深入理解JVM中的類加載機制

{"type":"doc","content":[{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#135ce0","name":"user"}}],"text":"0、引言","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"現如今，各種IDE越來越智能，我們程序員的日常開發基本上都是在IDE上完成的，它可以幫助我們將更多的注意力放在實際的業務處理中，隨着這種安逸的編碼生活的持續，我們慢慢也就忘記了代碼運行的底層原理。如果不學習，好像也沒啥問題，畢竟我們的關注重點是代碼邏輯實現上，當出現問題了，百度，谷歌一下，或者問問公司的狠人，問題好像也能愉快的解決，自己好像也理解了似的。但事實上呢，依此週而復始，仍舊不理解，學習一門技術，只有我們真正懂得了其底層原理，才能更好的解決問題。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#135ce0","name":"user"}}],"text":"1、類加載概述","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我們在前面幾篇文章中分別講解了","attrs":{}},{"type":"link","attrs":{"href":"https://mp.weixin.qq.com/s/of3CbfuAGzMevevwkno60w","title":null},"content":[{"type":"text","text":"類文件結構","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#036aca","name":"user"}}]},{"type":"text","text":"，","attrs":{}},{"type":"link","attrs":{"href":"https://mp.weixin.qq.com/s/kDnHV0D6aA-tUjK-fVco5w","title":null},"content":[{"type":"text","text":"JVM內存管理","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#036aca","name":"user"}}]},{"type":"text","text":"。這兩篇文章詳細描述了Class文件存儲格式的具體細節及JVM運行時數據區。而今天這篇文章將會講解Class文件中的信息進入到虛擬機中會發生什麼變化。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"**先來個官方敘述：**類加載是Java虛擬機把描述類的數據從Class文件加載到內存，並對數據進行校驗、轉換解析和初始化、最終形成可以被虛擬機直接使用的Java類型。通俗來講，就是我們在完成代碼的編寫後，編譯器會將我們的java文件編譯成對應的class文件(二進制字節碼文件)，通過類載器將這些class的時候將其加載到JVM中，生成對應的class對象。下面，讓我們詳細來分析下類加載過程。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#135ce0","name":"user"}}],"text":"2、類加載過程","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"對於任意一個類，類加載過程可以分爲","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"加載","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#1394d8","name":"user"}}],"attrs":{}},{"type":"text","text":"、","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"驗證","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#1394d8","name":"user"}}],"attrs":{}},{"type":"text","text":"、","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"準備","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#1394d8","name":"user"}}],"attrs":{}},{"type":"text","text":"、","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"解析","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#1394d8","name":"user"}}],"attrs":{}},{"type":"text","text":"、","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"使用","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#1394d8","name":"user"}}],"attrs":{}},{"type":"text","text":"和","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"卸載","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#1394d8","name":"user"}}],"attrs":{}},{"type":"text","text":"七個階段，如下圖所示：","attrs":{}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/1a/1afd8cde74443ab0ac42011ae27c10a7.png","alt":"image-20210128214026739","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"圖中的","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"加載","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#1394d8","name":"user"}}],"attrs":{}},{"type":"text","text":"、","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"驗證","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#1394d8","name":"user"}}],"attrs":{}},{"type":"text","text":"、","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"準備","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#1394d8","name":"user"}}],"attrs":{}},{"type":"text","text":"、","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"初始化","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#1394d8","name":"user"}}],"attrs":{}},{"type":"text","text":"和","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"卸載","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#1394d8","name":"user"}}],"attrs":{}},{"type":"text","text":"這五個階段的順序是確定的，而","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"解析","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#1394d8","name":"user"}}],"attrs":{}},{"type":"text","text":"則不一定，爲了支持java語言的","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"運行時綁定特性","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#1394d8","name":"u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