Handler源码分析
转载请注明来源 https://blog.csdn.net/u011453163/article/details/80162281
Handler的基本使用
一般情况下 我们都是在主线程初始化Handler的，主要是用Handler来处理UI相关的操作。
mHandler=new Handler(){
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
}
};然后在需要操作的时候
mHandler.sendMessage(msg);如果是在工作线程中初始化Handler 以上的方法就行不通了
new Thread(){
@Override
public void run() {
super.run();
mThreadHandler=new Handler(){
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
}
};
}
}.start();初始化的时候直接就报错了
java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()这是因为在Handler需要和Looper一起才能正常工作
new Thread(){
@Override
public void run() {
super.run();
Looper.prepare();
mThreadHandler=new Handler(){
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
}
};
Looper.loop();
}
}.start();加上 Looper就能正常使用了
Looper.prepare();
Looper.loop();为什么在主线程初始化Handler的时候不需要构建Looper呢?
因为主线程的Looper在ActivityThread 的main 方法就已经初始化了。
Handler源码分析
- 从构建一个handler对象开始
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;
}
以上代码可以直接看出 handler和looper的关系非常密切，hanlder的创建必须存在looper。这也是为什么在工作线程直接构建hanlder会崩溃的原因。
- Looper从何而来
mLooper = Looper.myLooper(); public static @Nullable Looper myLooper() {
return sThreadLocal.get();
} static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>(); 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));
}以上是Looper类里 关于looper如何构建和获取的源码。Looper本身是个单例类，所以数据可以全局共享。
大概逻辑是这样的
Looper的prepare构建并把looper存储在ThreadLocal里。这里创建的looper因为是在Looper类内部创建的所以都是互不关联的独立对象。ThreadLocal是个存储的集合（其实是数组），然后在使用到looper的时候通过sThreadLocal.get();获取对应的looper。
- 现在Looper 有了，满足了Handler的构建条件。该发送message了
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
} 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);
}发送Message的最终结果是执行了
queue.enqueueMessage(msg, uptimeMillis);- MessageQueue队列出场
queue.enqueueMessage(msg, uptimeMillis); boolean enqueueMessage(Message msg, long when) {
.....
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;
}
.....
return true;
}MessageQueue的enqueueMessage方法将message加到链表中，Message是一个链表块。所以Handler发送消息实际上就是将消息加到链表中。
- 消息有入链也就有出链 Looper.loop();
public static void loop() {
final Looper me = myLooper();
if (me == null) {
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();
for (;;) {
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 slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
final long traceTag = me.mTraceTag;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
final long end;
try {
msg.target.dispatchMessage(msg);
end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
......
}
}这是loop方法的关键方法，方法的逻辑是起一个无限循环从MessageQueue中取出message
try {
msg.target.dispatchMessage(msg);
end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}这里的msg.target就是handler对象，而dispatchMessage最终也是调用了handleMessage方法
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}target实在handler发送message message加入链表的时候赋值，就是发送者本身。
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}到这里 handler从构建 到发送message（sendMessage） 到 处理message(handleMessage)整个过程就打通了。
- 线程切换的核心ThreadLocal 和 ThreadLocalMap
ThreadLocalMap是ThreadLocal 的一个静态内部类。ThreadLocalMap的存储容器是数组，而且自己有一套扩容机制。ThreadLocal的大致作用是给Thread里的ThreadLocalMap容器添加对象，操作的都是Thread里的ThreadLocalMap容器。
使用代码
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();
}
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
ThreadLocalMap getMap(Thread t) {
return t.threadLocals;
}Handler之所以能切换线程 是因为Looper在构建的时候就绑定到了构建是所在的线程中了。
Looper.prepare();
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));
}Looper.loop();
public static void loop() {
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
......
}
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
} 所以Looper一直运行在它被构建时的线程中，并且Looper时持有MessageQueue的，所其他的线程发送的message会存储到Looper的MessageQueue中，并且在Looper所在的线程中得到执行，达到线程切换的效果。
总结
hanlder机制示例图大概是这样的
Handler机制几个相关类之间的关系
Message(消息体)
(一个链表块)
持有handler 对象 用于调用handleMessage
MessageQueue(消息队列)
存储消息方式：链表
持有Message
作用 消息的入链 和 出链
Looper (搬运消息的)
持有MessageQueue
无限循环 取消息并且调用handler的 dispatchMessage->handleMessage方法
Handler
发送Message (sendMessage)
处理Message (handleMessage)
以上是我对handler的一些理解 有什么不对的欢迎指点。
上一篇： 2019/11/20