parameter before a method is invoked.

Any help appreciated!