dex文件结构

文件头(File Header)

Dex文件头主要包括校验和以及其他结构的偏移地址和长度信息。

字段名称	偏移值	长度	描述
magic	0x0	8	'Magic'值，即魔数字段，格式如”dex/n035/0”，其中的035表示结构的版本。
checksum	0x8	4	校验码。
signature	0xC	20	SHA-1签名。
file_size	0x20	4	Dex文件的总长度。
header_size	0x24	4	文件头长度，009版本=0x5C,035版本=0x70。
endian_tag	0x28	4	标识字节顺序的常量,根据这个常量可以判断文件是否交换了字节顺序,缺省情况下=0x78563412。
link_size	0x2C	4	连接段的大小，如果为0就表示是静态连接。
link_off	0x30	4	连接段的开始位置，从本文件头开始算起。如果连接段的大小为0，这里也是0。
map_off	0x34	4	map数据基地址。
string_ids_size	0x38	4	字符串列表的字符串个数。
string_ids_off	0x3C	4	字符串列表表基地址。
type_ids_size	0x40	4	类型列表里类型个数。
type_ids_off	0x44	4	类型列表基地址。
proto_ids_size	0x48	4	原型列表里原型个数。
proto_ids_off	0x4C	4	原型列表基地址。
field_ids_size	0x50	4	字段列表里字段个数。
field_ids_off	0x54	4	字段列表基地址。
method_ids_size	0x58	4	方法列表里方法个数。
method_ids_off	0x5C	4	方法列表基地址。
class_defs_size	0x60	4	类定义类表中类的个数。
class_defs_off	0x64	4	类定义列表基地址。
data_size	0x68	4	数据段的大小，必须以4字节对齐。
data_off	0x6C	4	数据段基地址

魔数字段

     魔数字段，主要就是Dex文件的标识符，它占用4个字节，在目前的源码里是 “dex\n”，它的作用主要是用来标识dex文件的，比如有一个文件也以dex为后缀名，仅此并不会被认为是Davlik虚拟机运行的文件，还要判断这 四个字节。另外Davlik虚拟机也有优化的Dex，也是通过个字段来区分的，当它是优化的Dex文件时，它的值就变成”dey\n”了。根据这四个字 节，就可以识别不同类型的Dex文件了。

      跟在“dex\n”后面的是版本字段，主要用来标识Dex文件的版本。目前支持的版本号为“035\0”，不管是否优化的版本，都是使用这个版本号。

检验码字段

     主要用来检查从这个字段开始到文件结尾，这段数据是否完整，有没有人修改过，或者传送过程中是否有出错等等。通常用来检查数据是否完整的算法，有 CRC32、有SHA128等，但这里采用并不是这两类，而采用一个比较特别的算法，叫做adler32，这是在开源zlib里常用的算法，用来检查文件 是否完整性。该算法由MarkAdler发明，其可靠程度跟CRC32差不多，不过还是弱一点点，但它有一个很好的优点，就是使用软件来计算检验码时比较 CRC32要快很多。可见Android系统，就算法上就已经为移动设备进行优化了。

     Java中可使用java.util.zip.Adler32类做校验操作

 

SHA-1签名字段

     dex文件头里，前面已经有了面有一个4字节的检验字段码了，为什么还会有SHA-1签名字段呢？不是重复了吗？可是仔细考虑一下，这样设计自有道理。因 为dex文件一般都不是很小，简单的应用程序都有几十K，这么多数据使用一个4字节的检验码，重复的机率还是有的，也就是说当文件里的数据修改了，还是很 有可能检验不出来的。这时检验码就失去了作用，需要使用更加强大的检验码，这就是SHA-1。SHA-1校验码有20个字节，比前面的检验码多了16个字 节，几乎不会不同的文件计算出来的检验是一样的。设计两个检验码的目的，就是先使用第一个检验码进行快速检查，这样可以先把简单出错的dex文件丢掉了，
接着再使用第二个复杂的检验码进行复杂计算，验证文件是否完整，这样确保执行的文件完整和安全。

