本篇內(nèi)容介紹了“java對(duì)象的內(nèi)存布局分為哪幾個(gè)區(qū)域”的有關(guān)知識(shí)，在實(shí)際案例的操作過(guò)程中，不少人都會(huì)遇到這樣的困境，接下來(lái)就讓小編帶領(lǐng)大家學(xué)習(xí)一下如何處理這些情況吧！希望大家仔細(xì)閱讀，能夠?qū)W有所成！
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HotSpot虛擬機(jī)中，對(duì)象在內(nèi)存中的布局分為三塊區(qū)域：對(duì)象頭、實(shí)例數(shù)據(jù)和對(duì)齊填充。
1）對(duì)象頭：包括標(biāo)記字段和類(lèi)型指針兩部分內(nèi)容(注:如果是數(shù)組對(duì)象，則包含三部分內(nèi)容)：
1)Mark Word(標(biāo)記字段)：用于存儲(chǔ)運(yùn)行時(shí)對(duì)象自身的數(shù)據(jù)。 1>占用內(nèi)存大小與虛擬機(jī)位長(zhǎng)一致，在運(yùn)行期間，考慮到JVM的空間效率，Mark Word被設(shè)計(jì)成為一個(gè)非固定的數(shù)據(jù)結(jié)構(gòu)，以便存儲(chǔ)更多有效的數(shù)據(jù)。 2>存儲(chǔ)運(yùn)行時(shí)對(duì)象自身的數(shù)據(jù)： 哈希碼(hash) GC分代年齡(age) 鎖標(biāo)識(shí)位： 01 無(wú)鎖 01 偏向鎖 00 輕量級(jí)鎖 10 重量級(jí)鎖 偏向鎖標(biāo)識(shí)位(biased_lock) 0 無(wú)鎖 1 偏向鎖 偏向線程ID(JavaThread*) 偏向時(shí)間戳(epoch) 說(shuō)明：鎖標(biāo)識(shí)位、偏向鎖標(biāo)識(shí)位、偏向線程ID等的具體實(shí)現(xiàn)均是在monitor對(duì)象中完成的。(源碼中的ObjectMonitor對(duì)象) 3>Mark Word里存儲(chǔ)的數(shù)據(jù)會(huì)隨著鎖標(biāo)志位的變化而變化，即不同的鎖狀態(tài)，存儲(chǔ)著不同的數(shù)據(jù)： 鎖狀態(tài) 存儲(chǔ)內(nèi)容 鎖標(biāo)識(shí)位 偏向鎖標(biāo)識(shí)位(是否是偏向鎖) -------- ------------------------------ -------- -------- 無(wú)鎖狀態(tài) 哈希碼、GC分代年齡 01 0 偏向鎖 線程ID、偏向時(shí)間戳、GC分代年齡 01 1 輕量級(jí)鎖 指向棧中鎖記錄的指針 00 無(wú) 重量級(jí)鎖 指向monitor的指針 10 無(wú) GC標(biāo)記 無(wú) 11 無(wú) 2)Class Metadata Address(類(lèi)型指針)：指向?qū)ο蟮念?lèi)元數(shù)據(jù)(方法區(qū)的Class數(shù)據(jù))，虛擬機(jī)通過(guò)這個(gè)指針確定該對(duì)象是哪個(gè)類(lèi)的實(shí)例。 3)如果對(duì)象是數(shù)組類(lèi)型，則對(duì)象頭中還存儲(chǔ)著數(shù)組的長(zhǎng)度。
2）實(shí)例數(shù)據(jù)：存放類(lèi)的屬性數(shù)據(jù)信息，包括父類(lèi)的屬性信息。
3）對(duì)齊填充：由于虛擬機(jī)要求對(duì)象起始地址必須是8字節(jié)的整數(shù)倍，填充數(shù)據(jù)不是必須存在的，僅僅是為了字節(jié)對(duì)齊。
4）使用JOL(Java Object Layout)工具查看java對(duì)象的內(nèi)存布局：
maven依賴(lài)：代碼： public class TestClassLayout { public static void main(String[] args) { TestClass[] testClassObj = new TestClass[5]; // 查看對(duì)象內(nèi)存布局 System.out.println(ClassLayout.parseInstance(testClassObj).toPrintable()); } } 結(jié)果： 1 [Lcom.jxn.test.TestClass; object internals: 2 OFFSET SIZE TYPE DESCRIPTION VALUE 3 0 4 (object header) 01 00 00 00 (00000001 00000000 00000000 00000000) (1) 4 4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0) 5 8 4 (object header) 82 c1 00 f8 (10000010 11000001 00000000 11111000) (-134168190) 6 12 4 (object header) 05 00 00 00 (00000101 00000000 00000000 00000000) (5) 7 16 20 com.jxn.test.TestClass TestClass;. org.openjdk.jol jol-core 0.14 N/A 8 36 4 (loss due to the next object alignment) 9 Instance size: 40 bytes 10 Space losses: 0 bytes internal + 4 bytes external = 4 bytes total 對(duì)象頭： 第3行+第4行： Mark Word(標(biāo)記字段) 第5行： 類(lèi)型指針 第6行： 數(shù)組長(zhǎng)度 實(shí)例數(shù)據(jù)： 第7行： 數(shù)組中存儲(chǔ)了5個(gè)TestClass對(duì)象的引用(開(kāi)啟指針壓縮后，類(lèi)型引用占4個(gè)字節(jié))，故占用5*4=20個(gè)字節(jié)。 對(duì)齊填充： 第8行： 對(duì)象頭+實(shí)例數(shù)據(jù) 占用的內(nèi)存為36字節(jié)(不是8的整數(shù)倍)，故需要4字節(jié)的對(duì)齊填充。 對(duì)象占用的總空間： 第9行 內(nèi)存浪費(fèi)的總空間： 第10行： 對(duì)齊填充消耗了4個(gè)字節(jié)。 說(shuō)明：若數(shù)組的長(zhǎng)度改為6，則 對(duì)象頭+實(shí)例數(shù)據(jù) 占用的內(nèi)存為40字節(jié)(8的整數(shù)倍)，故不會(huì)出現(xiàn)對(duì)齊填充： [Lcom.jxn.test.TestClass; object internals: OFFSET SIZE TYPE DESCRIPTION VALUE 0 4 (object header) 01 00 00 00 (00000001 00000000 00000000 00000000) (1) 4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0) 8 4 (object header) 82 c1 00 f8 (10000010 11000001 00000000 11111000) (-134168190) 12 4 (object header) 06 00 00 00 (00000110 00000000 00000000 00000000) (6) 16 24 com.jxn.test.TestClass TestClass;. N/A Instance size: 40 bytes Space losses: 0 bytes internal + 0 bytes external = 0 bytes total
5）hotspot/src/share/vm/oops/markOop.hpp 源碼中的說(shuō)明：
