動圖圖解GC算法 - 讓垃圾回收動起來！

{"type":"doc","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"提到Java中的垃圾回收，我相信很多小夥伴和我一樣，第一反應就是面試必問了，你要是沒背過點GC算法、收集器什麼的知識，出門都不敢說自己背過八股文。說起來還真是有點尷尬，工作中實際用到這方面知識的場景真是不多，並且這東西學起來也很枯燥，但是奈何面試官就是愛問，我們能有什麼辦法呢？","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"既然已經卷成了這樣，不學也沒有辦法，Hydra犧牲了週末時間，給大家畫了幾張動圖，希望通過這幾張圖，能夠幫助大家對垃圾收集算法有個更好的理解。廢話不多說，首先還是從基礎問題開始，看看怎麼判斷一個對象是否應該被回收。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"判斷對象存活","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"垃圾回收的根本目的是利用一些算法進行內存的管理，從而有效的利用內存空間，在進行垃圾回收前，需要判斷對象的存活情況，在jvm中有兩種判斷對象的存活算法，下面分別進行介紹。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"1、引用計數算法","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在對象中添加一個引用計數器，每當有一個地方引用它時計數器就加 1，當引用失效時計數器減 1。當計數器爲0的時候，表示當前對象可以被回收。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這種方法的原理很簡單，判斷起來也很高效，但是存在兩個問題：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"堆中對象每一次被引用和引用清除時，都需要進行計數器的加減法操作，會帶來性能損耗","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"當兩個對象相互引用時，計數器永遠不會0。也就是說，即使這兩個對象不再被程序使用，仍然沒有辦法被回收，通過下面的例子看一下循環引用時的計數問題：","attrs":{}}]}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public void reference(){\n A a = new A();\n B b = new B();\n a.instance = b;\n b.instance = a; \n}\n","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"引用計數的變化過程如下圖所示：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/80/809a124417a539d64d424bdc1dec7459.gif","alt":null,"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}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"可以看到，在方法執行完成後，棧中的引用被釋放，但是留下了兩個對象在堆內存中循環引用，導致了兩個實例最後的引用計數都不爲0，最終這兩個對象的內存將一直得不到釋放，也正是因爲這一缺陷，使引用計數算法並沒有被實際應用在gc過程中。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"2、可達性分析算法","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"可達性分析算法是jvm默認使用的尋找垃圾的算法，需要注意的是，雖然說的是","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"尋找垃圾","attrs":{}},{"type":"text","text":"，但實際上可達性分析算法尋找的是仍然存活的對象。至於這樣設計的理由，是因爲如果直接尋找沒有被引用的垃圾對象，實現起來相對複雜、耗時也會比較長，反過來標記存活的對象會更加省時。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"可達性分析算法的基本思路就是，以一系列被稱爲","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"GC Roots","attrs":{}},{"type":"text","text":"的對象作爲起始點，從這些節點開始向下搜索，搜索所走過的路徑稱爲","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"引用鏈","attrs":{}},{"type":"text","text":"，當一個對象到GC Roots沒有任何引用鏈相連時，證明該對象不再存活，可以作爲垃圾被回收。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/2f/2f9e3c8f60828c8a985bcfdc1e719965.gif","alt":null,"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}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在java中，可作爲GC Roots的對象有以下幾種：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在虛擬機棧（棧幀的本地變量表）中引用的對象","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在方法區中靜態屬性引用的對象","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在方法區中常量引用的對象","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在本地方法棧中JNI（","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"native","attrs":{}}],"attrs":{}},{"type":"text","text":"方法）引用的對象","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"jvm內部的引用，如基本數據類型對應的Class對象、一些常駐異常對象等，及系統類加載器","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"被同步鎖","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"synchronized","attrs":{}}],"attrs":{}},{"type":"text","text":"持有的對象引用","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"反映jvm內部情況的 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"JMXBean","attrs":{}}],"attrs":{}},{"type":"text","text":"、","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"JVMTI ","attrs":{}}],"attrs":{}},{"type":"text","text":"中註冊的回調本地代碼緩存等","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"此外還有一些","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"臨時性","attrs":{}},{"type":"text","text":"的GC Roots，這是因爲垃圾收集大多采用","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"分代收集","attrs":{}},{"type":"text","text":"和","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"局部回收","attrs":{}},{"type":"text","text":"，考慮到跨代或跨區域引用的對象時，就需要將這部分關聯的對象也添加到GC Roots中以確保準確性","attrs":{}}]}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"其中比較重要、同時提到的比較多的還是前面4種，其他的簡單瞭解一下即可。在瞭解了jvm是如何尋找垃圾對象之後，我們來看一看不同的垃圾收集算法的執行過程是怎樣的。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"垃圾收集算法","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"1、標記-清除算法","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"標記清除算法是一種非常基礎的垃圾收集算法，當堆中的有效內存空間耗盡時，會觸發STW（","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"stop the world","attrs":{}}],"attrs":{}},{"type":"text","text":"），然後分","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"標記","attrs":{}},{"type":"text","text":"和","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"清除","attrs":{}},{"type":"text","text":"兩階段來進行垃圾收集工作：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"標記：從GC 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