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属性在android中非常重要，我们基本的不多介绍了，主要说下其用法，原理等。
在java层主要通过SystemProperties这个类来访问Android的系统属性，通过一系列的native函数。
public class SystemProperties
{
......
public static String get(String key) {
if (key.length() > PROP_NAME_MAX) {
throw new IllegalArgumentException("key.length > " + PROP_NAME_MAX);
}
return native_get(key);
}
public static String get(String key, String def) {
if (key.length() > PROP_NAME_MAX) {
throw new IllegalArgumentException("key.length > " + PROP_NAME_MAX);
}
return native_get(key, def);
}
public static int getInt(String key, int def) {
if (key.length() > PROP_NAME_MAX) {
throw new IllegalArgumentException("key.length > " + PROP_NAME_MAX);
}
return native_get_int(key, def);
}
public static long getLong(String key, long def) {
if (key.length() > PROP_NAME_MAX) {
throw new IllegalArgumentException("key.length > " + PROP_NAME_MAX);
}
return native_get_long(key, def);
}
public static boolean getBoolean(String key, boolean def) {
if (key.length() > PROP_NAME_MAX) {
throw new IllegalArgumentException("key.length > " + PROP_NAME_MAX);
}
return native_get_boolean(key, def);
}
public static void set(String key, String val) {
if (key.length() > PROP_NAME_MAX) {
throw new IllegalArgumentException("key.length > " + PROP_NAME_MAX);
}
if (val != null && val.length() > PROP_VALUE_MAX) {
throw new IllegalArgumentException("val.length > " +
PROP_VALUE_MAX);
}
native_set(key, val);
}
我们再来看下android_os_SystemProperties.cpp中的这些native函数，注意都是静态的，因为在java层也是静态调用。
static jboolean SystemProperties_get_boolean(JNIEnv *env, jobject clazz,
jstring keyJ, jboolean defJ)
{
int len;
const char* key;
char buf[PROPERTY_VALUE_MAX];
jboolean result = defJ;
if (keyJ == NULL) {
jniThrowNullPointerException(env, "key must not be null.");
goto error;
}
key = env->GetStringUTFChars(keyJ, NULL);
len = property_get(key, buf, "");
if (len == 1) {
char ch = buf[0];
if (ch == '0' || ch == 'n')
result = false;
else if (ch == '1' || ch == 'y')
result = true;
} else if (len > 1) {
if (!strcmp(buf, "no") || !strcmp(buf, "false") || !strcmp(buf, "off")) {
result = false;
} else if (!strcmp(buf, "yes") || !strcmp(buf, "true") || !strcmp(buf, "on")) {
result = true;
}
}
env->ReleaseStringUTFChars(keyJ, key);
error:
return result;
}
static void SystemProperties_set(JNIEnv *env, jobject clazz,
jstring keyJ, jstring valJ)
{
int err;
const char* key;
const char* val;
if (keyJ == NULL) {
jniThrowNullPointerException(env, "key must not be null.");
return ;
}
key = env->GetStringUTFChars(keyJ, NULL);
if (valJ == NULL) {
val = ""; /* NULL pointer not allowed here */
} else {
val = env->GetStringUTFChars(valJ, NULL);
}
err = property_set(key, val);
env->ReleaseStringUTFChars(keyJ, key);
if (valJ != NULL) {
env->ReleaseStringUTFChars(valJ, val);
}
if (err < 0) {
jniThrowException(env, "java/lang/RuntimeException",
"failed to set system property");
}
}
最后是调用了system/core/libcutils/properties.c文件中的下面函数
int property_set(const char *key, const char *value)
