源码中：
主线程：
在程序启动的时候，系统已经帮我们自动调用了Looper.prepare()方法。查看ActivityThread中的main()
public static void main(String[] args) {
SamplingProfilerIntegration.start();
CloseGuard.setEnabled(false);
Environment.initForCurrentUser();
EventLogger.setReporter(new EventLoggingReporter());
Process.setArgV0("<pre-initialized>");
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
AsyncTask.init();
if (false) {
Looper.myLooper().setMessageLogging(new LogPrinter(Log.DEBUG, "ActivityThread"));
}
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
请注意Looper.prepareMainLooper()：
public static final void prepareMainLooper() {
prepare();
setMainLooper(myLooper());
if (Process.supportsProcesses()) {
myLooper().mQueue.mQuitAllowed = false;
}
}
子线程：
new Thread(new Runnable() {
@Override
public void run() {
Looper.prepare()
handler2 = new Handler();
Looper.loop()
}
}).start();
如果没有Looper.prepare().会报错：
Can't create handler inside thread that has not called Looper.prepare()
因为没looper对象创建
looper.prepare()源码：
public static final void prepare() {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper());
}
