概述
本篇文章将通過源碼來分析Handler異步消息處理機制，主要包括Handler的使用方法、場景、線程安全等
Handler機制的簡單介紹
Android的消息異步處理機制，是每個Android程式員都應該掌握的。下面是這幾個類的官方介紹，簡單的翻譯下，大家都應該看得懂。
Handler
Handler對象允許你發送消息和Runnable對象到MessageQueue裡面，也可以處理這些從MessageQueue裡面取出來的消息和Runnable對象。每一個Handler執行個體都跟一個線程和MessageQueue綁定。當你建立一個新的Handler時，handler對象将會跟建立時所在的線程的線程執行個體和MessageQueue綁定。從這點可以知道，handler将會把消息和runnable執行個體發送到和它綁定的messagequeue，并且執行或處理從這些隊列分發出來的消息和Runnable對象。
Looper
每個線程最多隻有一個Looper對象。使用Looper時需要在所在的線程調用一下靜态方法Looper.prepare()，這個方法會建立目前線程所屬的Looper對象和Messagequeue對象。然後調一下Looper.loop方法開啟一個死循環不斷的從MessageQueue中取出消息交給Handler去處理。
MessageQueue
簡單的說，MessageQueue本質上内部有個Message的單連結清單，通過Handler對象發送Message對象到連結清單裡面，又Looper.loop()方法裡面分發出來。可以通過靜态方法Looper.myQueue拿到目前線程的MessageQueue。
Message
Message類描述了消息的類型以及它所承載的資料。建立Message最好的方法是調用 Message.obtain()或者Handler.obtainMessage()，這兩個方法是從一個Message對象的回收池中擷取對象。避免了頻繁地建立Message對象産生額外的開銷。
Handler機制大概原理如下圖所示，首先Handler把Message發送到MessageQueue，同時Looper不斷的從MessageQueue取出待處理的Message，然後回調Handler的dispatchMessage方法，把消息交給Handler去處理。
Handler機制的一般使用場景
public class HandlerActivity extends AppCompatActivity {
private TextView textView;
Handler handler = new MyHandler(this);
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main2);
textView = findViewById(R.id.text);
new Thread(new Runnable() {
@Override
public void run() {
//模拟從網絡擷取消息
String text = getTextFromNet();
Message message = Message.obtain();
message.what = 0;
message.obj = text;
handler.sendMessage(message);
}
}).start();
}
private void setText(String text){
textView.setText(text);
}
private String getTextFromNet(){
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
return "text text text";
}
static class MyHandler extends Handler{
private WeakReference<HandlerActivity> activityRef;
private MyHandler(HandlerActivity activity){
this.activityRef = new WeakReference<>(activity);
}
@Override
public void handleMessage(Message msg) {
if (activityRef!=null) {
HandlerActivity activity = activityRef.get();
if (activity != null) {
switch (msg.what) {
case 0:
String text = (String) msg.obj;
activity.setText(text);
break;
}
}
}
}
}
}
以上是一個模拟在子線程去通路網絡擷取一個文本資訊，然後通過主線程把它顯示到UI上面。下面從源碼分析Handler的工作原理。
源碼分析
Looper
想要在一個線程中使用Handler機制處理消息，首先要調用Looper.prepare()方法
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));
}
sThreadLocal是ThreadLocal的一個執行個體，它用于儲存目前線程變量 。可以看到一個線程隻能調用一次Looper.prepare()方法，否則将抛出RuntimeException。prepare()方法内部建立了一個Looper執行個體
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
Looper的構造方法裡面建立了一個MessageQueue對象，并且通過給Looper的mThread對象指派的方式綁定了目前線程。注意Looper的構造方法為私有方法，它隻在prepare()方法内部調用，而一個線程隻能調用一次prepare()方法，是以一個線程隻有一個Looper執行個體和一個MessageQueue執行個體。
然後我們看Looper.Looper()方法。
public static void loop() {
final Looper me = myLooper();//擷取目前線程的Looper對象
if (me == null) {//loop()要在prepare()之後調用
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
final MessageQueue queue = me.mQueue;
// Make sure the identity of this thread is that of the local process,
// and keep track of what that identity token actually is.
