文章目錄
源碼分析Handler消息模型
Handler
職責
從開發者角度來看,可分爲
- Message的發送者
- Message的處理者
- Message的移除者
首先我們老看Handler的成員變量
public class Handler {
@UnsupportedAppUsage
final Looper mLooper;//當前線程對應的Looper對象
final MessageQueue mQueue;//Looper裏維護的對應MessageQueue
@UnsupportedAppUsage
final Callback mCallback;//當message沒有callback的時候,會判斷走當前的Callback回調
final boolean mAsynchronous;//是否是異步執行
@UnsupportedAppUsage
IMessenger mMessenger;//用於跨進程發送消息
}
//Handler.java
public class Handler {
public Handler(@Nullable Callback callback, boolean async) {
//爲false,用於檢測是否是弱引用或者靜態變量,檢測內存泄露
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());
}
}
//創建當前線程Looper
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread " + Thread.currentThread()
+ " that has not called Looper.prepare()");
}
//從Looper中取出MessageQueue
mQueue = mLooper.mQueue;
//回調賦值
mCallback = callback;
//是否異步
mAsynchronous = async;
}
}
//Looper.java
public static @Nullable Looper myLooper() {
return sThreadLocal.get();//從當前線程中獲取對應Looper,具體Looper後面詳細講解
}
1.Message的發送者
public class Handler{
//發送消息
public final boolean sendMessage(@NonNull Message msg) {
return sendMessageDelayed(msg, 0);//執行延時爲0的發送
}
//發送延遲處理消息
public final boolean sendMessageDelayed(@NonNull Message msg, long delayMillis) {
if (delayMillis < 0) {//容錯判斷
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);//發送相對於當前時間的任務消息
}
public boolean sendMessageAtTime(@NonNull Message msg, long uptimeMillis) {
//難道構造函數中獲取的MessageQueue
MessageQueue queue = mQueue;
if (queue == null) {//爲空則拋出異常
RuntimeException e = new RuntimeException(
this + " sendMessageAtTime() called with no mQueue");
Log.w("Looper", e.getMessage(), e);
return false;
}
//將消息加入隊列
return enqueueMessage(queue, msg, uptimeMillis);
}
private boolean enqueueMessage(@NonNull MessageQueue queue, @NonNull Message msg,
long uptimeMillis) {
msg.target = this;//將當前Handler對象賦值給Message的target
msg.workSourceUid = ThreadLocalWorkSource.getUid();//返回該線程當前執行的代碼的UID。
//設置異步執行
if (mAsynchronous) {
msg.setAsynchronous(true);
}
//交由MessageQueue處理具體的隊列邏輯
return queue.enqueueMessage(msg, uptimeMillis);
}
}
//MessageQueue.java
public final class MessageQueue{
boolean enqueueMessage(Message msg, long when) {
//hanler爲null,拋出異常
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
//相同message已經被處理,拋出異常
if (msg.isInUse()) {
throw new IllegalStateException(msg + " This message is already in use.");
}
synchronized (this) {
//調用了quit方法取消終止循環,就return掉
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;
}
//更改Message使用標記
msg.markInUse();
//更新執行時間值
msg.when = when;
//當前MessageQueue的Message鏈表
Message p = mMessages;
//需要喚醒
boolean needWake;
//如果當前Messages沒有 || 立即執行 || 當前時間比Message鏈表頭結點時間更短,則將當前節點作爲第一個結點加入
if (p == null || when == 0 || when < p.when) {
// New head, wake up the event queue if blocked.
