Handler作为Android开发中,线程间通信的常用方案,其内部的机制原理到底是怎样的呢?我们这篇文章,来一探究竟。
一、情景分析:
一般来说,当我们需要执行耗时操作,此时我们开启工作线程,当我们完成耗时操作之后,我们需要同步主线程,也就是UI线程,进行刷新。此时我们可以考虑使用Handler。
当然了,也可以主线程同步子线程使用Handler,不过一般比较常用的上面所说的场景。
二、主角登场:
Message、Handler、MessageQueue、Looper
先用一句话把这四个要素串联起来:
在工作线程中完成耗时操作后,将我们需要传递给主线程的数据,用Message包装起来,然后通过Handler作为信使发送Message到MessageQueue消息队列。另一边,主线程的Looper会不断循环地从MessageQueue中取Message,当取到Message之后,会将该Message分发给对应的Handler进行处理。
三、Message:
位于:
package android.os;
构造方法:
/** Constructor (but the preferred way to get a Message is to call {@link #obtain() Message.obtain()}).
*/
public Message() {
}
obtain方法:
/**
* Return a new Message instance from the global pool. Allows us to
* avoid allocating new objects in many cases.
*/
public static Message obtain()
/**
* Same as {@link #obtain()}, but copies the values of an existing
* message (including its target) into the new one.
* @param orig Original message to copy.
* @return A Message object from the global pool.
*/
public static Message obtain(Message orig)
/**
* Same as {@link #obtain()}, but sets the value for the <em>target</em> member on the Message returned.
* @param h Handler to assign to the returned Message object's <em>target</em> member.
* @return A Message object from the global pool.
*/
public static Message obtain(Handler h)
/**
* Same as {@link #obtain(Handler)}, but assigns a callback Runnable on
* the Message that is returned.
* @param h Handler to assign to the returned Message object's <em>target</em> member.
* @param callback Runnable that will execute when the message is handled.
* @return A Message object from the global pool.
*/
public static Message obtain(Handler h, Runnable callback)
/**
* Same as {@link #obtain()}, but sets the values for both <em>target</em> and
* <em>what</em> members on the Message.
* @param h Value to assign to the <em>target</em> member.
* @param what Value to assign to the <em>what</em> member.
* @return A Message object from the global pool.
*/
public static Message obtain(Handler h, int what)
/**
* Same as {@link #obtain()}, but sets the values of the <em>target</em>, <em>what</em>, and <em>obj</em>
* members.
* @param h The <em>target</em> value to set.
* @param what The <em>what</em> value to set.
* @param obj The <em>object</em> method to set.
* @return A Message object from the global pool.
*/
public static Message obtain(Handler h, int what, Object obj)
/**
* Same as {@link #obtain()}, but sets the values of the <em>target</em>, <em>what</em>,
* <em>arg1</em>, and <em>arg2</em> members.
*
* @param h The <em>target</em> value to set.
* @param what The <em>what</em> value to set.
* @param arg1 The <em>arg1</em> value to set.
* @param arg2 The <em>arg2</em> value to set.
* @return A Message object from the global pool.
*/
public static Message obtain(Handler h, int what, int arg1, int arg2)
/**
* Same as {@link #obtain()}, but sets the values of the <em>target</em>, <em>what</em>,
* <em>arg1</em>, <em>arg2</em>, and <em>obj</em> members.
*
* @param h The <em>target</em> value to set.
* @param what The <em>what</em> value to set.
* @param arg1 The <em>arg1</em> value to set.
* @param arg2 The <em>arg2</em> value to set.
* @param obj The <em>obj</em> value to set.
* @return A Message object from the global pool.
*/
public static Message obtain(Handler h, int what,
int arg1, int arg2, Object obj)
源码中,这些obtain方法中,都是调用了下面的obtain方法:
/**
* Return a new Message instance from the global pool. Allows us to
* avoid allocating new objects in many cases.
