Virtual Bean Factory

A simple API to compose bean behavior based on annotations.

A virtual bean is an interface that describes

• properties as getters and setters
• services as abstract or default methods

The BeanFactory API (javadoc) cleanly decouples the virtual bean definition, from the semantics of the annotation defined in terms of implementors and interceptors.

This class offers capabilities similar to Spring, CDI or Guice but decomposes the concept of interceptor into 2 different parts: interceptors that can be composed and implementors that are unique for an abstract method (thus non composable). In order to be composable, interceptors are less powerful than classical interceptors, they return void, they can not change the arguments or the return value of a call, and do not explicitly call each others. Unlike Spring or CDI, there is no annotations with a predefined semantics, the semantics of an annotation is fully defined by the code of the implementors and interceptors registered.

Conceptually, there are only three operations:

• create(interface) takes a virtual bean (the interface) and returns an instance of that interface with all the property initialize to their default values.

• registerImplementor(annotation, implementor)that registers a lambda that will be called to ask for an implementation (a method handle) for any abstract methods annotated by the annotation

• registerInterceptor(annotation, filter, interceptor) that register a lambda that will be called to get method handle that should run before (PRE) and after (POST) a method call.

There are several helper methods that allows to register InvocationHandlers and Advices instead of respectively Implementors and Interceptor that have an easier semantics but are less performant because their API requires method arguments to be boxed in an array.

Also interceptors can be unregistered using unregisterInterceptor(annotation, interceptor) allowing to dynamically add/remove pre and post code.

The beauty of all of this is that the clean separation between the virtual bean, implementors and interceptors does not hinder performance, but actually helps

• the implementation is fully lazy, if a method of the virtual bean is never called the runtime cost is zero
• implementors, invocation handlers, interceptors and advices are resolved once per call
  and fully inlined
• if there are several interceptors for a call, there are called one after the other, and not one on top of the others, so no gigantic stracktraces

There are some examples below.

How to use it

Get the latest binary distribution via JitPack

Maven

  <repositories>
      <repository>
          <id>jitpack.io</id>
          <url>https://jitpack.io</url>
      </repository>
  </repositories>
  <dependency>
      <groupId>com.github.forax</groupId>
      <artifactId>virtual-bean</artifactId>
      <version>1.0</version>
  </dependency>

Gradle

  repositories {
      ...
      maven { url 'https://jitpack.io' }
  }
  dependencies {
      compile 'com.github.forax:virtual-bean:1.0'
  }

All your parameter are belong to us

The virtual bean HelloManager defines a method sayHello annotated with an annotation @ParametersNonNull to say that the parameter should not be null

@Retention(RetentionPolicy.RUNTIME)
@interface ParametersNonNull { }

interface HelloManager {
  @ParametersNonNull
  default void sayHello(String text)  {
    System.out.println("hello " + text);
  }
}

The BeanFactory API let you define the semantics of the annotation @ParametersNonNull using an advice with registerAdvice() and automatically provides an implementation of any virtual beans with create().

So we register an advice for the annotation class ParametersNonNull that calls Objects.requireNonNull on all arguments and test by creating a HelloManager.

  public static void main(String[] args) {
    var lookup = MethodHandles.lookup();
    var beanFactory = new BeanFactory(lookup);

    beanFactory.registerAdvice(ParametersNonNull.class, new Advice() {
      public void pre(Method method, Object bean, Object[] args) {
        System.out.println("pre " + Arrays.toString(args));
        for (int i = 0; i < args.length; i++) {
          Objects.requireNonNull(args[i], "argument " + i + " of " + method + " is null");
        }
      }

      public void post(Method method, Object bean, Object[] args) {
        System.out.println("post " + Arrays.toString(args));
      }
    });

    var helloManager = beanFactory.create(HelloManager.class);
    helloManager.sayHello("Bob");
    helloManager.sayHello(null);
  }

If you run this code, the last call to sayHello will throw a NullPointException because the argument is null. It's not the most efficient code tho, mostly because for each call, arguments are boxed in an array. The example below, explains how to alleviate that issue.

The full code is available here: https://github.com/forax/virtual-bean/blob/master/src/test/java/com/github/forax/virtualbean/example/Example.java

All your parameter are belong to us (the sequel)

We can improve the efficiency of the code above by using an interceptor instead of an advice. But this came at the price of having to figure out how the java.lang.invoke API really works.

