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Schema-based AOP Support

If you prefer an XML-based format, Spring also offers support for defining aspects using the aop namespace tags. The exact same pointcut expressions and advice kinds as when using the @AspectJ style are supported. Hence, in this section we focus on that syntax and refer the reader to the discussion in the previous section (@AspectJ support) for an understanding of writing pointcut expressions and the binding of advice parameters.

To use the aop namespace tags described in this section, you need to import the spring-aop schema, as described in XML Schema-based configuration . See the AOP schema for how to import the tags in the aop namespace.

Within your Spring configurations, all aspect and advisor elements must be placed within an <aop:config> element (you can have more than one <aop:config> element in an application context configuration). An <aop:config> element can contain pointcut, advisor, and aspect elements (note that these must be declared in that order).

The <aop:config> style of configuration makes heavy use of Spring’s auto-proxying mechanism. This can cause issues (such as advice not being woven) if you already use explicit auto-proxying through the use of BeanNameAutoProxyCreator or something similar. The recommended usage pattern is to use either only the <aop:config> style or only the AutoProxyCreator style and never mix them.

Declaring an Aspect

When you use the schema support, an aspect is a regular Java object defined as a bean in your Spring application context. The state and behavior are captured in the fields and methods of the object, and the pointcut and advice information are captured in the XML.

You can declare an aspect by using the <aop:aspect> element, and reference the backing bean by using the ref attribute, as the following example shows:

<aop:config>
	<aop:aspect id="myAspect" ref="aBean">
		...
	</aop:aspect>
</aop:config>

<bean id="aBean" class="...">
	...
</bean>

The bean that backs the aspect (aBean in this case) can of course be configured and dependency injected just like any other Spring bean.

Declaring a Pointcut

You can declare a named pointcut inside an <aop:config> element, letting the pointcut definition be shared across several aspects and advisors.

A pointcut that represents the execution of any business service in the service layer can be defined as follows:

<aop:config>

	<aop:pointcut id="businessService"
		expression="execution(* com.xyz.service.*.*(..))" />

</aop:config>

Note that the pointcut expression itself uses the same AspectJ pointcut expression language as described in @AspectJ support. If you use the schema based declaration style, you can also refer to named pointcuts defined in @Aspect types within the pointcut expression. Thus, another way of defining the above pointcut would be as follows:

<aop:config>

	<aop:pointcut id="businessService"
		expression="com.xyz.CommonPointcuts.businessService()" /> (1)

</aop:config>

1	References the `businessService` named pointcut defined in Sharing Named Pointcut Definitions.

Declaring a pointcut inside an aspect is very similar to declaring a top-level pointcut, as the following example shows:

<aop:config>

	<aop:aspect id="myAspect" ref="aBean">

		<aop:pointcut id="businessService"
			expression="execution(* com.xyz.service.*.*(..))"/>

		...
	</aop:aspect>

</aop:config>

In much the same way as an @AspectJ aspect, pointcuts declared by using the schema based definition style can collect join point context. For example, the following pointcut collects the this object as the join point context and passes it to the advice:

<aop:config>

	<aop:aspect id="myAspect" ref="aBean">

		<aop:pointcut id="businessService"
			expression="execution(* com.xyz.service.*.*(..)) &amp;&amp; this(service)"/>

		<aop:before pointcut-ref="businessService" method="monitor"/>

		...
	</aop:aspect>

</aop:config>

The advice must be declared to receive the collected join point context by including parameters of the matching names, as follows:

Java
Kotlin

public void monitor(Object service) {
	// ...
}

fun monitor(service: Any) {
	// ...
}

When combining pointcut sub-expressions, && is awkward within an XML document, so you can use the and, or, and not keywords in place of &&, ||, and !, respectively. For example, the previous pointcut can be better written as follows:

<aop:config>

	<aop:aspect id="myAspect" ref="aBean">

		<aop:pointcut id="businessService"
			expression="execution(* com.xyz.service.*.*(..)) and this(service)"/>

		<aop:before pointcut-ref="businessService" method="monitor"/>

		...
	</aop:aspect>

</aop:config>

Note that pointcuts defined in this way are referred to by their XML id and cannot be used as named pointcuts to form composite pointcuts. The named pointcut support in the schema-based definition style is thus more limited than that offered by the @AspectJ style.

Declaring Advice

The schema-based AOP support uses the same five kinds of advice as the @AspectJ style, and they have exactly the same semantics.