*/
public static Message obtain() {
synchronized (sPoolSync) {
if (sPool != null) {
Message m = sPool;
sPool = m.next;
m.next = null;
m.flags = 0; // clear in-use flag
sPoolSize--;
return m;
}
}
return new Message();
}
public static final Object sPoolSync = new Object();
private static Message sPool;
解析如下:
sPoolSync是一个同步锁对象
sPool是一个全局池消息实例
这个方法其实就是,从全局池返回一个消息实例,这样子我们在很多情况下可以避免分配新对象。
因此,使用obtain方法,在大多数情况下,可以避免消耗更多的内存资源。
Message的三个重要的包间成员变量:
/*package*/ Handler target;
/*package*/ Runnable callback;
// sometimes we store linked lists of these things
/*package*/ Message next;
Message的四个公开的变量what、arg1、arg2、obj,这些一般是用来存储消息进行传递。
四、Handler:
位于:
package android.os;
构造方法:
/**
* Default constructor associates this handler with the {@link Looper} for the
* current thread.
*
* If this thread does not have a looper, this handler won't be able to receive messages
* so an exception is thrown.
*
* @deprecated Implicitly choosing a Looper during Handler construction can lead to bugs
* where operations are silently lost (if the Handler is not expecting new tasks and quits),
* crashes (if a handler is sometimes created on a thread without a Looper active), or race
* conditions, where the thread a handler is associated with is not what the author
* anticipated. Instead, use an {@link java.util.concurrent.Executor} or specify the Looper
* explicitly, using {@link Looper#getMainLooper}, {link android.view.View#getHandler}, or
* similar. If the implicit thread local behavior is required for compatibility, use
* {@code new Handler(Looper.myLooper())} to make it clear to readers.
*
*/
@Deprecated
public Handler() {
this(null, false);
}
@Deprecated
public Handler(@Nullable Callback callback) {
this(callback, false);
}
public Handler(@NonNull Looper looper) {
this(looper, null, false);
}
public Handler(@NonNull Looper looper, @Nullable Callback callback) {
this(looper, callback, false);
}
@UnsupportedAppUsage(maxTargetSdk = Build.VERSION_CODES.R, trackingBug = 170729553)
public Handler(boolean async) {
this(null, async);
}
/**
* 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(@Nullable 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(); //返回与当前线程关联的Looper对象
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread " + Thread.currentThread()
+ " that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue; //返回Looper对象的消息队列
mCallback = callback;
mAsynchronous = async;
}
/**
* Use the provided {@link Looper} instead of the default one and take a callback
* interface in which to handle messages. Also 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 conditions such as display vsync.
*
* @param looper The looper, must not be null.
* @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
*/
@UnsupportedAppUsage
public Handler(@NonNull Looper looper, @Nullable Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
总结下创建Handler实例的时候,主要做了以下事情:
1、获取当前Handler实例所在线程的Looper对象。
2、如果Looper不为空,则获取Looper的消息队列,并赋值给Handler的成员变量mQueue。
3、可以设置Callback来处理消息回调。
Handler是进行消息处理,但是处理消息的不是它,真正来处理消息的是Looper。
流程:Handler发送消息,然后排队等待,等Looper处理消息,Looper处理完消息通知Handler来领取消息,Handler领取到消息后进行分发处理。
总结出两条结论:
1、Handler有且只能绑定一个线程的Looper。
2、Handler将消息发送给Looper的消息队列MessageQueue,等待处理。
sendMessage方法:
sendEmptyMessage--->sendEmptyMessageDelayed--->sendMessageDelayed
sendMessage--->sendMessageDelayed
sendMessageDelayed--->sendMessageAtTime
sendEmptyMessageAtTime--->sendMessageAtTime
sendMessageAtTime--->enqueueMessage
/**
* Enqueue a message into the message queue after all pending messages
* before the absolute time (in milliseconds) <var>uptimeMillis</var>.
* <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b>
* Time spent in deep sleep will add an additional delay to execution.
* You will receive it in {@link #handleMessage}, in the thread attached
* to this handler.
*
* @param uptimeMillis The absolute time at which the message should be
* delivered, using the
* {@link android.os.SystemClock#uptimeMillis} time-base.
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting. Note that a
* result of true does not mean the message will be processed -- if
* the looper is quit before the delivery time of the message
* occurs then the message will be dropped.
