In addition to the provided advice classes described earlier, you can implement your own advice classes. While you can provide any implementation of org.aopalliance.aop.Advice (usually org.aopalliance.intercept.MethodInterceptor), we generally recommend that you subclass o.s.i.handler.advice.AbstractRequestHandlerAdvice. This has the benefit of avoiding the writing of low-level aspect-oriented programming code as well as providing a starting point that is specifically tailored for use in this environment.spring-doc.cn

Subclasses need to implement the doInvoke() method, the definition of which follows:spring-doc.cn

/**
 * Subclasses implement this method to apply behavior to the {@link MessageHandler} callback.execute()
 * invokes the handler method and returns its result, or null).
 * @param callback Subclasses invoke the execute() method on this interface to invoke the handler method.
 * @param target The target handler.
 * @param message The message that will be sent to the handler.
 * @return the result after invoking the {@link MessageHandler}.
 * @throws Exception
 */
protected abstract Object doInvoke(ExecutionCallback callback, Object target, Message<?> message) throws Exception;

The callback parameter is a convenience to avoid subclasses that deal with AOP directly. Invoking the callback.execute() method invokes the message handler.spring-doc.cn

The target parameter is provided for those subclasses that need to maintain state for a specific handler, perhaps by maintaining that state in a Map keyed by the target. This feature allows the same advice to be applied to multiple handlers. The RequestHandlerCircuitBreakerAdvice uses advice this to keep circuit breaker state for each handler.spring-doc.cn

The message parameter is the message sent to the handler. While the advice cannot modify the message before invoking the handler, it can modify the payload (if it has mutable properties). Typically, an advice would use the message for logging or to send a copy of the message somewhere before or after invoking the handler.spring-doc.cn

The return value would normally be the value returned by callback.execute(). However, the advice does have the ability to modify the return value. Note that only AbstractReplyProducingMessageHandler instances return values. The following example shows a custom advice class that extends AbstractRequestHandlerAdvice:spring-doc.cn

public class MyAdvice extends AbstractRequestHandlerAdvice {

    @Override
    protected Object doInvoke(ExecutionCallback callback, Object target, Message<?> message) throws Exception {
        // add code before the invocation
        Object result = callback.execute();
        // add code after the invocation
        return result;
    }
}

In addition to the execute() method, ExecutionCallback provides an additional method: cloneAndExecute(). This method must be used in cases where the invocation might be called multiple times within a single execution of doInvoke(), such as in the RequestHandlerRetryAdvice. This is required because the Spring AOP org.springframework.aop.framework.ReflectiveMethodInvocation object maintains state by keeping track of which advice in a chain was last invoked. This state must be reset for each call.spring-doc.cn

For more information, see the ReflectiveMethodInvocation Javadoc.spring-doc.cn

In addition to the execute() method, ExecutionCallback provides an additional method: cloneAndExecute(). This method must be used in cases where the invocation might be called multiple times within a single execution of doInvoke(), such as in the RequestHandlerRetryAdvice. This is required because the Spring AOP org.springframework.aop.framework.ReflectiveMethodInvocation object maintains state by keeping track of which advice in a chain was last invoked. This state must be reset for each call.spring-doc.cn

For more information, see the ReflectiveMethodInvocation Javadoc.spring-doc.cn