Android Property实现介绍

1. property应用例子
Java:
import android.os.SystemProperties;
SystemProperties.get("persist.sys.usb.config", "mtp,adb");
SystemProperties.getBoolean("ro.adb.secure", false);
SystemProperties.set("sys.usb.config", config);
SystemProperties.set需要有system权限，可以这么做
a. 在Android.mk中，加上LOCAL_CERTIFICATE :=platform
b. 在AndroidManifest.xml中，加上android:sharedUserId="android.uid.system"

Native C:
#include
LOCAL_SHARED_LIBRARIES := \
libcutils

int property_set(const char *key, const char *value)
int property_get(const char *key, char *value, const char *default_value)
property_set("service.adb.root", "1");
property_get("ro.debuggable", value, "")

2. 设置属性的调用过程
SystemProperties.set in Java
set frameworks/base/core/java/android/os/SystemProperties.java
     native_set(key, val)
       SystemProperties_set frameworks/base/core/jni/android_os_SystemProperties.cpp
          property_set    /system/core/libcutils/properties.c

setprop in .rc/.sh file
setprop_main system/core/toolbox/setprop.c
property_set /system/core/libcutils/properties.c

property_set    /system/core/libcutils/properties.c
HAVE_LIBC_SYSTEM_PROPERTIES #define HAVE_LIBC_SYSTEM_PROPERTIES 1 in build/core/combo/include/arch/linux-arm/AndroidConfig.h
__system_property_set   bionic/libc/bionic/system_properties.c
     send_prop_msg(PROP_MSG_SETPROP)
       handle_property_set_onfd(case PROP_MSG_SETPROP) system/core/init/property_service.c
         检查权限
         if "ctl." 开头 handle_control_message()   system/core/init/init.c
              ctrl.start   msg_start()     启动service
              ctrl.stop    msg_stop()      停止service
              ctrl.restart msg_restart()   重启service
         else property_set()
             如果能找到且以ro开头，就返回-1；能找到且不以ro开头调用 __system_property_update()
             如果找不到则调用__system_property_add()
             如果是"net."开头，调用property_set("net.change", name)
             如果以"persist."开头，调用write_persistent_property();
             property_changed(name, value);
除了"persist."开头的property，关机后值会消失；以"persist."开头的会保存在/data/property目录下一文件里，可以看write_persistent_property()

3. 属性的初始值
从下面一些文件可以得到属性的初始值
void start_property_service(void)
{
...
    load_properties_from_file(PROP_PATH_SYSTEM_BUILD);
    load_properties_from_file(PROP_PATH_SYSTEM_DEFAULT);
    load_override_properties();
    /* Read persistent properties after all default values have been loaded. */
    load_persistent_properties();
...
#define PROP_PATH_RAMDISK_DEFAULT   "/default.prop"
#define PROP_PATH_SYSTEM_BUILD     "/system/build.prop"
#define PROP_PATH_SYSTEM_DEFAULT   "/system/default.prop"
#define PROP_PATH_LOCAL_OVERRIDE   "/data/local.prop"

4. 属性的权限
如果是serice, 调用check_control_perms()检查权限,可以看看control_perms[]
如果是其他属性，调用check_perms()检查权限，可以看看property_perms[]

5. property_changed() system/core/init/init.c
void property_changed(const char *name, const char *value)
{
    if (property_triggers_enabled)
        queue_property_triggers(name, value);
}

queue_property_triggers()
   action_add_queue_tail(act)

设置完属性后，会调用on property相应的action, 可以看看init.rc

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