什麼是JVM？深入解析JVM原理！

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Frame，非基本類型的對象在JVM棧上僅存放一個指向堆的地址。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"6、堆（Heap）","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"它是JVM用來存儲對象實例以及數組值的區域，可以認爲Java中所有通過new創建的對象的內存都在此分配，Heap中的對象的內存需要等待GC進行回收。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"堆在JVM啓動的時候就被創建，堆中儲存了各種對象，這些對象被自動管理內存系統（Automatic Storage Management System），也就是常說的“Garbage Collector（垃圾回收器）”管理。這些對象無需、也無法顯示地被銷燬。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"JVM將Heap分爲兩塊：新生代New Generation和舊生代Old 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Buffer），其大小由JVM根據運行的情況計算而得，在TLAB上分配對象時不需要加鎖，因此JVM在給線程對象分配內存時會盡量的在TLAB上分配，在這種情況下JVM中分配對象內存的性能和C基本是一樣的，但如果對象過大的話則仍然要直接使用堆空間分配。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"TLAB僅作用於新生代的Eden Space，因此在編寫Java程序時，通常多個小的對象比大的對象分配起來更加高效。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"所有新創建的Object都將會存儲在新生代Young Generation中。如果Young Generation的數據在一次或多次GC後存活下來，那麼將被轉移到OldGeneration。新的Object總是創建在Eden Space。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"7、方法區域（Method Area）","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在Sun JDK中這塊區域對應的爲PermanetGeneration，又稱爲持久代。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"方法區域存放所加載類的信息（名稱、修飾符等）、類中的靜態變量、類中定義爲final類型的常量、類中的Field信息、類中的方法信息，當開發人員在程序中通過Class對象中的getName，isInstance等方法來獲取信息時，這些數據都來源於方法區域，同時方法區域也是","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"全局共享","attrs":{}},{"type":"text","text":"的，在一定條件下它也會被GC，當方法區域需要使用的內存超過其允許的大小時，就會拋出OutOfMemory的錯誤信息。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"8、運行時常量池（Runtime Constant Pool）","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"存放的爲類中的固定常量信息、方法和Field的引用信息等，其空間從方法區域中分配。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"9、本地方法堆棧（Native Method Stacks）","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"JVM採用本地方法堆來支持native方法的執行，此區域用於存儲每個native方法調用的狀態。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"10、JVM垃圾回收","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"GC的基本原理","attrs":{}},{"type":"text","text":"：將內存中不再被使用的對象進行回收，GC中用於回收的方法稱爲收集器，由於GC需要消耗一些資源和時間，Java在對對象生命週期特徵進行分析後，按照新生代、舊生代的方式來對對象進行收集，以儘可能的縮短GC對應用造成的暫停。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"對新生代的對象收集稱爲minor GC","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"對舊生代的對象收集稱爲Full GC","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"程序中主動調用System.gc（）強制執行的GC爲Full GC。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"不同的對象引用類型，GC會採用不同的方法進行回收，JVM對象的引用分爲了四種類型","attrs":{}},{"type":"text","text":"：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"①強引用：默認情況下，對象採用的均爲強引用（這個對象的實例沒有其他對象引用時， GC時纔會被回收）","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"②軟引用：軟引用是Java中提供的一種比較適合於緩存場景的應用（只有內存不夠的情況下才會被GC）","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"③弱引用：在GC時一定會被GC回收。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"④虛引用：虛引用只是用來得知對象是否被GC。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"​","attrs":{}}]}]}