resources.arsc解析

2017-08-31  本文已影响366人  第八区

文章中所使用软件和代码资源

示例apk
示例代码
binary view二进制文件查看工具:
android 6.0系统源码（网上搜索下载，这里暂不提供资源）

chunk

整个文件是由一系列的chunk构成的，算是整个文件划分的基本单位吧，实际上就是把整个文件无差别的划分成多个模块，每个模块就是一个chunk，结构更加清晰。每个chunk是最前面是一个ResChunk_header的结构体，描述这个chunk的信息。

所有的chunk的header定义在android-6.0.0_r1\frameworks\base\include\androidfw\ResourceTypes.h

/** ********************************************************************
 *  Base Types
 *
 *  These are standard types that are shared between multiple specific
 *  resource types.
 *
 *********************************************************************** */

/**
 * Header that appears at the front of every data chunk in a resource.
 */
struct ResChunk_header
{
    // Type identifier for this chunk.  The meaning of this value depends
    // on the containing chunk.
    uint16_t type;

    // Size of the chunk header (in bytes).  Adding this value to
    // the address of the chunk allows you to find its associated data
    // (if any).
    uint16_t headerSize;

    // Total size of this chunk (in bytes).  This is the chunkSize plus
    // the size of any data associated with the chunk.  Adding this value
    // to the chunk allows you to completely skip its contents (including
    // any child chunks).  If this value is the same as chunkSize, there is
    // no data associated with the chunk.
    uint32_t size;
};

enum {
    RES_NULL_TYPE               = 0x0000,
    RES_STRING_POOL_TYPE        = 0x0001,
    RES_TABLE_TYPE              = 0x0002,
    RES_XML_TYPE                = 0x0003,

    // Chunk types in RES_XML_TYPE
    RES_XML_FIRST_CHUNK_TYPE    = 0x0100,
    RES_XML_START_NAMESPACE_TYPE= 0x0100,
    RES_XML_END_NAMESPACE_TYPE  = 0x0101,
    RES_XML_START_ELEMENT_TYPE  = 0x0102,
    RES_XML_END_ELEMENT_TYPE    = 0x0103,
    RES_XML_CDATA_TYPE          = 0x0104,
    RES_XML_LAST_CHUNK_TYPE     = 0x017f,
    // This contains a uint32_t array mapping strings in the string
    // pool back to resource identifiers.  It is optional.
    RES_XML_RESOURCE_MAP_TYPE   = 0x0180,

    // Chunk types in RES_TABLE_TYPE
    RES_TABLE_PACKAGE_TYPE      = 0x0200,
    RES_TABLE_TYPE_TYPE         = 0x0201,
    RES_TABLE_TYPE_SPEC_TYPE    = 0x0202,
    RES_TABLE_LIBRARY_TYPE      = 0x0203
};

如注释解释，每个chunk的头都会包含ResChunk_header,它是一个基础类型。上面的枚举定义了chunk的类型。ResChunk_header的成员变量解释：

文件header（ResTable_header）

resources.arsc整个文件内容也是一个chunk，是ResTable。头部为ResTable_header。我们看到定义

/** ********************************************************************
 *  RESOURCE TABLE
 *
 *********************************************************************** */

/**
 * Header for a resource table.  Its data contains a series of
 * additional chunks:
 *   * A ResStringPool_header containing all table values.  This string pool
 *     contains all of the string values in the entire resource table (not
 *     the names of entries or type identifiers however).
 *   * One or more ResTable_package chunks.
 *
 * Specific entries within a resource table can be uniquely identified
 * with a single integer as defined by the ResTable_ref structure.
 */
struct ResTable_header
{
    struct ResChunk_header header;

    // The number of ResTable_package structures.
    uint32_t packageCount;
};

这是整个资源表的头，资源中的chunk也是按照一定的顺序进行存储。

ResTable_header

从ResTable_header的结构体定义我们进行解析：

这里的二进制的存储是小端对齐。如果不懂的可以google补习一下。对uint32_t、uint16_t不太了解的也可以搜一下。这里我们就不跑题了。

到这里第一个chunk，也就是最外层的chunk的头我们已经解析完成了。

全局字符串池

紧接着是Global String Pool，全局字符串池，这也是Resources.arsc存在最重要的一个原因之一，就是把所有字符串放到这个池子里，大家复用这些字符串，可以很大的减小APK包的尺寸。

