During development XML is used for specifying Android manifests and resources. Both manifests and resources are subsequently accessed by the runtime.
In the case of the SystemServer, it is actually the PackageManagerService which first accesses a manifest as it scans the framework directory (on a device this would be /system/framework but in our case isn’t) and encounters the
framework-res.apk
package.
It creates an instance of android.content.res.PackageParser to parse the package, and this in turn calls the android.content.res.AssetManager method
public final XmlResourceParser openXmlResourceParser(
int cookie,
String fileName)
throws
IOException
passing
AndroidManifest.xml
as the fileName argument.
This is a problem for us, because, yes you’ve guessed it, its a native method. Not much of a problem surely ? Java XML parsers are two a penny. True, but by the time the runtime encounters an XML file it is no longer in the de-facto XML format, it has in fact been transformed into a form of binary XML.
Fortunately the format is specified in the file
$(ANDROID_SRC)/frameworks/base/include/utils/ResourceTypes.h
along with a bunch of other stuff which we will be making use of later on.
At this point the interesting stuff starts at line 467. To begin with its not entirely obvious what’s going on, but after a litle bit of experimentation it becomes clear.
For example, a start tag is represented by a
struct ResXMLTree_node
followed by a
struct ResXMLTree_attrExt
followed by zero or more
struct ResXMLTree_attribute
so what it actually looks like in the file is
|
struct ResXMLTree_node | |
|
struct ResChunk_header | |
| type (0×0102) | uint16_t |
| headerSize | uint16_t |
| size | uint32_t |
| lineNumber | uint32_t |
| comment | uint32_t |
|
struct ResXMLTree_attrExt | |
| namespace | uint32_t |
| name | uint32_t |
| attributeStart | uint16_t |
| attributeSize | uint16_t |
| idAttributeIndex | uint16_t |
| classAttributeIndex | uint16_t |
| styleAttributeIndex | uint16_t |
|
struct ResXMLTree_attribute[0] | |
| namespace | uint32_t |
| name | uint32_t |
| rawValue | uint32_t |
| typedValue | Value |
|
… | |
Once the format is clear then the parser itself is straightforward.
Copyright (c) 2009 By Simon Lewis. All Rights Reserved.
