Handler之源码再解析
实例
handler用法已经用法和内存泄漏已经有很多博客写的很好,再次不在赘述,此处仅仅是自己看别人博客时感觉不解的地方再次分析记录,希望看本篇文章的时候结合api会更有收获。
先写一个例子(这个例子是随便从别的地方拷贝的):
public class MainActivity extends AppCompatActivity {
private ProgressBar progress_bar = null;
private Button start = null;
Handler mHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
//在此处更新UI
progress_bar.setProgress(msg.arg1);
}
};
Runnable mRunable = new Runnable(){
int m = 0;
public void run() {
// TODO Auto-generated method stub
m += 10;
Message msg = mHandler.obtainMessage();
msg.arg1 = m;
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
// TODO: handle exception
e.printStackTrace();
}
//发送消息
mHandler.sendMessage(msg);
if(m == 100)
//从线程队列中移除线程
mHandler.removeCallbacks(mRunable);
}
};
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
progress_bar = (ProgressBar)findViewById(R.id.progress_bar);
start = (Button)findViewById(R.id.send_msg);
start.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
//让进度条显示出来
progress_bar.setVisibility(View.VISIBLE);
//将线程加入到handler的线程队列中
mHandler.post(mRunable);
}
});
}
}
Looper
分析Handler首先从Looper开始,
public final class Looper {
。。。
}
从类的定义来看,Looper是一个final类型的类,final类型的类有以下特点:
final类不能被继承,没有子类,final类中的方法默认是final的。
final方法不能被子类的方法覆盖,但可以被继承。
final成员变量表示常量,只能被赋值一次,赋值后值不再改变。
final不能用于修饰构造方法。
注意:父类的private成员方法是不能被子类方法覆盖的,因此private类型的方法默认是final类型的。
1、final类
final类不能被继承,因此final类的成员方法没有机会被覆盖,默认都是final的。在设计类时候,如果这个类不需要有子类,类的实现细节不允许改变,并且确信这个类不会载被扩展,那么就设计为final类。
2、final方法
如果一个类不允许其子类覆盖某个方法,则可以把这个方法声明为final方法。使用final方法的原因有二:
①把方法锁定,防止任何继承类修改它的意义和实现。
②高效,编译器在遇到调用final方法时候会转入内嵌机制,大大提高执行效率。
3、looper的构造方法中初始化了一个队列还有当前线程
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
UI线程也就是主线程默认维持一个Looper也就是mainLooper,所以调用Handler的时候无需手动调用looper,但是其他子线程使用handler之前需要先调用Looper.prepare
(),之后调用Looper.loop(),让我们看看这两个方法到底做了什么。
Looper.prepare()
/** Initialize the current thread as a looper.
* This gives you a chance to create handlers that then reference
* this looper, before actually starting the loop. Be sure to call
* {@link #loop()} after calling this method, and end it by calling
* {@link #quit()}.
*/
public static void prepare() {
prepare(true);
}
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));
}
此处注意prepare中使用一个sThreadLocal新建一个Looper并且set进去,也就是很多博客说的关联一个looper到该线程,实际就是以此线程为键值,新建一个Looper作为value放到ThreadLocal中。
ThreadLocal的作用简单说一下,是多线程并发处理的一种,把共享变量拷贝一份到线程空间,每个线程有一份数据,所以不存在多线程同步问题,和synchronize以时间换空间方法不同,它是一种以空间换时间的机制。
Looper.loop()
Looper.loop()和prepare一样都是一个静态方法,关键代码,请看下面的汉语注释。
/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*/
public static void loop() {
//此处是获取该线程的looper,实际上就是从ThreadLocal中把prepare时候放进去的Looper对象取出来,追入myLooper()就可以发现。
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
//这个地方很关键,me代表的是当前looper,而当前Looper中的mQueue正是Looper初始化时候的队列
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 (;;) {
//死循环,不断地从队列中取出message,而这个message也正是关键部分,后面会一一解惑。
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 {
//这一步调用的是Handler中的dispatchMessage,也正是这一步完成了msg从子线程到主线程的蜕变,追进去可看到,实际调用的是handleMesage方法。而这个msg.target是如何初始化的,请继续看下面Handler部分分析
msg.target.dispatchMessage(msg);
end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
if (slowDispatchThresholdMs > 0) {
final long time = end - start;
if (time > slowDispatchThresholdMs) {
Slog.w(TAG, "Dispatch took " + time + "ms on "
+ Thread.currentThread().getName() + ", h=" +
msg.target + " cb=" + msg.callback + " msg=" + msg.what);
}
}
if (logging != null) {
logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
}
// Make sure that during the course of dispatching the
// identity of the thread wasn't corrupted.
final long newIdent = Binder.clearCallingIdentity();
if (ident != newIdent) {
Log.wtf(TAG, "Thread identity changed from 0x"
+ Long.toHexString(ident) + " to 0x"
+ Long.toHexString(newIdent) + " while dispatching to "
+ msg.target.getClass().getName() + " "
+ msg.callback + " what=" + msg.what);
}
//这一步是在分发完成Message之后,标记将该Message标记为 *正在使用*
msg.recycleUnchecked();
}
}
dispatchMessage方法源码
/**
* Handle system messages here.
*/
public void dispatchMessage(Message msg) {
//此处的msg正是我们sendMessage的时候设置进去的,这一步也完成了主线程中的方法handleMessage回调,所以这里面可以进行UI更新操作。
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
Handler
我们来看一下Handler,同样注意下面的汉语注释
/**
* Use the {@link Looper} for the current thread with the specified callback interface
* and set whether the handler should be asynchronous.
*
* Handlers are synchronous by default unless this constructor is used to make
* one that is strictly asynchronous.
*
* Asynchronous messages represent interrupts or events that do not require global ordering
* with respect to synchronous messages. Asynchronous messages are not subject to
* the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
*
* @param callback The callback interface in which to handle messages, or null.
* @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
* each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
*
* @hide
*/
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());
}
}
//同样先获取looper此处的looper是UI线程
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
//初始化消息队列
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
什么时候子线程中的msg.target,完成赋值
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
正是在这一步,而这一步是所有sendMessage,sendMesageDelay等等发送消息的接口全部都会调用到这个方法
总结
综上,可以知道,Looper.prepare()的作用是new一个子线程专属的Looper,并且与之关联。
Looper.loop()的作用是先循环的从队列中获取消息,然后通过dispatchMessage,调用handleMessage回调,完成子线程到主线程的信息发送。
Handler构造的作用是先完成自身队列和looper的初始化,
sendMessage,把Looper中的msg.target赋值为当前handler。