android中ThreadLocal的作用以及loop的作用handler是如何处理线程安全的

简书链接:android中ThreadLocal的作用以及loop的作用handler是如何处理线程安全的
文章字数：460,阅读全文大约需要1分钟
http://blog.51cto.com/lavasoft/51926

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("<pre-initialized>");
        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");
    }
}

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();
    }
}