Before Advice

Before advice runs before a matched method execution. It is declared inside an <aop:aspect> by using the <aop:before> element, as the following example shows:

<aop:aspect id="beforeExample" ref="aBean">

	<aop:before
		pointcut-ref="dataAccessOperation"
		method="doAccessCheck"/>

	...

</aop:aspect>

In the example above, dataAccessOperation is the id of a named pointcut defined at the top (<aop:config>) level (see Declaring a Pointcut).

As we noted in the discussion of the @AspectJ style, using named pointcuts can significantly improve the readability of your code. See Sharing Named Pointcut Definitions for details.

To define the pointcut inline instead, replace the pointcut-ref attribute with a pointcut attribute, as follows:

<aop:aspect id="beforeExample" ref="aBean">

	<aop:before
		pointcut="execution(* com.xyz.dao.*.*(..))"
		method="doAccessCheck"/>

	...

</aop:aspect>

The method attribute identifies a method (doAccessCheck) that provides the body of the advice. This method must be defined for the bean referenced by the aspect element that contains the advice. Before a data access operation is performed (a method execution join point matched by the pointcut expression), the doAccessCheck method on the aspect bean is invoked.

After Returning Advice

After returning advice runs when a matched method execution completes normally. It is declared inside an <aop:aspect> in the same way as before advice. The following example shows how to declare it:

<aop:aspect id="afterReturningExample" ref="aBean">

	<aop:after-returning
		pointcut="execution(* com.xyz.dao.*.*(..))"
		method="doAccessCheck"/>

	...
</aop:aspect>

As in the @AspectJ style, you can get the return value within the advice body. To do so, use the returning attribute to specify the name of the parameter to which the return value should be passed, as the following example shows:

<aop:aspect id="afterReturningExample" ref="aBean">

	<aop:after-returning
		pointcut="execution(* com.xyz.dao.*.*(..))"
		returning="retVal"
		method="doAccessCheck"/>

	...
</aop:aspect>

The doAccessCheck method must declare a parameter named retVal. The type of this parameter constrains matching in the same way as described for @AfterReturning. For example, you can declare the method signature as follows:

Java
Kotlin

public void doAccessCheck(Object retVal) {...

fun doAccessCheck(retVal: Any) {...

After Throwing Advice

After throwing advice runs when a matched method execution exits by throwing an exception. It is declared inside an <aop:aspect> by using the after-throwing element, as the following example shows:

<aop:aspect id="afterThrowingExample" ref="aBean">

	<aop:after-throwing
		pointcut="execution(* com.xyz.dao.*.*(..))"
		method="doRecoveryActions"/>

	...
</aop:aspect>

As in the @AspectJ style, you can get the thrown exception within the advice body. To do so, use the throwing attribute to specify the name of the parameter to which the exception should be passed as the following example shows:

<aop:aspect id="afterThrowingExample" ref="aBean">

	<aop:after-throwing
		pointcut="execution(* com.xyz.dao.*.*(..))"
		throwing="dataAccessEx"
		method="doRecoveryActions"/>

	...
</aop:aspect>

The doRecoveryActions method must declare a parameter named dataAccessEx. The type of this parameter constrains matching in the same way as described for @AfterThrowing. For example, the method signature may be declared as follows:

Java
Kotlin

public void doRecoveryActions(DataAccessException dataAccessEx) {...

fun doRecoveryActions(dataAccessEx: DataAccessException) {...

After (Finally) Advice

After (finally) advice runs no matter how a matched method execution exits. You can declare it by using the after element, as the following example shows:

<aop:aspect id="afterFinallyExample" ref="aBean">

	<aop:after
		pointcut="execution(* com.xyz.dao.*.*(..))"
		method="doReleaseLock"/>

	...
</aop:aspect>

Around Advice

The last kind of advice is around advice. Around advice runs "around" a matched method’s execution. It has the opportunity to do work both before and after the method runs and to determine when, how, and even if the method actually gets to run at all. Around advice is often used if you need to share state before and after a method execution in a thread-safe manner – for example, starting and stopping a timer.

Always use the least powerful form of advice that meets your requirements.

For example, do not use around advice if before advice is sufficient for your needs.

You can declare around advice by using the aop:around element. The advice method should declare Object as its return type, and the first parameter of the method must be of type ProceedingJoinPoint. Within the body of the advice method, you must invoke proceed() on the ProceedingJoinPoint in order for the underlying method to run. Invoking proceed() without arguments will result in the caller’s original arguments being supplied to the underlying method when it is invoked. For advanced use cases, there is an overloaded variant of the proceed() method which accepts an array of arguments (Object[]). The values in the array will be used as the arguments to the underlying method when it is invoked. See Around Advice for notes on calling proceed with an Object[].