{
return __system_property_set(key, value);
}
int property_get(const char *key, char *value, const char *default_value)
{
int len;
len = __system_property_get(key, value);
if(len > 0) {
return len;
}
if(default_value) {
len = strlen(default_value);
if (len >= PROPERTY_VALUE_MAX) {
len = PROPERTY_VALUE_MAX - 1;
}
memcpy(value, default_value, len);
value[len] = '\0';
}
return len;
}
最后在bionic/libc/bionic/system_properties.cpp中调用如下函数write都是通过socket来往init写属性的，然后在init中调用__system_property_update和__system_property_add来往共享内存中写属性，但是获取属性应该是通过共享内存读取的。int __system_property_get(const char *name, char *value)
{
const prop_info *pi = __system_property_find(name);//从共享内存上获取相应的属性内存
if (pi != 0) {
return __system_property_read(pi, 0, value);//从属性内存中读取属性内容
} else {
value[0] = 0;
return 0;
}
}
int __system_property_set(const char *key, const char *value)
{
if (key == 0) return -1;
if (value == 0) value = "";
if (strlen(key) >= PROP_NAME_MAX) return -1;
if (strlen(value) >= PROP_VALUE_MAX) return -1;
prop_msg msg;
memset(&msg, 0, sizeof msg);
msg.cmd = PROP_MSG_SETPROP;
strlcpy(msg.name, key, sizeof msg.name);
strlcpy(msg.value, value, sizeof msg.value);
const int err = send_prop_msg(&msg);
if (err < 0) {
return err;
}
return 0;
}
c层获取属性我们就是通过上面的property_set和property_get方法
int property_set(const char *key, const char *value)
{
return __system_property_set(key, value);
}
int property_get(const char *key, char *value, const char *default_value)
{
int len;
len = __system_property_get(key, value);
if(len > 0) {
return len;
}
if(default_value) {
len = strlen(default_value);
if (len >= PROPERTY_VALUE_MAX) {
len = PROPERTY_VALUE_MAX - 1;
}
memcpy(value, default_value, len);
value[len] = '\0';
}
return len;
}
系统中的每个进程都可以调用这些函数来读取和修改属性。读取属性值对任何进程都是没有限制的，直接由本进程从共享区中读取；但是修改属性值则必须通过init进程完成，同时进程还需要检查请求的进程是否有权限修改该属性值。
属性值成功修改后，init进程会检查init.rc中是否定义了该属性值的触发器。如果有定义，就执行该触发器下的命令。看下面：
on property:sys.lc.amtmode=0
class_start core
class_start main
class_start late_start
start lc-oms-sa
我们看下属性的一些分类：
1.ro前缀的，"ro."这样的属性是只读属性，一旦设置，属性值不能再改变了。
2.persist前缀的,"persist."这样的属性改变会写入目录data/property下与属性名相同的文件中。再次开机时这些值会被init进程读取出来，因此关机再启动也是生效的。
3.net前缀的，"net."这样的属性当它改变时，属性"net.change"将会被自动设置为最后修改的属性名
4.属性"ctl.start" "ctl.stop"和 "ctl.restart"属性控制类属性，用于启动和停止服务的。使用ctl.start启动服务时，系统将会启动结果放在名为"init.svc.服务名”属性中。
property_init 主要是在__system_property_area_init函数中创建了共享内存
void property_init() {
if (property_area_initialized) {
return;
}
property_area_initialized = true;
if (__system_property_area_init()) {
return;
}
pa_workspace.size = 0;
pa_workspace.fd = open(PROP_FILENAME, O_RDONLY | O_NOFOLLOW | O_CLOEXEC);
if (pa_workspace.fd == -1) {
ERROR("Failed to open %s: %s\n", PROP_FILENAME, strerror(errno));
return;
}
}
我们来看__system_property_area_init函数，最后是在map_prop_area_rw函数中调用了mmap创建了共享内存
int __system_property_area_init()
{
return map_prop_area_rw();
}static int map_prop_area_rw()
{
/* dev is a tmpfs that we can use to carve a shared workspace
* out of, so let's do that...