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
//插入隊列中間。 通常,除非隊列的開頭有障礙並且消息是隊列中最早的異步消息,否則我們不必喚醒事件隊列。
needWake = mBlocked && p.target == null && msg.isAsynchronous();
Message prev;
//根據when尋找插入位置並插入
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;
}
// 我們可以假設mPtr!= 0,因爲mQuitting爲false。
if (needWake) {
//native方法喚醒時間隊列
nativeWake(mPtr);
}
}
return true;
}
}
上面小節介紹的是Handler作爲發送者的具體做的事情,Handler通過sendMessage等其他方法將消息Message加入MessageQueue裏面的消息鏈表裏
2.Message的處理者
再Looper.loop()中循環取消息調用msg.target.dispatchMessage(msg);來處理時間,其中的target有講過,是對應當前的Handler對象,所以我們來看dispatchMessage(msg)方法
public class Handler{
//處理消息
public void dispatchMessage(@NonNull Message msg) {
//如果message中設置了callback,則回調callback。
//callback是通過post方法發送消息賦值的
if (msg.callback != null) {
handleCallback(msg);
} else {
//如果成員變量mCallback已設置,則回調mCallback.handleMessage處理
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
//如果上面都沒有,使用handleMessage處理消息,該方法是一個空實現,需要使用者重寫
handleMessage(msg);
}
}
//子類必須實現
public void handleMessage(@NonNull Message msg) {
}
}
所以Handler也是消息的處理者,是消息的入口與出口,這樣的設計讓我們在使用時無需關心Looper和MessageQueue詳細細節是怎麼處理的,同時可以有多個handler來向消息隊列里加入消息並處理,方便在各個地方使用。
3.Message的移除者
三個方法都是調用MessageQueue對應的remove方法
public class Handler{
public final void removeMessages(int what) {
mQueue.removeMessages(this, what, null);
}
public final void removeMessages(int what, @Nullable Object object) {
mQueue.removeMessages(this, what, object);
}
public final void removeCallbacksAndMessages(@Nullable Object token) {
mQueue.removeCallbacksAndMessages(this, token);
}
}
都是調用MessageQueue來移除隊列
Looper
職責
負責無限循環從MessageQueue中取出消息
Looper的創建
Looper是私有構造函數,所以需要使用Looper.prepare來創建Looper對象
public final class Looper {
//Looper是一個私有的構造函數,在裏面創建了MessageQueue隊列和獲取當前線程
private Looper(boolean quitAllowed) {
//獲取當前線程,參數quitAllowed,主線程不能退出傳的false,子線程傳的true
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
//無參方法,創建一個允許退出的Looper
public static void prepare() {
prepare(true);
}
private static void prepare(boolean quitAllowed) {
//sThreadLocal是一個存儲Looper的線程對象,重複創建Looper對象,則拋錯
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
//創建Looper對象並設置給sThreadLocal
sThreadLocal.set(new Looper(quitAllowed));
}
//創建主線程Looper
public static void prepareMainLooper() {
//主線程不允許退出,所以傳false
prepare(false);
synchronized (Looper.class) {
//成員變量保存主線程Looper,重複創建拋出異常
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
//從sThreadLocal獲取Looper,賦值成員變量
sMainLooper = myLooper();
}
}
//獲取當前線程Looper
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
}
Looper循環
Looper顧名思義,是用來循環的,其中具體操作在loop()方法中
public final class Looper {
/**
* 在此線程中運行消息隊列。 確保調用{@link #quit()}以結束循環
**/
public static void loop() {
//獲取當前線程Looper
final Looper me = myLooper();
//未調用prepare創建,拋出異常
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
//獲取到Looper中創建的消息隊列
final MessageQueue queue = me.mQueue;
//確保此線程的身份是本地進程的身份,並跟蹤該身份令牌實際上是什麼.
Binder.clearCallingIdentity();
final long ident = Binder.clearCallingIdentity();
// 允許使用系統屬性覆蓋閾值。例如
// adb shell 'setprop log.looper.1000.main.slow 1 && stop && start'
final int thresholdOverride =
SystemProperties.getInt("log.looper."
+ Process.myUid() + "."
