Android開發App啟動流程與訊息機制詳解

2022-08-24 18:01:38

引言

相信很多人對這個問題不陌生，但是大家回答的都比較簡單，如談到app啟動流程有人就會是app的生命週期去了，談到訊息機制有人就會說looper迴圈訊息進行分發，如果是面試可能面試官不會滿意，今天我們搞一篇完善的原始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 是一個執行緒內部的資料儲存類，通過它可以在指定的執行緒中儲存資料，資料儲存以後，只有再指定執行緒中可以獲取到儲存的資料，對於其他執行緒來說則無法獲取到資料，是共用資料變數儲存，通俗的來講，就是儲存每個執行緒的資料，肯定大家都沒聽懂，沒事的，接下來我們通過程式碼來解釋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主執行緒-->  "+threadLocal.get());
        Thread t1 = new Thread(()->{
        });
        t1.start();
        threadLocal.set("data--2");
        System.out.println("2主執行緒-->  "+threadLocal.get());
        Thread t2 = new Thread(()->{
            System.out.println("執行緒2--->  "+threadLocal.get());
        });
        t2.start();
        Thread t3 = new Thread(()->{
            threadLocal.set("data-->3");
            System.out.println("執行緒3--->  "+threadLocal.get());
        });
        t3.start();
        System.out.println("3主執行緒-->  "+threadLocal.get());
        Thread.sleep(1000);
    }

列印結果

1主執行緒--> data--1
2主執行緒--> data--2
執行緒2---> data--1
3主執行緒--> data--2
執行緒3---> data-->3

從上面的例子我們可以看到，ThreadLocal儲存一個String這個變數，這個變數初始化會有一個值，在接下來的執行緒種，每個執行緒都會擁有一個初始值，這個初始值在主執行緒中，一旦這個初始值發生改變，如果是在主執行緒種改變如進行set，則後面的子執行緒獲取的都是這個改變後的值，但是如果子執行緒種也改變了這個值，則只在當前子執行緒種有此值沒其子執行緒還是獲取的主執行緒種那個值，我們來簡單畫個圖給大家

ThreadLocal種的三個重要方法

//預設情況下initialValue是返回為空的
    protected T initialValue() {
        return null;
    }
//在get的時候如果沒有呼叫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();
    }
    //這個方法在在呼叫的時候實際上getMap(t)是為空的，所以就會呼叫createMap，這個方法會把當前的執行緒作為值，保證getMap再呼叫就不會為空
    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，初始化改變了，後面的也改變，但是自己設定的，還是用自己的，就這麼簡單，好了，接下來進行下一步

2、App的啟動流程

我們看下Android的原始碼

//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("&lt;pre-initialized&gt;");
        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();
        }
    }

我們再點選去看，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();
    }

很驚訝的看見了 sThreadLocal，這裡是呼叫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儲存的是Looper這個物件

    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));
    }

這裡呼叫了set方法，建立了一個全域性唯一的Looper

    private Looper(boolean quitAllowed) {
        mQueue = new MessageQueue(quitAllowed);
        mThread = Thread.currentThread();
    }

建立了一個全域性唯一的主執行緒訊息佇列

3、Activity中建立Handler

建立一個handler，重寫handleMessage方法

    private Handler handler = new Handler(){
        @Override
        public void handleMessage(Message msg) {
            super.handleMessage(msg);
        }
    };

傳送訊息

        Message message = new Message();
        handler.sendMessage(message);
	//點選去
    public final boolean sendMessage(Message msg)
    {
        return sendMessageDelayed(msg, 0);
    }
	//點選去
	    public final boolean sendMessageDelayed(Message msg, long delayMillis)
    {
        if (delayMillis &lt; 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);
    }

    //我們看到了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;
    }

到這裡我們明白了，也就是再app啟動後那個唯一的Queue，好了我們整理下Handler的訊息機制