【Android源码分析】Threadlocal与looper+handler

前言

从一个异常出发开始

new Thread(new Runnable() {
@Override
public void run() {
new Handler();
}
}).start();

java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()

new Handler()

Looper.prepare()

new Handler()

Looper.prepareMainLooper();

ActivityThread thread = new ActivityThread();
thread.attach(false);

if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}

if (false) {
Looper.myLooper().setMessageLogging(
new LogPrinter(Log.DEBUG, "ActivityThread"));
}

// End of event ActivityThreadMain.
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
Looper.loop();

Looper.prepareMainLooper()

寻找那个异常

Looper.prepare()

prepare()

/*
Initialize the current thread as a looper. This gives you a chance to create handlers that then reference this looper, before actually starting the loop. Be sure to call {@link #loop()} after calling this method, and end it by calling{@link #quit()}.
*/
public static void prepare() {
prepare(true);
}

private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper(quitAllowed));
}
...
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}

sThreadLocal

Looper

Looper

Looper.loop()

loop()

繁忙的

loop()

/** Return the Looper object associated with the current thread.  Returns null if the calling thread is not associated with a Looper.*/
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}

/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*/
public static void loop() {
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
final MessageQueue queue = me.mQueue;
...
for (;;) {
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
...
try {
msg.target.dispatchMessage(msg);
end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
...
msg.recycleUnchecked();
}
}

loop()

for(;;)

/**
* Handle system messages here.
*/
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}

handleMessage(msg)

Looper.prepare()

Looper.loop()

HandlerThread

子线程处理msg的封装HandlerThread

@Override
public void run() {
mTid = Process.myTid();
Looper.prepare();
synchronized (this) {
mLooper = Looper.myLooper();
notifyAll();
}
Process.setThreadPriority(mPriority);
onLooperPrepared();
Looper.loop();
mTid = -1;
}

getThreadHandler()

小结

ThreadLocal是什么

static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();

public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}

public T get() {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null) {
@SuppressWarnings("unchecked")
T result = (T)e.value;
return result;
}
}
return setInitialValue();
}

ThreadLocal.ThreadLocalMap

sThreadLocal

set()

public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}

小结