We have open-sourced a new Java library Proxycian. It is a small Java library for generating dynamic proxies on top of ByteBuddy or Javassist. You can generate data transfer objects, rich traits or even whole implicit DAO implementations dynamically at runtime easily. This library solves the complex stuff, so you can focus on application logic. Serializability and cloning are already solved by us. We also aim for transparent and easily debuggable proxies, because as we know proxies is usually part of “magic” for the team. Hence, the name of this library - Proxycian as a magician for the proxies ;)


Developers in FG Forrest use dynamic proxies successfully for over 10 years. Our initial implementation was based on the Spring framework abstractions, but we quickly realized that their implementation is overly complex and prone to subtle errors leading to memory leaks as well as being poorly observable / debuggable. Our second implementation took advantage of JBoss Javassist but it still kept many unnecessary abstractions and principles we learned from the Spring implementation.

This implementation is our third take on proxies that is the leanest and the most opinionated one so far. It was designed from scratch and based on current and actively developed libraries and with the emphasis on:

  • simplicity
  • clear and transparent classes / method implementation caching
  • debuggability - you just want your debugger to step in the dynamic implementation without much fuss around
  • transparency - you can easily find out why the library chose the implementation it chose

Let us know if we achieved our goals or not. Opinions and feedback is welcome.

What we do solve with Proxycian

Let’s see a few practical examples on what you can do with Proxycian:

1. stateful and dynamic traits

Java doesn’t have multiple inheritance, but you can imitate it to certain level with default methods on interfaces, but there will be always limitations. Classes might implement multiple interfaces, but these interfaces can’t have fields and keep data in them. Also, you cannot decide which traits your class will have in runtime.

Proxycian allows us to create self-sustainable traits keeping both logic and data that don’t require any orchestration or cooperation with the main class which they are attached to. Also, we can easily create specialized class with set of traits selected in runtime, usually on some text-based configuration or user interaction with the application.

2. interception / delegation

We use Proxycian to wrap external classes - such as Spring ReloadableResourceBundle or JDBC DataSource to intercept calls in the development environment to provide information for the developers - reporting which message codes / messages were used while rendering the page, or which (and how many) SQL queries were executed through the DataSource.

There are many use-cases where the interception might come handy. You can make a class wrapping original and delegate method calls to it by hand, but it quickly gets incomprehensible and maintaining is costly. AOP with dynamic proxies offer a more clever and shorter way to achieve the same.

3. mocking contracts

There are situations when your application works with some interface for which the real implementation is not yet known, but eventually it will be resolved. In our modular system the modules export and require some interfaces. In order to fulfill them we needed to configure modules, how they depended on each other. This was the tricky for part of our developers and soon there were situations when two modules might have needed each other (circular dependency). This is a sign of poorly architected modules, but life brings situations when a circular dependency might resemble the least of all evils.

Proxycian allows us to create a dynamic proxy, which implements required interfaces and use it for immediate wiring by the dependency injection mechanism. The module has its requirement fulfilled and can be started. Any call to the proxy method will end with an exception, but we usually need to call method after entire system starts. As soon the other module that provides the interface starts, the proxy internal state is filled with a reference to the implementation and delegates each method call to it.

4. implicit DAO/Service interface implementation

Do you know Ruby’s Active Record or Spring Data libraries? You can easily implement your own using Proxycian. It’s matter of a few lines of code.


  • JDK 1.8 (currently, only JDK 8 is supported. We plan to support also JDK 11 and 17 in near future)
  • Log4J 2 (2.17+)
  • Apache Commons Langs (2.6+)
  • ByteBuddy / Javassist are bundled in our library, there will be no conflict with possible existing libraries on your classpath in different version

How to compile

Use standard Maven 3 command:

mvn clean install

How to run tests

Run your tests in an IDE or run:

mvn clean test

Help us maintain at least 80% code coverage!

How to use

How to integrate to your application

Include the Proxycian library in your Maven descriptor (pom.xml):


Or Gradle:

dependencies {
    compile 'one.edee.oss:proxycian_bytebuddy:1.0.2'

Or use proxycian_javassist if you prefer this implementation (it has much smaller memory JAR size). Otherwise, ByteBuddy is preferred implementation because it’s actively maintained and supports the newest JDK version.

How to generate a dynamic proxy class

Note: In this documentation we stick to ByteBuddy implementation in examples, but you can easily translate all of them to Javassist implementation by replacing word ByteBuddy with Javassist. The contracts are identical in Proxycian.