Binder.clearCallingIdentity();
final long ident = Binder.clearCallingIdentity();
// Allow overriding a threshold with a system prop. e.g.
// 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);
boolean slowDeliveryDetected = false;
for (;;) {//循環從MessageQueue中取出Message
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
// 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);
}
final long traceTag = me.mTraceTag;
long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
long slowDeliveryThresholdMs = me.mSlowDeliveryThresholdMs;
if (thresholdOverride > 0) {
slowDispatchThresholdMs = thresholdOverride;
slowDeliveryThresholdMs = thresholdOverride;
}
final boolean logSlowDelivery = (slowDeliveryThresholdMs > 0) && (msg.when > 0);
final boolean logSlowDispatch = (slowDispatchThresholdMs > 0);
final boolean needStartTime = logSlowDelivery || logSlowDispatch;
final boolean needEndTime = logSlowDispatch;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
final long dispatchStart = needStartTime ? SystemClock.uptimeMillis() : 0;
final long dispatchEnd;
try {
msg.target.dispatchMessage(msg);//這裡分發消息
dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
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);
}
// Make sure that during the course of dispatching the
// identity of the thread wasn't corrupted.
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);
}
msg.recycleUnchecked();
}
}
Looper.loop()方法内部首先調用myLooper()方法拿到目前線程的Looper對象
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
從looper方法裡面可以看到，如果目前線程looper對象為空就會抛出錯誤。是以loop()方法要在prepare()方法後面調用。接下來可以看到Loope()方法内23行有個for循環，不斷MessageQueue對象的next方法中不斷取出Message，該方法是一個阻塞方法，沒有擷取到Message對象就會阻塞，然後通過第57行語句msg.target.dispatchMessage(msg)分發Message對象。msg.target實際是Handler對象，後面會具體介紹到。
Handler
接着我們看Handler
public Handler() {
this(null, false);
}
public Handler(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) {
throw new RuntimeException(
"Can't create handler inside thread " + Thread.currentThread()
+ " that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
在Handler的構造方法内會先拿到目前線程的Looper對象，然後判斷是否等于空，如果等于空就抛出錯誤。在這裡，handler對象會跟目前線程的Looper對象和MessageQueue對象綁定，即持有他們的引用。
在使用Handler對象發送Message的時候，我們一般會調用handler.sendMessage(msg)或者sendMessageDelayed()方法，不過在方法内部最終都會調用sendMessageAtTime()方法。
public boolean sendMessageAtTime(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);
}
在sendMessageAtTime()方法裡面又調用enqueueMessage(queue, msg, uptimeMillis);
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
可以看到msg.target = this;把目前對象賦給了msg.target，也就是我們在介紹Looper.loop()方法時候提到的msg.target是Handler對象。最後調用了MessageQueue的enqueueMessage()方法把消息存到MessageQueue對象裡面，MessageQueue的enqueueMessage()方法
boolean enqueueMessage(Message msg, long when) {
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
if (msg.isInUse()) {
throw new IllegalStateException(msg + " This message is already in use.");
}
synchronized (this) {
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) {
// New head, wake up the event queue if blocked.
