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基于Jdk1.8
刚从事Android就开始学习handler，当时学习了原理，觉得好精妙，但是细节并没有很好的掌握，所以回过头来再次学习、总结一下。
一、原理
1、Looper.java里面维护了一个单链表MessageQueue,然后for循环从MessageQueue里面读取Message执行,而我们要执行的任务就存放在Message里面。
2、有任务就取出来执行，没有任务就会阻塞,阻塞是通过调用natave方法netavePollOnce()来实现的，唤醒通过nataveWeak()实现。
3、Handler 提交的任务都会以Message的形式存入MessageQueue,是按照执行时间顺序存入链表的，执行时间timeMillis = SystemClock.uptimeMillis() + delayMillis。
二、学习之后的疑惑
1、Looper、MessageQueue关系？
2、任务延时是如何实现的
3、handler 如何获取looper?
4、loop()中是个死循环，主线程为什么不会卡死
5、如何用handler检测卡顿
6、已经有一个延时20s的任务在等待，此时再次提交一个延时5s的任务，内部流程？
7、handler 中循环阻塞、唤醒分别是怎么实现的。
8、主线程为什么可以直接创建handler,而其它线程却不可以呢
9、ThreadLocal工作原理
10、boolean handleMessage() 返回值分别代表什么
三、详细解析
1、 Looper初始化的时候会创建单链表MessageQueue
// Looper.java
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();
}
2、提交延时任务后调用顺序如下，最后会调用enqueueMessage, 其中when = System.currentMills() + delayTime,将时间设置到message,并将message插入到messageQueue。
public final boolean postDelayed(@NonNull Runnable r, long delayMillis) {
return sendMessageDelayed(getPostMessage(r), delayMillis);
}
public final boolean sendMessageDelayed(@NonNull Message msg, long delayMillis) {
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
public boolean sendMessageAtTime(@NonNull Message msg, long uptimeMillis) {
MessageQueue queue = mQueue;
if (queue == null) {
RuntimeException e = new RuntimeException(
this + " sendMessageAtTime() called with no mQueue");
Log.w("Looper", e.getMessage(), e);
return false;
}
return enqueueMessage(queue, msg, uptimeMillis);
};
private boolean enqueueMessage(@NonNull MessageQueue queue, @NonNull Message msg,
long uptimeMillis) {
msg.target = this;
msg.workSourceUid = ThreadLocalWorkSource.getUid();
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
插入消息时最终会调用如下方法，有2种情况，一种是插入messageQueue第一个位置（注释1，消息队列为空、任务无延时、延时小于队列里头消息的延时）， 此时会唤醒阻塞继续执行loop()中的循环；另外一种是插入messageQueue其它位置（注释2），不会唤醒阻塞。
// MessageQueue.java
boolean enqueueMessage(Message msg, long when) {
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
synchronized (this) {
if (msg.isInUse()) {
throw new IllegalStateException(msg + " This message is already in use.");
}
if (mQuitting) {
IllegalStateException e = new IllegalStateException(
msg.target + " sending message to a Handler on a dead thread");
Log.w(TAG, e.getMessage(), e);
msg.recycle();
return false;
}
msg.markInUse();
msg.when = when;
Message p = mMessages;
boolean needWake;
if (p == null || when == 0 || when < p.when) {//注释1 会唤醒阻塞
// New head, wake up the event queue if blocked.
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {//注释2 不会唤醒阻塞
// Inserted within the middle of the queue. Usually we don't have to wake
// up the event queue unless there is a barrier at the head of the queue
// and the message is the earliest asynchronous message in the queue.
needWake = mBlocked && p.target == null && msg.isAsynchronous();
Message prev;
for (;;) {
prev = p;
p = p.next;
if (p == null || when < p.when) {
break;
}
if (needWake && p.isAsynchronous()) {
needWake = false;
}
}
msg.next = p; // invariant: p == prev.next
prev.next = msg;
}
// We can assume mPtr != 0 because mQuitting is false.
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}
唤醒阻塞时会走如下方法，注释3 的方法将不再阻塞。然后会将目标时间与当前时间求差值（注释4），然后继续阻塞在注释3，阻塞时间为差值，这样就实现了延时功能。
// MessageQueue.java
Message next() {
// Return here if the message loop has already quit and been disposed.
// This can happen if the application tries to restart a looper after quit
// which is not supported.
final long ptr = mPtr;
if (ptr == 0) {
return null;
}
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
nativePollOnce(ptr, nextPollTimeoutMillis);//注释3
synchronized (this) {
// Try to retrieve the next message. Return if found.