+ Thread.currentThread().getName()
+ ".slow", 0);
//標誌是否緩慢
boolean slowDeliveryDetected = false;
//開始無限循環取出消息
for (;;) {
// 可能阻塞,具體去取出消息,在後續MessageQueue中講解
Message msg = queue.next();
if (msg == null) {
// 沒有消息表示消息隊列正在退出。
return;
}
// 如果UI事件設置了記錄器,則必須在局部變量中
final Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
// 確保觀察者在處理交易時不會改變,可以通過set方法監聽
final Observer observer = sObserver;
//獲取到緩慢或發送日誌的閾值
final long traceTag = me.mTraceTag;
long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
long slowDeliveryThresholdMs = me.mSlowDeliveryThresholdMs;
if (thresholdOverride > 0) {
slowDispatchThresholdMs = thresholdOverride;
slowDeliveryThresholdMs = thresholdOverride;
}
//都大於0,則緩慢發送
final boolean logSlowDelivery = (slowDeliveryThresholdMs > 0) && (msg.when > 0);
//分發閾值大於0,則日誌緩慢分發
final boolean logSlowDispatch = (slowDispatchThresholdMs > 0);
//如果是緩慢發送,則需要記錄開始時間
final boolean needStartTime = logSlowDelivery || logSlowDispatch;
//如果緩慢分發,則需要記錄結束時間
final boolean needEndTime = logSlowDispatch;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
//分發開始時間
final long dispatchStart = needStartTime ? SystemClock.uptimeMillis() : 0;
final long dispatchEnd;
Object token = null;
//告訴觀察者開始分發立即調用
if (observer != null) {
token = observer.messageDispatchStarting();
}
//設置消息資源Uid
long origWorkSource = ThreadLocalWorkSource.setUid(msg.workSourceUid);
try {
//實際交由消息對應Hander的dispatchMessage處理
msg.target.dispatchMessage(msg);
//告訴觀察者,消息已經分發
if (observer != null) {
observer.messageDispatched(token, msg);
}
//記錄消息結束時間
dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
} catch (Exception exception) {
//告訴觀察者分發出現異常
if (observer != null) {
observer.dispatchingThrewException(token, msg, exception);
}
throw exception;
} finally {
//重置狀態
ThreadLocalWorkSource.restore(origWorkSource);
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
//下面一系列記錄日誌
if (logSlowDelivery) {
if (slowDeliveryDetected) {
if ((dispatchStart - msg.when) <= 10) {
Slog.w(TAG, "Drained");
slowDeliveryDetected = false;
}
} else {
if (showSlowLog(slowDeliveryThresholdMs, msg.when, dispatchStart, "delivery",
msg)) {
// Once we write a slow delivery log, suppress until the queue drains.
slowDeliveryDetected = true;
}
}
}
if (logSlowDispatch) {
showSlowLog(slowDispatchThresholdMs, dispatchStart, dispatchEnd, "dispatch", msg);
}
if (logging != null) {
logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
}
//確保在分派線程的過程中沒有破壞線程的身份.
final long newIdent = Binder.clearCallingIdentity();
if (ident != newIdent) {
Log.wtf(TAG, "Thread identity changed from 0x"
+ Long.toHexString(ident) + " to 0x"
+ Long.toHexString(newIdent) + " while dispatching to "
+ msg.target.getClass().getName() + " "
+ msg.callback + " what=" + msg.what);
}
//回收message,主要是重置成員變量值並加入消息池,方便後續繼續複用該Message
msg.recycleUnchecked();
}
}
}
MessageQueue
職責
使用鏈表存儲消息
構造函數和部分成員變量
//MessageQueue.java
public final class MessageQueue {
//是否允許退出
private final boolean mQuitAllowed;
//當前消息,數據結構是鏈表
Message mMessages;
//是否正在推出
private boolean mQuitting;
// 指示next()是否被阻止在pollOnce()中以非零超時等待
private boolean mBlocked;
MessageQueue(boolean quitAllowed) {
mQuitAllowed = quitAllowed;
//過native方法初始化消息隊列,其中mPtr是供native代碼使用
mPtr = nativeInit();
}
}
//native代碼
static void android_os_MessageQueue_nativeInit(JNIEnv* env, jobject obj) {
//創建出MessageQueue對象
NativeMessageQueue* nativeMessageQueue = new NativeMessageQueue();
//未能創建成功拋出異常
if (!nativeMessageQueue) {
jniThrowRuntimeException(env, "Unable to allocate native queue");
return;
}
//添加強引用指針
nativeMessageQueue->incStrong(env);
//設置messageQueue