*/
const int fd = open(property_filename,//文件/dev/__properties__，应该是匿名映射，没有实际文件
O_RDWR | O_CREAT | O_NOFOLLOW | O_CLOEXEC | O_EXCL, 0444);
if (fd < 0) {
if (errno == EACCES) {
/* for consistency with the case where the process has already
* mapped the page in and segfaults when trying to write to it
*/
abort();
}
return -1;
}
if (ftruncate(fd, PA_SIZE) < 0) {
close(fd);
return -1;
}
pa_size = PA_SIZE;
pa_data_size = pa_size - sizeof(prop_area);
compat_mode = false;
void *const memory_area = mmap(NULL, pa_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);//内存映射
if (memory_area == MAP_FAILED) {
close(fd);
return -1;
}
prop_area *pa = new(memory_area) prop_area(PROP_AREA_MAGIC, PROP_AREA_VERSION);
/* plug into the lib property services */
__system_property_area__ = pa;
close(fd);
return 0;
}
共享内存使用名称为如下的设备文件创建。
#define PROP_FILENAME "/dev/__properties__"
在init进程中的主函数中：在解析init.rc之前，先调用了start_property_service函数
property_load_amt_defaults(amt_mode);
property_load_boot_defaults();
start_property_service();
init_parse_config_file("/init.rc");
start_property_service函数创建了socket，然后监听，并且调用register_epoll_handler函数把socket的fd放入了epoll中
void start_property_service() {
property_set_fd = create_socket(PROP_SERVICE_NAME, SOCK_STREAM | SOCK_CLOEXEC | SOCK_NONBLOCK,
0666, 0, 0, NULL);
if (property_set_fd == -1) {
ERROR("start_property_service socket creation failed: %s\n", strerror(errno));
exit(1);
}
listen(property_set_fd, 8);
register_epoll_handler(property_set_fd, handle_property_set_fd);
}
register_epoll_handler函数就是把fd放入epoll中
void register_epoll_handler(int fd, void (*fn)()) {
epoll_event ev;
ev.events = EPOLLIN;
ev.data.ptr = reinterpret_cast<void*>(fn);
if (epoll_ctl(epoll_fd, EPOLL_CTL_ADD, fd, &ev) == -1) {
ERROR("epoll_ctl failed: %s\n", strerror(errno));
}
}
先我们就来解析下handle_property_set_fd函数
static void handle_property_set_fd()
{
prop_msg msg;
int s;
int r;
struct ucred cr;
struct sockaddr_un addr;
socklen_t addr_size = sizeof(addr);
socklen_t cr_size = sizeof(cr);
char * source_ctx = NULL;
struct pollfd ufds[1];
const int timeout_ms = 2 * 1000; /* Default 2 sec timeout for caller to send property. */
int nr;
if ((s = accept(property_set_fd, (struct sockaddr *) &addr, &addr_size)) < 0) {//获取对端socket的fd
return;
}
/* Check socket options here */
if (getsockopt(s, SOL_SOCKET, SO_PEERCRED, &cr, &cr_size) < 0) {
close(s);
ERROR("Unable to receive socket options\n");
return;
}
ufds[0].fd = s;
ufds[0].events = POLLIN;
ufds[0].revents = 0;
nr = TEMP_FAILURE_RETRY(poll(ufds, 1, timeout_ms));
if (nr == 0) {
ERROR("sys_prop: timeout waiting for uid=%d to send property message.\n", cr.uid);
close(s);
return;
} else if (nr < 0) {
ERROR("sys_prop: error waiting for uid=%d to send property message: %s\n", cr.uid, strerror(errno));
close(s);
return;
}
r = TEMP_FAILURE_RETRY(recv(s, &msg, sizeof(msg), MSG_DONTWAIT));//获取socket数据
if(r != sizeof(prop_msg)) {
ERROR("sys_prop: mis-match msg size received: %d expected: %zu: %s\n",
r, sizeof(prop_msg), strerror(errno));
close(s);
return;
}
switch(msg.cmd) {
case PROP_MSG_SETPROP:
msg.name[PROP_NAME_MAX-1] = 0;
msg.value[PROP_VALUE_MAX-1] = 0;
if (!is_legal_property_name(msg.name, strlen(msg.name))) {
ERROR("sys_prop: illegal property name. Got: \"%s\"\n", msg.name);
close(s);
return;
}
getpeercon(s, &source_ctx);
if(memcmp(msg.name,"ctl.",4) == 0) {//ctl类型
// Keep the old close-socket-early behavior when handling
// ctl.* properties.
