Using the `ProxyFactoryBean` to Create AOP Proxies

If you use the Spring IoC container (an ApplicationContext or BeanFactory) for your business objects (and you should be!), you want to use one of Spring’s AOP FactoryBean implementations. (Remember that a factory bean introduces a layer of indirection, letting it create objects of a different type.)

The Spring AOP support also uses factory beans under the covers.

The basic way to create an AOP proxy in Spring is to use the org.springframework.aop.framework.ProxyFactoryBean. This gives complete control over the pointcuts, any advice that applies, and their ordering. However, there are simpler options that are preferable if you do not need such control.

Basics

The ProxyFactoryBean, like other Spring FactoryBean implementations, introduces a level of indirection. If you define a ProxyFactoryBean named foo, objects that reference foo do not see the ProxyFactoryBean instance itself but an object created by the implementation of the getObject() method in the ProxyFactoryBean . This method creates an AOP proxy that wraps a target object.

One of the most important benefits of using a ProxyFactoryBean or another IoC-aware class to create AOP proxies is that advice and pointcuts can also be managed by IoC. This is a powerful feature, enabling certain approaches that are hard to achieve with other AOP frameworks. For example, an advice may itself reference application objects (besides the target, which should be available in any AOP framework), benefiting from all the pluggability provided by Dependency Injection.

JavaBean Properties

In common with most FactoryBean implementations provided with Spring, the ProxyFactoryBean class is itself a JavaBean. Its properties are used to:

Specify the target you want to proxy.
Specify whether to use CGLIB (described later and see also JDK- and CGLIB-based proxies).

Some key properties are inherited from org.springframework.aop.framework.ProxyConfig (the superclass for all AOP proxy factories in Spring). These key properties include the following:

proxyTargetClass: true if the target class is to be proxied, rather than the target class’s interfaces. If this property value is set to true, then CGLIB proxies are created (but see also JDK- and CGLIB-based proxies).
optimize: Controls whether or not aggressive optimizations are applied to proxies created through CGLIB. You should not blithely use this setting unless you fully understand how the relevant AOP proxy handles optimization. This is currently used only for CGLIB proxies. It has no effect with JDK dynamic proxies.
frozen: If a proxy configuration is frozen, changes to the configuration are no longer allowed. This is useful both as a slight optimization and for those cases when you do not want callers to be able to manipulate the proxy (through the Advised interface) after the proxy has been created. The default value of this property is false, so changes (such as adding additional advice) are allowed.
exposeProxy: Determines whether or not the current proxy should be exposed in a ThreadLocal so that it can be accessed by the target. If a target needs to obtain the proxy and the exposeProxy property is set to true, the target can use the AopContext.currentProxy() method.

Other properties specific to ProxyFactoryBean include the following:

proxyInterfaces: An array of String interface names. If this is not supplied, a CGLIB proxy for the target class is used (but see also JDK- and CGLIB-based proxies).
interceptorNames: A String array of Advisor, interceptor, or other advice names to apply. Ordering is significant, on a first come-first served basis. That is to say that the first interceptor in the list is the first to be able to intercept the invocation.

The names are bean names in the current factory, including bean names from ancestor factories. You cannot mention bean references here, since doing so results in the ProxyFactoryBean ignoring the singleton setting of the advice.

You can append an interceptor name with an asterisk (*). Doing so results in the application of all advisor beans with names that start with the part before the asterisk to be applied. You can find an example of using this feature in Using “Global” Advisors.
singleton: Whether or not the factory should return a single object, no matter how often the getObject() method is called. Several FactoryBean implementations offer such a method. The default value is true. If you want to use stateful advice - for example, for stateful mixins - use prototype advice along with a singleton value of false.

JDK- and CGLIB-based proxies

This section serves as the definitive documentation on how the ProxyFactoryBean chooses to create either a JDK-based proxy or a CGLIB-based proxy for a particular target object (which is to be proxied).

