JVM又曾放過誰，垃圾終將被回收！

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此時，空閒區間已經與活動區間交換，而垃圾對象現在已經全部留在了原來的活動區間，也就是現在的空閒區間。事實上，在活動區間轉換爲空間區間的同時，垃圾對象已經被一次性全部回收。","attrs":{}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/ab/ab1f07e29f1e2bb42e9a080921e8c6da.png","alt":"image-20210205175603999","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","marks":[{"type":"strong","attrs":{}}],"text":"標記整理算法","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"標記整理算法與標記清除算法很相似，也是分爲兩個階段：標記和整理，。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"標記","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#1394d8","name":"user"}}],"attrs":{}},{"type":"text","text":"：它的第一個階段與標記/清除算法是一模一樣的，均是遍歷GC Roots，然後將存活的對象標記。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"整理","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#1394d8","name":"user"}}],"attrs":{}},{"type":"text","text":"：移動所有存活的對象，且按照內存地址次序依次排列，然後將末端內存地址以後的內存全部回收。因此，第二階段才稱爲整理階段","attrs":{}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/ce/cef2662d3234e0eee1f960e66296b823.png","alt":"image-20210205195350250","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","marks":[{"type":"strong","attrs":{}}],"text":"分代回收算法","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"目前大多數JVM垃圾收集器採用的算法都是分代回收算法，其思想是根據對象存活的生命週期將內存劃分成三個區域：","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"新生代","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#1394d8","name":"user"}}],"attrs":{}},{"type":"text","text":"、","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"老年代","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#1394d8","name":"user"}}],"attrs":{}},{"type":"text","text":"和","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"永久代","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#1394d8","name":"user"}}],"attrs":{}},{"type":"text","text":"。劃分區域可以參考堆的內存結構，新生代和老年代和堆中的劃分是一一對應，而永久代在Java8中已經用元空間代替了。","attrs":{}}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"新生代回收算法","attrs":{}}]}],"attrs":{}}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"新生代主要存放生命週期較短的對象，所有新生成的對象首先都應該放在新生代中的Eden區，回收時將Eden區存活對象複製到To Survivor區，然後清空eden區。當To Survivor區也存放滿了時，則將Eden區和To Survivor區存活對象複製到From Survivor區，然後清空Eden和這個From Survivor區，此時To Survivor區爲空，然後將To Survivor區和From Survivor區交換，即保持From Survivor區爲空， 如此往復。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"當From Survivor區不足以存放 Eden區和To Survivor區的存活對象時，就將存活對象直接存放到老年代。若是老年代也滿了就會觸發一次Full GC，也就是新生代、老年代都進行回收。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"一般情況下，新生代發生的GC也叫做Minor GC，MinorGC發生頻率比較高(不一定等Eden區滿了才觸發)。","attrs":{}}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"老年代回收算法","attrs":{}}]}],"attrs":{}}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在新生代中經歷了N次垃圾回收後仍然存活的對象，就會被放到年老代中。因此，可以認爲年老代中存放的都是一些生命週期較長的對象。老年代的內存比新生代也大很多(大概比例是1:2)，當老年代內存滿時觸發Major GC即Full GC，Full GC發生頻率比較低，老年代對象存活時間比較長，存活率標記高。","attrs":{}}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"永久代回收算法","attrs":{}}]}],"attrs":{}}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"永久代用於存放靜態文件，如Java類、方法等。永久代對垃圾回收沒有顯著影響，但是有些應用可能動態生成或者調用一些class，例如Hibernate 等，在這種時候需要設置一個比較大的永久代空間來存放這些運行過程中新增的類。在JDK1.8之前，永久代稱方法區，在JDK1.8後，永久代被稱爲元空間。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"堆結構中的不同分區中的算法選擇","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"事實上，JVM對堆中的不同區域（新生代和老年代）採用不同的算法。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"新生代比較適合複製算法，新生代有Eden、From Survivor和To Survivor三個區，因爲Eden區中對象會被複制到To Survivor，且From Survivor和To Survivor相互交換比較頻繁，所以採用複製算法。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"老年代中的對象的生命週期比較長，所以不適合複製算法，在老年代一般採用的是標記-整理/清除算法。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"參考文獻","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"[1]https://www.sohu.com/a/359103068_675634","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"[2]https://www.cnblogs.com/widget90/p/12932172.html","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"[3]https://baijiahao.baidu.com/s?id=1663281048126158605&wfr=spider&for=pc","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"[4]https://www.cnblogs.com/jichi/p/11139437.html","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"[5]深入理解JVM虛擬機值(第三版).周志華","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}}]}