遭遇类加载冲突

 

 

OSGI Bundle和Web容器加载类冲突问题的分析和解决

http://blog.csdn.net/achilles12345/article/details/7260000

 

上面这篇文章解释的算是比较清楚的。我自己遭遇的问题如下（HSF使用了log4j,我自己也使用了log4j,然后问题就来了）：

 

信息: Dual registration of jndi stream handler: factory already defined
log4j:WARN No appenders could be found for logger (com.taobao.remoting.common).
log4j:WARN Please initialize the log4j system properly.
DeployServer started, listen at: 12201
log4j:ERROR Could not create an Appender. Reported error follows.
java.lang.ClassCastException: org.apache.log4j.DailyRollingFileAppender cannot b
e cast to org.apache.log4j.Appender
        at org.apache.log4j.xml.DOMConfigurator.parseAppender(DOMConfigurator.ja
va:175)
        at org.apache.log4j.xml.DOMConfigurator.findAppenderByName(DOMConfigurat
or.java:150)
        at org.apache.log4j.xml.DOMConfigurator.findAppenderByReference(DOMConfi
gurator.java:163)
        at org.apache.log4j.xml.DOMConfigurator.parseChildrenOfLoggerElement(DOM
Configurator.java:425)
        at org.apache.log4j.xml.DOMConfigurator.parseCategory(DOMConfigurator.ja
va:345)
        at org.apache.log4j.xml.DOMConfigurator.parse(DOMConfigurator.java:827)
        at org.apache.log4j.xml.DOMConfigurator.doConfigure(DOMConfigurator.java
:712)
        at org.apache.log4j.xml.DOMConfigurator.doConfigure(DOMConfigurator.java
:618)
        at org.apache.log4j.xml.DOMConfigurator.configure(DOMConfigurator.java:7
43)
        at com.taobao.config.client.ConfigClientLogger.initialize(ConfigClientMa
nager.java:52)
        at com.taobao.config.client.ConfigClientLogger.<clinit>(ConfigClientMana
ger.java:102)
        at com.taobao.config.client.SubscriberRegistrar.<clinit>(SubscriberRegis
trar.java:72)
        at com.taobao.hsf.model.metadata.component.MetadataComponent.subscriber(
MetadataComponent.java:106)
        at com.taobao.hsf.process.component.ProcessComponent.consume(ProcessComp
onent.java:107)
        at com.taobao.hsf.app.spring.util.HSFSpringConsumerBean.init(HSFSpringCo
nsumerBean.java:251)
        at com.taobao.hsf.app.spring.util.HSFSpringConsumerBean.afterPropertiesS
et(HSFSpringConsumerBean.java:264)
        at org.springframework.beans.factory.support.AbstractAutowireCapableBean
Factory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:1514)
        at org.springframework.beans.factory.support.AbstractAutowireCapableBean
Factory.initializeBean(AbstractAutowireCapableBeanFactory.java:1452)
        at org.springframework.beans.factory.support.AbstractAutowireCapableBean
Factory.doCreateBean(AbstractAutowireCapableBeanFactory.java:519)
        at org.springframework.beans.factory.support.AbstractAutowireCapableBean
Factory.createBean(AbstractAutowireCapableBeanFactory.java:456)
        at org.springframework.beans.factory.support.AbstractBeanFactory$1.getOb
ject(AbstractBeanFactory.java:294)
        at org.springframework.beans.factory.support.DefaultSingletonBeanRegistr
y.getSingleton(DefaultSingletonBeanRegistry.java:225)
        at org.springframework.beans.factory.support.AbstractBeanFactory.doGetB
e
an(AbstractBeanFactory.java:291)
        at org.springframework.beans.factory.support.AbstractBeanFactory.getBean
(AbstractBeanFactory.java:193)
        at org.springframework.beans.factory.support.DefaultListableBeanFactory.
preInstantiateSingletons(DefaultListableBeanFactory.java:591)
        at org.springframework.context.support.AbstractApplicationContext.finish
BeanFactoryInitialization(AbstractApplicationContext.java:918)
        at org.springframework.context.support.AbstractApplicationContext.refres
h(AbstractApplicationContext.java:469)
        at org.springframework.web.context.ContextLoader.configureAndRefreshWebA
pplicationContext(ContextLoader.java:383)
        at org.springframework.web.context.ContextLoader.initWebApplicationConte
xt(ContextLoader.java:283)
        at org.springframework.web.context.ContextLoaderListener.contextInitiali
zed(ContextLoaderListener.java:111)
        at org.apache.catalina.core.StandardContext.listenerStart(StandardContex
t.java:3934)
        at org.apache.catalina.core.StandardContext.start(StandardContext.java:4
429)
        at org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1045)

