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
String native_get(String key);
2
private
static
native
String native_get(String key, String def);
3
private
static
native
void
native_set(String key, String def);
4
public
static
void
set(String key, String val) {
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, jobject clazz, jstring keyJ, jstring valJ)
2
{
3
int
err;
4
const
char
*
key;
5
const
char
*
val;
6
key
=
env
->
GetStringUTFChars(keyJ, NULL);
7
if
(valJ
==
NULL) {
8
val
=
""
;
/*
NULL pointer not allowed here
*/
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_ROOT 0 /* traditional unix root user */
2
#define
AID_SYSTEM 1000 /* system server */
3
#define
AID_RADIO 1001 /* telephony subsystem, RIL */
4
#define
AID_DHCP 1014 /* dhcp client */
5
#define
AID_SHELL 2000 /* adb and debug shell user */
6
#define
AID_CACHE 2001 /* cache access */
7
#define
AID_APP 10000 /* first app user */
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
/*
Read persistent properties after all default values have been loaded.
*/
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: Unable to %s service ctl [%s] uid: %d pid:%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: permission denied uid:%d name:%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！