/**
 * Definition for a pool of strings.  The data of this chunk is an
 * array of uint32_t providing indices into the pool, relative to
 * stringsStart.  At stringsStart are all of the UTF-16 strings
 * concatenated together; each starts with a uint16_t of the string's
 * length and each ends with a 0x0000 terminator.  If a string is >
 * 32767 characters, the high bit of the length is set meaning to take
 * those 15 bits as a high word and it will be followed by another
 * uint16_t containing the low word.
 *
 * If styleCount is not zero, then immediately following the array of
 * uint32_t indices into the string table is another array of indices
 * into a style table starting at stylesStart.  Each entry in the
 * style table is an array of ResStringPool_span structures.
 */
struct ResStringPool_header
{
    struct ResChunk_header header;

    // Number of strings in this pool (number of uint32_t indices that follow
    // in the data).
    uint32_t stringCount;

    // Number of style span arrays in the pool (number of uint32_t indices
    // follow the string indices).
    uint32_t styleCount;

    // Flags.
    enum {
        // If set, the string index is sorted by the string values (based
        // on strcmp16()).
        SORTED_FLAG = 1<<0,

        // String pool is encoded in UTF-8
        UTF8_FLAG = 1<<8
    };
    uint32_t flags;

    // Index from header of the string data.
    uint32_t stringsStart;

    // Index from header of the style data.
    uint32_t stylesStart;
};

同样全局池的head里有一个ResChunk_header。我们继续解析resources.arsc文件


ResStringPool_header

除了ResStringPool_header头部、字符串内容块和字符串样式内容块之外，还有两个偏移数组，分别是字符串偏移数组和字符串样式偏移数组，这两个偏移数组的大小就分别等于字符串的数量stringCount和styleCount的值，而每一个元素都是一个无符号整数。整个字符中资源池的组成就如图13所示：


9.jpg

上面我们解析到stringCount=1704 ,1704*4=6816 而6816+28=6844正好等于stringsStart。

11.png

我们解析第一个字符串。在String Offset Array的前四个字节为0。所以它是从String Content的起始位置开始存储。由前面计算知道String content相对head为6844。则相对绝对地址为6844+12=6856=0x1AC8。上面截图显示。

无论是UTF8，还是UTF16的字符串编码，每一个字符串的前面都有2个字节表示其长度，而且后面以一个NULL字符结束。对于UTF8编码的字符串来说，NULL字符使用一个字节的0x00来表示，而对于UTF16编码的字符串来说，NULL字符使用两个字节的0x0000来表示。

的到的结果就是res/anim/abc_fade_out.xml。这个我们用到的appcompat兼容库中包含这个文件。


12.png

同样的方法再往后解析就是res/drawable/abc_list_selector_background_transition_holo_dark.xml，这个同样在兼容库中。

abc_list_selector_background_transition_holo_dark.xml

后面的解析以此类推，我就不做过多的介绍了

Package解析

/**
 * A collection of resource data types within a package.  Followed by
 * one or more ResTable_type and ResTable_typeSpec structures containing the
 * entry values for each resource type.
 */
struct ResTable_package
{
    struct ResChunk_header header;

    // If this is a base package, its ID.  Package IDs start
    // at 1 (corresponding to the value of the package bits in a
    // resource identifier).  0 means this is not a base package.
    uint32_t id;

    // Actual name of this package, \0-terminated.
    uint16_t name[128];

    // Offset to a ResStringPool_header defining the resource
    // type symbol table.  If zero, this package is inheriting from
    // another base package (overriding specific values in it).
    uint32_t typeStrings;

    // Last index into typeStrings that is for public use by others.
    uint32_t lastPublicType;

    // Offset to a ResStringPool_header defining the resource
    // key symbol table.  If zero, this package is inheriting from
    // another base package (overriding specific values in it).
    uint32_t keyStrings;

    // Last index into keyStrings that is for public use by others.
    uint32_t lastPublicKey;

    uint32_t typeIdOffset;
};

前面我们计算知道ResTable_header的大小为12，紧接着后面就是全局字符串chunk。大小为 59452。所以package前有59452+12=59464=0xE848。我们跳转到resources.arsc的这个地址处。

ResTable_package 6.png
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