我已经从另一个答案中提到的java.lang.instrument.Instrumentation方法中获得了非常好的结果。有关它的使用示例，请参阅java专家通讯和java。SourceForge上的sizeOf库。

关于占用多少内存的规则取决于JVM实现和CPU架构(例如32位和64位)。

关于SUN JVM的详细规则，请查看我的旧博客

问候 马克

不，100个小对象比一个大对象需要更多的信息(内存)。

Mindprod指出，这不是一个简单的问题:

A JVM is free to store data any way it pleases internally, big or little endian, with any amount of padding or overhead, though primitives must behave as if they had the official sizes. For example, the JVM or native compiler might decide to store a boolean[] in 64-bit long chunks like a BitSet. It does not have to tell you, so long as the program gives the same answers. It might allocate some temporary Objects on the stack. It may optimize some variables or method calls totally out of existence replacing them with constants. It might version methods or loops, i.e. compile two versions of a method, each optimized for a certain situation, then decide up front which one to call. Then of course the hardware and OS have multilayer caches, on chip-cache, SRAM cache, DRAM cache, ordinary RAM working set and backing store on disk. Your data may be duplicated at every cache level. All this complexity means you can only very roughly predict RAM consumption.

测量方法

您可以使用Instrumentation.getObjectSize()来获得对象消耗的存储空间的估估值。

要可视化实际的对象布局、占用空间和引用，可以使用JOL (Java对象布局)工具。

对象头文件和对象引用

在现代64位JDK中，对象具有12字节的报头，填充为8字节的倍数，因此最小对象大小为16字节。对于32位jvm，开销为8字节，填充为4字节的倍数。(来自Dmitry Spikhalskiy的回答、Jayen的回答和JavaWorld。)

通常，引用在32位平台上为4字节，在64位平台上为-Xmx32G;32Gb以上8字节(-Xmx32G)。(参见压缩对象引用。)

因此，64位JVM通常需要30-50%以上的堆空间。我应该使用32位的JVM还是64位的JVM?， 2012, JDK 1.7)

盒装类型、数组和字符串

与原始类型相比，盒装包装器的开销更大(来自JavaWorld):

Integer: The 16-byte result is a little worse than I expected because an int value can fit into just 4 extra bytes. Using an Integer costs me a 300 percent memory overhead compared to when I can store the value as a primitive type Long: 16 bytes also: Clearly, actual object size on the heap is subject to low-level memory alignment done by a particular JVM implementation for a particular CPU type. It looks like a Long is 8 bytes of Object overhead, plus 8 bytes more for the actual long value. In contrast, Integer had an unused 4-byte hole, most likely because the JVM I use forces object alignment on an 8-byte word boundary.

其他容器也很昂贵:

Multidimensional arrays: it offers another surprise. Developers commonly employ constructs like int[dim1][dim2] in numerical and scientific computing. In an int[dim1][dim2] array instance, every nested int[dim2] array is an Object in its own right. Each adds the usual 16-byte array overhead. When I don't need a triangular or ragged array, that represents pure overhead. The impact grows when array dimensions greatly differ. For example, a int[128][2] instance takes 3,600 bytes. Compared to the 1,040 bytes an int[256] instance uses (which has the same capacity), 3,600 bytes represent a 246 percent overhead. In the extreme case of byte[256][1], the overhead factor is almost 19! Compare that to the C/C++ situation in which the same syntax does not add any storage overhead. String: a String's memory growth tracks its internal char array's growth. However, the String class adds another 24 bytes of overhead. For a nonempty String of size 10 characters or less, the added overhead cost relative to useful payload (2 bytes for each char plus 4 bytes for the length), ranges from 100 to 400 percent.

对齐

考虑这个例子对象:

class X {                      // 8 bytes for reference to the class definition
   int a;                      // 4 bytes
   byte b;                     // 1 byte
   Integer c = new Integer();  // 4 bytes for a reference
}

naïve和表明X的一个实例将使用17个字节。然而，由于对齐(也称为填充)，JVM以8字节的倍数分配内存，因此它将分配24字节而不是17字节。

在32位系统上，每个对象的开销似乎是16字节(在64位系统上是24字节)。

http://algs4.cs.princeton.edu/14analysis/是一个很好的信息来源。下面是一个很好的例子。

http://www.cs.virginia.edu/kim/publicity/pldi09tutorials/memory-efficient-java-tutorial.pdf的信息也非常丰富，例如:

每个对象对于其关联的监视器和类型信息以及字段本身都有一定的开销。除此之外，字段可以按照JVM认为合适的方式进行布局(我相信)——但正如另一个答案所示，至少有些JVM会相当紧密地打包。考虑这样一个类:

public class SingleByte
{
    private byte b;
}

vs

public class OneHundredBytes
{
    private byte b00, b01, ..., b99;
}

On a 32-bit JVM, I'd expect 100 instances of SingleByte to take 1200 bytes (8 bytes of overhead + 4 bytes for the field due to padding/alignment). I'd expect one instance of OneHundredBytes to take 108 bytes - the overhead, and then 100 bytes, packed. It can certainly vary by JVM though - one implementation may decide not to pack the fields in OneHundredBytes, leading to it taking 408 bytes (= 8 bytes overhead + 4 * 100 aligned/padded bytes). On a 64 bit JVM the overhead may well be bigger too (not sure).

编辑:见下面的评论;显然HotSpot的边界是8字节，而不是32字节，因此每个SingleByte实例将占用16字节。

无论哪种方式，“单个大对象”至少与多个小对象一样有效——对于像这样的简单情况。