extensions/assistedinject/src/com/google/inject/assistedinject/FactoryProvider2.java
@Override
  public Object invoke(Object proxy, final Method method, final Object[] args) throws Throwable {
    // If we setup a method handle earlier for this method, call it.
    // This is necessary for default methods that java8 creates, so we
    // can call the default method implementation (and not our proxied version of it).
    if (methodHandleByMethod.containsKey(method)) {
      return methodHandleByMethod.get(method).invokeWithArguments(args);
    }

    if (method.getDeclaringClass().equals(Object.class)) {
      if ("equals".equals(method.getName())) {
        return proxy == args[0];
      } else if ("hashCode".equals(method.getName())) {
        return System.identityHashCode(proxy);
      } else {
        return method.invoke(this, args);
      }
    }

    AssistData data = assistDataByMethod.get(method);
    checkState(data != null, "No data for method: %s", method);
    Provider<?> provider;
    if (data.cachedBinding != null) { // Try to get optimized form...
      provider = data.cachedBinding.getProvider();
    } else {
      provider = getBindingFromNewInjector(method, args, data).getProvider();
    }
    try {
      int p = 0;
      for (ThreadLocalProvider tlp : data.providers) {
        tlp.set(args[p++]);
      }
      return provider.get();
    } catch (ProvisionException e) {
      // if this is an exception declared by the factory method, throw it as-is
      if (e.getErrorMessages().size() == 1) {
        Message onlyError = getOnlyElement(e.getErrorMessages());
        Throwable cause = onlyError.getCause();
        if (cause != null && canRethrow(method, cause)) {
          throw cause;
        }
      }
      throw e;
    } finally {
      for (ThreadLocalProvider tlp : data.providers) {
        tlp.remove();
      }
    }
  }
@Override
  public Object invoke(Object proxy, final Method method, final Object[] args) throws Throwable {
    // If we setup a method handle earlier for this method, call it.
    // This is necessary for default methods that java8 creates, so we
    // can call the default method implementation (and not our proxied version of it).
    if (methodHandleByMethod.containsKey(method)) {
      return methodHandleByMethod.get(method).invokeWithArguments(args);
    }

    if (method.getDeclaringClass().equals(Object.class)) {
      if ("equals".equals(method.getName())) {
        return proxy == args[0];
      } else if ("hashCode".equals(method.getName())) {
        return System.identityHashCode(proxy);
      } else {
        return method.invoke(this, args);
      }
    }

    AssistData data = assistDataByMethod.get(method);
    checkState(data != null, "No data for method: %s", method);
    Provider<?> provider;
    if (data.cachedBinding != null) { // Try to get optimized form...
      provider = data.cachedBinding.getProvider();
    } else {
      provider = getBindingFromNewInjector(method, args, data).getProvider();
    }
    try {
      int p = 0;
      for (ThreadLocalProvider tlp : data.providers) {
        tlp.set(args[p++]);
      }
      return provider.get();
    } catch (ProvisionException e) {
      // if this is an exception declared by the factory method, throw it as-is
      if (e.getErrorMessages().size() == 1) {
        Message onlyError = getOnlyElement(e.getErrorMessages());
        Throwable cause = onlyError.getCause();
        if (cause != null && canRethrow(method, cause)) {
          throw cause;
        }
      }
      throw e;
    } finally {
      for (ThreadLocalProvider tlp : data.providers) {
        tlp.remove();
      }
    }
  }
When a factory method is invoked, we create a child injector that binds all parameters, then
use that to get an instance of the return type.