      SHA（Secure Hash Algorithm, 安全散列算法）是美国国家安全局设计，美国国家标准与技术研究院发布的一系列密码散列函数。SHA-1看起来和MD5算法很像，也许是Ron Rivest在SHA-1的设计中起了一定的作用。SHA-1的内部比MD5更强，其摘要比MD5的16字节长4个字节，这个算法成功经受了密码分析专家 的攻击，也因而受到密码学界的广泛推崇。这个算法在目前网络上的签名，BT软件里就有大量使用，比如在BT里要计算是否同一个种子时，就是利用文件的签名
来判断的。同一份8G的电影从几千BT用户那里下载，也不会出现错误的数据，导致电影不播放。

map_off字段

这个字段主要保存map开始位置，就是从文件头开始到map数据的长度，通过这个索引就可以找到map数据。map的数据结构如下：

名称	大小	说明
size	4字节	map里项的个数
list	变长	每一项定义为12字节，项的个数由上面项大小决定。

map数据排列结构定义如下：

 <wbr>
<span style="word-wrap: normal; word-break: normal; line-height: 20px;">typedef</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">struct</span> DexMapList <span style="word-wrap: normal; word-break: normal; line-height: 20px;">{</span>
u4 size<span style="word-wrap: normal; word-break: normal; line-height: 20px;">;</span>
DexMapItem list<span style="word-wrap: normal; word-break: normal; line-height: 20px;">[</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">1</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">]</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">;</span>
<span style="word-wrap: normal; word-break: normal; line-height: 20px;">}</span>DexMapList<span style="word-wrap: normal; word-break: normal; line-height: 20px;">;</span>
</wbr>

每一个map项的结构定义如下：

 <wbr>
<span style="word-wrap: normal; word-break: normal; line-height: 20px;">typedef</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">struct</span> DexMapItem <span style="word-wrap: normal; word-break: normal; line-height: 20px;">{</span>
u2 type<span style="word-wrap: normal; word-break: normal; line-height: 20px;">;</span>
u2 unused<span style="word-wrap: normal; word-break: normal; line-height: 20px;">;</span>
u4 size<span style="word-wrap: normal; word-break: normal; line-height: 20px;">;</span>
u4 offset<span style="word-wrap: normal; word-break: normal; line-height: 20px;">;</span>
<span style="word-wrap: normal; word-break: normal; line-height: 20px;">}</span>DexMapItem<span style="word-wrap: normal; word-break: normal; line-height: 20px;">;</span>
</wbr>

DexMapItem结构定义每一项的数据意义：类型、类型个数、类型开始位置。

其中的类型定义如下：

<span style="word-wrap: normal; word-break: normal; line-height: 20px;">enum</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">{</span>
kDexTypeHeaderItem <span style="word-wrap: normal; word-break: normal; line-height: 20px;">=</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">0x0000</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">,</span>
kDexTypeStringIdItem <span style="word-wrap: normal; word-break: normal; line-height: 20px;">=</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">0x0001</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">,</span>
kDexTypeTypeIdItem <span style="word-wrap: normal; word-break: normal; line-height: 20px;">=</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">0x0002</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">,</span>
kDexTypeProtoIdItem <span style="word-wrap: normal; word-break: normal; line-height: 20px;">=</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">0x0003</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">,</span>
kDexTypeFieldIdItem <span style="word-wrap: normal; word-break: normal; line-height: 20px;">=</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">0x0004</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">,</span>
kDexTypeMethodIdItem <span style="word-wrap: normal; word-break: normal; line-height: 20px;">=</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">0x0005</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">,</span>
kDexTypeClassDefItem <span style="word-wrap: normal; word-break: normal; line-height: 20px;">=</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">0x0006</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">,</span>
kDexTypeMapList <span style="word-wrap: normal; word-break: normal; line-height: 20px;">=</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">0x1000</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">,</span>
kDexTypeTypeList <span style="word-wrap: normal; word-break: normal; line-height: 20px;">=</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">0x1001</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">,</span>
kDexTypeAnnotationSetRef<wbr>List <span style="word-wrap: normal; word-break: normal; line-height: 20px;">=</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">0x1002</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">,</span>
kDexTypeAnnotationSetIte<wbr>m <span style="word-wrap: normal; word-break: normal; line-height: 20px;">=</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">0x1003</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">,</span>
kDexTypeClassDataItem <span style="word-wrap: normal; word-break: normal; line-height: 20px;">=</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">0x2000</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">,</span>
kDexTypeCodeItem <span style="word-wrap: normal; word-break: normal; line-height: 20px;">=</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">0x2001</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">,</span>
kDexTypeStringDataItem <span style="word-wrap: normal; word-break: normal; line-height: 20px;">=</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">0x2002</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">,</span>
kDexTypeDebugInfoItem <span style="word-wrap: normal; word-break: normal; line-height: 20px;">=</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">0x2003</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">,</span>
kDexTypeAnnotationItem <span style="word-wrap: normal; word-break: normal; line-height: 20px;">=</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">0x2004</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">,</span>
kDexTypeEncodedArrayItem<wbr> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">=</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">0x2005</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">,</span>
kDexTypeAnnotationsDirec<wbr>toryItem <span style="word-wrap: normal; word-break: normal; line-height: 20px;">=</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">0x2006</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">,</span>
<span style="word-wrap: normal; word-break: normal; line-height: 20px;">}</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">;</span>
</wbr></wbr></wbr></wbr>