The behavior of the ProxyFactoryBean with regard to creating JDK- or CGLIB-based proxies changed between versions 1.2.x and 2.0 of Spring. The ProxyFactoryBean now exhibits similar semantics with regard to auto-detecting interfaces as those of the TransactionProxyFactoryBean class.

If the class of a target object that is to be proxied (hereafter simply referred to as the target class) does not implement any interfaces, a CGLIB-based proxy is created. This is the easiest scenario, because JDK proxies are interface-based, and no interfaces means JDK proxying is not even possible. You can plug in the target bean and specify the list of interceptors by setting the interceptorNames property. Note that a CGLIB-based proxy is created even if the proxyTargetClass property of the ProxyFactoryBean has been set to false. (Doing so makes no sense and is best removed from the bean definition, because it is, at best, redundant, and, at worst confusing.)

If the target class implements one (or more) interfaces, the type of proxy that is created depends on the configuration of the ProxyFactoryBean.

If the proxyTargetClass property of the ProxyFactoryBean has been set to true, a CGLIB-based proxy is created. This makes sense and is in keeping with the principle of least surprise. Even if the proxyInterfaces property of the ProxyFactoryBean has been set to one or more fully qualified interface names, the fact that the proxyTargetClass property is set to true causes CGLIB-based proxying to be in effect.

If the proxyInterfaces property of the ProxyFactoryBean has been set to one or more fully qualified interface names, a JDK-based proxy is created. The created proxy implements all of the interfaces that were specified in the proxyInterfaces property. If the target class happens to implement a whole lot more interfaces than those specified in the proxyInterfaces property, that is all well and good, but those additional interfaces are not implemented by the returned proxy.

If the proxyInterfaces property of the ProxyFactoryBean has not been set, but the target class does implement one (or more) interfaces, the ProxyFactoryBean auto-detects the fact that the target class does actually implement at least one interface, and a JDK-based proxy is created. The interfaces that are actually proxied are all of the interfaces that the target class implements. In effect, this is the same as supplying a list of each and every interface that the target class implements to the proxyInterfaces property. However, it is significantly less work and less prone to typographical errors.

Proxying Interfaces

Consider a simple example of ProxyFactoryBean in action. This example involves:

A target bean that is proxied. This is the personTarget bean definition in the example.
An Advisor and an Interceptor used to provide advice.
An AOP proxy bean definition to specify the target object (the personTarget bean), the interfaces to proxy, and the advice to apply.

The following listing shows the example:

<bean id="personTarget" class="com.mycompany.PersonImpl">
	<property name="name" value="Tony"/>
	<property name="age" value="51"/>
</bean>

<bean id="myAdvisor" class="com.mycompany.MyAdvisor">
	<property name="someProperty" value="Custom string property value"/>
</bean>

<bean id="debugInterceptor" class="org.springframework.aop.interceptor.DebugInterceptor">
</bean>

<bean id="person"
	class="org.springframework.aop.framework.ProxyFactoryBean">
	<property name="proxyInterfaces" value="com.mycompany.Person"/>

	<property name="target" ref="personTarget"/>
	<property name="interceptorNames">
		<list>
			<value>myAdvisor</value>
			<value>debugInterceptor</value>
		</list>
	</property>
</bean>

Note that the interceptorNames property takes a list of String, which holds the bean names of the interceptors or advisors in the current factory. You can use advisors, interceptors, before, after returning, and throws advice objects. The ordering of advisors is significant.

You might be wondering why the list does not hold bean references. The reason for this is that, if the singleton property of the ProxyFactoryBean is set to false, it must be able to return independent proxy instances. If any of the advisors is itself a prototype, an independent instance would need to be returned, so it is necessary to be able to obtain an instance of the prototype from the factory. Holding a reference is not sufficient.