*/
public boolean sendMessageAtTime(@NonNull 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(@NonNull MessageQueue queue, @NonNull Message msg,
long uptimeMillis) {
msg.target = this;
msg.workSourceUid = ThreadLocalWorkSource.getUid();
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
boolean enqueueMessage(Message msg, long when) {
//判断msg的所属Handler
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
//因为是队列,有先后之分,因此使用了同步机制
synchronized (this) {
if (msg.isInUse()) {
throw new IllegalStateException(msg + " This message is already in use.");
}
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;
//队列中排在最后的那个Message
Message p = mMessages;
boolean needWake;
//如果队列为空,或者等待时间为0,或者比前面的Message的等待时间要短,就插队
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;
//此处for循环是为了取出一个空的或者when比当前message长的一个消息,然后进行插入
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;
}
dispatchMessage方法:
/**
* Handle system messages here.
*/
public void dispatchMessage(@NonNull Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
/**
* Subclasses must implement this to receive messages.
*/
public void handleMessage(@NonNull Message msg) {
}
五、MessageQueue:
package android.os;
enqueueMessage方法:
boolean enqueueMessage(Message msg, long when) {
//判断msg的所属Handler
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
//因为是队列,有先后之分,因此使用了同步机制
synchronized (this) {
if (msg.isInUse()) {
throw new IllegalStateException(msg + " This message is already in use.");
}
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;
//队列中排在最后的那个Message
Message p = mMessages;
boolean needWake;
//如果队列为空,或者等待时间为0,或者比前面的Message的等待时间要短,就插队
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;
//此处for循环是为了取出一个空的或者when比当前message长的一个消息,然后进行插入
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;
}
next方法:
@UnsupportedAppUsage
Message next() {
// Return here if the message loop has already quit and been disposed.
// This can happen if the application tries to restart a looper after quit
// which is not supported.
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();
}
//当没有消息的时候,会调用本地方法nativePollOnce进入阻塞状态,此时线程会释放CPU资源进入休眠状态
nativePollOnce(ptr, nextPollTimeoutMillis);
synchronized (this) {
// Try to retrieve the next message. Return if found.
//获取从开机到现在的毫秒数,设备睡眠的时间不包括在内
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) {
//如果当前时间未到队列第一个消息的执行时间,继续执行循环
// 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 {
// 否则,跳出循环,返回队列顶部的消息
// 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.
//队列中没有消息了,继续循坏
nextPollTimeoutMillis = -1;
}
// Process the quit message now that all pending messages have been handled.
//外部调用quit命令,跳出循坏
if (mQuitting) {
dispose();
return null;
}
// If first time idle, then get the number of idlers to run.
// Idle handles only run if the queue is empty or if the first message
// in the queue (possibly a barrier) is due to be handled in the future.
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);
}
}
}
// Reset the idle handler count to 0 so we do not run them again.
pendingIdleHandlerCount = 0;
// While calling an idle handler, a new message could have been delivered
// so go back and look again for a pending message without waiting.
nextPollTimeoutMillis = 0;
}
}
MessageQueue是一个数据结构,一般我们叫它消息队列,但是其实它不是一个队列,它的数据结构是单向链表。 它的作用就是存储Message,消息进入MessageQueue,然后又从MessageQueue中被取出来。
六、Looper:
package android.os;
Looper是循坏处理消息的发动机,它不断从消息队列中取出的消息,处理,然后分发处理事件。
每个线程有且仅有一个Looper。
loop方法:
/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*/
public static void loop() {
//获取当前的Looper
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
//获取当前Looper的消息队列
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();
// Allow overriding a threshold with a system prop. e.g.
// adb shell 'setprop log.looper.1000.main.slow 1 && stop && start'
final int thresholdOverride =
SystemProperties.getInt("log.looper."
+ Process.myUid() + "."