 

从上面的类型可知，它包括了在dex文件里可能出现的所有类型。可以看出这里的类型与文件头里定义的类型有很多是一样的，这里的类型其实就是文件头里定义 的类型。其实这个map的数据，就是头里类型的重复，完全是为了检验作用而存在的。当Android系统加载dex文件时，如果比较文件头类型个数与 map里类型不一致时，就会停止使用这个dex文件

 

string_ids_size/off字段

 

这两个字段主要用来标识字符串资源。源程序编译后，程序里用到的字符串都保存在这个数据段里，以便解释执行这个dex文件使用。其中包括调用库函数里的类名称描述，用于输出显示的字符串等。

string_ids_size标识了有多少个字符串，string_ids_off标识字符串数据区的开始位置。字符串的存储结构如下：

<span style="word-wrap: normal; word-break: normal; line-height: 20px;">typedef</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">struct</span> DexStringId <span style="word-wrap: normal; word-break: normal; line-height: 20px;">{</span>
u4  stringDataOff<span style="word-wrap: normal; word-break: normal; line-height: 20px;">;</span>
<span style="word-wrap: normal; word-break: normal; line-height: 20px;">}</span> DexStringId<span style="word-wrap: normal; word-break: normal; line-height: 20px;">;</span>

可以看出这个数据区保存的只是字符串表的地址索引。如果要找到字符串的实际数据，还需要通过个地址索引找到文件的相应开始位置，然后才能得到字符串数据。 每一个字符串项的索引占用4个字节，因此这个数据区的大小就为4*string_ids_size。实际数据区中的字符串采用UTF8格式保存。

例如,如果dex文件使用16进制显示出来内容如下：

063c 696e 6974 3e00

其实际数据则是”\0”

另外这段数据中不仅包括字符串的字符串的内容和结束标志，在最开头的位置还标明了字符串的长度。上例中第一个字节06就是表示这个字符串有6个字符。

关于字符串的长度有两点需要注意的地方：

1、关于长度的编码格式

dex文件里采用了变长方式表示字符串长度。一个字符串的长度可能是一个字节（小于256）或者4个字节（1G大小以上）。字符串的长度大多数都是小于 256个字节，因此需要使用一种编码，既可以表示一个字节的长度，也可以表示4个字节的长度，并且1个字节的长度占绝大多数。能满足这种表示的编码方式有 很多，但dex文件里采用的是uleb128方式。leb128编码是一种变长编码，每个字节采用７位来表达原来的数据，最高位用来表示是否有后继字节。

