UEventObserver是android Java层利用uevent与获取Kernel层状态变化的机制。

通过grep发现framework有如下模块使用UEventObserver的功能来提供服务：

电池状态：services/java/com/android/server/BatteryService.java

耳机状态：services/java/com/android/server/HeadsetObserver.java

DOCK状态：services/java/com/android/server/DockObserver.java

USB状态：services/java/com/android/server/usb/UsbService.java

它们全部继承自UEventObserver，先看看这个类的构造和原理：

./core/java/android/os/UEventObserver.java

|

[ native_setup(), next_event() ]

\|/

./core/jni/android_os_UEventObserver.cpp

|

[ uevent_init(),uevent_next_event() ]

\|/

/hardware/libhardware_legacy/uevent/uevent.c

| [userspace]

---------------------[socket]-----------------------------------------------------------------------------

|

\|/ [kernel]

socket(PF_NETLINK, SOCK_DGRAM, NETLINK_KOBJECT_UEVENT)

下面用HeadsetObserver作为例子说明如何使用UEventObserver来监听kernel的uevent。

继承UEventObserver的类必须实现自己的public abstract void onUEvent(UEvent event)：

[java] view plain copy

@Override
public void onUEvent(UEventObserver.UEvent event) {
if (LOG) Slog.v(TAG, "Headset UEVENT: " + event.toString());
try {
update(event.get("SWITCH_NAME"), Integer.parseInt(event.get("SWITCH_STATE"))); // update中处理事务
} catch (NumberFormatException e) {
Slog.e(TAG, "Could not parse switch state from event " + event);
}
}

这个函数会在UEventObserver接收到event的时候由UEventObserver来回调，HeadsetObserver使用startObserving("DEVPATH=/devices/virtual/switch/h2w") 来开始监听，这个API会确保sThread已经运行并且以字串参数作为匹配参数增加一个observer:

public final synchronized void startObserving(String match) {
        ensureThreadStarted();
        sThread.addObserver(match, this);
    }

UEventObserver的核心部分就是sThread的run:

[java] view plain copy

public void run() {
native_setup(); //调用uevent.c中的API来打开socket
byte[] buffer = new byte[1024];
int len;
while (true) {
len = next_event(buffer); // 在uevent.c中用poll调用来获取event,会阻塞
if (len > 0) {
String bufferStr = new String(buffer, 0, len); // easier to search a String
synchronized (mObservers) {
for (int i = 0; i < mObservers.size(); i += 2) {
if (bufferStr.indexOf((String)mObservers.get(i)) != -1) { // 找到匹配的match参数，说明某个observer的监听的event发生了
((UEventObserver)mObservers.get(i+1))
.onUEvent(new UEvent(bufferStr)); // 调用这个observer的onUEvent函数
}
}
}
}
}
}

Android UEventObserver