What does it mean to "proxy a bean"?

Question

At work and online I keep hearing the term "proxy" with respect to enterprise Java development. For example, metrics-spring uses this phrase:

This module does the following things:

Creates metrics and proxies beans which contain methods annotated with @Timed, @Metered, @ExceptionMetered, and @Counted [emphasis mine]

I'm unfamiliar with a lot of the language in the Java ecosystem of frameworks and libraries. I feel like I have a good understanding of what a bean is, but I'm still not clear about how one would proxy a bean.

What does it mean to proxy a bean?

Answer 1

Typically, you have a bean like

Bean bean = new Bean(); // actually created by the context

With this, you can do anything that the Bean class declares as behavior (invoke its methods).

There are times where it would be nice if you could, for example, track how long a method invocation takes.

You could do

long start = .. // get start time
bean.invoke();
long end = .. // get end time
// end - start

But doing this for each method invocation sucks. So, instead, patterns, architectures, and styles like Aspect Oriented Programming exist.

Instead of the Bean above, you'd have

Bean bean = new TimingBean(new Bean()); // again done by the context

where the TimingBean is a proxy type that extends and implements all the types that Bean extends and implements. For all intents and purposes it is a Bean, but it adds a bunch of additional behavior before delegating each invocation to the Bean object. In this case, it would track how long each of Bean's method's took to execute.

Basic Spring uses JDK proxies and CGLIB proxies. Here are some differences between them.

It uses this for its scheduling and asynchronous invocations. It uses it for transactional support with databases. It uses it for caching. It even uses it for its Java based container configuration.