handler原子锁_android 中ThreadLocal的作用以及loop()的作用 handler是如何处理线程安全的...

悠悠 2023-01-01 06:54 89阅读 0赞

ThreadLocal使用场合主要解决多线程中数据数据因并发产生不一致问题。ThreadLocal为每个线程的中并发访问的数据提供一个副本，通过访问副本来运行业务，这样的结果是耗费了内存，单大大减少了线程同步所带来性能消耗，也减少了线程并发控制的复杂度。

ThreadLocal不能使用原子类型，只能使用Object类型。ThreadLocal的使用比synchronized要简单得多。

ThreadLocal和Synchonized都用于解决多线程并发访问。但是ThreadLocal与synchronized有本质的区别。synchronized是利用锁的机制，使变量或代码块在某一时该只能被一个线程访问。而ThreadLocal为每一个线程都提供了变量的副本，使得每个线程在某一时间访问到的并不是同一个对象，这样就隔离了多个线程对数据的数据共享。而Synchronized却正好相反，它用于在多个线程间通信时能够获得数据共享。

Synchronized用于线程间的数据共享，而ThreadLocal则用于线程间的数据隔离。

当然ThreadLocal并不能替代synchronized,它们处理不同的问题域。Synchronized用于实现同步机制，比ThreadLocal更加复杂。

ok，那么再看看activityThread，也就是 安卓应用的main方法入口

public static void main(String\[\] args) \{

Trace.traceBegin(Trace.TRACE\_TAG\_ACTIVITY\_MANAGER, "ActivityThreadMain");

SamplingProfilerIntegration.start();

CloseGuard.setEnabled(false);

Environment.initForCurrentUser();

// Set the reporter for event logging in libcore

EventLogger.setReporter(new EventLoggingReporter());

// Make sure TrustedCertificateStore looks in the right place for CA certificates

final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());

TrustedCertificateStore.setDefaultUserDirectory(configDir);

Process.setArgV0("");

Looper.prepareMainLooper();

ActivityThread thread = new ActivityThread();

thread.attach(false);

if (sMainThreadHandler == null) \{

sMainThreadHandler = thread.getHandler();

if (false) \{

Looper.myLooper().setMessageLogging(new

// End of event ActivityThreadMain.

Looper.loop();

throw new RuntimeException("Main thread loop unexpectedly exited");

![Image 1][]

image.png

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));

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 (;;) \{//死循环

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 traceTag = me.mTraceTag;

if (traceTag != 0 && Trace.isTagEnabled(traceTag)) \{

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();

可以看出来做了一个死循环，但是这个死循环是可以退出的

Message msg = queue.next(); // 消息为空的时候退出，那么android的应用main()方法是不是一直不为空呢

[Image 1]:

handler原子锁_android 中ThreadLocal的作用以及loop()的作用 handler是如何处理线程安全的...

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