android property system

转自：http://rxwen.blogspot.com/2010/01/android-property-system.html

Property system is an important feature on android. It runs as a service and manages system configurations and status. All these configurations and status are properties. A property is a key/value pair, both of which are of string type.
From the sense of function, it's very similar to . Many android applications and libraries directly or indirectly relies on this feature to determine their runtime behavior. For example, adbd process queries property service to check if it's running in emulator. Another example is the  returns the value stored in property service.

How property system works
The high level architecture of property system is shown as following.

In the figure, there are three processes, a group of persistent property files and a shared memory block. The shared memory block is the container of all property records. Only the property service process can write to the shared memory block. It'll load property records from persistent the save them in the shared memory.
The consumer process loads the shared memory in its own virtual space and access properties directly. The setter process also loads the shared memory in its virtual space, but it can't write to the memory directly. When the setter tries to add or update a property, it sends the property to property service via . The property service will write the property to shared memory on behalf of the setter process, as well as to the persistent file.

Property service runs inside . The init process first creates a shared memory region and stores a fd to the region. Then init process maps the region into its virtual space with  with MAP_SHARED flag, as a result, any updates to this area can be seen by all processes. This fd and region size are saved in a environment variable named "ANDROID_PROPERTY_WORKSPACE". Any other processes like consumer and setter will use this environment variable to get the fd and size, so that they can mmap this region into its own virtual space. The layout of the shared memory is shown below.

After that, init process will load properties from following files:
/default.prop
/system/build.prop
/system/default.prop
/data/local.prop

The next step is start property service. In this step, a unix domain socket server is created. This socket's pathname is "/dev/socket/property_service" which is well known to other client processes.
Finally, init process calls poll to wait for connect event on the socket.

On the consumer side, when it initializes libc(bionic/libc/bionic/libc_common.c __libc_init_common function). It will retrieve the fd and size from environment variable, and map the shared memory into its own space(bionic/libc/bionic/system_properties.c __system_properties_init function). After that, libcutils can read property just as normal memory for the consumer.

Currently, properties can't be removed. That's to say, once a property has been added, it can't be removed, neither can its key be changed.

How to get/set properties

There are three main means to get/set properies on android.

1. native code
When writing native applications, property_get and property_set APIs can be used to get/set properties. To use them, we need to includecutils/properties.h and link against libcutils.

2. java code
Android also provides  and  functions in java library, our java application can use them to get/set properties. 

But it's important to note that although these java APIs are semantically equal to native version, java version store data in a totally different place. Actually, a hashtable is employed by dalvik VM to store properties. So, java properties are separated, it can't get or set native properties, and neither vice versa.

Update:  mentioned that android.os.SystemProperties class can manipulate native properties, though it's intended for internal usage only. It calls through jni into native property library to get/set properties.

3. shell script
Android provides  and  command line tool to retrieve and update properties. They can be used in shell script. They are implemented on top of libcutils.