前言
Android的消息机制主要说的是Handler的运行机制,相信大家对Handler已经非常熟悉了,Handler可以轻松的将一个任务切换到Handler所在的线程中去执行。最熟悉的就是我们只能在UI线程中更新UI,所以我们经常来用Handler来更新UI,但Handler并不是专门用来更新UI的。本文源码基于Android8.0。
一、为什么只能在主线程中访问UI
可能我们每个人都知道在Android中只能在主线程中访问UI,但是为什么是这样呢,难道谷歌当时设计的时候不会考虑这个问题?首先Android的UI控件不是线程安全的,如果我们在多个线程中可以同时操作UI,那么UI控件会处于不可预期的状态,如果在设计的时候对访问UI加上锁机制,一方面会让访问UI的逻辑变得复杂,得不偿失,另一方便锁机制会使得访问UI效率降低,所以综合考虑上述方面使用Handler是最为简单的方式了。
UI操作的验证由ViewRootImpl来完成的,源码如下:
void checkThread() { if (mThread != Thread.currentThread()) { throw new CalledFromWrongThreadException( "Only the original thread that created a view hierarchy can touch its views."); } }
所以我们经常可以看到如果在子线程中处理UI报错Only the original thread that created a view hierarchy can touch its views.的异常。
二、Android的消息机制
Android的消息机制主要说的是Handler的运行机制,Handler的运行需要MessageQueue和Looper,MessageQueue就是消息队列,它是采用单链表的数据结构来存储消息列表。而Looper会不断的查看MessageQueue中是否有消息,当有消息的时候就取出来。Hanlder在创建的时候会采用当前线程的Looper来构造消息循环系统,但是默认的线程是没有Looper的,比如我们在子线程创建一个Hanlder(这里只是便于演示所以不采用线程池了,建议标准开发中使用线程池)
/** * 子线程使用Hanlder测试方法 */ private void ThreadHandlerTest() { new Thread(new Runnable() { @Override public void run() { Handler handler = new Handler(); } }).start(); }
运行结果报错如下:
java.lang.RuntimeException: Can't create handler inside thread Thread[Thread-2,5,main] that has not called Looper.prepare()
at android.os.Handler.<init>(Handler.java:206)
at android.os.Handler.<init>(Handler.java:119)
at lonbon.com.hanlderlooperdemo.MainActivity$1.run(MainActivity.java:24)
at java.lang.Thread.run(Thread.java:764)
这只因为默认的线程中没有Looper,所以我们要为当前线程创建Looper对象:
Looper.prepare(); Handler handler = new Handler(); Looper.loop();
我们通过Looper.prepare()方法创建Looper对象,.loop()方法开启消息循环。关于looper详细的下面讲解。
那么我们可能有疑问了,我们在主线程中使用Handler的时候没有创建looper对象也可以正常使用,那是因为默认的UI线程创建的时候默认创建了looper对象。
创建完Handler之后,通过handler的send或者post方法发送消息,这个消息会被存储到消息队列,Looper发现消息队列中有新的消息便会处理这个消息,然后handlermessage方法或者Runable方法会被调用,大致过程如图所示。
三、ThreadLocal
ThreadLocal是Looper中的特殊概念,用来在当前线程中存储数据,我们获取当前线程的Looper也是通过ThreadLocal操作的,当然,日常开发中我们能使用的ThreadLocal的地方并不多。比如我们在两个不同线程中进行如下操作:
首先我们声明一个String类型的ThreadLocal变量,创建两个线程分别使用set方法赋值,然后打印。
private ThreadLocal<String> threadLocal = new ThreadLocal<>();
/** * 测试线程1 */ private void ThreadTest1() { new Thread(new Runnable() { @Override public void run() { threadLocal.set("BigYellow"); Log.d(TAG,threadLocal.get()); } }).start(); }
/** * 测试线程2 */ private void ThreadTest2() { new Thread(new Runnable() { @Override public void run() { threadLocal.set("大黄"); Log.d(TAG,threadLocal.get()); } }).start(); }
运行打印,日志如下:
02-12 10:21:37.961 11719-12135/? D/TAG: BigYellow
02-12 10:21:37.966 11719-12136/? D/TAG: 大黄
我们可以看到取出的分别是各自线程对应的值,如果我们在主线程中呢?显然是null因为我们没有在主线程中存值。
接下来我们从源码的角度来分析ThreadLocal的存取值过程,首先我们看set方法。
public void set(T value) { Thread t = Thread.currentThread(); ThreadLocalMap map = getMap(t); if (map != null) map.set(this, value); else createMap(t, value); }
首先通过getMap方法获取当前线程的ThreadLocalMap,如果map不为空就通过map的set方法将值存储,如果为空则创建map,
我们来看下ThreadLocalMap,ThreadLocalMap是一个存储当前线程数据的Map集合,set方法源码如下所示:
