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這篇文章主要講解了“Android怎么開發(fā)App啟動流程與消息機制”，文中的講解內(nèi)容簡單清晰，易于學習與理解，下面請大家跟著小編的思路慢慢深入，一起來研究和學習“Android怎么開發(fā)App啟動流程與消息機制”吧！
什么是ThreadLocal呢，專業(yè)的來講，ThreadLocal 是一個線程內(nèi)部的數(shù)據(jù)存儲類，通過它可以在指定的線程中存儲數(shù)據(jù)，數(shù)據(jù)存儲以后，只有再指定線程中可以獲取到存儲的數(shù)據(jù)，對于其他線程來說則無法獲取到數(shù)據(jù)，是共享數(shù)據(jù)變量存儲，通俗的來講，就是保存每個線程的數(shù)據(jù)，肯定大家都沒聽懂，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(fā)生改變，如果是在主線程種改變?nèi)邕M行set，則后面的子線程獲取的都是這個改變后的值，但是如果子線程種也改變了這個值，則只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家
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(diào)用set方法 getMap(t);是返回為空的，所以，返回的是setInitialValue(),這些方法請看后面的介紹，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(diào)用的時候?qū)嶋H上getMap(t)是為空的，所以就會調(diào)用createMap，這個方法會把當前的線程作為值，保證getMap再調(diào)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，初始化改變了，后面的也改變，但是自己設置的，還是用自己的，就這么簡單，好了，接下來進行下一步
我們看下Android的源碼
//這是main函數(shù)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();
}很驚訝的看見了 sThreadLocal，這里是調(diào)用get方法
// sThreadLocal.get() will return null unless you've called prepare(). static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
這里我們可以看到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));
}這里調(diào)用了set方法，創(chuàng)建了一個全局唯一的Looper
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}創(chuàng)建了一個全局唯一的主線程消息隊列
創(chuàng)建一個handler，重寫handleMessage方法
private Handler handler = new Handler(){
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
}
};發(fā)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(xù)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在哪里獲取的
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;
}到這里我們明白了，也就是再app啟動后那個唯一的Queue，好了我們整理下Handler的消息機制
hander發(fā)送消息的時候，調(diào)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
感謝各位的閱讀，以上就是“Android怎么開發(fā)App啟動流程與消息機制”的內(nèi)容了，經(jīng)過本文的學習后，相信大家對Android怎么開發(fā)App啟動流程與消息機制這一問題有了更深刻的體會，具體使用情況還需要大家實踐驗證。這里是億速云，小編將為大家推送更多相關知識點的文章，歡迎關注！
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