// The markOop describes the header of an object. // // Note that the mark is not a real oop but just a word. // It is placed in the oop hierarchy for historical reasons. // // Bit-format of an object header (most significant first, big endian layout below): // // 32 bits: // -------- // hash:25 ------------>| age:4 biased_lock:1 lock:2 (normal object) // JavaThread*:23 epoch:2 age:4 biased_lock:1 lock:2 (biased object) // size:32 ------------------------------------------>| (CMS free block) // PromotedObject*:29 ---------->| promo_bits:3 ----->| (CMS promoted object) // // 64 bits: // -------- // unused:25 hash:31 -->| unused:1 age:4 biased_lock:1 lock:2 (normal object) // JavaThread*:54 epoch:2 unused:1 age:4 biased_lock:1 lock:2 (biased object) // PromotedObject*:61 --------------------->| promo_bits:3 ----->| (CMS promoted object) // size:64 ----------------------------------------------------->| (CMS free block) // // unused:25 hash:31 -->| cms_free:1 age:4 biased_lock:1 lock:2 (COOPs && normal object) // JavaThread*:54 epoch:2 cms_free:1 age:4 biased_lock:1 lock:2 (COOPs && biased object) // narrowOop:32 unused:24 cms_free:1 unused:4 promo_bits:3 ----->| (COOPs && CMS promoted object) // unused:21 size:35 -->| cms_free:1 unused:7 ------------------>| (COOPs && CMS free block) // // - hash contains the identity hash value: largest value is // 31 bits, see os::random(). Also, 64-bit vm's require // a hash value no bigger than 32 bits because they will not // properly generate a mask larger than that: see library_call.cpp // and c1_CodePatterns_sparc.cpp. // // - the biased lock pattern is used to bias a lock toward a given // thread. When this pattern is set in the low three bits, the lock // is either biased toward a given thread or "anonymously" biased, // indicating that it is possible for it to be biased. When the // lock is biased toward a given thread, locking and unlocking can // be performed by that thread without using atomic operations. // When a lock's bias is revoked, it reverts back to the normal // locking scheme described below. // // Note that we are overloading the meaning of the "unlocked" state // of the header. Because we steal a bit from the age we can // guarantee that the bias pattern will never be seen for a truly // unlocked object. // // Note also that the biased state contains the age bits normally // contained in the object header. Large increases in scavenge // times were seen when these bits were absent and an arbitrary age // assigned to all biased objects, because they tended to consume a // significant fraction of the eden semispaces and were not // promoted promptly, causing an increase in the amount of copying // performed. The runtime system aligns all JavaThread* pointers to // a very large value (currently 128 bytes (32bVM) or 256 bytes (64bVM)) // to make room for the age bits & the epoch bits (used in support of // biased locking), and for the CMS "freeness" bit in the 64bVM (+COOPs). // // [JavaThread* | epoch | age | 1 | 01] lock is biased toward given thread // [0 | epoch | age | 1 | 01] lock is anonymously biased // // - the two lock bits are used to describe three states: locked/unlocked and monitor. // // [ptr | 00] locked ptr points to real header on stack // [header | 0 | 01] unlocked regular object header // [ptr | 10] monitor inflated lock (header is wapped out) // [ptr | 11] marked used by markSweep to mark an object not valid at any other time // // We assume that stack/thread pointers have the lowest two bits cleared.
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