The start of the code is the same, we create an annotation and using it on a method of a virtual bean, but we also create a method handle (a function pointer) on Objects.requireNonNull

  @Retention(RetentionPolicy.RUNTIME)
  @interface ParametersNonNull { }

  interface HelloManager {
    @ParametersNonNull
    default void sayHello(String text)  {
      System.out.println("hello " + text);
    }
  }
  
  private static final MethodHandle REQUIRE_NON_NULL;
  static {
    try {
      REQUIRE_NON_NULL = MethodHandles.lookup().findStatic(Objects.class,
          "requireNonNull", MethodType.methodType(Object.class, Object.class, String.class));
    } catch (NoSuchMethodException | IllegalAccessException e) {
      throw new AssertionError(e);
    }
  }

We register an interceptor that will be called only once per method call and return a method handle that will check that if a parameter is an object, the method handle corresponding to requireNonNull must be called.

  public static void main(String[] args) {
    var lookup = MethodHandles.lookup();
    var beanFactory = new BeanFactory(lookup);

    beanFactory.registerInterceptor(ParametersNonNull.class, (kind, method, type) -> {
      if (kind == POST) {
        return null;
      }
      var parameterTypes = method.getParameterTypes();
      var filters = new MethodHandle[parameterTypes.length];
      for(var i = 0; i < parameterTypes.length; i++) {
        var parameterType = parameterTypes[i];
        if (parameterType.isPrimitive()) {
          continue;
        }
        var requireNonNull = MethodHandles.insertArguments(REQUIRE_NON_NULL, 1,
            "argument " + i + " of " + method + " is null");
        var filter = requireNonNull.asType(MethodType.methodType(parameterType, parameterType));
        filters[i] = filter;
      }
      var empty = MethodHandles.empty(type);
      return MethodHandles.filterArguments(empty, 1, filters);
    });

    var helloManager = beanFactory.create(HelloManager.class);
    helloManager.sayHello("Bob");
    helloManager.sayHello(null);
  }

The behavior of this code is identical as the previous solution, but it performs better because method arguments are not boxed anymore.

The full code is available here: https://github.com/forax/virtual-bean/blob/master/src/test/java/com/github/forax/virtualbean/example/Example2.java

Invocation handler and object injection

A BeanFactory also provides implementations of abstract method by registering implementors. We can use that to implement a simple dependency injection.

First we create an annotation @Inject and a class Injector that associate a class to a supplier of instances of that class.

  @Retention(RetentionPolicy.RUNTIME)
  @interface Inject { }

  static class Injector {
    private final HashMap<Class<?>, Supplier<?>> map = new HashMap<>();

    public <T> void bind(Class<T> type, Supplier<? extends T> supplier) {
      map.put(type, supplier);
    }
    public <T> T getInstance(Class<T> type) {
      return type.cast(map.get(type).get());
    }
  }

Then we register an invocation handler that will be called when abstract methods annotated by Inject is called.`

In this example, each time the method Clock.current() is called, the invocation_handler asks the injector to supply an instance of LocalTime, calling LocalTime.now().

  public static void main(String[] arguments) {
    var lookup = MethodHandles.lookup();
    var beanFactory = new BeanFactory(lookup);

    var injector = new Injector();
    beanFactory.registerInvocationHandler(Inject.class
        (method, bean, args) -> injector.getInstance(method.getReturnType()));

    interface Clock {
      @Inject
      LocalTime current();
    }

    var clock = beanFactory.create(Clock.class);
    injector.bind(LocalTime.class, LocalTime::now);
    System.out.println(clock.current());
    System.out.println(clock.current());
  }

The full code is available here: https://github.com/forax/virtual-bean/blob/master/src/test/java/com/github/forax/virtualbean/example/Example3.java

Dynamically add/remove an interceptor

The class BeanFactory API allows not only to add interceptors but also to remove them dynamically. In that case, all method calls optimized by the VM using an interceptor will be trashed, and will be re-optimize later with the new interceptors when the JIT will kick in again.

Here, we define an annotation Log that will log all calls of methods annotated with that annotation, and a virtual bean HelloManager with a method sayHello annotated with @Log.

  @Retention(RetentionPolicy.RUNTIME)
  @interface Log { }

  interface HelloManager {
    @Log
    default void sayHello(String text) {
      System.out.println("hello " + text);
    }
  }

The main shows an example of registering and then unregistering the logging interceptor

  public static void main(String[] args) {
    var lookup = MethodHandles.lookup();
    var beanFactory = new BeanFactory(lookup);

    var interceptor = new Advice() {
      public void pre(Method method, Object bean, Object[] args) {
        System.out.println("enter " + method);
      }

      public void post(Method method, Object bean, Object[] args) {
        System.out.println("exit " + method);
      }
    }.asInterceptor();

    var helloManager = beanFactory.create(HelloManager.class);
    helloManager.sayHello("no log");

    beanFactory.registerInterceptor(Log.class, interceptor);
    helloManager.sayHello("with log");

    beanFactory.unregisterInterceptor(Log.class, interceptor);
    helloManager.sayHello("with no log anymore");
  }

The full code is available here: https://github.com/forax/virtual-bean/blob/master/src/test/java/com/github/forax/virtualbean/example/Example4.java

There are more ...

There are more examples, all available here: https://github.com/forax/virtual-bean/blob/master/src/test/java/com/github/forax/virtualbean/example