使用Java的一個好處就是你可以不用親自來管理內存的分配和釋放。當你用new關鍵字來實例化一個對象時，它所需的內存會自動的在Java堆中分配。堆會被垃圾回收器進行管理，并且它會在對象超出作用域時進行內存回收。但是在JVM中有一個‘后門’可以讓你訪問不在堆中的本地內存(native memory)。在這篇文章中，我會給你演示一個對象是怎樣以連續的字節碼的方式在內存中進行存儲，并且告訴你是應該怎樣存儲這些字節，是在Java堆中還是在本地內存中。最后我會就怎樣從JVM中訪問內存更快給一些結論：是用Java堆還是本地內存。

使用Unsafe來分配和回收內存

sun.misc.Unsafe可以讓你在Java中分配和回收本地內存，就像C語言中的malloc和free。通過它分配的內存不在Java堆中，并且不受垃圾回收器的管理，因此在它被使用完的時候你需要自己來負責釋放和回收。下面是我寫的一個使用Unsafe來管理本地內存的一個工具類：

public class Direct implements Memory {private static Unsafe unsafe;private static boolean AVAILABLE = false;static {try {Field field = Unsafe.class.getDeclaredField("theUnsafe");field.setAccessible(true);unsafe = (Unsafe)field.get(null);AVAILABLE = true;} catch(Exception e) {// NOOP: throw exception later when allocating memory}}public static boolean isAvailable() {return AVAILABLE;}private static Direct INSTANCE = null;public static Memory getInstance() {if (INSTANCE == null) {INSTANCE = new Direct();}return INSTANCE;}private Direct() {}@Overridepublic long alloc(long size) {if (!AVAILABLE) {throw new IllegalStateException("sun.misc.Unsafe is not accessible!");}return unsafe.allocateMemory(size);}@Overridepublic void free(long address) {unsafe.freeMemory(address);}@Overridepublic final long getLong(long address) {return unsafe.getLong(address);}@Overridepublic final void putLong(long address, long value) {unsafe.putLong(address, value);}@Overridepublic final int getInt(long address) {return unsafe.getInt(address);}@Overridepublic final void putInt(long address, int value) {unsafe.putInt(address, value);}}

在本地內存中分配一個對象
讓我們來將下面的Java對象放到本地內存中：

public class SomeObject {private long someLong;private int someInt;public long getSomeLong() {return someLong;}public void setSomeLong(long someLong) {this.someLong = someLong;}public int getSomeInt() {return someInt;}public void setSomeInt(int someInt) {this.someInt = someInt;}}

我們所做的僅僅是把對象的屬性放入到Memory中：

public class SomeMemoryObject {private final static int someLong_OFFSET = 0;private final static int someInt_OFFSET = 8;private final static int SIZE = 8 + 4; // one long + one intprivate long address;private final Memory memory;public SomeMemoryObject(Memory memory) {this.memory = memory;this.address = memory.alloc(SIZE);}@Overridepublic void finalize() {memory.free(address);}public final void setSomeLong(long someLong) {memory.putLong(address + someLong_OFFSET, someLong);}public final long getSomeLong() {return memory.getLong(address + someLong_OFFSET);}public final void setSomeInt(int someInt) {memory.putInt(address + someInt_OFFSET, someInt);}public final int getSomeInt() {return memory.getInt(address + someInt_OFFSET);}}

現在我們來看看對兩個數組的讀寫性能：其中一個含有數百萬的SomeObject對象，另外一個含有數百萬的SomeMemoryObject對象。

// with JIT: Number of Objects:? 1,000???? 1,000,000???? 10,000,000??? 60,000,000 Heap Avg Write:????? 107???????? 2.30????????? 2.51???????? 2.58?????? Native Avg Write:??? 305???????? 6.65????????? 5.94???????? 5.26 Heap Avg Read:?????? 61????????? 0.31????????? 0.28???????? 0.28 Native Avg Read:???? 309???????? 3.50????????? 2.96???????? 2.16// without JIT: (-Xint) Number of Objects:? 1,000???? 1,000,000???? 10,000,000??? 60,000,000 Heap Avg Write:????? 104???????? 107?????????? 105???????? 102?????? Native Avg Write:??? 292???????? 293?????????? 300???????? 297 Heap Avg Read:?????? 59????????? 63??????????? 60????????? 58 Native Avg Read:???? 297???????? 298?????????? 302???????? 299

訪問一大塊的連續內存空間

結論：跨越JVM的屏障來讀本地內存大約會比直接讀Java堆中的內存慢10倍，而對于寫操作會慢大約2倍。但是需要注意的是，由于每一個SomeMemoryObject對象所管理的本地內存空間都是獨立的，因此讀寫操作都不是連續的。那么我們接下來就來對比下讀寫連續的內存空間的性能。

這個測試分別在堆中和一大塊連續本地內存中包含了相同的測試數據。然后我們來做多次的讀寫操作看看哪個更快。并且我們會做一些隨機地址的訪問來對比結果。

// with JIT and sequential access: Number of Objects:? 1,000???? 1,000,000???? 1,000,000,000 Heap Avg Write:????? 12????????? 0.34?????????? 0.35 Native Avg Write:??? 102???????? 0.71?????????? 0.69 Heap Avg Read:?????? 12????????? 0.29?????????? 0.28 Native Avg Read:???? 110???????? 0.32?????????? 0.32// without JIT and sequential access: (-Xint) Number of Objects:? 1,000???? 1,000,000????? 10,000,000 Heap Avg Write:????? 8?????????? 8????????????? 8 Native Avg Write:??? 91????????? 92???????????? 94 Heap Avg Read:?????? 10????????? 10???????????? 10 Native Avg Read:???? 91????????? 90???????????? 94// with JIT and random access: Number of Objects:? 1,000???? 1,000,000???? 1,000,000,000 Heap Avg Write:????? 61????????? 1.01?????????? 1.12 Native Avg Write:??? 151???????? 0.89?????????? 0.90 Heap Avg Read:?????? 59????????? 0.89?????????? 0.92 Native Avg Read:???? 156???????? 0.78?????????? 0.84// without JIT and random access: (-Xint) Number of Objects:? 1,000???? 1,000,000????? 10,000,000 Heap Avg Write:????? 55????????? 55????????????? 55 Native Avg Write:??? 141???????? 142???????????? 140 Heap Avg Read:?????? 55????????? 55????????????? 55 Native Avg Read:???? 138???????? 140???????????? 138

結論:在做連續訪問的時候，Java堆內存通常都比本地內存要快。對于隨機地址訪問，堆內存僅僅比本地內存慢一點點，并且是針對大塊連續數據的時候，而且沒有慢很多。

最后的結論

在Java中使用本地內存有它的意義，比如當你要操作大塊的數據時(>2G)并且不想使用垃圾回收器(GC)的時候。從延遲的角度來說，直接訪問本地內存不會比訪問Java堆快。這個結論其實是有道理的，因為跨越JVM屏障肯定是有開銷的。這樣的結論對使用本地還是堆的ByteBuffer同樣適用。使用本地ByteBuffer的速度提升不在于訪問這些內存，而是它可以直接與操作系統提供的本地IO進行操作。

總結

以上是生活随笔為你收集整理的哪个更快：Java 堆还是本地内存的全部內容，希望文章能夠幫你解決所遇到的問題。

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