To create new a class with the requested contracts, just use:

final Class<?> theInstance = ByteBuddyProxyGenerator.getProxyClass(

If you call it for the first time, a new class extending Person.class and implementing Trait1.class and Trait2.class is created for you. If you call it second time, you’ll receive the previously created (cached) class with that contract. The cache is kept in the static field of the generator and might be anytime cleared by calling ByteBuddyProxyGenerator.clearClassCache().

If you want to extend some class, it must be stated as the first class of the proxy contract, but you might also create proxies based on a bunch of interfaces and no superclass (then the java.lang.Object becomes the superclass of the proxy).

You can also create proxies based on superclasses without a default constructor (i.e. having only one constructor with one or more arguments). Imagine that the Person.class has the only constructor protected Person(String firstName, String lastName):

final Class<?> theInstance = ByteBuddyProxyGenerator.getProxyClass(
	new Class<?>[]{
	new Class<?>[]{

You can also specify a classloader that will maintain the created class, but this is usually not necessary. Proxycian uses by default the same classloader that loads ByteBuddyProxyGenerator.class itself.

But this is not the way Proxycian was meant to be used - read the next chapter for a general usage scenario.

How to generate a dynamic proxy instance

Creating classes is not the common way how you’ll create proxies. You usually want the instance of the proxy and not the class. You can achieve this in a single call, when both classes and a first instance is created at once:

final Object theInstance = ByteBuddyProxyGenerator.instantiate(
	new ProxyRecipe(
		new Class[] {Trait1.class, Trait2.class}, 
		new Advice[]{LocalDataStoreAdvice.INSTANCE}
	new GenericBucket()

Proxycian uses an abstraction of ProxyRecipe that is used to wrap the definition of the contract (i.e. what interfaces will the proxy have) as well as the logic that will handle the calls to methods of the contract (interfaces). Advice is an abstraction for implementation of the logic and call filtering logic - i.e. which method calls will be serviced by this particular Advice.

Second argument - in our case GenericBucket is a single object maintaining the state of the proxy. It’s the state you’ll provide and control.

Note: It’s recommended to cache the ProxyRecipe and not create it again and again with each call of instantiate method as you see in the example.


Advice is strictly stateless and if you create a new Advice, we recommend defining the no instance fields, creating private non-args constructor and providing a single public static field INSTANCE, which provides access to the advice instance (as you see in the example: LocalDataStoreAdvice.INSTANCE).

There are two different kind of Advices:

  • regular Advice: the advice only defines the filters for methods it intercepts, and the implementation of them; see example advice one.edee.oss.proxycian.trait.beanMemoryStore.BeanMemoryStoreAdvice
  • IntroductionAdvice: is the same as regular advice but also introduces a new interface (or set of interfaces) to the proxy; this means that it is not necessary to state this interface in the ProxyRecipe explicitly, but it will be automatically added to the proxy contract whenever the IntroductionAdvice is part of the recipe

Each advice may require a state object to implement certain contracts, so that they can keep the necessary state in it ( remember, Advices are stateless). If they don’t work with the state, they just require generic Object.class contract to be fulfilled by the state object which matches everything.

The simple Advice may look like this:

public class ExampleAdvice implements Advice<Object> {
	private static final long serialVersionUID = 4100044042153442374L;

	public Class<Object> getRequestedStateContract() {
		return Object.class;

	public List<MethodClassification<?, Object>> getMethodClassification() {
		return Collections.singletonList(
			new PredicateMethodClassification<>(
				/* description */   "Hello world method",
				/* matcher */       (method, proxyState) -> 
					"helloWorld".equals(method.getName()) && 
						String.class.equals(method.getReturnType()) && 
						method.getParameterCount() == 1 &&
				/* methodContext */ MethodClassification.noContext(),
				/* invocation */    (proxy, method, args, methodContext, proxyState, invokeSuper) -> 
						"Hello world, " + args[0]


This advice reacts to a method call with following the signature String helloWorld(String myName); and when calling proxy.helloWorld("Jan") returns Hello world, Jan in response. Advice provides a set of so called MethodClassification that are used to intercept the proper methods on a proxy interface. There are two types of method classifications:


The classification consists of 4 parts:

1. description - a simple string description of the classification, it’s used only for developer orientation in debugging sessions, it has no other real usage in the Proxycian.

2. matcher - represents a simple predicate that accepts java.lang.Method and a reference to the proxy state object, and returns TRUE if this method classifier intercepts this method call.