        at org.apache.catalina.core.StandardHost.start(StandardHost.java:722)
        at org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1045)

        at org.apache.catalina.core.StandardEngine.start(StandardEngine.java:443
)
        at org.apache.catalina.core.StandardService.start(StandardService.java:5
16)
        at org.apache.catalina.core.StandardServer.start(StandardServer.java:710
)
        at org.apache.catalina.startup.Catalina.start(Catalina.java:583)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.
java:39)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAcces
sorImpl.java:25)
        at java.lang.reflect.Method.invoke(Method.java:597)
        at org.apache.catalina.startup.Bootstrap.start(Bootstrap.java:288)
        at org.apache.catalina.startup.Bootstrap.main(Bootstrap.java:413)
log4j:ERROR Could not parse url [bundleresource://2/configclient.log4j.xml].
java.lang.NullPointerException
        at java.util.Hashtable.put(Hashtable.java:394)
        at org.apache.log4j.xml.DOMConfigurator.findAppenderByName(DOMConfigurat
or.java:151)
        at org.apache.log4j.xml.DOMConfigurator.findAppenderByReference(DOMConfi
gurator.java:163)
        at org.apache.log4j.xml.DOMConfigurator.parseChildrenOfLoggerElement(DOM
Configurator.java:425)
        at org.apache.log4j.xml.DOMConfigurator.parseCategory(DOMConfigurator.ja
va:345)
        at org.apache.log4j.xml.DOMConfigurator.parse(DOMConfigurator.java:827)
        at org.apache.log4j.xml.DOMConfigurator.doConfigure(DOMConfigurator.java
:712)
        at org.apache.log4j.xml.DOMConfigurator
.doConfigure(DOMConfigurator.java
:618)
        at org.apache.log4j.xml.DOMConfigurator.configure(DOMConfigurator.java:7
43)
        at com.taobao.config.client.ConfigClientLogger.initialize(ConfigClientMa
nager.java:52)
        at com.taobao.config.client.ConfigClientLogger.<clinit>(ConfigClientMana
ger.java:102)
        at com.taobao.config.client.SubscriberRegistrar.<clinit>(SubscriberRegis
trar.java:72)
        at com.taobao.hsf.model.metadata.component.MetadataComponent.subscriber(
MetadataComponent.java:106)
        at com.taobao.hsf.process.component.ProcessComponent.consume(ProcessComp
onent.java:107)
        at com.taobao.hsf.app.spring.util.HSFSpringConsumerBean.init(HSFSpringCo
nsumerBean.java:251)
        at com.taobao.hsf.app.spring.util.HSFSpringConsumerBean.afterPropertiesS
et(HSFSpringConsumerBean.java:264)
        at org.springframework.beans.factory.support.AbstractAutowireCapableBean
Factory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:1514)
        at org.springframework.beans.factory.support.AbstractAutowireCapableBean
Factory.initializeBean(AbstractAutowireCapableBeanFactory.java:1452)
        at org.springframework.beans.factory.support.AbstractAutowireCapableBean
Factory.doCreateBean(AbstractAutowireCapableBeanFactory.java:519)
        at org.springframework.beans.factory.support.AbstractAutowireCapableBean
Factory.createBean(AbstractAutowireCapableBeanFactory.java:456)
        at org.springframework.beans.factory.support.AbstractBeanFactory$1.getOb
ject(AbstractBeanFactory.java:294)
        at org.springframework.beans.factory.support.DefaultSingletonBeanRegistr
y.getSingleton(DefaultSingletonBeanRegistry.java:225)
        at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBe
an(AbstractBeanFactory.java:291)
        at org.springframework.beans.factory.support.AbstractBeanFactory.getBean
(AbstractBeanFactory.java:193)
        at org.springframework.beans.factory.support.DefaultListableBeanFactory.
preInstantiateSingletons(DefaultListableBeanFactory.java:591)
        at org.springframework.context.support.AbstractApplicationContext.finish
BeanFactoryInitialization(AbstractApplicationContext.java:918)
        at org.springframework.context.support.AbstractApplicationContext.refres
h(AbstractApplicationContext.java:469)
        at org.springframework.web.context.ContextLoader.configureAndRefreshWebA
pplicationContext(ContextLoader.java:383)
        at org.springframework.web.context.ContextLoader.initWebApplicationConte
xt(ContextLoader.java:283)
        at org.springframework.web.context.ContextLoaderListener.contextInitiali
zed(ContextLoaderListener.java:111)
        at org.apache.catalina.core.StandardContext.listenerStart(StandardContex
t.java:3934)
        at org.apache.catalina.core.StandardContext.start(StandardContext.java:4
429)
        at org.apache.catalina.core.C
ontainerBase.start(ContainerBase.java:1045)

