Spring proxies are based on the *Proxy design pattern* and serve as intermediaries to other resources, offering extra features at a slight performance penalty.
For example, they facilitate lazy resource initialization and data caching.

The `@Configuration` annotation enables this mechanism by default through the `proxyBeanMethods` attribute set to `true`.
This ensures that the `@Bean` methods are proxied in order to enforce bean lifecycle behavior, e.g. to return shared singleton bean instances even in case of direct `@Bean` method calls in user code.
This functionality is achieved via method interception, implemented through a runtime-generated *https://github.com/cglib/cglib/wiki[CGLIB]* subclass.

== Why is this an issue?

When setting the `proxyBeanMethods` attribute to `false` the `@Bean` methods are not proxied and this is similar to removing the `@Configuration` stereotype.
In this scenario, `@Bean` methods within the `@Configuration` annotated class operate in https://docs.spring.io/spring-framework/docs/current/javadoc-api/org/springframework/context/annotation/Bean.html[_lite mode_], resulting in a new bean creation each time the method is invoked.

For `Singleton` beans, this could cause unexpected outcomes as the bean is created multiple times instead of being created once and cached.

The rule raises an issue when the `proxyBeanMethods` attribute is set to `false` and the `@Bean` method of a `Singleton` bean is directly invoked in the `@Configuration` annotated class code.

== How to fix it

The issue can be fixed in the following ways:

* Not invoking the `@Bean` method directly, but rather injecting the bean in the context and using it, by means of `@Bean`  https://docs.spring.io/spring-framework/reference/core/beans/java/bean-annotation.html#beans-java-dependencies[method parameters].

* If the performance penalty is negligible, consider not disabling the `proxyBeanMethods` attribute, so that the `@Bean` methods are proxied and the bean lifecycle is enforced.

=== Code examples

==== Noncompliant code example

In the example below, every instance of `PrototypeBean` will have a different instance of `SingletonBean`, as `singletonBean()` is called directly from `prototypeBean()`.

[source,java,diff-id=1,diff-type=noncompliant]
----
@Configuration(proxyBeanMethods = false)
class ConfigurationExample {
  @Bean
  public SingletonBean singletonBean() {
    return new SingletonBean();
  }

  @Bean
  @Scope("prototype")
  public PrototypeBean prototypeBean() {
    return new PrototypeBean(singletonBean()); // Noncompliant, the singletonBean is created every time a prototypeBean is created
  }

  class SingletonBean {
    // ...
  }

  class PrototypeBean {
    // ...

    public PrototypeBean(SingletonBean singletonBean) {
      // ...
    }

    // ...
  }
}
----

==== Compliant solution

The compliant solution relies on the `@Bean` method parameter to automatically inject the `SingletonBean` from the `ApplicationContext`.
This way every instance of `PrototypeBean` will have the same instance of `SingletonBean`.

[source,java,diff-id=1,diff-type=compliant]
----
@Configuration(proxyBeanMethods = false)
class ConfigurationExample {
  @Bean
  public SingletonBean singletonBean() {
    return new SingletonBean();
  }

  @Bean
  @Scope("prototype")
  public PrototypeBean prototypeBean(SingletonBean singletonBean) { // Compliant, the singletonBean is injected in the context and used by every prototypeBean
    return new PrototypeBean(singletonBean);
  }

  class SingletonBean {
    // ...
  }

  class PrototypeBean {
    // ...

    public PrototypeBean(SingletonBean singletonBean) {
      // ...
    }

    // ...
  }
}
----

== Resources
=== Documentation

* Spring - https://docs.spring.io/spring-framework/docs/current/javadoc-api/org/springframework/context/annotation/Configuration.html#proxyBeanMethods()[Configuration - proxyBeanMethods]

* Spring - https://docs.spring.io/spring-framework/reference/core/aop/proxying.html[Proxying Mechanisms]

* Spring - https://docs.spring.io/spring-framework/reference/core/beans/java/bean-annotation.html#beans-java-dependencies[Bean Annotation - Dependencies]

* GitHub - https://github.com/cglib/cglib/wiki[CGLIB]

=== Articles & blog posts

* Medium - https://blog.devgenius.io/demystifying-proxy-in-spring-3ab536046b11[Demystifying Proxy in Spring]