close(s);
if (check_control_mac_perms(msg.value, source_ctx)) {
handle_control_message((char*) msg.name + 4, (char*) msg.value);
} else {
ERROR("sys_prop: Unable to %s service ctl [%s] uid:%d gid:%d pid:%d\n",
msg.name + 4, msg.value, cr.uid, cr.gid, cr.pid);
}
} else {
if (check_perms(msg.name, source_ctx)) {//检查权限
property_set((char*) msg.name, (char*) msg.value);//设置属性
} else {
ERROR("sys_prop: permission denied uid:%d name:%s\n",
cr.uid, msg.name);
}
// Note: bionic's property client code assumes that the
// property server will not close the socket until *AFTER*
// the property is written to memory.
close(s);
}
freecon(source_ctx);
break;
default:
close(s);
break;
}
}
我们来看property_set函数调用了property_set_impl函数来设置属性
int property_set(const char* name, const char* value) {
int rc = property_set_impl(name, value);
if (rc == -1) {
ERROR("property_set(\"%s\", \"%s\") failed\n", name, value);
}
return rc;
}property_set_impl函数主要讲属性值写入，或者更新到共享内存中，然后当属性是net类型的，把net类型的属性名写入net.change属性，persist属性写入文件，最后调用property_changed函数来处理，属性改变后的触发器事件。static int property_set_impl(const char* name, const char* value) {
size_t namelen = strlen(name);
size_t valuelen = strlen(value);
if (!is_legal_property_name(name, namelen)) return -1;
if (valuelen >= PROP_VALUE_MAX) return -1;
if (strcmp("selinux.reload_policy", name) == 0 && strcmp("1", value) == 0) {
if (selinux_reload_policy() != 0) {
ERROR("Failed to reload policy\n");
}
} else if (strcmp("selinux.restorecon_recursive", name) == 0 && valuelen > 0) {
if (restorecon_recursive(value) != 0) {
ERROR("Failed to restorecon_recursive %s\n", value);
}
}
prop_info* pi = (prop_info*) __system_property_find(name);
if(pi != 0) {
/* ro.* properties may NEVER be modified once set */
if(!strncmp(name, "ro.", 3)) return -1;//ro文件，直接退出
__system_property_update(pi, value, valuelen);//更新属性数据到共享内存
} else {
int rc = __system_property_add(name, namelen, value, valuelen);//增加属性
if (rc < 0) {
return rc;
}
}
/* If name starts with "net." treat as a DNS property. */
if (strncmp("net.", name, strlen("net.")) == 0) {
if (strcmp("net.change", name) == 0) {
return 0;
}
/*
* The 'net.change' property is a special property used track when any
* 'net.*' property name is updated. It is _ONLY_ updated here. Its value
* contains the last updated 'net.*' property.
*/
property_set("net.change", name);//net类型的属性，改变后需要写属性到net.change
} else if (persistent_properties_loaded &&
strncmp("persist.", name, strlen("persist.")) == 0) {
/*
* Don't write properties to disk until after we have read all default properties
* to prevent them from being overwritten by default values.
