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Systemproperties类在android.os下，但这个类是隐藏的，上层程序开发无法直接使用，用Java的反射机制就可以了。int __system_property_get(const char *name, char *value)
{
//数据已经存储在共享内存中，通过__system_property_area__ 即可获取到，之后等待读取完返回
const prop_info *pi = __system_property_find(name);
return __system_property_read(pi, 0, value);//阻塞式读取
}设置属性（客户端）：int __system_property_set(const char *key, const char *value)
{
msg.cmd = PROP_MSG_SETPROP;
strlcpy(msg.name, key, sizeof msg.name);
strlcpy(msg.value, value, sizeof msg.value);
err = send_prop_msg(&msg);
}
//发送消息通知property_service去启动服务或者设置属性
static int send_prop_msg(prop_msg *msg)
{
//与/dev/socket/property_service通信
s = socket(AF_LOCAL, SOCK_STREAM, 0);
connect(s, (struct sockaddr *) &addr, alen)
send(s, msg, sizeof(prop_msg), 0)
close(s);
}可以看出读取属性比较简单，就是通过__system_property_read直接读取共享内存中的数据即可，而设置属性则是通过客户端发送socket消息让property_service去启动服务或者设置属性。int main(int argc, char **argv)
{
//加入到action queue队列
queue_builtin_action(property_service_init_action, "property_service_init");
...
//将属性系统初始化函数加入action queue
queue_builtin_action(property_init_action, "property_init");
...
for(;;)
//执行action queue队列
//接收通过socket向property service发送的数据
nr = poll(ufds, fd_count, timeout);
……
handle_property_set_fd();
}
static int property_service_init_action(int nargs, char **args)
{
start_property_service();
}
void property_changed(const char *name, const char *value)
{
if (property_triggers_enabled) {
queue_property_triggers(name, value);
drain_action_queue();
}
}\system\core\init\property_service.c：void property_init(bool load_defaults)
{
//初始化共享内存空间
init_property_area();
//加载属性文件
load_properties_from_file(PROP_PATH_RAMDISK_DEFAULT);
}\bionic\libc\bionic\system_properties.c
static int init_property_area(void)
{
//创建匿名内存空间PA_SIZE = 32768
init_workspace(&pa_workspace, PA_SIZE)
//将内存区域分成两部分：属性系统基本信息和属性键值对
pa_info_array = (void*) (((char*) pa_workspace.data) + PA_INFO_START);
//初始化属性系统信息
pa = pa_workspace.data;
memset(pa, 0, PA_SIZE);
pa->magic = PROP_AREA_MAGIC;
pa->version = PROP_AREA_VERSION;
/* plug into the lib property services 每个进程都会使用此变量，指向系统属性共享内存区域，访问系统属性，很重要*/
__system_property_area__ = pa;
}
static int init_workspace(workspace *w, size_t size)
{
//dev is a tmpfs是一种虚拟内存文件系统
int fd = open("/dev/__properties__", O_RDWR | O_CREAT, 0600);
//将文件映射为共享进程空间内存 使其可以与操作内存方式一致
void *data = mmap(NULL, size, PROT_READ | PROT_WRITE,
MAP_SHARED, fd, 0);
close(fd);
//删除文件
fd = open("/dev/__properties__", O_RDONLY);
unlink("/dev/__properties__");
//保存fd size 将作为环境变量传递给每个进程
w->data = data; w->size = size; w->fd = fd;
}
static void load_properties_from_file(const char *fn)
{
//读取系统属性键值对数据写入到共享内存中
data = read_file(fn, &sz);
load_properties(data);
}
void start_property_service(void)
{
//加载属性配置文件，加载的属性将覆盖原先的值。这些属性加载之后，最后加载的属性会被保持在/data/property中。
load_properties_from_file(PROP_PATH_SYSTEM_BUILD);
load_properties_from_file(PROP_PATH_SYSTEM_DEFAULT);
load_properties_from_file(PROP_PATH_LOCAL_OVERRIDE);
load_persistent_properties();
//创建socket资源 并绑定
fd = create_socket(PROP_SERVICE_NAME, SOCK_STREAM, 0666, 0, 0);
//监听
listen(fd, 8);
}
void handle_property_set_fd()
{
//等待建立通信
s = accept(property_set_fd, (struct sockaddr *) &addr, &addr_size);
//获取套接字相关信息 uid gid
getsockopt(s, SOL_SOCKET, SO_PEERCRED, &cr, &cr_size);
//接收属性设置请求消息
recv(s, &msg, sizeof(msg), 0);
//处理消息
switch(msg.cmd) {
case PROP_MSG_SETPROP:
//通过设置系统属性 处理ctl.开头消息
if(memcmp(msg.name,"ctl.",4) == 0)
{
//权限检测
if (check_control_perms(msg.value, cr.uid, cr.gid))
{