private void set(ThreadLocal<?> key, Object value) { // We don't use a fast path as with get() because it is at // least as common to use set() to create new entries as // it is to replace existing ones, in which case, a fast // path would fail more often than not. Entry[] tab = table; int len = tab.length; int i = key.threadLocalHashCode & (len-1); for (Entry e = tab[i]; e != null; e = tab[i = nextIndex(i, len)]) { ThreadLocal<?> k = e.get(); if (k == key) { e.value = value; return; } if (k == null) { replaceStaleEntry(key, value, i); return; } } tab[i] = new Entry(key, value); int sz = ++size; if (!cleanSomeSlots(i, sz) && sz >= threshold) rehash(); }
首先定义了一个Entry类型的数组,我们主要来看for循环中的操作,for循环主要做的就是为插入值得位置找到合适的位置,通过不断到table数组中去寻找,直到存放的entry为null
if (k == key) { e.value = value; return; }
如果key的值相同说明该线程曾经设置过Threadlocal,直接赋值即可。
if (k == null) { replaceStaleEntry(key, value, i); return; }
Entry继承的是WeakReference,这是弱引用带来的坑
所以要判断是否为null,如果为null就进行置换操作,即
replaceStaleEntry(key, value, i);
private void replaceStaleEntry(ThreadLocal<?> key, Object value, int staleSlot) { Entry[] tab = table; int len = tab.length; Entry e; // Back up to check for prior stale entry in current run. // We clean out whole runs at a time to avoid continual // incremental rehashing due to garbage collector freeing // up refs in bunches (i.e., whenever the collector runs). int slotToExpunge = staleSlot; for (int i = prevIndex(staleSlot, len); (e = tab[i]) != null; i = prevIndex(i, len)) if (e.get() == null) slotToExpunge = i; // Find either the key or trailing null slot of run, whichever // occurs first for (int i = nextIndex(staleSlot, len); (e = tab[i]) != null; i = nextIndex(i, len)) { ThreadLocal<?> k = e.get(); // If we find key, then we need to swap it // with the stale entry to maintain hash table order. // The newly stale slot, or any other stale slot // encountered above it, can then be sent to expungeStaleEntry // to remove or rehash all of the other entries in run. if (k == key) { e.value = value; tab[i] = tab[staleSlot]; tab[staleSlot] = e; // Start expunge at preceding stale entry if it exists if (slotToExpunge == staleSlot) slotToExpunge = i; cleanSomeSlots(expungeStaleEntry(slotToExpunge), len); return; } // If we didn't find stale entry on backward scan, the // first stale entry seen while scanning for key is the // first still present in the run. if (k == null && slotToExpunge == staleSlot) slotToExpunge = i; } // If key not found, put new entry in stale slot tab[staleSlot].value = null; tab[staleSlot] = new Entry(key, value); // If there are any other stale entries in run, expunge them if (slotToExpunge != staleSlot) cleanSomeSlots(expungeStaleEntry(slotToExpunge), len); }
当table数组中存储的ThreadLocal对应的值还在但是key不存在了,就认为Entry过期了,
int slotToExpunge = staleSlot; for (int i = prevIndex(staleSlot, len); (e = tab[i]) != null; i = prevIndex(i, len)) if (e.get() == null) slotToExpunge = i;
上述代码检查脏数据,清理整个table,否则会因为GC问题导致很严重的后果。
if (k == key) { e.value = value; tab[i] = tab[staleSlot]; tab[staleSlot] = e; // Start expunge at preceding stale entry if it exists if (slotToExpunge == staleSlot) slotToExpunge = i; cleanSomeSlots(expungeStaleEntry(slotToExpunge), len); return; }
如果找到key了我们需要进行替换,将过期数据进行删除刷新。源代码中注释的很清楚了,这里就不一一解释了。
个人感觉和之前早期版本(6.0之前)的set方法变化很大。
我们接下来来看ThreadLocal的get方法,首先同样的获取当前线程的ThreadLocalMap,获取map的entry对象,如果不为空的话就从中取值即可。如果map为空就回到setInitialValue初始化方法.
