Android 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