什麼是handler？
熟悉Android開發的一定都知道Handler對於Android開發的重要性吧，Android主執行緒(UI執行緒)阻塞5s以上就會ANR,所以通常情況下耗時操作都是在子執行緒完成，當子執行緒完成耗時操作後，在通過Handler通知主執行緒去更新UI,最常見的使用場景就是在網路請求完成的時候，將網路請求的資料傳給主執行緒，所以Handler的作用就是完成執行緒間通訊。
最簡單的執行緒間通訊
我們想一下Android中Handler的使用步驟吧。
- 主執行緒中定義一個Handler，並覆寫它的handlerMessage方法。
private Handler mHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
//update UI
}
};
複製程式碼- 在子執行緒耗時操作完成後建立一個message物件，並通過呼叫handler.sendMessage方法將這個message傳送給主執行緒
new Thread(new Runnable() {
@Override
public void run() {
// do someting
Message message = Message.obtain();
mHandler.sendMessage(message);
}
}).start();
複製程式碼Java程式中要實現執行緒間通訊要如何做呢？
- 先定義一個Message物件，這裡我們只在Message中定義一個字串
public class Message {
public String msg;
}
複製程式碼- 再定義一個IHandler介面
public interface IHandler {
void handleMessage(Message msg);
}
複製程式碼- 下面我們寫一段普通的java程式碼
public class Main {
public static void main(String[] args) {
IHandler handler = new IHandler() {
@Override
public void handleMessage(Message message) {
System.out.println("main thread receive a message: " + message.msg);
}
};
new SubThread(handler).start();
}
private static class SubThread extends Thread {
private IHandler mHandler;
public SubThread(IHandler handler) {
mHandler = handler;
}
@Override
public void run() {
super.run();
while (true) {
try {
Thread.sleep(1000);
Message message = new Message();
message.msg = "hello";
mHandler.handleMessage(message);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
}
複製程式碼執行結果：
main thread receive a message: hello
main thread receive a message: hello
main thread receive a message: hello
······
······
複製程式碼這就是最簡單的執行緒通訊，通過介面回撥將子執行緒中的Message傳遞給主執行緒去處理。Android中的Handler當然不是用這種方式實現的了，但是對於類與類之間的通訊，介面回撥是最簡單最通用的方法了
下面我們參照Android中的方式實現一個Handler工具吧。Android的Handler在Java層包括了Handler,Looper,Message和MessageQueue四個類。其中Handler的主要作用是傳送和處理訊息，sendMessage會將新的Message加入到MessageQueue中。Looper是一個輪詢器，檢查MessageQueue中是否有Message，如果有Message就取出來分發給對應的Handler去處理。
- Message不用多做處理，只需要增加一個目標Handler就可以了
public class Message {
public Handler target;
public String msg;
}
複製程式碼- MessageQueue是一個先進先出的佇列，為了方便實現我們使用java提供的LinkedBlockingQueue來實現
public class MessageQueue {
private LinkedBlockingQueue<Message> messageList;
public MessageQueue() {
messageList = new LinkedBlockingQueue<>();
}
public void enqueueMessage(Message msg) {
messageList.add(msg);
}
public Message next() {
return messageList.poll();
}
}
複製程式碼- Looper是和執行緒繫結的，一個執行緒裡只能有一個Looper，所以Looper應該儲存在ThreadLocal中。Looper的初始化應該放到Looper.prepare中去完成。
public class Looper {
private static ThreadLocal<Looper> sThreadLocal = new ThreadLocal<>();
final MessageQueue mQueue;
final Thread mThread;
public Looper() {
mQueue = new MessageQueue();
mThread = Thread.currentThread();
}
public static void prepare() {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper());
}
public static Looper myLooper() {
return sThreadLocal.get();
}
public static void loop() {
final Looper me = myLooper();
final MessageQueue queue = me.mQueue;
for (; ; ) {
Message msg = queue.next();