*/
write_persistent_property(name, value);//persist类型的属性写入到data/property目录下以属性名命名的文件
}
property_changed(name, value);
return 0;
}
我们先看下write_persistent_property函数，将属性在data/property目录下创建以属性名命名的文件，然后写入属性值。写入方式是先做了一个临时文件，成功后改名。
static void write_persistent_property(const char *name, const char *value)
{
char tempPath[PATH_MAX];
char path[PATH_MAX];
int fd;
snprintf(tempPath, sizeof(tempPath), "%s/.temp.XXXXXX", PERSISTENT_PROPERTY_DIR);
fd = mkstemp(tempPath);//做临时文件
if (fd < 0) {
ERROR("Unable to write persistent property to temp file %s: %s\n", tempPath, strerror(errno));
return;
}
write(fd, value, strlen(value));//写入数据
fsync(fd);
close(fd);
snprintf(path, sizeof(path), "%s/%s", PERSISTENT_PROPERTY_DIR, name);
if (rename(tempPath, path)) {//改名
unlink(tempPath);
ERROR("Unable to rename persistent property file %s to %s\n", tempPath, path);
}
}
property_changed函数就是看有哪些满足属性的触发器，然后放入执行队列中。最后在init的循环中，执行触发器相应的命令
void property_changed(const char *name, const char *value)
{
if (property_triggers_enabled)
queue_property_triggers(name, value);
}void queue_property_triggers(const char *name, const char *value)
{
struct listnode *node, *node2;
struct action *act;
struct trigger *cur_trigger;
bool match;
int name_length;
list_for_each(node, &action_list) {
act = node_to_item(node, struct action, alist);
match = !name;
list_for_each(node2, &act->triggers) {
cur_trigger = node_to_item(node2, struct trigger, nlist);
if (!strncmp(cur_trigger->name, "property:", strlen("property:"))) {
const char *test = cur_trigger->name + strlen("property:");
if (!match) {
name_length = strlen(name);
if (!strncmp(name, test, name_length) &&
test[name_length] == '=' &&
(!strcmp(test + name_length + 1, value) ||
!strcmp(test + name_length + 1, "*"))) {
match = true;
continue;
}
}
const char* equals = strchr(test, '=');
if (equals) {
char prop_name[PROP_NAME_MAX + 1];
char value[PROP_VALUE_MAX];
int length = equals - test;
if (length <= PROP_NAME_MAX) {
int ret;
memcpy(prop_name, test, length);
prop_name[length] = 0;
/* does the property exist, and match the trigger value? */
ret = property_get(prop_name, value);
if (ret > 0 && (!strcmp(equals + 1, value) ||
!strcmp(equals + 1, "*"))) {
continue;
}
}
}
}
match = false;
break;
}
if (match) {
action_add_queue_tail(act);//最后将满足的触发器加入执行队列中
}
}
}
我们先来看init.rc中的下面触发器
on load_system_props_action
load_system_props
而load_system_props_action是在late-init中触发的
on late-init
trigger early-fs
trigger fs
trigger post-fs
# Load properties from /system/ + /factory after fs mount. Place
# this in another action so that the load will be scheduled after the prior
# issued fs triggers have completed.
trigger load_system_props_action
我们再来看load_system_props的处理，其中PROP_PATH_SYSTEM_BUILD就是/system/build.prop文件，load_properties_from_file函数最后会调用property_set函数设置属性
void load_system_props() {
load_properties_from_file(PROP_PATH_SYSTEM_BUILD, NULL);
load_properties_from_file(PROP_PATH_VENDOR_BUILD, NULL);
load_properties_from_file(PROP_PATH_FACTORY, "ro.*");
load_recovery_id_prop();
}
同样persist类型的属性如下：
on load_persist_props_action
load_persist_props
start logd
start logd-reinit
也是在late-init触发，最后调用load_persist_props
on late-init
trigger early-fs
trigger fs
trigger post-fs
# Load properties from /system/ + /factory after fs mount. Place
# this in another action so that the load will be scheduled after the prior
# issued fs triggers have completed.
trigger load_system_props_action
# Now we can mount /data. File encryption requires keymaster to decrypt
# /data, which in turn can only be loaded when system properties are present
trigger post-fs-data
trigger load_persist_props_action
load_persist_props函数调用了load_override_properties load_persistent_properties来去读属性值
void load_persist_props(void) {
load_override_properties();
/* Read persistent properties after all default values have been loaded. */
load_persistent_properties();//读取data/property/下面persist类型的属性
}load_override_properties函数，如果ro.debuggable为1.从文件/data/local.prop来读取属性。/data/local.prop文件时为了覆盖系统缺省的属性值。build.prop文件放在system目录下，修改不是很方便，如果希望测试某个属性，可以在/data/local.prop文件中修改，可以覆盖build.prop中的定义。static void load_override_properties() {
if (ALLOW_LOCAL_PROP_OVERRIDE) {
char debuggable[PROP_VALUE_MAX];
int ret = property_get("ro.debuggable", debuggable);
if (ret && (strcmp(debuggable, "1") == 0)) {
load_properties_from_file(PROP_PATH_LOCAL_OVERRIDE, NULL);
}
}
} 标签：
原文地址：http://blog.csdn.net/kc58236582/article/details/51939322