The person bean definition shown earlier can be used in place of a Person implementation, as follows:

Java
Kotlin

Person person = (Person) factory.getBean("person");

val person = factory.getBean("person") as Person

Other beans in the same IoC context can express a strongly typed dependency on it, as with an ordinary Java object. The following example shows how to do so:

<bean id="personUser" class="com.mycompany.PersonUser">
	<property name="person"><ref bean="person"/></property>
</bean>

The PersonUser class in this example exposes a property of type Person. As far as it is concerned, the AOP proxy can be used transparently in place of a “real” person implementation. However, its class would be a dynamic proxy class. It would be possible to cast it to the Advised interface (discussed later).

You can conceal the distinction between target and proxy by using an anonymous inner bean. Only the ProxyFactoryBean definition is different. The advice is included only for completeness. The following example shows how to use an anonymous inner bean:

<bean id="myAdvisor" class="com.mycompany.MyAdvisor">
	<property name="someProperty" value="Custom string property value"/>
</bean>

<bean id="debugInterceptor" class="org.springframework.aop.interceptor.DebugInterceptor"/>

<bean id="person" class="org.springframework.aop.framework.ProxyFactoryBean">
	<property name="proxyInterfaces" value="com.mycompany.Person"/>
	<!-- Use inner bean, not local reference to target -->
	<property name="target">
		<bean class="com.mycompany.PersonImpl">
			<property name="name" value="Tony"/>
			<property name="age" value="51"/>
		</bean>
	</property>
	<property name="interceptorNames">
		<list>
			<value>myAdvisor</value>
			<value>debugInterceptor</value>
		</list>
	</property>
</bean>

Using an anonymous inner bean has the advantage that there is only one object of type Person. This is useful if we want to prevent users of the application context from obtaining a reference to the un-advised object or need to avoid any ambiguity with Spring IoC autowiring. There is also, arguably, an advantage in that the ProxyFactoryBean definition is self-contained. However, there are times when being able to obtain the un-advised target from the factory might actually be an advantage (for example, in certain test scenarios).

Proxying Classes

What if you need to proxy a class, rather than one or more interfaces?

Imagine that in our earlier example, there was no Person interface. We needed to advise a class called Person that did not implement any business interface. In this case, you can configure Spring to use CGLIB proxying rather than dynamic proxies. To do so, set the proxyTargetClass property on the ProxyFactoryBean shown earlier to true. While it is best to program to interfaces rather than classes, the ability to advise classes that do not implement interfaces can be useful when working with legacy code. (In general, Spring is not prescriptive. While it makes it easy to apply good practices, it avoids forcing a particular approach.)

If you want to, you can force the use of CGLIB in any case, even if you do have interfaces.

CGLIB proxying works by generating a subclass of the target class at runtime. Spring configures this generated subclass to delegate method calls to the original target. The subclass is used to implement the Decorator pattern, weaving in the advice.

CGLIB proxying should generally be transparent to users. However, there are some issues to consider:

final classes cannot be proxied, because they cannot be extended.
final methods cannot be advised, because they cannot be overridden.
private methods cannot be advised, because they cannot be overridden.
Methods that are not visible, typically package private methods in a parent class from a different package, cannot be advised because they are effectively private.

There is no need to add CGLIB to your classpath. CGLIB is repackaged and included in the spring-core JAR. In other words, CGLIB-based AOP works "out of the box", as do JDK dynamic proxies.

There is little performance difference between CGLIB proxies and dynamic proxies. Performance should not be a decisive consideration in this case.

Using “Global” Advisors

By appending an asterisk to an interceptor name, all advisors with bean names that match the part before the asterisk are added to the advisor chain. This can come in handy if you need to add a standard set of “global” advisors. The following example defines two global advisors:

<bean id="proxy" class="org.springframework.aop.framework.ProxyFactoryBean">
	<property name="target" ref="service"/>
	<property name="interceptorNames">
		<list>
			<value>global*</value>
		</list>
	</property>
</bean>

<bean id="global_debug" class="org.springframework.aop.interceptor.DebugInterceptor"/>
<bean id="global_performance" class="org.springframework.aop.interceptor.PerformanceMonitorInterceptor"/>

Using the ProxyFactoryBean to Create AOP Proxies