Handle circular dependencies in servlet connections
---------------------------------------------------

                 Key: COCOON-2036
                 URL: https://issues.apache.org/jira/browse/COCOON-2036
             Project: Cocoon
          Issue Type: Bug
          Components: - Servlet service framework
    Affects Versions: 2.2-dev (Current SVN)
            Reporter: Alexander Klimetschek


Circular dependencies in servlet connections lead to a Spring exception [1]  in 
[2] that does not provide any help. The previous implementation (block:) did 
allow circular dependencies because they were not handled by spring but by 
custom code.

Solution would be either to allow them (probably difficult to implement with 
spring) or, if not, to provide a helpful warning message, that skips this 
problem. The latter could be a check for embeddedServlet==null and, if not, 
throw an exception saying "you might have a circular dependency in 
<servlet-foobar>".

---

[1]:

The exception is thrown after ServletFactoryBean.getObject() tries to create a 
proxy for the embeddedServlet, which is null in the case of a circular 
dependency (one of the circle endpoints is created, but the other will be null).

Caused by: org.springframework.aop.framework.AopConfigException: Can't proxy 
null object
        at 
org.springframework.aop.framework.ProxyFactory.<init>(ProxyFactory.java:49)
        at 
org.apache.cocoon.servletservice.spring.ServletFactoryBean.getObject(ServletFactoryBean.java:194)
        at 
org.springframework.beans.factory.support.AbstractBeanFactory.getObjectFromFactoryBean(AbstractBeanFactory.java:1211)

[2]:
    public Object getObject() throws Exception {
        ProxyFactory proxyFactory = new ProxyFactory(this.embeddedServlet);
        proxyFactory.addAdvice(new ServiceInterceptor());
        if (this.mountPath != null) {
            proxyFactory.addAdvisor(new MountableMixinAdvisor());
        }
        proxyFactory.addAdvisor(new ServletServiceContextMixinAdvisor());
        return proxyFactory.getProxy();
    } 



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