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(); }
四、Looper
Looper我们上面说了是用来构建消息循环系统,我们通过ThreadLocal来获取当前线程的Looper对象.我们上面也说到了如何在子线程中创建looper,通过Looper的prepare方法为当前线程创建一个looper,通过loop方法开启消息循环。在主线程中创建Looper是通过
Looper.prepareMainLooper();
方法,因为UI线程的Looper比较特殊是默认创建好的,所以我们可以通过下列代码来获取主线程的looper
Looper.getMainLooper();
我们可以开启looper肯定也可以关闭looper,关闭looper有这个方法,一个是
getMainLooper().quit();
public void quit() { mQueue.quit(false); }
quit方法会直接退出looper,另一种方法是
getMainLooper().quitSafely();
和quit方法不同的是quitSafely方法调用后在消息队列中的消息处理完成之后在退出,就像方法名一样是安全退出。所以如果我们在子线程中手动创建了looper,记得在执行完线程后调用退出方法,否则子线程会一直处于等待状态,影响性能。
接下来我们看looper是如何通过loop方法开启消息循环的,loop方法源码如下所示:
/** * Run the message queue in this thread. Be sure to call * {@link #quit()} to end the 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); } } 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); } msg.recycleUnchecked(); } }
从中我们可以看到looper会不断的调用
queue.next()
方法从消息队列中取出消息,如果为空就一直等待,如果有消息就调用
msg.target.dispatchMessage(msg);
方法处理msg.target就是发送这条消息的Handler对象,而handler的dispatchMessage方法又是在创建Handler所在的Looper执行的,所以这样就将消息交给指定的线程去处理了。
五、消息队列MessageQueue与Handler
消息队列MessageQueue主要有插入和读取两个操作,读取成功后也就相当于删除。
插入方法对应的enqueueMessage源码如下:
boolean enqueueMessage(Message msg, long when) { if (msg.target == null) { throw new IllegalArgumentException("Message must have a target."); } if (msg.isInUse()) { throw new IllegalStateException(msg + " This message is already in use."); } synchronized (this) { if (mQuitting) { IllegalStateException e = new IllegalStateException( msg.target + " sending message to a Handler on a dead thread"); Log.w(TAG, e.getMessage(), e); msg.recycle(); return false; } msg.markInUse(); msg.when = when; Message p = mMessages; boolean needWake; 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; } // We can assume mPtr != 0 because mQuitting is false. if (needWake) { nativeWake(mPtr); } } return true; }
我们上面也说了消息队列其实是一个单链表,所以就相当于单链表的插入操作。读取方法是next方法,读取后将消息移除,这里就不作过多解释了。
而我们日常开发中最常用的就是创建Hanlder的匿名内部类方式(这种方式记得处理内存泄漏),然后通过hanlder.send方法发送消息,而send方法最终又会调用sendMessageAtTime方法,源码如下:
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); }
我们看最终返回值就可以看到其实调用handler的send方法就是调用enqueueMessage方法往消息队列中插入了一条消息,然后不断循环的looper进去取出又交给handler处理,这样就构成了Android的消息机制。
上文源码基于Android8.0,如有纰漏欢迎指出探讨。