if (msg == null) {
continue;
}
msg.target.dispatchMessage(msg);
}
}
}
複製程式碼- 最後看一下Handler的實現
public class Handler {
private Looper mLooper;
private MessageQueue mQueue;
public Handler() {
mLooper = Looper.myLooper();
mQueue = mLooper.mQueue;
}
public void sendMessage(Message msg) {
msg.target = this;
mQueue.enqueueMessage(msg);
}
public void dispatchMessage(Message msg) {
handleMessage(msg);
}
public void handleMessage(Message msg) {
}
}
複製程式碼我們依然用之前例子裡的程式碼來測試
public class Main {
public static void main(String[] args) {
Looper.prepare();
Handler handler = new Handler() {
@Override
public void handleMessage(Message message) {
super.handleMessage(message);
System.out.println("main thread receive a message: " + message.msg);
}
};
new SubThread(handler).start();
Looper.loop();
}
private static class SubThread extends Thread {
private Handler mHandler;
public SubThread(Handler handler) {
mHandler = handler;
}
@Override
public void run() {
super.run();
while (true) {
try {
Thread.sleep(1000);
Message message = new Message();
message.msg = "hello";
mHandler.handleMessage(message);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
}
複製程式碼輸出結果
main thread receive a message: hello
main thread receive a message: hello
main thread receive a message: hello
······
······
複製程式碼一個簡單的Handler模擬程式就這樣完成了。
深入分析
上面我們用幾十行程式碼實現的那個玩具Handler只是為了讓我們更好的理解Handler機制，Android實際中的Handler要複雜的多，更重要的是Android中Handler的實現不止包含了java程式碼，還包含了native呼叫
private native static long nativeInit();
private native static void nativeDestroy(long ptr);
private native void nativePollOnce(long ptr, int timeoutMillis); /*non-static for callbacks*/
private native static void nativeWake(long ptr);
private native static boolean nativeIsPolling(long ptr);
private native static void nativeSetFileDescriptorEvents(long ptr, int fd, int events);
複製程式碼這幾個native方法主要的作用是什麼呢？在之前的Demo程式我們的loop方法實現是如果Message為空的時候就continue,這就造成了明明訊息佇列裡沒有訊息，Looper依然在那裡空轉浪費cpu資源。native方法就是解決這個問題的。
MessageQueue的建立
這幾個native方法都是在MessageQueue中定義，所以我們就先來研究一下MessageQueue建立。
構造方法
MessageQueue(boolean quitAllowed) {
mQuitAllowed = quitAllowed;
mPtr = nativeInit();
}
複製程式碼mQuitAllowed表示這個訊息佇列能不能退出，除了主執行緒外，其他執行緒的訊息佇列都是可以退出的。
mPtr儲存了native方法nativeInit()方法的返回值，這個值我們後面會用到，先看下nativeInit()做了什麼吧。
nativeInit呼叫的是android_os_MessageQueue.cpp裡面的方法
static jlong android_os_MessageQueue_nativeInit(JNIEnv* env, jclass clazz) {
NativeMessageQueue* nativeMessageQueue = new NativeMessageQueue();
if (!nativeMessageQueue) {
jniThrowRuntimeException(env, "Unable to allocate native queue");
return 0;
}
nativeMessageQueue->incStrong(env);
return reinterpret_cast<jlong>(nativeMessageQueue);
}
複製程式碼從上面的程式碼我們知道，nativeInit建立了一個NativeMessageQueue物件，並將它的指標強轉成了Java中的long型別儲存在mPtr中，我們可以這樣理解，MessageQueue.java中儲存了一份NativeMessageQueue的指標，在需要的時候，mPtr可以傳給native方法並轉換成NativeMessageQueue物件。
NativeMessageQueue的構造方法
NativeMessageQueue::NativeMessageQueue() :
mPollEnv(NULL), mPollObj(NULL), mExceptionObj(NULL) {
mLooper = Looper::getForThread();
