Java 内存中字节的大小

好的，已经进行了很多讨论，而没有很多代码:)

这是一个快速基准测试。谈到这种事情，通常会有一些警告-测试内存由于JITting等原因而有些奇怪，但是如果数量合适，反正还是很有用的。它有两种类型，每种类型都有80个成员-LotsOfBytes具有80个字节，LotsOfInts具有80个int。我们构建了很多它们，确保它们没有被GC处理，并检查内存使用情况：

class LotsOfBytes{    byte a0, a1, a2, a3, a4, a5, a6, a7, a8, a9, aa, ab, ac, ad, ae, af;    byte b0, b1, b2, b3, b4, b5, b6, b7, b8, b9, ba, bb, bc, bd, be, bf;    byte c0, c1, c2, c3, c4, c5, c6, c7, c8, c9, ca, cb, cc, cd, ce, cf;    byte d0, d1, d2, d3, d4, d5, d6, d7, d8, d9, da, db, dc, dd, de, df;    byte e0, e1, e2, e3, e4, e5, e6, e7, e8, e9, ea, eb, ec, ed, ee, ef;}class LotsOfInts{    int a0, a1, a2, a3, a4, a5, a6, a7, a8, a9, aa, ab, ac, ad, ae, af;    int b0, b1, b2, b3, b4, b5, b6, b7, b8, b9, ba, bb, bc, bd, be, bf;    int c0, c1, c2, c3, c4, c5, c6, c7, c8, c9, ca, cb, cc, cd, ce, cf;    int d0, d1, d2, d3, d4, d5, d6, d7, d8, d9, da, db, dc, dd, de, df;    int e0, e1, e2, e3, e4, e5, e6, e7, e8, e9, ea, eb, ec, ed, ee, ef;}public class Test{    private static final int SIZE = 1000000;    public static void main(String[] args) throws Exception    {     LotsOfBytes[] first = new LotsOfBytes[SIZE];        LotsOfInts[] second = new LotsOfInts[SIZE];        System.gc();        long startMem = getMemory();        for (int i=0; i < SIZE; i++)        { first[i] = new LotsOfBytes();        }        System.gc();        long endMem = getMemory();        System.out.println ("Size for LotsOfBytes: " + (endMem-startMem));        System.out.println ("Average size: " + ((endMem-startMem) / ((double)SIZE)));        System.gc();        startMem = getMemory();        for (int i=0; i < SIZE; i++)        { second[i] = new LotsOfInts();        }        System.gc();        endMem = getMemory();        System.out.println ("Size for LotsOfInts: " + (endMem-startMem));        System.out.println ("Average size: " + ((endMem-startMem) / ((double)SIZE)));        // Make sure nothing gets collected        long total = 0;        for (int i=0; i < SIZE; i++)        { total += first[i].a0 + second[i].a0;        }        System.out.println(total);    }    private static long getMemory()    {        Runtime runtime = Runtime.getRuntime();        return runtime.totalMemory() - runtime.freeMemory();    }}

在我的盒子上输出：

Size for LotsOfBytes: 88811688Average size: 88.811688Size for LotsOfInts: 327076360Average size: 327.076360

因此，显然存在一些开销-从外观上看为8个字节，尽管不知何故LotsOfInts只有7个字节（如我所说，这里有些奇怪）-但问题是字节字段似乎被打包成LotsOfBytes的样子， （除去开销后）仅占用LotsOfInts内存的四分之一。