tomcat类加载顺序：
- Bootstrap classes of your JVM:jdk相关类
- System class loader classes：tomcat相关类
- /WEB-INF/classes of your web application：应用程序classes下相关类
- /WEB-INF/lib/*.jar of your web application：应用程序lib下相关类
- Common class loader classes ：tomcat中的公共类tomcat\lib
tomcat类加载的双亲委派模型：
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Bootstrap
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System
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Common
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Webapp1 Webapp2 ...
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在什么时候一个java class/interface会被卸载呢？Sun公司的原话是 这么说的："class or interface may be unloaded if and only if its class loader is unreachable. Classes loaded by the bootstrap loader may not be unloaded."