if (mLooper == NULL) {
mLooper = new Looper(false);
Looper::setForThread(mLooper);
}
}
複製程式碼上面的程式碼的作用很簡單，如果ThreadLocal中有Looper物件就返回。如果沒有就new一個Looper物件，並將其儲存在ThreadLocal中。
Looper.cpp的建構函式
Looper::Looper(bool allowNonCallbacks) :
mAllowNonCallbacks(allowNonCallbacks), mSendingMessage(false),
mPolling(false), mEpollFd(-1), mEpollRebuildRequired(false),
mNextRequestSeq(0), mResponseIndex(0), mNextMessageUptime(LLONG_MAX) {
mWakeEventFd = eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC);
LOG_ALWAYS_FATAL_IF(mWakeEventFd < 0, "Could not make wake event fd: %s",
strerror(errno));
AutoMutex _l(mLock);
rebuildEpollLocked();
}
複製程式碼上面的程式碼有一個關鍵的方法eventfd,這個具體的作用可參看Linux程式間通訊-eventfd，簡單講的話這個就是linux程式間通訊的一種方式，Linux核心空間維護了一個64位的計數器，可以一個程式呼叫write寫入一個數，另一個程式呼叫read讀出來。
最後一個函式
void Looper::rebuildEpollLocked() {
// Close old epoll instance if we have one.
if (mEpollFd >= 0) {
close(mEpollFd);
}
// Allocate the new epoll instance and register the wake pipe.
mEpollFd = epoll_create(EPOLL_SIZE_HINT);
LOG_ALWAYS_FATAL_IF(mEpollFd < 0, "Could not create epoll instance: %s", strerror(errno));
struct epoll_event eventItem;
memset(& eventItem, 0, sizeof(epoll_event)); // zero out unused members of data field union
eventItem.events = EPOLLIN;
eventItem.data.fd = mWakeEventFd;
int result = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, mWakeEventFd, & eventItem);
LOG_ALWAYS_FATAL_IF(result != 0, "Could not add wake event fd to epoll instance: %s",
strerror(errno));
for (size_t i = 0; i < mRequests.size(); i++) {
const Request& request = mRequests.valueAt(i);
struct epoll_event eventItem;
request.initEventItem(&eventItem);
int epollResult = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, request.fd, & eventItem);
if (epollResult < 0) {
ALOGE("Error adding epoll events for fd %d while rebuilding epoll set: %s",
request.fd, strerror(errno));
}
}
}
複製程式碼這個裡面也有兩個關鍵的方法epoll_create和epoll_ctl，epoll的具體講解可以看這個。簡單說就是epoll可以監控很多個檔案描述符，並註冊想要監控的事件。mWakeEventFd是之前我們建立的那個eventfd的檔案描述符，EPOLLIN表示寫事件。所以
int result = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, mWakeEventFd, & eventItem)
複製程式碼這行程式碼的含義就是當eventfd有寫入事件時觸發。
sendMessage的流程
我們先來看一下Handler中sendMessage大致的時序圖
- Handler呼叫自身的方法
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
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);
}
複製程式碼- MessageQueue呼叫enqueueMessage方法
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) {
// 第一個訊息；需要喚醒Looper
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
// 按照時間排序找到合適的插入點，這種情況通常不需要喚醒Looper，但是如果同步分隔欄且是第一個非同步訊息就需要喚醒Looper
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;
}
複製程式碼上面程式碼是Message加入佇列的過程，Message入隊過程並不像我們Demo裡寫的那樣先進先出的，而是按照執行時間進行排序，如果這個Message是佇列裡面的第一個Message，則需要喚醒Looper。我們假設我們傳送的Message就是第一個，那麼就會觸發nativeWake方法，傳入的引數就是我們在MessageQueue建構函式中儲存的指標mPtr，這個指標指向NativeMessageQueue。
- android_os_MessageQueue.cpp中的android_os_MessageQueue_nativeWake
static void android_os_MessageQueue_nativeWake(JNIEnv* env, jclass clazz, jlong ptr) {
NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
nativeMessageQueue->wake();
}
複製程式碼- NativeMessageQueue呼叫wake方法
void NativeMessageQueue::wake() {
mLooper->wake();
}
複製程式碼- Looper.cpp呼叫wake方法
void Looper::wake() {
#if DEBUG_POLL_AND_WAKE
ALOGD("%p ~ wake", this);
#endif
uint64_t inc = 1;
// 向eventfd中寫入1
ssize_t nWrite = TEMP_FAILURE_RETRY(write(mWakeEventFd, &inc, sizeof(uint64_t)));
if (nWrite != sizeof(uint64_t)) {
if (errno != EAGAIN) {
LOG_ALWAYS_FATAL("Could not write wake signal to fd %d: %s",
mWakeEventFd, strerror(errno));
}
}
}
複製程式碼mWakeEventFd就是在Looper的構造方法中建立的eventfd的檔案描述符。上述程式碼的作用就是向eventfd中寫入一個1。還記得Looper構造方法中的epoll嗎？epoll正在監聽eventfd的寫事件，現在已經觸發了。觸發了什麼事呢？我們接著往下分析。
handleMessage的流程
我們先來看一下Handler中handleMessage大致的時序圖
- Looper中的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;
Binder.clearCallingIdentity();
final long ident = Binder.clearCallingIdentity();
for (;;) {
//訊息佇列為空的時候會阻塞
Message msg = queue.next();
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 traceTag = me.mTraceTag;
if (traceTag != 0) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
try {
msg.target.dispatchMessage(msg);
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
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();
}
}
複製程式碼無限迴圈取Message，取到Message後呼叫msg.target.dispatchMessage(msg)分發給對應的Handler處理，看著似乎跟我們Demo裡是一樣的，但關鍵點就在於
Message msg = queue.next();
複製程式碼這個操作是會阻塞的，還記得Looper.cpp裡面的wake有個喚醒操作嗎？這個喚醒操作就是為了喚醒queue.next()的。
- MessageQueue的next方法
Message next() {
final long ptr = mPtr;
if (ptr == 0) {
return null;
}
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
nativePollOnce(ptr, nextPollTimeoutMillis);
synchronized (this) {
// Try to retrieve the next message. Return if found.
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;
if (msg != null && msg.target == null) {
// Stalled by a barrier. Find the next asynchronous message in the queue.
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
if (msg != null) {
if (now < msg.when) {
// Next message is not ready. Set a timeout to wake up when it is ready.
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
} else {
// Got a message.
mBlocked = false;
if (prevMsg != null) {
prevMsg.next = msg.next;
} else {
mMessages = msg.next;
}
msg.next = null;
if (DEBUG) Log.v(TAG, "Returning message: " + msg);
msg.markInUse();
return msg;
}
} else {
// No more messages.
nextPollTimeoutMillis = -1;
}
// Process the quit message now that all pending messages have been handled.
if (mQuitting) {
dispose();
return null;
}
if (pendingIdleHandlerCount < 0
&& (mMessages == null || now < mMessages.when)) {
pendingIdleHandlerCount = mIdleHandlers.size();
}
if (pendingIdleHandlerCount <= 0) {
// No idle handlers to run. Loop and wait some more.
mBlocked = true;
continue;
}
if (mPendingIdleHandlers == null) {
mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
}
mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
}
// Run the idle handlers.
