Java对象在Android上是如何在内存中布局的

Question

我对HotSpot中堆上对象的布局相当熟悉，但对Android不太熟悉。

例如，在32位HotSpot JVM中，堆上的一个对象被实现为一个8字节的头，后面是对象的字段（一个字节用于boolean，四个字节用于引用，以及预期的其他所有字段），按特定顺序排列（对于超类中的字段有一些特殊规则），并填充为8字节的倍数。

我做了一些研究，但找不到任何安卓系统的特定信息。

（我感兴趣的是优化一些非常广泛使用的数据结构，以最大限度地减少Android上的内存消耗。）

Answer 1

dalvik/vm/oo/Object.h是您的朋友。struct Object的评论说：

/*
 * There are three types of objects:
 *  Class objects - an instance of java.lang.Class
 *  Array objects - an object created with a "new array" instruction
 *  Data objects - an object that is neither of the above
 *
 * We also define String objects.  At present they're equivalent to
 * DataObject, but that may change.  (Either way, they make some of the
 * code more obvious.)
 *
 * All objects have an Object header followed by type-specific data.
 */

java.lang.Class对象是特殊的；它们的布局由CCD_ 6中的CCD_。数组对象很简单：

struct ArrayObject : Object {
    /* number of elements; immutable after init */
    u4              length;
    /*
     * Array contents; actual size is (length * sizeof(type)).  This is
     * declared as u8 so that the compiler inserts any necessary padding
     * (e.g. for EABI); the actual allocation may be smaller than 8 bytes.
     */
    u8              contents[1];
};

对于阵列，宽度以vm/oo/Array.cpp为单位。布尔值的宽度为1，对象的长度为sizeof(Object*)（通常为4），所有其他基元类型都有其预期的（压缩的）长度。

数据对象非常简单：

/*
 * Data objects have an Object header followed by their instance data.
 */
struct DataObject : Object {
    /* variable #of u4 slots; u8 uses 2 slots */
    u4              instanceData[1];
};

DataObject（所有非Class类实例）的布局由vm/oo/Class.cpp中的computeFieldOffsets控制。根据那里的评论：

/*
 * Assign instance fields to u4 slots.
 *
 * The top portion of the instance field area is occupied by the superclass
 * fields, the bottom by the fields for this class.
 *
 * "long" and "double" fields occupy two adjacent slots.  On some
 * architectures, 64-bit quantities must be 64-bit aligned, so we need to
 * arrange fields (or introduce padding) to ensure this.  We assume the
 * fields of the topmost superclass (i.e. Object) are 64-bit aligned, so
 * we can just ensure that the offset is "even".  To avoid wasting space,
 * we want to move non-reference 32-bit fields into gaps rather than
 * creating pad words.
 *
 * In the worst case we will waste 4 bytes, but because objects are
 * allocated on >= 64-bit boundaries, those bytes may well be wasted anyway
 * (assuming this is the most-derived class).
 *
 * Pad words are not represented in the field table, so the field table
 * itself does not change size.
 *
 * The number of field slots determines the size of the object, so we
 * set that here too.
 *
 * This function feels a little more complicated than I'd like, but it
 * has the property of moving the smallest possible set of fields, which
 * should reduce the time required to load a class.
 *
 * NOTE: reference fields *must* come first, or precacheReferenceOffsets()
 * will break.
 */

因此，超类字段排在第一位（和往常一样），后面是引用类型字段，后面是一个32位字段（如果可用，如果因为有奇数个32位引用字段而需要填充），然后是64位字段。后面是常规的32位字段。请注意，所有字段都是32位或64位（较短的基元被填充）。特别是，在这个时候，虚拟机不会使用少于4个字节来存储byte/char/short/boolean字段，尽管它在理论上肯定可以支持这一点。

请注意，所有这些都是基于在提交43241340（2013年2月6日）时读取Dalvik源代码。由于虚拟机的这一方面似乎没有公开的文档，您不应该依赖它来稳定地描述虚拟机的对象布局：它可能会随着时间的推移而变化。