handle_control_message((char*) msg.name + 4, (char*) msg.value);
}
} else
{
//更改系统属性值
if (check_perms(msg.name, cr.uid, cr.gid))
{
property_set((char*) msg.name, (char*) msg.value);
}
}
break;
}
close(s);
}
void handle_control_message(const char *msg, const char *arg)
{
if (!strcmp(msg,"start")) {
msg_start(arg);
} else if (!strcmp(msg,"stop")) {
msg_stop(arg);
} else if (!strcmp(msg,"restart")) {
msg_stop(arg);
msg_start(arg);
}
}
static void msg_start(const char *name)
{
service_start(svc, args);
}
void service_start(struct service *svc, const char *dynamic_args)
{
//创建进程
pid = fork();
if (pid == 0) {
if (properties_inited()) {
//获取系统属性空间文件描述
get_property_workspace(&fd, &sz);
//dup最小的可用文件描述符
sprintf(tmp, "%d,%d", dup(fd), sz);
//加入ANDROID_PROPERTY_WORKSPACE环境变量到ENV
//包含共享内存fd
add_environment("ANDROID_PROPERTY_WORKSPACE", tmp);
}
//执行程序 传递环境变量ENV
execve(svc->args[0], (char**) svc->args, (char**) ENV)
//设置Service系统属性
notify_service_state(svc->name, "running");
}
}
void get_property_workspace(int *fd, int *sz)
{
*fd = pa_workspace.fd;
*sz = pa_workspace.size;
}在bionic\libc\bionic\libc_init_dynamic.c中：//将系统属性内存空间映射到当前进程虚拟空间，进程在启动时，会加载动态库bionic libc库：
void __attribute__((constructor)) __libc_preinit(void);
void __libc_preinit(void)
{
__libc_init_common(elfdata);
}
void __libc_init_common(uintptr_t *elfdata)
{
__system_properties_init();
}
int __system_properties_init(void)
{
prop_area *pa; int s, fd; unsigned sz; char *env;
//获取环境变量ANDROID_PROPERTY_WORKSPACE
//与上面init进程中设置对应
env = getenv("ANDROID_PROPERTY_WORKSPACE");
//共享内存文件描述符 内存大小
fd = atoi(env);
sz = atoi(env + 1);
//将文件描述符映射到当前进程虚拟空间内存，实现共享内存
pa = mmap(0, sz, PROT_READ, MAP_SHARED, fd, 0);
//全局变量指向共享系统属性内存首地址
__system_property_area__ = pa;
}static int check_perms(const char *name, unsigned int uid, unsigned int gid)
{
//进行权限检测
for (i = 0; property_perms[i].prefix; i++) {
int tmp;
if (strncmp(property_perms[i].prefix, name,
strlen(property_perms[i].prefix)) == 0) {
if ((uid && property_perms[i].uid == uid) ||
(gid && property_perms[i].gid == gid)) {
return 1;
}
}
}
return 0;
}
property_perms[] = {
{ "net.rmnet0.", AID_RADIO, 0 },
{ "net.gprs.", AID_RADIO, 0 },
{ "net.ppp", AID_RADIO, 0 },
{ "ril.", AID_RADIO, 0 },
{ "gsm.", AID_RADIO, 0 },
{ "persist.radio", AID_RADIO, 0 },
{ "net.dns", AID_RADIO, 0 },
{ "net.", AID_SYSTEM, 0 },
{ "dev.", AID_SYSTEM, 0 },
{ "runtime.", AID_SYSTEM, 0 },
{ "hw.", AID_SYSTEM, 0 },
{ "sys.", AID_SYSTEM, 0 },
{ "service.", AID_SYSTEM, 0 },
{ "wlan.", AID_SYSTEM, 0 },
{ "dhcp.", AID_SYSTEM, 0 },
{ "dhcp.", AID_DHCP, 0 },
{ "vpn.", AID_SYSTEM, 0 },
{ "vpn.", AID_VPN, 0 },
{ "debug.", AID_SHELL, 0 },
{ "log.", AID_SHELL, 0 },
{ "service.adb.root", AID_SHELL, 0 },
{ "persist.sys.", AID_SYSTEM, 0 },
{ "persist.service.", AID_SYSTEM, 0 },
{ "persist.security.", AID_SYSTEM, 0 },
{ NULL, 0, 0 }
};
int property_set(const char *name, const char *value)
{
property_changed(name, value);
return 0;
}以上使用到的相关的宏定义在这里：#define PROP_SERVICE_NAME "property_service" #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"
还有这里 system/core/include/private/android_filesystem_config.h
#define AID_SYSTEM 1000 /* system server */ #define AID_RADIO 1001 /* telephony subsystem, RIL */ #define AID_DHCP 1014 /* dhcp client */ #define AID_VPN 1016 /* vpn system */ #define AID_SHELL 2000 /* adb and debug shell user */
代码有点长，具体来解释下这段代码都做了神马~
Init main()
Android SystemProperties系统属性详解
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原文地址：http://blog.csdn.net/wdong_love_cl/article/details/52404692
