Android(安卓)的系统属性(SystemProperties)设置分析

Android 的系统属性包括两部分：文件保存的持久属性和每次开机导入的cache属性。前者主要保存在下面几个文件中：

bionic / libc / include / sys / _system_properties.h 1 #define PROP_SERVICE_NAME"property_service"
2 #define PROP_PATH_RAMDISK_DEFAULT"/default.prop"
3 #define PROP_PATH_SYSTEM_BUILD"/system/build.prop"
4 #define PROP_PATH_SYSTEM_DEFAULT"/system/default.prop"
5 #define PROP_PATH_LOCAL_OVERRIDE"/data/local.prop"

后者则通过frameworks/base/core/java/android/os/SystemProperties.java的接口定义，

1 private static native Stringnative_get(Stringkey);
2 private static native Stringnative_get(Stringkey,Stringdef);
3 private static native void native_set(Stringkey,Stringdef);
4 public static void set(Stringkey,Stringval){
5 if (key.length() > PROP_NAME_MAX){
6 throw new IllegalArgumentException( " key.length> " + PROP_NAME_MAX);
7 }
8 if (val != null && val.length() > PROP_VALUE_MAX){
9 throw new IllegalArgumentException( " val.length> " +
10 PROP_VALUE_MAX);
11 }
12 native_set(key,val);
13 }

该接口类在初始化运行环境中注册对应的cpp接口android_os_SystemProperties.cpp，实际操作通过JNI调用的是cpp文件对应的接口：

frameworks/base/core/jni/AndroidRuntime.cpp

1 namespace android{
2 extern int register_android_os_SystemProperties(JNIEnv * env);
3 }

frameworks/base/core/jni/android_os_SystemProperties.cpp

1 static void SystemProperties_set(JNIEnv * env,jobjectclazz,jstringkeyJ,jstringvalJ)
2 {
3 int err;
4 const char * key;
5 const char * val;
6 key = env -> GetStringUTFChars(keyJ,NULL);
7 if (valJ == NULL){
8 val = "" ; /* NULLpointernotallowedhere */
9 } else {
10 val = env -> GetStringUTFChars(valJ,NULL);
11 }
12 err = property_set(key,val);
13 env -> ReleaseStringUTFChars(keyJ,key);
14 if (valJ != NULL){
15 env -> ReleaseStringUTFChars(valJ,val);
16 }
17 }

设置key的value时，需要作鉴权，根据设置程序所在进程的fd获知uid值，比如system server进程可以设置net打头的key，不可以设置gsm打头的key，相关的定义如下：

system/core/include/private/android_filesystem_config.h

1 #define AID_ROOT0/*traditionalunixrootuser*/
2 #define AID_SYSTEM1000/*systemserver*/
3 #define AID_RADIO1001/*telephonysubsystem,RIL*/
4 #define AID_DHCP1014/*dhcpclient*/
5 #define AID_SHELL2000/*adbanddebugshelluser*/
6 #define AID_CACHE2001/*cacheaccess*/
7 #define AID_APP10000/*firstappuser*/

system/core/init/property_service.c

1 #define PERSISTENT_PROPERTY_DIR"/data/property"
2 struct {
3 const char * prefix;
4 unsigned int uid;
5 }property_perms[] = {
6 { " net.rmnet0. " ,AID_RADIO},
7 { " net.gprs. " ,AID_RADIO},
8 { " ril. " ,AID_RADIO},
9 { " gsm. " ,AID_RADIO},
10 { " net.dns " ,AID_RADIO},
11 { " net.usb0 " ,AID_RADIO},
12 { " net. " ,AID_SYSTEM},
13 { " dev. " ,AID_SYSTEM},
14 { " runtime. " ,AID_SYSTEM},
15 { " hw. " ,AID_SYSTEM},
16 { " sys. " ,AID_SYSTEM},
17 { " service. " ,AID_SYSTEM},
18 { " wlan. " ,AID_SYSTEM},
19 { " dhcp. " ,AID_SYSTEM},
20 { " dhcp. " ,AID_DHCP},
21 { " debug. " ,AID_SHELL},
22 { " log. " ,AID_SHELL},
23 { " service.adb.root " ,AID_SHELL},
24 { " persist.sys. " ,AID_SYSTEM},
25 { " persist.service. " ,AID_SYSTEM},
26 {NULL, 0 }
27 };
28 int property_set( const char * name, const char * value)
29 {
30 property_changed(name,value);
31 return 0 ;
32 }
33 int start_property_service( void )
34 {
35 int fd;
36
37 load_properties_from_file(PROP_PATH_SYSTEM_BUILD);
38 load_properties_from_file(PROP_PATH_SYSTEM_DEFAULT);
39 load_properties_from_file(PROP_PATH_LOCAL_OVERRIDE);
40 /* Readpersistentpropertiesafteralldefaultvalueshavebeenloaded. */
41 load_persistent_properties();
42
43 fd = create_socket(PROP_SERVICE_NAME,SOCK_STREAM, 0666 , 0 , 0 );
44 if (fd < 0 ) return - 1 ;
45 fcntl(fd,F_SETFD,FD_CLOEXEC);
46 fcntl(fd,F_SETFL,O_NONBLOCK);
47
48 listen(fd, 8 );
49 return fd;
50 }
51 void handle_property_set_fd( int fd)
52 {
53 switch (msg.cmd){
54 case PROP_MSG_SETPROP:
55 msg.name[PROP_NAME_MAX - 1 ] = 0 ;
56 msg.value[PROP_VALUE_MAX - 1 ] = 0 ;
57
58 if (memcmp(msg.name, " ctl. " , 4 ) == 0 ){
59 if (check_control_perms(msg.value,cr.uid)){
60 handle_control_message(( char * )msg.name + 4 ,( char * )msg.value);
61 } else {
62 ERROR( " sys_prop:Unableto%sservicectl[%s]uid:%dpid:%d/n " ,
63 msg.name + 4 ,msg.value,cr.uid,cr.pid);
64 }
65 } else {
66 if (check_perms(msg.name,cr.uid)){
67 property_set(( char * )msg.name,( char * )msg.value);
68 } else {
69 ERROR( " sys_prop:permissiondenieduid:%dname:%s/n " ,
70 cr.uid,msg.name);
71 }
72 }
73 break ;
74
75 default :
76 break ;
77 }
78 }

