How does Java's Dynamic Proxy actually work?

Question

I understand how to use Dynamic Proxies in Java but what I don't understand is how the VM actually creates a dynamic proxy. Does it generate bytecode and load it? Or something else? Thanks.

Answer 1

The proxy class is generated on-the fly(hence dynamic proxy) and loaded by the classloader. That's why if you debug applications that relies on JDK proxying you'll see bunch of classes named 'com.sun.proxy.$Proxy0'.

To test my theory you can use an example from Dynamic proxy classes along with the VM parameter -verbose:class which will tell you the loaded classes by the classloader and you shall notice among the classes loaded the com.sun.proxy.$Proxy0.

Answer 2

At least for Sun's implementation, if you look at the source code of java.lang.reflect.Proxy you'll see that yes, it generates the byte code on-the-fly (using the class sun.misc.ProxyGenerator).

Answer 3

I suggest that you read Dynamic Proxy Classes:

The Proxy.getProxyClass method returns the java.lang.Class object for a proxy class given a class loader and an array of interfaces. The proxy class will be defined in the specified class loader and will implement all of the supplied interfaces. If a proxy class for the same permutation of interfaces has already been defined in the class loader, then the existing proxy class will be returned; otherwise, a proxy class for those interfaces will be generated dynamically and defined in the class loader. [emphasis mine]