android_os_MessageQueue_setNativeMessageQueue(env, obj, nativeMessageQueue);
}
static void android_os_MessageQueue_setNativeMessageQueue(JNIEnv* env, jobject messageQueueObj,
NativeMessageQueue* nativeMessageQueue) {
//將隊列地址設置保存在mPtr中
env->SetIntField(messageQueueObj, gMessageQueueClassInfo.mPtr,
reinterpret_cast<jint>(nativeMessageQueue));
}
//創建nativeMessageQueue
NativeMessageQueue::NativeMessageQueue() : mInCallback(false), mExceptionObj(NULL) {
//獲取到線程對應Looper
mLooper = Looper::getForThread();
//如果未創建,則創建Looper並設置進線程中
if (mLooper == NULL) {
mLooper = new Looper(false);
Looper::setForThread(mLooper);
}
}
消息入隊列
Handler獲取的MessageQueue調用了enqueueMessage方法將消息入隊列,已經在上面的Handler中講解了,這裏再列舉一遍
public final class MessageQueue {
boolean enqueueMessage(Message msg, long when) {
//hanler爲null,拋出異常
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
//相同message已經被處理,拋出異常
if (msg.isInUse()) {
throw new IllegalStateException(msg + " This message is already in use.");
}
synchronized (this) {
//調用了quit方法取消終止循環,就return掉
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;
}
//標記消息已使用
msg.markInUse();
//更新執行時間值
msg.when = when;
//當前MessageQueue的Message鏈表
Message p = mMessages;
//需要喚醒
boolean needWake;
//如果當前Messages沒有 || 立即執行 || 當前時間比Message鏈表頭結點時間更短,則將當前節點作爲第一個結點加入
if (p == null || when == 0 || when < p.when) {
// New head, wake up the event queue if blocked.
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
//插入隊列中間。 通常,除非隊列的開頭有障礙並且消息是隊列中最早的異步消息,否則我們不必喚醒事件隊列。
needWake = mBlocked && p.target == null && msg.isAsynchronous();
Message prev;
//根據when尋找插入位置並插入
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;
}
// 我們可以假設mPtr!= 0,因爲mQuitting爲false。
if (needWake) {
//native方法喚醒時間隊列
nativeWake(mPtr);
}
}
return true;
}
}
消息出隊列
Looper調用MessageQueue的next()方法去除消息
public final class MessageQueue {
Message next() {
// 如果消息循環已經退出並被處理,請返回此處。
// 如果應用程序嘗試退出後不支持的循環程序,則會發生這種情況
final long ptr = mPtr;
//當消息循環已經退出,則直接返回
if (ptr == 0) {
return null;
}
int pendingIdleHandlerCount = -1; //只有第一次賦值爲-1
//初始化下一次poll喚醒延時
int nextPollTimeoutMillis = 0;
for (;;) {
//只要延時不爲0,刷新待定命令
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
//阻塞操作,當等待nextPollTimeoutMillis時長,或者消息隊列被喚醒,都會返回,
//***這就是主線程爲什麼一直不阻塞主線程
//實際調用的是c++層ALooer::pollOnce方法,通過 mCondition.wait來等待
//mCondition.waitRelative(mLock, waitUntilNs - nowNs);
nativePollOnce(ptr, nextPollTimeoutMillis);
synchronized (this) {
// 嘗試檢索下一條消息。 如果找到則返回。
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;
if (msg != null && msg.target == null) {
// 查詢MessageQueue中的下一條異步消息
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
if (msg != null) {
if (now < msg.when) {
// 設置下一次輪詢消息的超時時間
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
} else {
// 獲取一條消息,並返回 zz
mBlocked = false;
if (prevMsg != null) {
prevMsg.next = msg.next;
} else {
mMessages = msg.next;
}
msg.next = null;
if (DEBUG) Log.v(TAG, "Returning message: " + msg);
//標記消息已使用
msg.markInUse();
return msg;
}
} else {
//沒有消息
nextPollTimeoutMillis = -1;
}
// 現在已處理所有掛起的消息,請處理退出消息。
if (mQuitting) {
dispose();
return null;
}
// 如果是第一次空閒,則獲取要運行的惰輪數量。
//空閒句柄僅在隊列爲空或將來要處理隊列中的第一條消息(可能是屏障)時才運行。
if (pendingIdleHandlerCount < 0
&& (mMessages == null || now < mMessages.when)) {
pendingIdleHandlerCount = mIdleHandlers.size();
}
if (pendingIdleHandlerCount <= 0) {
// 沒有空閒的處理程序可以運行。 循環並等待更多。
mBlocked = true;
continue;
}
if (mPendingIdleHandlers == null) {
mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
}
mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
}
// 只有第一次循環時,會運行idle handlers,執行完成後,重置pendingIdleHandlerCount爲0.
