Android Property System

看了有中文翻译的，觉得还是看英文的比较直观，我没有说我的英文有多好，做技术都应该有这个体会

Every property has a name and value. Both name and value are text strings. Property is heavily used in Android to record system setting or exchange information between processes. The property is globally visible in the whole system. Every process can get/set a property.

On system initialization, Android will allocates a block of shared memory for storing the properties. This is done in “init” daemon whose source code is at: device/system/init. The “init” daemon will start a Property Service. The Property Service is running in the process of “init” daemon. Every client that wants to SET property needs to connect to the Property Service and send message to Property Service. Property Service will update/create the property in shared memory. Any client that wants to GET property can read the property from the shared memory directly. This promotes the read performance.

The client application can invoke the API function exposed from libcutils to GET/SET a property. The source code of libcutils locates at: device/libs/cutils.

The API function is:

int property_get(const char *key, char *value, const char *default_value);

int property_set(const char *key, const char *value);

The libcutils is in turn calling the __system_property_xxx function in libc to get a property from the shared memory. The source code of libc is at: device/system/bionic.

The Property Service is also in turn calling the __system_property_init function in libc to initiate the shared memory for properties. When starting the Property Service will load the default properties from below files:

/default.prop

/system/build.prop

/system/default.prop

/data/local.prop

The properties are loaded in the above order. Later loaded properties will override the previous values. After those properties are loaded, the last loaded is the persistent properties which is persisted in /data/property.

Special Properties

If a property’s name begins with “ro.”, then this property is treated as a read-only property. Once set, the value of the property can’t be changed.

If a property’s name begins with “persist.”, then when setting this property, the value will be written to /data/property, too.

If a property’s name begins with “net.”, when when setting this property, the “net.change” property will be set automatically to contain the name of the last updated property. (It’s tricky. The netresolve module uses this property to track if there is any change on the net.* properties.)

The property “ctrl.start” and “ctrl.stop” is used to start and stop a service. Every service must be defined in /init.rc. On system startup, the init daemon will parse the init.rc and start the Property Service. Once received a request to set the property of “ctrl.start”, the Property Service will use the property value as the service name to find the service and then start the service. The service starting result is then put to the property “init.svc.”. The client application can poll the value of that property to determine the result.

Android toolbox

The Android toolbox provides two applets: setprop and getprop to get and set properties. The usage is:

getprop

setprop  

Java

The java application can use the System.getProperty() and System.setProperty() function to Get and Set the property.

Action

By default the set property will only cause "init" daemon to write to shared memory, it won't execute any script or binary. But you can add your actions to correspond to property change in init.rc. For example, in the default init.rc, you can find.

# adbd on at boot in emulator
on property:ro.kernel.qemu=1
    start adbd

on property:persist.service.adb.enable=1
    start adbd

on property:persist.service.adb.enable=0
    stop adbd

So if you set persist.service.adb.enable to 1, the "init" daemon knows it has actions to do, then it will start adbd service.