+ Thread.currentThread().getName()
+ ".slow", 0);
boolean slowDeliveryDetected = false;
for (;;) {
//取出队头的消息
Message msg = queue.next(); // might block
//如果消息为空,则直接return
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;
long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
long slowDeliveryThresholdMs = me.mSlowDeliveryThresholdMs;
if (thresholdOverride > 0) {
slowDispatchThresholdMs = thresholdOverride;
slowDeliveryThresholdMs = thresholdOverride;
}
final boolean logSlowDelivery = (slowDeliveryThresholdMs > 0) && (msg.when > 0);
final boolean logSlowDispatch = (slowDispatchThresholdMs > 0);
final boolean needStartTime = logSlowDelivery || logSlowDispatch;
final boolean needEndTime = logSlowDispatch;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
final long dispatchStart = needStartTime ? SystemClock.uptimeMillis() : 0;
final long dispatchEnd;
try {
//调用消息的对应的Handler的dispatchMessage方法
msg.target.dispatchMessage(msg);
dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
if (logSlowDelivery) {
if (slowDeliveryDetected) {
if ((dispatchStart - msg.when) <= 10) {
Slog.w(TAG, "Drained");
slowDeliveryDetected = false;
}
} else {
if (showSlowLog(slowDeliveryThresholdMs, msg.when, dispatchStart, "delivery",
msg)) {
// Once we write a slow delivery log, suppress until the queue drains.
slowDeliveryDetected = true;
}
}
}
if (logSlowDispatch) {
showSlowLog(slowDispatchThresholdMs, dispatchStart, dispatchEnd, "dispatch", msg);
}
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);
}
//回收可能正在使用的消息
msg.recycleUnchecked();
}
}
基本上就是,拿出消息,然后分发消息,最后回收消息。
七、总结与扩展:
1、我们new了一个Handler,然后我们通过handler.sendMessage发送消息,然后我们在handler的handleMessage方法中拿到消息并处理。
------这到底是怎么做到的?
2、为什么在主线程中可以直接new Handler并实现线程通信,而在子线程中不行?
ActivityThread的部分源码:
public static void main(String[] args) {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
// Install selective syscall interception
AndroidOs.install();
// 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();
// Make sure TrustedCertificateStore looks in the right place for CA certificates
final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
TrustedCertificateStore.setDefaultUserDirectory(configDir);
// Call per-process mainline module initialization.
initializeMainlineModules();
Process.setArgV0("<pre-initialized>");
Looper.prepareMainLooper();
// Find the value for {@link #PROC_START_SEQ_IDENT} if provided on the command line.
// It will be in the format "seq=114"
long startSeq = 0;
if (args != null) {
for (int i = args.length - 1; i >= 0; --i) {
if (args[i] != null && args[i].startsWith(PROC_START_SEQ_IDENT)) {
startSeq = Long.parseLong(
args[i].substring(PROC_START_SEQ_IDENT.length()));
}
}
}
ActivityThread thread = new ActivityThread();
thread.attach(false, startSeq);
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");
}
3、既然一个线程因为Looper.loop变成了永动机,那为什么不会出现ANR呢?
------拿主线程举例,主线程本身也是一个线程,线程是有生命周期的,任务执行结束,或者在执行任务过程中抛出了异常,线程就结束了。恰恰正是由于Looper维护的这个死循环才能保证主线程不退出。
ANR是由于在主线程执行耗时操作在规定时间没没有完成,触发了Android系统的ANR机制。这个与Looper维护的死循环没有任何关系。
当然loop也不是一直在循环处理任务,如果没有任务,线程会休眠。
4、一个线程可以有几个Handler,一个线程可以有几个Looper,一个Hanlder可以对应几个Looper?
--- ThreadLocal:
ThreadLocal和Synchronized都是为了解决多线程中相同变量的访问冲突问题,不同的点是:
Synchronized是通过线程等待,牺牲时间来解决访问冲突。
ThreadLocal是通过每个线程单独一份存储空间,牺牲空间来解决冲突,并且相比Synchronized,ThreadLocal具有线程隔离的效果,只有在线程内才能获取到对应的值,线程外不能访问到想要的值。
5、Handler会造成内存泄漏吗?为什么会?怎么解决?
--- 非静态内部类,或者匿名内部类。使得Handler默认持有外部类的引用。因此在Activity销毁时,由于Handler可能有未执行完/正在执行的Message。导致Handler持有Activity的引用。进而导致GC无法回收Activity,从而产生了内存泄漏。
--- Activity销毁的时候,也就是在回调onDestroy生命周期方法的时候,清空Handler中未执行完或正在执行的Callback以及Message。
--- Handler的内存泄漏是可能会出现,并且它不是会一直泄漏。