The following example shows how to declare around advice in XML:

<aop:aspect id="aroundExample" ref="aBean">

	<aop:around
		pointcut="execution(* com.xyz.service.*.*(..))"
		method="doBasicProfiling"/>

	...
</aop:aspect>

The implementation of the doBasicProfiling advice can be exactly the same as in the @AspectJ example (minus the annotation, of course), as the following example shows:

Java
Kotlin

public Object doBasicProfiling(ProceedingJoinPoint pjp) throws Throwable {
	// start stopwatch
	Object retVal = pjp.proceed();
	// stop stopwatch
	return retVal;
}

fun doBasicProfiling(pjp: ProceedingJoinPoint): Any? {
	// start stopwatch
	val retVal = pjp.proceed()
	// stop stopwatch
	return pjp.proceed()
}

Advice Parameters

The schema-based declaration style supports fully typed advice in the same way as described for the @AspectJ support — by matching pointcut parameters by name against advice method parameters. See Advice Parameters for details. If you wish to explicitly specify argument names for the advice methods (not relying on the detection strategies previously described), you can do so by using the arg-names attribute of the advice element, which is treated in the same manner as the argNames attribute in an advice annotation (as described in Determining Argument Names). The following example shows how to specify an argument name in XML:

<aop:before
	pointcut="com.xyz.Pointcuts.publicMethod() and @annotation(auditable)" (1)
	method="audit"
	arg-names="auditable" />

1	References the `publicMethod` named pointcut defined in Combining Pointcut Expressions.

The arg-names attribute accepts a comma-delimited list of parameter names.

The following slightly more involved example of the XSD-based approach shows some around advice used in conjunction with a number of strongly typed parameters:

Java
Kotlin

package com.xyz.service;

public interface PersonService {

	Person getPerson(String personName, int age);
}

public class DefaultPersonService implements PersonService {

	public Person getPerson(String name, int age) {
		return new Person(name, age);
	}
}

package com.xyz.service

interface PersonService {

	fun getPerson(personName: String, age: Int): Person
}

class DefaultPersonService : PersonService {

	fun getPerson(name: String, age: Int): Person {
		return Person(name, age)
	}
}

Next up is the aspect. Notice the fact that the profile(..) method accepts a number of strongly-typed parameters, the first of which happens to be the join point used to proceed with the method call. The presence of this parameter is an indication that the profile(..) is to be used as around advice, as the following example shows:

Java
Kotlin

package com.xyz;

import org.aspectj.lang.ProceedingJoinPoint;
import org.springframework.util.StopWatch;

public class SimpleProfiler {

	public Object profile(ProceedingJoinPoint call, String name, int age) throws Throwable {
		StopWatch clock = new StopWatch("Profiling for '" + name + "' and '" + age + "'");
		try {
			clock.start(call.toShortString());
			return call.proceed();
		} finally {
			clock.stop();
			System.out.println(clock.prettyPrint());
		}
	}
}

package com.xyz

import org.aspectj.lang.ProceedingJoinPoint
import org.springframework.util.StopWatch

class SimpleProfiler {

	fun profile(call: ProceedingJoinPoint, name: String, age: Int): Any? {
		val clock = StopWatch("Profiling for '$name' and '$age'")
		try {
			clock.start(call.toShortString())
			return call.proceed()
		} finally {
			clock.stop()
			println(clock.prettyPrint())
		}
	}
}

Finally, the following example XML configuration effects the execution of the preceding advice for a particular join point:

<beans xmlns="http://www.springframework.org/schema/beans"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xmlns:aop="http://www.springframework.org/schema/aop"
	xsi:schemaLocation="
		http://www.springframework.org/schema/beans
		https://www.springframework.org/schema/beans/spring-beans.xsd
		http://www.springframework.org/schema/aop
		https://www.springframework.org/schema/aop/spring-aop.xsd">

	<!-- this is the object that will be proxied by Spring's AOP infrastructure -->
	<bean id="personService" class="com.xyz.service.DefaultPersonService"/>

	<!-- this is the actual advice itself -->
	<bean id="profiler" class="com.xyz.SimpleProfiler"/>

	<aop:config>
		<aop:aspect ref="profiler">

			<aop:pointcut id="theExecutionOfSomePersonServiceMethod"
				expression="execution(* com.xyz.service.PersonService.getPerson(String,int))
				and args(name, age)"/>

			<aop:around pointcut-ref="theExecutionOfSomePersonServiceMethod"
				method="profile"/>