它的编码算法如下：

DEX_INLINE u1<span style="word-wrap: normal; word-break: normal; line-height: 20px;">*</span> writeUnsignedLeb128<span style="word-wrap: normal; word-break: normal; line-height: 20px;">(</span>u1<span style="word-wrap: normal; word-break: normal; line-height: 20px;">*</span> ptr<span style="word-wrap: normal; word-break: normal; line-height: 20px;">,</span> u4 data<span style="word-wrap: normal; word-break: normal; line-height: 20px;">)</span>
<span style="word-wrap: normal; word-break: normal; line-height: 20px;">{</span>
<span style="word-wrap: normal; word-break: normal; line-height: 20px;">while</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">(</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">true</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">)</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">{</span>
u1 out <span style="word-wrap: normal; word-break: normal; line-height: 20px;">=</span> data <span style="word-wrap: normal; word-break: normal; line-height: 20px;">&</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">0x7f</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">;</span>
<span style="word-wrap: normal; word-break: normal; line-height: 20px;">if</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">(</span>out <span style="word-wrap: normal; word-break: normal; line-height: 20px;">!=</span> data<span style="word-wrap: normal; word-break: normal; line-height: 20px;">)</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">{</span>
<span style="word-wrap: normal; word-break: normal; line-height: 20px;">*</span>ptr<span style="word-wrap: normal; word-break: normal; line-height: 20px;">++</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">=</span> out <span style="word-wrap: normal; word-break: normal; line-height: 20px;">|</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">0x80</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">;</span>
data <span style="word-wrap: normal; word-break: normal; line-height: 20px;">>>=</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">7</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">;</span>
<span style="word-wrap: normal; word-break: normal; line-height: 20px;">}</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">else</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">{</span>
<span style="word-wrap: normal; word-break: normal; line-height: 20px;">*</span>ptr<span style="word-wrap: normal; word-break: normal; line-height: 20px;">++</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">=</span> out<span style="word-wrap: normal; word-break: normal; line-height: 20px;">;</span>
<span style="word-wrap: normal; word-break: normal; line-height: 20px;">break</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">;</span>
<span style="word-wrap: normal; word-break: normal; line-height: 20px;">}</span>
<span style="word-wrap: normal; word-break: normal; line-height: 20px;">}</span>
<span style="word-wrap: normal; word-break: normal; line-height: 20px;">return</span> ptr<span style="word-wrap: normal; word-break: normal; line-height: 20px;">;</span>
<span style="word-wrap: normal; word-break: normal; line-height: 20px;">}</span>