在开机启动后的init操作中，会执行一个loop循环，当检测到有新的设置时，进入设置流程，鉴权失败会提示相关的异常，如sys_prop: permission denied uid:1000 name:gsm.phone.id

system/core/init/init.c

1 void property_changed( const char * name, const char * value)
2 {
3 if (property_triggers_enabled){
4 queue_property_triggers(name,value);
5 drain_action_queue();
6 }
7 }
8 int main( int argc, char ** argv)
9 {
10 parse_config_file( " /init.rc " );
11 qemu_init();
12 device_fd = device_init();
13 property_init();
14 fd = open(console_name,O_RDWR);
15 property_set_fd = start_property_service();
16 ufds[ 0 ].fd = device_fd;
17 ufds[ 0 ].events = POLLIN;
18 ufds[ 1 ].fd = property_set_fd;
19 ufds[ 1 ].events = POLLIN;
20 ufds[ 2 ].fd = signal_recv_fd;
21 ufds[ 2 ].events = POLLIN;
22 fd_count = 3 ;
23 for (;;){
24 if (ufds[ 0 ].revents == POLLIN)
25 handle_device_fd(device_fd);
26
27 if (ufds[ 1 ].revents == POLLIN)
28 handle_property_set_fd(property_set_fd);
29 if (ufds[ 3 ].revents == POLLIN)
30 handle_keychord(keychord_fd);
31 }
32 return 0 ;
33 }

OVER！

android源码开发中，常常要用到一些全局标志或者说变量，这时候我们可以给android系统添加自己想要的属性。

1.Settings.system

这种系统属性我们经常用到，例如飞行模式的开启和关闭，我们就是去改变Settings.system.AIRPLANE_MODE_ON 的值。

下面就介绍下我们怎样去定义一个系统属性。例如要添加一个属性名为“SILVAN_LIU”

路径：frameworks/base/core/java/android/provider/Settings.java

[java] view plain copy print ?

publicstaticfinalStringSILVAN_LIU="silvan_liu";
publicstaticfinalString[]SETTINGS_TO_BACKUP={
~
+SILVAN_LIU
~
}

public static final String SILVAN_LIU = "silvan_liu"; public static final String[] SETTINGS_TO_BACKUP ={ ~ +SILVAN_LIU ~ }这样添加后，你就是可以通过Settings.System.getInt(getContentResolver(), Settings.System.SILVAN_LIU,0) 和 Settings.System.getInt(getContentResolver(), Settings.System.SILVAN_LIU,0 )去得到和设定SILVAN_LIU的属性值。

2.SystemProperties

创建与修改android属性用Systemproperties.set(name, value)，获取android属性用Systemproperties.get(name)，需要注意的是android属性的名称是有一定的格式要求的，如下：前缀必须用system\core\init\property_service.c中定义的前缀，进行系统属性设置的程序也必须有system或root权限

如果我们要添加一个property：例如：silvan_liu

路径：system/core/rootdir/int.rc

在on post-fs-data 目录下

setprop persist.sys.silvan_liu 1//persist.sys 前缀名； 1为初始值

PS:不同前缀名权限不同，这里就不一一说明；还有为什么要加载on post-fs-data目录下，这和int.rc的语法有关。

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