		</aop:aspect>
	</aop:config>

</beans>

Consider the following driver script:

Java
Kotlin

public class Boot {

	public static void main(String[] args) {
		ApplicationContext ctx = new ClassPathXmlApplicationContext("beans.xml");
		PersonService person = ctx.getBean(PersonService.class);
		person.getPerson("Pengo", 12);
	}
}

fun main() {
	val ctx = ClassPathXmlApplicationContext("beans.xml")
	val person = ctx.getBean(PersonService.class)
	person.getPerson("Pengo", 12)
}

With such a Boot class, we would get output similar to the following on standard output:

StopWatch 'Profiling for 'Pengo' and '12': running time (millis) = 0
-----------------------------------------
ms     %     Task name
-----------------------------------------
00000  ?  execution(getFoo)

Advice Ordering

When multiple pieces of advice need to run at the same join point (executing method) the ordering rules are as described in Advice Ordering. The precedence between aspects is determined via the order attribute in the <aop:aspect> element or by either adding the @Order annotation to the bean that backs the aspect or by having the bean implement the Ordered interface.

In contrast to the precedence rules for advice methods defined in the same @Aspect class, when two pieces of advice defined in the same <aop:aspect> element both need to run at the same join point, the precedence is determined by the order in which the advice elements are declared within the enclosing <aop:aspect> element, from highest to lowest precedence.

For example, given an around advice and a before advice defined in the same <aop:aspect> element that apply to the same join point, to ensure that the around advice has higher precedence than the before advice, the <aop:around> element must be declared before the <aop:before> element.

As a general rule of thumb, if you find that you have multiple pieces of advice defined in the same <aop:aspect> element that apply to the same join point, consider collapsing such advice methods into one advice method per join point in each <aop:aspect> element or refactor the pieces of advice into separate <aop:aspect> elements that you can order at the aspect level.

Introductions

Introductions (known as inter-type declarations in AspectJ) let an aspect declare that advised objects implement a given interface and provide an implementation of that interface on behalf of those objects.

You can make an introduction by using the aop:declare-parents element inside an aop:aspect. You can use the aop:declare-parents element to declare that matching types have a new parent (hence the name). For example, given an interface named UsageTracked and an implementation of that interface named DefaultUsageTracked, the following aspect declares that all implementors of service interfaces also implement the UsageTracked interface. (In order to expose statistics through JMX for example.)

<aop:aspect id="usageTrackerAspect" ref="usageTracking">

	<aop:declare-parents
		types-matching="com.xyz.service.*+"
		implement-interface="com.xyz.service.tracking.UsageTracked"
		default-impl="com.xyz.service.tracking.DefaultUsageTracked"/>

	<aop:before
		pointcut="execution(* com.xyz..service.*.*(..))
			and this(usageTracked)"
			method="recordUsage"/>

</aop:aspect>

The class that backs the usageTracking bean would then contain the following method:

Java
Kotlin

public void recordUsage(UsageTracked usageTracked) {
	usageTracked.incrementUseCount();
}

fun recordUsage(usageTracked: UsageTracked) {
	usageTracked.incrementUseCount()
}

The interface to be implemented is determined by the implement-interface attribute. The value of the types-matching attribute is an AspectJ type pattern. Any bean of a matching type implements the UsageTracked interface. Note that, in the before advice of the preceding example, service beans can be directly used as implementations of the UsageTracked interface. To access a bean programmatically, you could write the following:

Java
Kotlin

UsageTracked usageTracked = context.getBean("myService", UsageTracked.class);

val usageTracked = context.getBean("myService", UsageTracked.class)

Aspect Instantiation Models

The only supported instantiation model for schema-defined aspects is the singleton model. Other instantiation models may be supported in future releases.

Advisors

The concept of "advisors" comes from the AOP support defined in Spring and does not have a direct equivalent in AspectJ. An advisor is like a small self-contained aspect that has a single piece of advice. The advice itself is represented by a bean and must implement one of the advice interfaces described in Advice Types in Spring. Advisors can take advantage of AspectJ pointcut expressions.

Spring supports the advisor concept with the <aop:advisor> element. You most commonly see it used in conjunction with transactional advice, which also has its own namespace support in Spring. The following example shows an advisor:

<aop:config>

	<aop:pointcut id="businessService"
		expression="execution(* com.xyz.service.*.*(..))"/>

	<aop:advisor
		pointcut-ref="businessService"
		advice-ref="tx-advice" />

</aop:config>

<tx:advice id="tx-advice">
	<tx:attributes>
		<tx:method name="*" propagation="REQUIRED"/>
	</tx:attributes>
</tx:advice>

As well as the pointcut-ref attribute used in the preceding example, you can also use the pointcut attribute to define a pointcut expression inline.