它的解码算法如下：

DEX_INLINE <span style="word-wrap: normal; word-break: normal; line-height: 20px;">int</span> readUnsignedLeb128<span style="word-wrap: normal; word-break: normal; line-height: 20px;">(</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">const</span> u1<span style="word-wrap: normal; word-break: normal; line-height: 20px;">**</span> pStream<span style="word-wrap: normal; word-break: normal; line-height: 20px;">)</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">{</span>
<span style="word-wrap: normal; word-break: normal; line-height: 20px;">const</span> u1<span style="word-wrap: normal; word-break: normal; line-height: 20px;">*</span> ptr <span style="word-wrap: normal; word-break: normal; line-height: 20px;">=</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">*</span>pStream<span style="word-wrap: normal; word-break: normal; line-height: 20px;">;</span>
<span style="word-wrap: normal; word-break: normal; line-height: 20px;">int</span> result <span style="word-wrap: normal; word-break: normal; line-height: 20px;">=</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">*</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">(</span>ptr<span style="word-wrap: normal; word-break: normal; line-height: 20px;">++</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">)</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">;</span>
<span style="word-wrap: normal; word-break: normal; line-height: 20px;">if</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">(</span>result <span style="word-wrap: normal; word-break: normal; line-height: 20px;">></span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">0x7f</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">)</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">{</span>
<span style="word-wrap: normal; word-break: normal; line-height: 20px;">int</span> cur <span style="word-wrap: normal; word-break: normal; line-height: 20px;">=</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">*</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">(</span>ptr<span style="word-wrap: normal; word-break: normal; line-height: 20px;">++</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">)</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">;</span>
result <span style="word-wrap: normal; word-break: normal; line-height: 20px;">=</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">(</span>result <span style="word-wrap: normal; word-break: normal; line-height: 20px;">&</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">0x7f</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">)</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">|</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">(</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">(</span>cur <span style="word-wrap: normal; word-break: normal; line-height: 20px;">&</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">0x7f</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">)</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;"><<</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">7</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">)</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">;</span>
<span style="word-wrap: normal; word-break: normal; line-height: 20px;">if</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">(</span>cur <span style="word-wrap: normal; word-break: normal; line-height: 20px;">></span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">0x7f</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">)</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">{</span>
cur <span style="word-wrap: normal; word-break: normal; line-height: 20px;">=</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">*</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">(</span>ptr<span style="word-wrap: normal; word-break: normal; line-height: 20px;">++</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">)</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">;</span>
result <span style="word-wrap: normal; word-break: normal; line-height: 20px;">|=</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">(</span>cur <span style="word-wrap: normal; word-break: normal; line-height: 20px;">&</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">0x7f</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">)</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;"><<</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">14</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">;</span>
<span style="word-wrap: normal; word-break: normal; line-height: 20px;">if</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">(</span>cur <span style="word-wrap: normal; word-break: normal; line-height: 20px;">></span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">0x7f</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">)</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">{</span>
cur <span style="word-wrap: normal; word-break: normal; line-height: 20px;">=</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">*</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">(</span>ptr<span style="word-wrap: normal; word-break: normal; line-height: 20px;">++</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">)</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">;</span>
result <span style="word-wrap: normal; word-break: normal; line-height: 20px;">|=</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">(</span>cur <span style="word-wrap: normal; word-break: normal; line-height: 20px;">&</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">0x7f</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">)</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;"><<</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">21</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">;</span>
<span style="word-wrap: normal; word-break: normal; line-height: 20px;">if</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">(</span>cur <span style="word-wrap: normal; word-break: normal; line-height: 20px;">></span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">0x7f</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">)</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">{</span>

cur <span style="word-wrap: normal; word-break: normal; line-height: 20px;">=</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">*</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">(</span>ptr<span style="word-wrap: normal; word-break: normal; line-height: 20px;">++</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">)</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">;</span>
result <span style="word-wrap: normal; word-break: normal; line-height: 20px;">|=</span> cur <span style="word-wrap: normal; word-break: normal; line-height: 20px;"><<</span> <span style="word-wrap: normal; word-break: normal; line-height: 20px;">28</span><span style="word-wrap: normal; word-break: normal; line-height: 20px;">;</span>
<span style="word-wrap: normal; word-break: normal; line-height: 20px;">}</span>
<span style="word-wrap: normal; word-break: normal; line-height: 20px;">}</span>
<span style="word-wrap: normal; word-break: normal; line-height: 20px;">}</span>
<span style="word-wrap: normal; word-break: normal; line-height: 20px;">}</span>
<span style="word-wrap: normal; word-break: normal; line-height: 20px;">*</span>pStream <span style="word-wrap: normal; word-break: normal; line-height: 20px;">=</span> ptr<span style="word-wrap: normal; word-break: normal; line-height: 20px;">;</span>
<span style="word-wrap: normal; word-break: normal; line-height: 20px;">return</span> result<span style="word-wrap: normal; word-break: normal; line-height: 20px;">;</span>
<span style="word-wrap: normal; word-break: normal; line-height: 20px;">}</span>

根据上面的算法分析上面例子字符串，取得第一个字节是06，最高位为0，因此没有后继字节，那么取出这个字节里7位有效数据，就是6，也就是说这个字符串是6个字节，但不包括结束字符“\0”。

2、关于长度的意义

由于字符串内容采用的是UTF-8格式编码，表示一个字符的字节数是不定的。即有时是一个字节表示一个字符，有时是两个、三个甚至四个字节表示一个字符。 而这里的长度代表的并不是整个字符串所占用的字节数，表示这个字符串包含的字符个数。所以在读取时需要注意，尤其是在包含中文字符时，往往会因为读取的长 度不正确导致字符串被截断。

转自： http://blog.csdn.net/androidsecurity/article/details/8664778