Android(安卓)时区设置以及设置系统属性的分析

在开发android系统设置的过程中会涉及许多内容。其中很简单的时区设定就包含很多内容。前面分析的设置时间自动同步的相关内容，下面接着分析一下系统中时区设定的相关内容。

以Android 5.1.1 LMY48M这个版本为例说明：
在时区设定里会调用到Settings\src\com\android\settings\ZonePicker.java这个文件其中：

  @Override    public void onListItemClick(ListView listView, View v, int position, long id) {        // Ignore extra clicks        if (!isResumed()) return;        final Map<?, ?> map = (Map<?, ?>)listView.getItemAtPosition(position);        final String tzId = (String) map.get(KEY_ID);        // Update the system timezone value        final Activity activity = getActivity();        final AlarmManager alarm = (AlarmManager) activity.getSystemService(Context.ALARM_SERVICE);        alarm.setTimeZone(tzId);        final TimeZone tz = TimeZone.getTimeZone(tzId);        if (mListener != null) {            mListener.onZoneSelected(tz);        } else {            getActivity().onBackPressed();        }    }

完成了时区的设定，这么看来完成这件事情的是AlarmManager这个类，为了知其然更要知其所以然我们继续跟进AlarmManager

    /**     * Set the system default time zone.     * Requires the permission android.permission.SET_TIME_ZONE.     *     * @param timeZone in the format understood by {@link java.util.TimeZone}     */    public void setTimeZone(String timeZone) {        try {            mService.setTimeZone(timeZone);        } catch (RemoteException ex) {        }    }

可见AlarmManager也是调用AlarmManagerService.java这个来实现的，继续跟进，在这个服务中

 public void setTimeZone(String tz) {        mContext.enforceCallingOrSelfPermission(                "android.permission.SET_TIME_ZONE",                "setTimeZone");        long oldId = Binder.clearCallingIdentity();        try {            if (TextUtils.isEmpty(tz)) return;            TimeZone zone = TimeZone.getTimeZone(tz);            // Prevent reentrant calls from stepping on each other when writing            // the time zone property            boolean timeZoneWasChanged = false;            synchronized (this) {                String current = SystemProperties.get(TIMEZONE_PROPERTY);                if (current == null || !current.equals(zone.getID())) {                    if (localLOGV) {                        Slog.v(TAG, "timezone changed: " + current + ", new=" + zone.getID());                    }                    timeZoneWasChanged = true;                    SystemProperties.set(TIMEZONE_PROPERTY, zone.getID());                }                // Update the kernel timezone information                // Kernel tracks time offsets as 'minutes west of GMT'                int gmtOffset = zone.getOffset(System.currentTimeMillis());                setKernelTimezone(mDescriptor, -(gmtOffset / 60000));            }            TimeZone.setDefault(null);            if (timeZoneWasChanged) {                Intent intent = new Intent(Intent.ACTION_TIMEZONE_CHANGED);                intent.addFlags(Intent.FLAG_RECEIVER_REPLACE_PENDING);                intent.putExtra("time-zone", zone.getID());                mContext.sendBroadcastAsUser(intent, UserHandle.ALL);            }        } finally {            Binder.restoreCallingIdentity(oldId);        }    }

其中 SystemProperties.set(TIMEZONE_PROPERTY, zone.getID());是实现问题的关键。我们继续查看SystemProperties.java这个文件，其中

    private static native String native_get(String key);    private static native String native_get(String key, String def);    private static native int native_get_int(String key, int def);    private static native long native_get_long(String key, long def);    private static native boolean native_get_boolean(String key, boolean def);    private static native void native_set(String key, String def);    private static native void native_add_change_callback();    /**     * Set the value for the given key.     * @throws IllegalArgumentException if the key exceeds 32 characters     * @throws IllegalArgumentException if the value exceeds 92 characters     */    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);    }