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;
if (msg != null && msg.target == null) {
// Stalled by a barrier. Find the next asynchronous message in the queue.
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
if (msg != null) {
if (now < msg.when) {//注释4
// Next message is not ready. Set a timeout to wake up when it is ready.
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
} else {
// Got a message.
mBlocked = false;
if (prevMsg != null) {
prevMsg.next = msg.next;
} else {
mMessages = msg.next;
}
msg.next = null;
if (DEBUG) Log.v(TAG, "Returning message: " + msg);
msg.markInUse();
return msg;
}
} else {
// No more messages.
nextPollTimeoutMillis = -1;
}
// Process the quit message now that all pending messages have been handled.
if (mQuitting) {
dispose();
return null;
}
// If first time idle, then get the number of idlers to run.
// Idle handles only run if the queue is empty or if the first message
// in the queue (possibly a barrier) is due to be handled in the future.
if (pendingIdleHandlerCount < 0
&& (mMessages == null || now < mMessages.when)) {
pendingIdleHandlerCount = mIdleHandlers.size();
}
if (pendingIdleHandlerCount <= 0) {
// No idle handlers to run. Loop and wait some more.
mBlocked = true;
continue;
}
if (mPendingIdleHandlers == null) {
mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
}
mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
}
// Run the idle handlers.
// We only ever reach this code block during the first iteration.
for (int i = 0; i < pendingIdleHandlerCount; i++) {
final IdleHandler idler = mPendingIdleHandlers[i];
mPendingIdleHandlers[i] = null; // release the reference to the handler
boolean keep = false;
try {
keep = idler.queueIdle();
} catch (Throwable t) {
Log.wtf(TAG, "IdleHandler threw exception", t);
}
if (!keep) {
synchronized (this) {
mIdleHandlers.remove(idler);
}
}
}
// Reset the idle handler count to 0 so we do not run them again.
pendingIdleHandlerCount = 0;
// While calling an idle handler, a new message could have been delivered
// so go back and look again for a pending message without waiting.
nextPollTimeoutMillis = 0;
}
}
3、直接调用handler.geLooper()就可以获得，mLooper在初始化handler的时候通过Looper.myLooper()赋值。
//Handler.java
public final Looper getLooper() {
return mLooper;
}
public Handler(@Nullable Callback callback, boolean async) {
if (FIND_POTENTIAL_LEAKS) {
final Class<? extends Handler> klass = getClass();
if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
(klass.getModifiers() & Modifier.STATIC) == 0) {
Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
klass.getCanonicalName());
}
}
mLooper = Looper.myLooper(); //注释1 赋值
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread " + Thread.currentThread()
+ " that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
那handler中为何可以通过Looper.myLooper()来获取mLooper呢,到Looper.java类里面看一下
/**
* 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();
}
looper()是在初始化的时候创建MessageQueue,并且将自己存到sThreadLocal里
//Looper.java
@UnsupportedAppUsage
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
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();
}
4、Loop的循环和导致ANR的本质不同。
Loop()循环：当没有消息处理时，loop()会阻塞在queue.next()方法，内部阻塞在nativePollOnce()方法，从而让线程进入休眠状态，但可以随时唤醒执行任务。
ANR:本质是对一个事件进行计时，超时则ANR。
输入事件没有响应：当输入时间超过5s没有响应
广播10s：广播10s没有处理完
5、用handler检测卡顿有2种方式，本质原理都一样，就是检测UI线程处理一个任务的时间是否超过一个阀值，当超过这个阀值就是肉眼可见的卡顿了。
(1)Looper.loop()在处理消息前、后均会打印log，利用这个可以检测执行前后是否超过了设定的阀值，超过了就是卡顿。
//Looper.loop()
// This must be in a local variable, in case a UI event sets the logger
final Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
//to do
msg.target.dispatchMessage(msg);//处理消息任务
//to do
if (logging != null) {//
logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
}
上面logging默认为null，需要自己设置一个printer进行打印、检测
h.getLooper().setMessageLogging(new Printer() {
@Override
public void println(String x) {
}
});
(2)新开启一个线程，设置一个内部变量。循环向主线程设置一个值，循环过程中如果发现上一次没有设置成功，说明期间发生了卡顿。
// UI thread
int flag = 0;
new Thread(new Runnable() {
@Override
public void run() {
Log.v(TAG,"开始检测卡顿:" + Thread.currentThread().getName());
int cur;
while (true){
if (flag >= Integer.MAX_VALUE)flag &= 0;
cur = flag;
h.post(new Runnable() {
@Override
public void run() {
flag += 1;
}
});
try {
Thread.sleep(200);
} catch (InterruptedException e) {
e.printStackTrace();
}
if (cur == flag)Log.e(TAG,"UI发生了卡顿");
}
}
},"observerThread").start();
6、因为5s早于20s, 这个任务会被放入messageQueue的第一个位置，并且唤醒阻塞，阻塞被唤醒后会重新计算新的阻塞时间并再次进行阻塞。到达唤醒时间后，取出message执行。
7、插入任务后,当满足唤醒阻塞条件时，会调用MessageQueue 中nativeWake(mPtr);
当需要阻塞时，在messageQueue类的next()方法中会调用nativePollOnce(ptr, nextPollTimeoutMillis), nextPollTimeoutMillis阻塞时长，注释1是计算过程
//MessageQueue.java
Message next() {
// Return here if the message loop has already quit and been disposed.