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
// 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;
}
從第22到25行代碼可以知道，Message的入隊方式是根據回調的時間決定入隊的位置的，也就是when參數，when參數小的排在隊列的前面，參數大的排在後面。通過這種方式實作了延時消息。
我們前面說過，Looper.loop()方法會不斷的從MessageQueue裡面取出待處理的Message對象然後分發給Handler對象去處理。也就是msg.target.dispatchMessage(msg)語句，那麼我們看下Handler的dispatchMessage(msg)裡面到底是怎樣實作的。
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);//情況一
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {//情況二
return;
}
}
handleMessage(msg);//情況三
}
}
可以看到，如果Message的callback不等于空，則優先處理交給它，該callback是Runnable對象；如果Message的callback等于空，判斷handler的callback是否等于空，如果不等于空則回調handler.mCallback的handleMessage(msg)方法來處理消息，最後才會交給handler的handleMessage(msg)方法處理。這裡需要注意一下，隻有handler.mCallback的handleMessage(msg)傳回false才會繼續交給handler的handleMessage(msg)方法。是以我們可以通過handler的mCallback的傳回值去攔截這個消息。
//if (msg.callback != null) 知識簡單的調了callback的run方法
private static void handleCallback(Message message) {
message.callback.run();
}
那麼什麼情況下msg.callback才不會為空呢？
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
可以看到當調用handler的post(runnable)方法的時候，會調用getPostMessage()方法，在該方法裡面，會把runnable參數賦給Message對象的callback屬性，并傳回Message對象。也就是調用handler的post(runnable)方法發送消息，在dispatchMessage方法裡走的是情況一。
同樣的，我們看下handler的callback
public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
在構造handler對象的時候，給它傳入callback就可以了，這時候走的是情況二。handler.mCallback.handleMessage(msg)如果傳回true則表示該方法已經處理消息，就不在交給handler.handleMessage(msg)方法，否則交給它處理，就會到情況三。
public void handleMessage(Message msg) {
}
可以看到handler.handleMessage(msg)是個空方法，裡面沒有實作。是以使用的時候要重寫該方法。
MessageQueue的線程安全
既然handler是做異步處理的，那MessageQueue在多線程情況下必然會出現線程安全問題。那麼MessageQueue是如何保證線程安全的呢
boolean enqueueMessage(Message msg, long when) {
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
if (msg.isInUse()) {
throw new IllegalStateException(msg + " This message is already in use.");
}
//通過synchronized 關鍵字加鎖來保證線程安全
synchronized (this) {
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) {
// New head, wake up the event queue if blocked.
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
// 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;
}
通過源碼可以看到，MessageQueue的入隊列操作是通過synchronized 關鍵字加鎖來保證線程安全。同樣的出隊列也是同樣的方法。
主線程Looper.prepare()
前面我們說到一個線程想要使用Handler機制處理消息，首先要調用Looper.prepare()方法。那麼我們沒有對主線程調用該方法，為什麼還是可以使用handler呢。原因在于主線程的Looper.prepare()方法是在ActivityThread.main()方法裡面調用的，main()方法是Android應用程式的入口方法。
public static void main(String[] args) {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
// CloseGuard defaults to true and can be quite spammy. We
// disable it here, but selectively enable it later (via
// StrictMode) on debug builds, but using DropBox, not logs.
CloseGuard.setEnabled(false);
Environment.initForCurrentUser();
// Set the reporter for event logging in libcore
EventLogger.setReporter(new EventLoggingReporter());
// Make sure TrustedCertificateStore looks in the right place for CA certificates
final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
TrustedCertificateStore.setDefaultUserDirectory(configDir);
Process.setArgV0("<pre-initialized>");
Looper.prepareMainLooper();//這裡調用了主線程的prepareMainLooper()方法
...
}
需要注意的地方
- 如果想要在子線程做更新UI操作，那麼可以使用handler在子線程發送消息交給主線程處理，但是這裡需要注意handler執行個體應該在主線程建立，或者建立handler給它傳入Looper.getMainLooper()傳回的主線程Looper對象。
Handler handler = new Handler(Looper.getMainLooper());
- 當要建立一個Message對象，可以調用new Message()的構造方法建立。但最好的辦法是使用Message.obtain() 或者 Handler.obtainMessage() 方法，因為兩個方法是從一個message對象回收池裡面擷取message對象的，節省了建立新的message對象所需要的消耗。
![與專家面對面：Android開發入門問與答[圖]](data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACwAAAAAAQABAAACAkQBADs=)