这就开始通过jni调到c 、cpp的代码了。在master/core/jni/android_os_SystemProperties.cpp中

static JNINativeMethod method_table[] = {    { "native_get", "(Ljava/lang/String;)Ljava/lang/String;",      (void*) SystemProperties_getS },    { "native_get", "(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String;",      (void*) SystemProperties_getSS },    { "native_get_int", "(Ljava/lang/String;I)I",      (void*) SystemProperties_get_int },    { "native_get_long", "(Ljava/lang/String;J)J",      (void*) SystemProperties_get_long },    { "native_get_boolean", "(Ljava/lang/String;Z)Z",      (void*) SystemProperties_get_boolean },    { "native_set", "(Ljava/lang/String;Ljava/lang/String;)V",      (void*) SystemProperties_set },};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");    }}

其中 err = property_set(key, val);这行代码是真正实现该功能的。进去继续查看。这代码声明在#include “cutils/properties.h”这个文件中。在说下面的代码逻辑之前先再说一点关于宏的一些概念。众所周知c语言中不存在重载的概念，更不能在同一个文件中同时定义同名的函数，为了实现这个功能源码中通过宏来指定不同条件下的编译选择。所以在 properties.c的代码中存在多份property_set。控制具体编译那个的宏是在/build/core/combo/include/arch 下面的某个AndroidConfig.h文件其中就有HAVE_LIBC_SYSTEM_PROPERTIES的定义，但是其中注意这个和在编译系统时候选择的编译版本有关系，如果是darwin-x86或者windows下面的文件就没有这个定义。其实现就是另外一套逻辑了。就我手机里的系统当时的编译选项而说的话是包含这个定义的。所以

#ifdef HAVE_LIBC_SYSTEM_PROPERTIES#define _REALLY_INCLUDE_SYS__SYSTEM_PROPERTIES_H_#include int property_set(const char *key, const char *value){    return __system_property_set(key, value);}

其中在libc/bionic/system_properties.cpp

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;}static int send_prop_msg(const prop_msg *msg){    const int fd = socket(AF_LOCAL, SOCK_STREAM | SOCK_CLOEXEC, 0);    if (fd == -1) {        return -1;    }    const size_t namelen = strlen(property_service_socket);    sockaddr_un addr;    memset(&addr, 0, sizeof(addr));    strlcpy(addr.sun_path, property_service_socket, sizeof(addr.sun_path));    addr.sun_family = AF_LOCAL;    socklen_t alen = namelen + offsetof(sockaddr_un, sun_path) + 1;    if (TEMP_FAILURE_RETRY(connect(fd, reinterpret_cast(&addr), alen)) < 0) {        close(fd);        return -1;    }    const int num_bytes = TEMP_FAILURE_RETRY(send(fd, msg, sizeof(prop_msg), 0));    int result = -1;    if (num_bytes == sizeof(prop_msg)) {        // We successfully wrote to the property server but now we        // wait for the property server to finish its work.  It        // acknowledges its completion by closing the socket so we        // poll here (on nothing), waiting for the socket to close.        // If you 'adb shell setprop foo bar' you'll see the POLLHUP        // once the socket closes.  Out of paranoia we cap our poll        // at 250 ms.        pollfd pollfds[1];        pollfds[0].fd = fd;        pollfds[0].events = 0;        const int poll_result = TEMP_FAILURE_RETRY(poll(pollfds, 1, 250 /* ms */));        if (poll_result == 1 && (pollfds[0].revents & POLLHUP) != 0) {            result = 0;        } else {            // Ignore the timeout and treat it like a success anyway.            // The init process is single-threaded and its property            // service is sometimes slow to respond (perhaps it's off            // starting a child process or something) and thus this            // times out and the caller thinks it failed, even though            // it's still getting around to it.  So we fake it here,            // mostly for ctl.* properties, but we do try and wait 250            // ms so callers who do read-after-write can reliably see            // what they've written.  Most of the time.            // TODO: fix the system properties design.            result = 0;        }    }    close(fd);    return result;}

其中send_prop_msg是发消息给property_service.c，这个文件是整个过程的终结点。在这个类中会将数据写入/data/property这个目录下有 persist.sys.timezone 这个文件中

ok整个过程分析完了，至于为什么用socket通信到property_service，请看老罗的：
http://blog.csdn.net/Luoshengyang/article/details/38102011

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