// This can happen if the application tries to restart a looper after quit
// which is not supported.
// to do others
for (;;) {
nativePollOnce(ptr, nextPollTimeoutMillis);
// to do others
// Next message is not ready. Set a timeout to wake up when it is ready.
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);//注释1
//to do others
}
// to do others
}
8、线程里创建handler必须首先通过Looper.prepare()来创建Looper，原因如下，否则会在注释2处报错
public Handler(@Nullable Callback callback, boolean async) {
if (FIND_POTENTIAL_LEAKS) {
final Class<? extends Handler> klass = getClass();
if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
(klass.getModifiers() & Modifier.STATIC) == 0) {
Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
klass.getCanonicalName());
}
}
mLooper = Looper.myLooper();
if (mLooper == null) { // 注释2
throw new RuntimeException(
"Can't create handler inside thread " + Thread.currentThread()
+ " that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
而主线程创建handler之所以没报错，是因为在线程创建之初已经创建了looper,
//ActivityThread.java
public static void main(String[] args) {
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false, startSeq);
Looper.loop();
}
9、Thread.java有个变量threadLocals，即ThreadLocalMap，是ThreadLocal的一个内部类，数组结构。
Looper.prepare()时会调用threadLocal.set()来存looper，键值为threadLocal, value为looper。在ThreadLocal.java内部会通过当前现场获取threadLocalMap，当threadLocalMap为null时则进行初始化，并将初始化后的threadLocalMap赋值到当前Thread的变量threadLocals。
通过Looper.myLooper()获取looper时，会调用threadLocal.get()获取looper()。
内部通过thread.threadLocals即ThreadLocalMap来取值。
源码如下：
//Thread.java
ThreadLocal.ThreadLocalMap threadLocals = null;
Looper有内部变量sThreadLocal, 并在初始化时将自己（Looper）存入sThreadLocal.
//Looper.java
@UnsupportedAppUsage
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
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));
}
看一下set()方法,最终将Looper通过map.set()存入了ThreadLocalMap
//ThreadLocal.java
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
看一下map.set()方法
static class ThreadLocalMap {
private void set(ThreadLocal<?> key, Object value) {
Entry[] tab = table;
int len = tab.length;
int i = key.threadLocalHashCode & (len-1);//注释1 计算储存位置
for (Entry e = tab[i];
e != null;
e = tab[i = nextIndex(i, len)]) {
ThreadLocal<?> k = e.get();
if (k == key) {//注释3 如果key相同，直接替换
e.value = value;
return;
}
if (k == null) {
replaceStaleEntry(key, value, i);
return;
}
}
tab[i] = new Entry(key, value);//注释2 没有值的话直接存入
int sz = ++size;
if (!cleanSomeSlots(i, sz) && sz >= threshold)
rehash();
}
}
存看完了，看一下怎么取Looper()
class Looper{
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
}
class ThreadLocal{
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();
}
ThreadLocalMap getMap(Thread t) {//直接通过thread来获取map
return t.threadLocals;
}
}
当getMap()获取到的map为null时，get()方法中会调用setInitialValue(),然后再调用createMap(),并将value设为Null；如果是set()则会直接调用createMap(t, value);
class ThreadLocal{
private T setInitialValue() {
T value = initialValue();
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
return value;
}
void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
}
}
ThreadLocalMap初始化
class ThreadLocalMap{
ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
table = new Entry[INITIAL_CAPACITY];//注释1 长度为16的entry数组
int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
table[i] = new Entry(firstKey, firstValue);
size = 1;
setThreshold(INITIAL_CAPACITY);
}
//todo others
}
2021年11月 younger
