Tomcat9.0（可能8.0也一樣）啓動過程中的配置文件加載，一部分是通過EngineConfig, HostConfig, ContextConfig來進行的
這3個類都實現自LifecycleListener，初始化過程中他們執行順序如下：
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add listener: EngineConfig(Digester 中 操作的)
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add listener: HostConfig(Digester 中 操作的)
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fire listener: EngineConfig, before_init
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fire listener: EngineConfig, after_init
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fire listener: EngineConfig, before_start
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fire listener: HostConfig, before_init
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fire listener: HostConfig, after_init
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fire listener: HostConfig, before_start
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fire listener: HostConfig, start
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add listener: ContextConfig(HostConfig中的deployWar時操作的)
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fire listener: ContextConfig, before_init
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fire listener: ContextConfig, after_init
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fire listener: ContextConfig, before_start
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fire listener: ContextConfig, configure_start
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fire listener: ContextConfig, start
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fire listener: ContextConfig, after_start
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fire listener: HostConfig, after_start
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fire listener: EngineConfig, start
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fire listener: EngineConfig, after_start
（畫圖很累了，我就不寫文字描述了）