for (int i = 0; i < pendingIdleHandlerCount; i++) {
final IdleHandler idler = mPendingIdleHandlers[i];
mPendingIdleHandlers[i] = null; // 釋放handler引用
boolean keep = false;
try {
keep = idler.queueIdle();
} catch (Throwable t) {
Log.wtf(TAG, "IdleHandler threw exception", t);
}
if (!keep) {
synchronized (this) {
mIdleHandlers.remove(idler);
}
}
}
// 將空閒處理程序計數重置爲0,這樣我們就不再運行它們。
pendingIdleHandlerCount = 0;
//調用空閒處理程序時,可能已經傳遞了一條新消息,因此請返回並再次查找未決消息,而無需等待。
nextPollTimeoutMillis = 0;
}
}
}
消息移除
通過Handler的MessageQueue.removeMessages移除消息
public final class MessageQueue {
void removeMessages(Handler h, int what, Object object) {
//handler不能爲空
if (h == null) {
return;
}
synchronized (this) {
Message p = mMessages;
// 當是當前消息時,移除頭結點
while (p != null && p.target == h && p.what == what
&& (object == null || p.obj == object)) {
Message n = p.next;
mMessages = n;
p.recycleUnchecked();
p = n;
}
// 移除頭結點之後的所有消息
while (p != null) {
Message n = p.next;
if (n != null) {
if (n.target == h && n.what == what
&& (object == null || n.obj == object)) {
Message nn = n.next;
n.recycleUnchecked();
p.next = nn;
continue;
}
}
p = n;
}
}
}
}
退出消息循環
public final class MessageQueue {
void quit(boolean safe) {
//主線程不能退出,否則拋出錯誤
if (!mQuitAllowed) {
throw new IllegalStateException("Main thread not allowed to quit.");
}
synchronized (this) {
//已經調用退出攔截
if (mQuitting) {
return;
}
//改變退出標識
mQuitting = true;
//是否安全退出
if (safe) {
//只移除大於當前時間的結點
removeAllFutureMessagesLocked();
} else {
//直接遍歷所有Message
removeAllMessagesLocked();
}
// We can assume mPtr != 0 because mQuitting was previously false.
nativeWake(mPtr);
}
}
//直接遍歷移除所有message結點
private void removeAllMessagesLocked() {
Message p = mMessages;
while (p != null) {
Message n = p.next;
p.recycleUnchecked();
p = n;
}
mMessages = null;
}
//移除大於當前時間結點
private void removeAllFutureMessagesLocked() {
final long now = SystemClock.uptimeMillis();
Message p = mMessages;
if (p != null) {
if (p.when > now) {
removeAllMessagesLocked();
} else {
Message n;
for (;;) {
n = p.next;
if (n == null) {
return;
}
if (n.when > now) {
break;
}
p = n;
}
p.next = null;
do {
p = n;
n = p.next;
p.recycleUnchecked();
} while (n != null);
}
}
}
}
Message
Message設計
複用
public final class Message implements Parcelable {
/** 構造函數 (但是最好調用{@link #obtain() Message.obtain()}).
*/
public Message() {
}
public static Message obtain() {
//首先Message池裏面取出消息,如果sPool沒有消息,接直接new
synchronized (sPoolSync) {
if (sPool != null) {
Message m = sPool;
sPool = m.next;
m.next = null;
m.flags = 0; // clear in-use flag
sPoolSize--;
return m;
}
}
return new Message();
}
}
Message設計了一個最大容量爲50的Message複用池,各個線程均從當前Message池中取出消息重複使用,防止過多的new操作,耗費性能
總結
Android的所有線程的消息處理,都是通過Handler來發送和處理,然後通過Looper循環從MessageQueue中取出消息,交由Handler處理