Note: you can use static helper methods in one.edee.oss.proxycian.util.ReflectionUtils interface in your predicate. For example if you want to check whether the called method is the same method in particular interface, you can use this expression: ReflectionUtils.isMethodDeclaredOn(method, LocalDataStore.class, "getLocalData", String.class) where method is the called method, LocalDataStore.class is the checked interface, "getLocalData" is the name of the method in the interface and String.class is the single method parameter. You can find more handy methods here as well.

3. method context - it represents function that takes java.lang.Method and references it to the proxy state object and returns a DTO object that contains extracted information from the method signature that is necessary for the implementation logic. The DTO is created only for the first call and cached so all additional method calls will reuse this method context. It may therefore contain rather complex logic without fear of affecting proxy method call performance.

4. method implementation - the last piece of puzzle will provide the final logic for the method. This is the only part executed everytime the proxy method is called

Note: if you want to invoke the original method (for example you want only to do something before / after the original method executes), use expression: return

The method call interception logic is straightforward - when a method on a proxy is called for the first time, we need to resolve the proper implementation. The Proxycian will iterate over all advices and within them, over all the method classifiers, the advice provides and selects the first method classifier, which predicate returns true. If predicates of your advices overlap (the very same method might be intercepted and handled by Advice1 as well as Advice2) , the Advice which is defined first wins. The predicates may overlap even within single advice, so even the order in which you specify method classifiers is crucial. There are also “system methods” that are automatically handled by the Proxycian and these have their own priority. The ordering of the method classifier is as follows:

  1. Object#hashCode()
  2. Object#equals()
  3. Object#toString()
  4. Object#clone()
  5. ProxyStateAccessor#getProxyState()
  6. all your method classifiers
  7. when method is still not classified, the original method will be invoked - if it is “abstract”, the call will fail

When method classifier is selected, a function that creates method context is called and its result is cached into the ByteBuddyProxyGenerator method cache for the key one.edee.oss.proxycian.cache.ClassMethodCacheKey. Finally, the implementation part is executed. Next time the same method is called (maybe on another instance of the same proxy class) , the implementation with method context is quickly retrieved from the internal hash map and executed. With each call on the proxy instance, you pay the price of a single lookup to the hash map and delegating a call to an associated implementation object.

Method cache can be reset at any time by calling ByteBuddyProxyGenerator.clearMethodClassificationCache(). Your method classification can also add custom data to the method cache key, should it be necessary.


This implementation is similar to PredicateMethodClassification in its principle. It just combines the predicate with the method context creation together. These two aspects of the method classification contract are some time very similar and might be quite expensive. It makes sense to support this approach as well, so that the same logic can be used for matching the method as well as creating method context for it.

The PredicateMethodClassification in our example can be easily translated to DirectMethodClassification and vice versa:

new DirectMethodClassification<>(
	/* description */   "Hello world method",

	/* matcher */       (method, proxyState) -> {
		if ("helloWorld".equals(method.getName()) && String.class.equals(method.getReturnType()) &&
			method.getParameterCount() == 1 && String.class.equals(method.getParameterTypes()[0])) {
			return (proxy, theMethod, args, theProxyState, invokeSuper) -> "Hello world, " + args[0];
		} else {
			return null;

So if your method signature analysis is complex, and you would need to do the same operations both in predicate and the execution function - use DirectMethodClassification otherwise stick to the PredicateMethodClassification.

State object

State object is the target for all Java base methods, such as equals, hashCode, toString and the serialization and clone facility. We have single state objects on purpose - it’s much easier to track, debug and control data this way. The state of the object must fulfill the contract required by all the Advices of the proxy (do not confuse the contract required by the advices with the contract of the proxy itself!). The state object lives with the proxy instance and gets garbage collected with it.

There is no required interface for the state object if you create NON-serializable instance of the class. However, if you need an instance, that can be serialized using default Java serialization facility, you must implement ProxyStateWithConstructorArgs interface. The state itself must be Serializable and must allow keeping the original constructor arguments used for instance creation, so that they can be reused in deserialization phase.

Instantiation callback

There are certain use-cases when you have to “prepare” the instance immediately after creation, even before the method classification logic gets in the way. For such case there is one.edee.oss.proxycian.OnInstantiationCallback. With it, you can implement and pass to the instantiation method. In this callback you can freely invoke method of the instance and no dynamic logic stated in advices will be executed. Of course, calling abstract methods will trigger an exception.

