如何在Android上的内存中放置Java对象？

Question

我非常熟悉HotSpot中堆上对象的布局,但对于Android来说并不是那么多.

例如,在32位HotSpot JVM中,堆上的对象实现为8字节头,后跟对象的字段(一个字节用于boolean引用,四个字节用于引用,其他所有内容按预期方式),布局以某种特定的顺序(对超类中的字段有一些特殊规则),并填充为8个字节的倍数.

我做了一些研究,但我找不到任何特定于Android的信息.

(我有兴趣优化一些极其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是你的朋友.评论struct Object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对象很特别; 它们的布局由ClassObject结构中的Object.h.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];
};

对于数组,宽度为vm/oo/Array.cpp.布尔值是宽度1,对象具有sizeof(Object*)长度(通常为4),并且所有其他基本类型具有其预期(打包)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(所有非类类实例)的布局由computeFieldOffsetsin控制vm/oo/Class.cpp.根据评论:

/*
 * 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,超类字段首先(像往常一样),然后是引用类型字段,后跟一个32位字段(如果可用,如果需要填充,因为有奇数个32位引用字段),后跟64比特字段.接下来是常规的32位字段.请注意,所有字段都是32位或64位(填充较短的基元).特别是,此时,VM不会使用少于4个字节存储字节/字符/短/布尔字段,但它理论上肯定可以支持这一点.

请注意,所有这些都是基于提交时读取Dalvik源代码43241340(2013年2月6日).由于VM的这个方面似乎没有公开记录,因此您不应该依赖它来对VM的对象布局进行稳定的描述:它可能会随着时间的推移而发生变化.