Android开发App启动流程与消息机制详解

引言

1、第一步了解 ThreadLocal

2、App的启动流程

3、Activity中创建Handler

引言

相信很多人对这个问题不陌生，但是大家回答的都比较简单，如谈到app启动流程有人就会是app的生命周期去了，谈到消息机制有人就会说looper循环消息进行分发，如果是面试可能面试官不会满意，今天我们搞一篇完善的源码解析来进行阐述上面的问题

1、第一步了解 ThreadLocal

什么是ThreadLocal呢，专业的来讲，ThreadLocal 是一个线程内部的数据存储类，通过它可以在指定的线程中存储数据，数据存储以后，只有再指定线程中可以获取到存储的数据，对于其他线程来说则无法获取到数据，是共享数据变量存储，通俗的来讲，就是保存每个线程的数据，肯定大家都没听懂，没事的，接下来我们通过代码来解释ThreadLocal的具体作用

首先看一个例子

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主线程-->  "+threadLocal.get());
        Thread t1 = new Thread(()->{
        });
        t1.start();
        threadLocal.set("data--2");
        System.out.println("2主线程-->  "+threadLocal.get());
        Thread t2 = new Thread(()->{
            System.out.println("线程2--->  "+threadLocal.get());
        });
        t2.start();
        Thread t3 = new Thread(()->{
            threadLocal.set("data-->3");
            System.out.println("线程3--->  "+threadLocal.get());
        });
        t3.start();
        System.out.println("3主线程-->  "+threadLocal.get());
        Thread.sleep(1000);
    }

打印结果

1主线程--> data--1
2主线程--> data--2
线程2---> data--1
3主线程--> data--2
线程3---> data-->3

从上面的例子我们可以看到，ThreadLocal保存一个String这个变量，这个变量初始化会有一个值，在接下来的线程种，每个线程都会拥有一个初始值，这个初始值在主线程中，一旦这个初始值发生改变，如果是在主线程种改变如进行set，则后面的子线程获取的都是这个改变后的值，但是如果子线程种也改变了这个值，则只在当前子线程种有此值没其子线程还是获取的主线程种那个值，我们来简单画个图给大家

ThreadLocal种的三个重要方法

//默认情况下initialValue是返回为空的
    protected T initialValue() {
        return null;
    }
//在get的时候如果没有调用set方法  getMap(t);是返回为空的，所以，返回的是setInitialValue(),这些方法请看后面的介绍，而setInitialValue方法返回的其实就是初始值   
    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();
    }
    //这个方法在在调用的时候实际上getMap(t)是为空的，所以就会调用createMap，这个方法会把当前的线程作为值，保证getMap再调用就不会为空
    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);
    }

简单来讲，就是自己有的，用自己的，自己没有就用初始化的，初始化改变了，后面的也改变，但是自己设置的，还是用自己的，就这么简单，好了，接下来进行下一步

2、App的启动流程

我们看下Android的源码

//这是main函数的入口
 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");
    }

我们重点看下 Looper.prepareMainLooper();这个方法

 /**
     * 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();
        }
    }

我们再点击去看，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();
    }

很惊讶的看见了 sThreadLocal，这里是调用get方法

  // sThreadLocal.get() will return null unless you've called prepare().
    static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();

这里我们可以看到ThreadLocal保存的是Looper这个对象

    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));
    }

这里调用了set方法，创建了一个全局唯一的Looper

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

创建了一个全局唯一的主线程消息队列

3、Activity中创建Handler

创建一个handler，重写handleMessage方法

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

发送消息

        Message message = new Message();
        handler.sendMessage(message);
//点击去
    public final boolean sendMessage(Message msg)
    {
        return sendMessageDelayed(msg, 0);
    }
//点击去
    public final boolean sendMessageDelayed(Message msg, long delayMillis)
    {
        if (delayMillis &lt; 0) {
            delayMillis = 0;
        }
        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
    }

//继续看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);
    }

    //我们看到了enqueueMessage，我们看这个queue在哪里获取的
        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;
    }

到这里我们明白了，也就是再app启动后那个唯一的Queue，好了我们整理下Handler的消息机制