开发那点事系列二 - ClassLoader trouble shooting references

         工作中需要解决一些servlet容器类加载器的问题，尤其是Jboss 4.x系列，比方说log4j版本冲突需要靠更改配置项Java2ClassLoadingCompliance，UseJBossWebLoader；ear包部署，出现NoSuchMethodError,NoSuchFieldError,NoClassDefFoundError等(二进制兼容错误)需要进行类隔离（在 JVM加上-XX:+TraceClassLoading  -XX:+TraceClassUnloading分析完类的加载卸载verbose）等。除此之外，还有一种ClassLoader问题比较常见。即：如果类库提供了 SPI 接口，并且利用线程上下文类加载器来加载 SPI 实现的 Java 类，有可能会找不到 Java 类。如果出现了 NoClassDefFoundError异常（初始类加载器，定义类加载器问题），则可以先检查当前线程的上下文类加载器是否正确。通过 Thread.currentThread().getContextClassLoader()就可以得到该类加载器。该类加载器应该是该模块对应的类加载器。如果不是的话，可以首先通过 class.getClassLoader()来得到模块对应的类加载器，再通过 Thread.currentThread().setContextClassLoader()来设置当前线程的上下文类加载器。关于上下文加载器，一个显著的使用场景是javax.xml.parsers.DocumentBuilderFactory类中的newInstance方法，跟进去后代码具体如下：

 /*
     * Try to find provider using Jar Service Provider Mechanism
     *
     * @return instance of provider class if found or null
     */
    private static Object findJarServiceProvider(String factoryId)
        throws ConfigurationError
    {
        String serviceId = "META-INF/services/" + factoryId;
        InputStream is = null;

        // First try the Context ClassLoader
        ClassLoader cl = ss.getContextClassLoader();
        boolean useBSClsLoader = false;
        if (cl != null) {
            is = ss.getResourceAsStream(cl, serviceId);

            // If no provider found then try the current ClassLoader
            if (is == null) {
                cl = FactoryFinder.class.getClassLoader();
                is = ss.getResourceAsStream(cl, serviceId);
                useBSClsLoader = true;
            }
        } else {
            // No Context ClassLoader, try the current ClassLoader
            cl = FactoryFinder.class.getClassLoader();
            is = ss.getResourceAsStream(cl, serviceId);
            useBSClsLoader = true;
        }

        if (is == null) {
            // No provider found
            return null;
        }

        if (debug) {    // Extra check to avoid computing cl strings
            dPrint("found jar resource=" + serviceId + " using ClassLoader: " + cl);
        }

        BufferedReader rd;
        try {
            rd = new BufferedReader(new InputStreamReader(is, "UTF-8"));
        }
        catch (java.io.UnsupportedEncodingException e) {
            rd = new BufferedReader(new InputStreamReader(is));
        }

        String factoryClassName = null;
        try {
            // XXX Does not handle all possible input as specified by the
            // Jar Service Provider specification
            factoryClassName = rd.readLine();
            rd.close();
        } catch (IOException x) {
            // No provider found
            return null;
        }

        if (factoryClassName != null && !"".equals(factoryClassName)) {
            dPrint("found in resource, value=" + factoryClassName);

            // Note: here we do not want to fall back to the current
            // ClassLoader because we want to avoid the case where the
            // resource file was found using one ClassLoader and the
            // provider class was instantiated using a different one.
            return newInstance(factoryClassName, cl, false, useBSClsLoader);
        }

        // No provider found
        return null;
    }

最后，将自己曾经参考过的文献资料分享给大家，希望能对大家有所帮助。

1. https://community.jboss.org/wiki/ClassLoadingConfiguration

2. http://docs.redhat.com/docs/en-US/JBoss_Enterprise_Application_Platform/4.2/html/Server_Configuration_Guide/Class_Loading_and_Types_in_Java-Inside_the_JBoss_Class_Loading_Architecture.html

3. https://community.jboss.org/wiki/JBossClassLoadingUseCases

4. https://community.jboss.org/wiki/ClassLoadingConfiguration

5. https://community.jboss.org/wiki/EnableClassloaderLogging

6. https://community.jboss.org/wiki/ClassLoadingConfiguration.pdf;jsessionid=EAFBE16C312C6DCCAF6F42840D80F4F6

7. https://community.jboss.org/wiki/SeparatingApplicationLogs

8. http://javarevisited.blogspot.com/2011/06/noclassdeffounderror-exception-in.html

9. http://www.artima.com/insidejvm/blurb.html

10. http://articles.qos.ch/classloader.html