Android開發App啟動流程與消息機制詳解
引言
相信很多人對這個問題不陌生,但是大傢回答的都比較簡單,如談到app啟動流程有人就會是app的生命周期去瞭,談到消息機制有人就會說looper循環消息進行分發,如果是面試可能面試官不會滿意,今天我們搞一篇完善的源碼解析來進行闡述上面的問題
1、第一步瞭解 ThreadLocal
什麼是ThreadLocal呢,專業的來講,ThreadLocal 是一個線程內部的數據存儲類,通過它可以在指定的線程中存儲數據,數據存儲以後,隻有再指定線程中可以獲取到存儲的數據,對於其他線程來說則無法獲取到數據,是共享數據變量存儲,通俗的來講,就是保存每個線程的數據,肯定大傢都沒聽懂,沒事的,接下來我們通過代碼來解釋ThreadLocal的具體作用
首先看一個例子
public static void main(String[] args) throws InterruptedException { ThreadLocal<String> threadLocal = new ThreadLocal<String>(){ @Override protected String initialValue() { return "data--1"; } }; System.out.println("1主線程--> "+threadLocal.get()); Thread t1 = new Thread(()->{ }); t1.start(); threadLocal.set("data--2"); System.out.println("2主線程--> "+threadLocal.get()); Thread t2 = new Thread(()->{ System.out.println("線程2---> "+threadLocal.get()); }); t2.start(); Thread t3 = new Thread(()->{ threadLocal.set("data-->3"); System.out.println("線程3---> "+threadLocal.get()); }); t3.start(); System.out.println("3主線程--> "+threadLocal.get()); Thread.sleep(1000); }
打印結果
1主線程–> data–1
2主線程–> data–2
線程2—> data–1
3主線程–> data–2
線程3—> data–>3
從上面的例子我們可以看到,ThreadLocal保存一個String這個變量,這個變量初始化會有一個值,在接下來的線程種,每個線程都會擁有一個初始值,這個初始值在主線程中,一旦這個初始值發生改變,如果是在主線程種改變如進行set,則後面的子線程獲取的都是這個改變後的值,但是如果子線程種也改變瞭這個值,則隻在當前子線程種有此值 沒其子線程還是獲取的主線程種那個值,我們來簡單畫個圖給大傢
ThreadLocal種的三個重要方法
//默認情況下initialValue是返回為空的 protected T initialValue() { return null; } //在get的時候如果沒有調用set方法 getMap(t);是返回為空的,所以,返回的是setInitialValue(),這些方法請看後面的介紹,而setInitialValue方法返回的其實就是初始值 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(); } //這個方法在在調用的時候實際上getMap(t)是為空的,所以就會調用createMap,這個方法會把當前的線程作為值,保證getMap再調用就不會為空 public void set(T value) { Thread t = Thread.currentThread(); ThreadLocalMap map = getMap(t); if (map != null) map.set(this, value); else createMap(t, value); }
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; } public void remove() { ThreadLocalMap m = getMap(Thread.currentThread()); if (m != null) m.remove(this); } ThreadLocalMap getMap(Thread t) { return t.threadLocals; } void createMap(Thread t, T firstValue) { t.threadLocals = new ThreadLocalMap(this, firstValue); }
簡單來講,就是自己有的,用自己的,自己沒有就用初始化的,初始化改變瞭,後面的也改變,但是自己設置的,還是用自己的,就這麼簡單,好瞭,接下來進行下一步
2、App的啟動流程
我們看下Android的源碼
//這是main函數的入口 public static void main(String[] args) { Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain"); SamplingProfilerIntegration.start(); // 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(); 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(); throw new RuntimeException("Main thread loop unexpectedly exited"); }
我們重點看下 Looper.prepareMainLooper();
這個方法
/** * Initialize the current thread as a looper, marking it as an * application's main looper. The main looper for your application * is created by the Android environment, so you should never need * to call this function yourself. See also: {@link #prepare()} */ public static void prepareMainLooper() { prepare(false); synchronized (Looper.class) { if (sMainLooper != null) { throw new IllegalStateException("The main Looper has already been prepared."); } sMainLooper = myLooper(); } }
我們再點擊去看,myLooper
/** * 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(); }
很驚訝的看見瞭 sThreadLocal
,這裡是調用get方法
// sThreadLocal.get() will return null unless you've called prepare(). static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
這裡我們可以看到ThreadLocal保存的是Looper這個對象
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)); }
這裡調用瞭set方法,創建瞭一個全局唯一的Looper
private Looper(boolean quitAllowed) { mQueue = new MessageQueue(quitAllowed); mThread = Thread.currentThread(); }
創建瞭一個全局唯一的主線程消息隊列
3、Activity中創建Handler
- 創建一個handler,重寫handleMessage方法
private Handler handler = new Handler(){ @Override public void handleMessage(Message msg) { super.handleMessage(msg); } };
- 發送消息
Message message = new Message(); handler.sendMessage(message); //點擊去 public final boolean sendMessage(Message msg) { return sendMessageDelayed(msg, 0); } //點擊去 public final boolean sendMessageDelayed(Message msg, long delayMillis) { if (delayMillis < 0) { delayMillis = 0; } return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis); }
//繼續看sendMessageAtTime 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); } private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) { msg.target = this; if (mAsynchronous) { msg.setAsynchronous(true); } return queue.enqueueMessage(msg, uptimeMillis); }
//我們看到瞭enqueueMessage,我們看這個queue在哪裡獲取的 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 that has not called Looper.prepare()"); } mQueue = mLooper.mQueue; mCallback = callback; mAsynchronous = async; }
到這裡我們明白瞭,也就是再app啟動後那個唯一的Queue,好瞭我們整理下Handler的消息機制
hander發送消息的時候,調用sendMessage方法,handler種會講消息放到全局的消息隊列中queue.enqueueMessage(msg, uptimeMillis)
接著就會在MessageQueue種賦值全局消息
消息處理
消息消費
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