Prepared traits ready to use


This advice will intercept all method calls that follow Java Beans contract and stores the date into internal HashMap in the proxy state. This map can be accessed via. expression: ((BeanMemoryStore)((ProxyStateAccessor)instance).getProxyState()).getLocalDataStoreIfPresent(). In addition to standard Java Beans contracts, there is support for adding and removing 1:N items one by one. See example interface:

public interface JavaBeanWithMultipleItems {
	List<String> getItems();

	void setItems(List<String> items);

	boolean addItem(String item);

	boolean removeItem(String item);

The behaviour describes following test case:

public void shouldProxyJavaBeanWithMultipleItems(){
final Object theInstance=ByteBuddyProxyGenerator.instantiateSerializable(
	new ProxyRecipe(
	new Class[]{JavaBeanWithMultipleItems.class},
	new Advice[]{BeanMemoryStoreAdvice.ALL_METHOD_INSTANCE}
	new GenericBucket()

	assertTrue(theInstance instanceof JavaBeanWithMultipleItems);
final JavaBeanWithMultipleItems proxy=(JavaBeanWithMultipleItems)theInstance;


	assertArrayEquals(new String[]{"A","B","C"},proxy.getItems().toArray(new String[0]));


	assertArrayEquals(new String[]{"A","C"},proxy.getItems().toArray(new String[0]));


This is your handy advice that allows you to store any data from execution functions, or even default methods of your interfaces. See contract of LocalDataStore interface for more information.

If you add this advice to your dynamic proxy, you can then define multiple other “traits” that are merely interfaces with default methods that can take advantage of the internal memory store to become “stateful”. See following example:

public interface ExpensiveComputer extends LocalDataStore {

	default double computePi() {
		return computeLocalDataIfAbsent("cachedPi", () -> {
			double pi = 0;
			for (int i = 1; i < 1_000_000; i++) {
				pi += Math.pow(-1, i + 1) / (2 * i - 1);
			return 4 * pi;


You can add ExpensiveComputed to any dynamic proxy, that have LocalDataStoreAdvice, and you will have an object that will return a computed PI to 1 mil. iterations. As you can see the expensive computation will happen only once on that instance of dynamic proxy, because next time you call that method you’ll receive memoized value from the first call.


This advice lets you delegate calls to all methods of single interface directly to the state object or the object that is reachable from the state object. This allows you to compose multiple interface delegations at once. Beware, this example is quite long (to shorten it a little bit we use Lombok annotations in it):

public interface NameInterface {

	String getFullName();

	String getFirstName();

	String getLastName();

	void setFirstName(String firstName);

	void setLastName(String lastName);


public static class NameImplementation implements Serializable, NameInterface {
	private static final long serialVersionUID = -3190496823269251991L;
	private String firstName;
	private String lastName;

	public String getFullName() {
		return firstName + " " + lastName;


public interface AgeInterface {

	int getAge();

	void setAge(int ageInYears);


public static class AgeImplementation implements Serializable, AgeInterface {
	private static final long serialVersionUID = -2520784598151746890L;
	private int age;

public interface PersonInterface {

	String getPersonDescription();


public static class CompositionState implements Serializable, PersonInterface {
	private static final long serialVersionUID = 3427985599976732264L;
	private final NameImplementation nameHolder = new NameImplementation();
	private final AgeImplementation ageHolder = new AgeImplementation();

	public String getPersonDescription() {
		return nameHolder.getFullName() + " of age " + ageHolder.getAge();

	public void ByteBuddyProxyRecipeGenerator_DelegateCallsOnSubProperty() {
		final Object theInstance = ByteBuddyProxyGenerator.instantiateSerializable(
			new ProxyRecipe(
				DelegateCallsAdvice.getInstance(NameInterface.class, o -> ((CompositionState) o).getNameHolder()),
				DelegateCallsAdvice.getInstance(AgeInterface.class, o -> ((CompositionState) o).getAgeHolder()),
			new CompositionState()

		assertTrue(theInstance instanceof NameInterface);
		final NameInterface nameProxyContract = (NameInterface) theInstance;

		assertTrue(theInstance instanceof AgeInterface);
		final AgeInterface ageProxyContract = (AgeInterface) theInstance;

		assertTrue(theInstance instanceof PersonInterface);
		assertEquals("Jan Novotný of age 43", ((PersonInterface) theInstance).getPersonDescription());