// We only ever reach this code block during the first iteration.
for (int i = 0; i < pendingIdleHandlerCount; i++) {
final IdleHandler idler = mPendingIdleHandlers[i];
mPendingIdleHandlers[i] = null; // release the reference to the handler
boolean keep = false;
try {
keep = idler.queueIdle();
} catch (Throwable t) {
Log.wtf(TAG, "IdleHandler threw exception", t);
}
if (!keep) {
synchronized (this) {
mIdleHandlers.remove(idler);
}
}
}
pendingIdleHandlerCount = 0;
nextPollTimeoutMillis = 0;
}
}
複製程式碼當MessageQueue裡面沒有訊息時nativePollOnce(ptr, nextPollTimeoutMillis)會阻塞
- android_os_MessageQueue.cpp中的nativePollOnce
static void android_os_MessageQueue_nativePollOnce(JNIEnv* env, jobject obj,
jlong ptr, jint timeoutMillis) {
NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
nativeMessageQueue->pollOnce(env, obj, timeoutMillis);
}
複製程式碼- NativeMessageQueue中的pollOnce方法
void NativeMessageQueue::pollOnce(JNIEnv* env, jobject pollObj, int timeoutMillis) {
mPollEnv = env;
mPollObj = pollObj;
mLooper->pollOnce(timeoutMillis);
mPollObj = NULL;
mPollEnv = NULL;
if (mExceptionObj) {
env->Throw(mExceptionObj);
env->DeleteLocalRef(mExceptionObj);
mExceptionObj = NULL;
}
}
複製程式碼- Looper.cpp中的pollOnce方法
int Looper::pollOnce(int timeoutMillis, int* outFd, int* outEvents, void** outData) {
int result = 0;
for (;;) {
while (mResponseIndex < mResponses.size()) {
const Response& response = mResponses.itemAt(mResponseIndex++);
int ident = response.request.ident;
if (ident >= 0) {
int fd = response.request.fd;
int events = response.events;
void* data = response.request.data;
if (outFd != NULL) *outFd = fd;
if (outEvents != NULL) *outEvents = events;
if (outData != NULL) *outData = data;
return ident;
}
}
if (result != 0) {
if (outFd != NULL) *outFd = 0;
if (outEvents != NULL) *outEvents = 0;
if (outData != NULL) *outData = NULL;
return result;
}
result = pollInner(timeoutMillis);
}
}
複製程式碼- Looper.cpp中的pollInner方法
int Looper::pollInner(int timeoutMillis) {
// Poll.
int result = POLL_WAKE;
mResponses.clear();
mResponseIndex = 0;
// We are about to idle.
mPolling = true;
struct epoll_event eventItems[EPOLL_MAX_EVENTS];
// 等待eventfd中的寫事件觸發
int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);
for (int i = 0; i < eventCount; i++) {
int fd = eventItems[i].data.fd;
uint32_t epollEvents = eventItems[i].events;
if (fd == mWakeEventFd) {
if (epollEvents & EPOLLIN) {
awoken();
} else {
ALOGW("Ignoring unexpected epoll events 0x%x on wake event fd.", epollEvents);
}
}
}
return result;
}
複製程式碼pollInner中程式碼很長，我刪去了一些不相關的程式碼，主要就是這行程式碼
struct epoll_event eventItems[EPOLL_MAX_EVENTS];
int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);
複製程式碼epoll_wait會一直等待eventfd中的寫事件觸發，如果沒有就會阻塞。
- 最後的awoken
void Looper::awoken() {
uint64_t counter;
TEMP_FAILURE_RETRY(read(mWakeEventFd, &counter, sizeof(uint64_t)));
}
複製程式碼讀取eventfd中儲存的資料，read操作後eventfd會置0，重新陷入阻塞
總結
- Looper呼叫prepare方法建立MessageQueue
- MessageQueue建立的時候同時呼叫jni方法建立了eventfd和epoll,epoll監聽eventfd上的寫事件。
- Looper呼叫loop方法進入迴圈。
- 執行到nativePollOnce時，由於eventfd計數器的值為0陷入阻塞。
- Handler呼叫sendMessage傳送訊息。
- message加入到MessageQueue，同時執行nativeWake方法向eventfd寫入一個數字1。
- nativePollOnce阻塞解除，分發Message給對應的Handler。
- 讀取eventfd中的資料，eventfd置0。nativePollOnce繼續阻塞。