To define the precedence of an advisor so that the advice can participate in ordering, use the order attribute to define the Ordered value of the advisor.

An AOP Schema Example

This section shows how the concurrent locking failure retry example from An AOP Example looks when rewritten with the schema support.

The execution of business services can sometimes fail due to concurrency issues (for example, a deadlock loser). If the operation is retried, it is likely to succeed on the next try. For business services where it is appropriate to retry in such conditions (idempotent operations that do not need to go back to the user for conflict resolution), we want to transparently retry the operation to avoid the client seeing a PessimisticLockingFailureException. This is a requirement that clearly cuts across multiple services in the service layer and, hence, is ideal for implementing through an aspect.

Because we want to retry the operation, we need to use around advice so that we can call proceed multiple times. The following listing shows the basic aspect implementation (which is a regular Java class that uses the schema support):

Java
Kotlin

public class ConcurrentOperationExecutor implements Ordered {

	private static final int DEFAULT_MAX_RETRIES = 2;

	private int maxRetries = DEFAULT_MAX_RETRIES;
	private int order = 1;

	public void setMaxRetries(int maxRetries) {
		this.maxRetries = maxRetries;
	}

	public int getOrder() {
		return this.order;
	}

	public void setOrder(int order) {
		this.order = order;
	}

	public Object doConcurrentOperation(ProceedingJoinPoint pjp) throws Throwable {
		int numAttempts = 0;
		PessimisticLockingFailureException lockFailureException;
		do {
			numAttempts++;
			try {
				return pjp.proceed();
			}
			catch(PessimisticLockingFailureException ex) {
				lockFailureException = ex;
			}
		} while(numAttempts <= this.maxRetries);
		throw lockFailureException;
	}
}

class ConcurrentOperationExecutor : Ordered {

	private val DEFAULT_MAX_RETRIES = 2

	private var maxRetries = DEFAULT_MAX_RETRIES
	private var order = 1

	fun setMaxRetries(maxRetries: Int) {
		this.maxRetries = maxRetries
	}

	override fun getOrder(): Int {
		return this.order
	}

	fun setOrder(order: Int) {
		this.order = order
	}

	fun doConcurrentOperation(pjp: ProceedingJoinPoint): Any? {
		var numAttempts = 0
		var lockFailureException: PessimisticLockingFailureException
		do {
			numAttempts++
			try {
				return pjp.proceed()
			} catch (ex: PessimisticLockingFailureException) {
				lockFailureException = ex
			}

		} while (numAttempts <= this.maxRetries)
		throw lockFailureException
	}
}

Note that the aspect implements the Ordered interface so that we can set the precedence of the aspect higher than the transaction advice (we want a fresh transaction each time we retry). The maxRetries and order properties are both configured by Spring. The main action happens in the doConcurrentOperation around advice method. We try to proceed. If we fail with a PessimisticLockingFailureException, we try again, unless we have exhausted all of our retry attempts.

This class is identical to the one used in the @AspectJ example, but with the annotations removed.

The corresponding Spring configuration is as follows:

<aop:config>

	<aop:aspect id="concurrentOperationRetry" ref="concurrentOperationExecutor">

		<aop:pointcut id="idempotentOperation"
			expression="execution(* com.xyz.service.*.*(..))"/>

		<aop:around
			pointcut-ref="idempotentOperation"
			method="doConcurrentOperation"/>

	</aop:aspect>

</aop:config>

<bean id="concurrentOperationExecutor"
	class="com.xyz.service.impl.ConcurrentOperationExecutor">
		<property name="maxRetries" value="3"/>
		<property name="order" value="100"/>
</bean>

Notice that, for the time being, we assume that all business services are idempotent. If this is not the case, we can refine the aspect so that it retries only genuinely idempotent operations, by introducing an Idempotent annotation and using the annotation to annotate the implementation of service operations, as the following example shows:

Java
Kotlin

@Retention(RetentionPolicy.RUNTIME)
// marker annotation
public @interface Idempotent {
}

@Retention(AnnotationRetention.RUNTIME)
// marker annotation
annotation class Idempotent

The change to the aspect to retry only idempotent operations involves refining the pointcut expression so that only @Idempotent operations match, as follows:

<aop:pointcut id="idempotentOperation"
		expression="execution(* com.xyz.service.*.*(..)) and
		@annotation(com.xyz.service.Idempotent)"/>