        at org.apache.catalina.core.StandardHost.start(StandardHost.java:722)
        at org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1045)

        at org.apache.catalina.core.StandardEngine.start(StandardEngine.java:443
)
        at org.apache.catalina.core.StandardService.start(StandardService.java:5
16)
        at org.apache.catalina.core.StandardServer.start(StandardServer.java:710
)
        at org.apache.catalina.startup.Catalina.start(Catalina.java:583)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.
java:39)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAcces
sorImpl.java:25)
        at java.lang.reflect.Method.invoke(Method.java:597)
        at org.apache.catalina.startup.Bootstrap.start(Bootstrap.java:288)
        at org.apache.catalina.startup.Bootstrap.main(Bootstrap.java:413)
[2012-12-14 10:07:52,121] [main] WARN  com.mchange.v2.c3p0.management.ActiveMana
gementCoordinator  - A C3P0Registry mbean is already registered. This probably m
eans that an application using c3p0 was undeployed, but not all PooledDataSource
s were closed prior to undeployment. This may lead to resource leaks over time.
Please take care to close all PooledDataSources.
2012-12-14 10:07:53 org.apache.coyote.http11.Http11Protocol start
信息: Starting Coyote HTTP/1.1 on http-8080
2012-12-14 10:07:53 org.apache.jk.common.ChannelSocket init
信息: JK: ajp13 listening on /0.0.0.0:8009
2012-12-14 10:07:53 org.apache.jk.server.JkMain start
信息: Jk running ID=0 time=0/16  config=null
2012-12-14 10:07:53 org.apache.catalina.startup.Catalina start
信息: Server startup in 6509 ms

 核心指示是这一句：

 

java.lang.ClassCastException: org.apache.log4j.DailyRollingFileAppender cannot b
e cast to org.apache.log4j.Appender

 

根源就是org.apache.log4j.DailyRollingFileAppender的一个实例无法转换成org.apache.log4j.Appender，

虽然，这两个类存在是子-父关系，但是，DailyRollingFileAppender的实例所对应的类与org.apache.log4j.Appender不是由同一个类加载器加载，这就导致无法转型。

注意，具体分析的话，需要调试，把类的加载器给弄出来，才能真正搞清楚。因为，因为当前类的上下文加载器是可以被更换的，所以，产生冲突的情形会更复杂，需要具体场景具体分析。

至于为什么会产生转型，多半是将对象放入了某个容器，然后不同的上下文将容器中的实例取出来，这个时候就会发生转型失败。因为类型是兼容的，所以，编译器无法把关。只有在运行时才会报错。

    稍微提一下，淘宝的HSF在反序列化时，可以访问应用自身的类，那是因为HSF使用web context classloader来载入这样的实体类，反序列化后，返回给HSF消费者。这时可以确保和消费者是同一个类的加载器加载，所以，就不再会发生转型失败。