Java tutorial
/* * Copyright (C) 2006 The Android Open Source Project * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */ package android.os; import android.annotation.NonNull; import android.annotation.Nullable; import android.annotation.UnsupportedAppUsage; import android.os.LooperProto; import android.util.Log; import android.util.Printer; import android.util.Slog; import android.util.proto.ProtoOutputStream; /** * Class used to run a message loop for a thread. Threads by default do * not have a message loop associated with them; to create one, call * {@link #prepare} in the thread that is to run the loop, and then * {@link #loop} to have it process messages until the loop is stopped. * * <p>Most interaction with a message loop is through the * {@link Handler} class. * * <p>This is a typical example of the implementation of a Looper thread, * using the separation of {@link #prepare} and {@link #loop} to create an * initial Handler to communicate with the Looper. * * <pre> * class LooperThread extends Thread { * public Handler mHandler; * * public void run() { * Looper.prepare(); * * mHandler = new Handler() { * public void handleMessage(Message msg) { * // process incoming messages here * } * }; * * Looper.loop(); * } * }</pre> */ public final class Looper { /* * API Implementation Note: * * This class contains the code required to set up and manage an event loop * based on MessageQueue. APIs that affect the state of the queue should be * defined on MessageQueue or Handler rather than on Looper itself. For example, * idle handlers and sync barriers are defined on the queue whereas preparing the * thread, looping, and quitting are defined on the looper. */ private static final String TAG = "Looper"; // sThreadLocal.get() will return null unless you've called prepare(). @UnsupportedAppUsage static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>(); @UnsupportedAppUsage private static Looper sMainLooper; // guarded by Looper.class @UnsupportedAppUsage final MessageQueue mQueue; final Thread mThread; @UnsupportedAppUsage private Printer mLogging; private long mTraceTag; /** * If set, the looper will show a warning log if a message dispatch takes longer than this. */ private long mSlowDispatchThresholdMs; /** * If set, the looper will show a warning log if a message delivery (actual delivery time - * post time) takes longer than this. */ private long mSlowDeliveryThresholdMs; /** Initialize the current thread as a looper. * This gives you a chance to create handlers that then reference * this looper, before actually starting the loop. Be sure to call * {@link #loop()} after calling this method, and end it by calling * {@link #quit()}. */ public static void prepare() { prepare(true); } 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)); } /** * Initialize the current thread as a looper, marking it as an * application's main looper. The main looper for your application * is created by the Android environment, so you should never need * to call this function yourself. See also: {@link #prepare()} */ public static void prepareMainLooper() { prepare(false); synchronized (Looper.class) { if (sMainLooper != null) { throw new IllegalStateException("The main Looper has already been prepared."); } sMainLooper = myLooper(); } } /** * Returns the application's main looper, which lives in the main thread of the application. */ public static Looper getMainLooper() { synchronized (Looper.class) { return sMainLooper; } } /** * 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(); // 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 (;;) { 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; 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 { 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); } msg.recycleUnchecked(); } } private static boolean showSlowLog(long threshold, long measureStart, long measureEnd, String what, Message msg) { final long actualTime = measureEnd - measureStart; if (actualTime < threshold) { return false; } // For slow delivery, the current message isn't really important, but log it anyway. Slog.w(TAG, "Slow " + what + " took " + actualTime + "ms " + Thread.currentThread().getName() + " h=" + msg.target.getClass().getName() + " c=" + msg.callback + " m=" + msg.what); return true; } /** * Return the Looper object associated with the current thread. Returns * null if the calling thread is not associated with a Looper. */ public static @Nullable Looper myLooper() { return sThreadLocal.get(); } /** * Return the {@link MessageQueue} object associated with the current * thread. This must be called from a thread running a Looper, or a * NullPointerException will be thrown. */ public static @NonNull MessageQueue myQueue() { return myLooper().mQueue; } private Looper(boolean quitAllowed) { mQueue = new MessageQueue(quitAllowed); mThread = Thread.currentThread(); } /** * Returns true if the current thread is this looper's thread. */ public boolean isCurrentThread() { return Thread.currentThread() == mThread; } /** * Control logging of messages as they are processed by this Looper. If * enabled, a log message will be written to <var>printer</var> * at the beginning and ending of each message dispatch, identifying the * target Handler and message contents. * * @param printer A Printer object that will receive log messages, or * null to disable message logging. */ public void setMessageLogging(@Nullable Printer printer) { mLogging = printer; } /** {@hide} */ @UnsupportedAppUsage public void setTraceTag(long traceTag) { mTraceTag = traceTag; } /** * Set a thresholds for slow dispatch/delivery log. * {@hide} */ public void setSlowLogThresholdMs(long slowDispatchThresholdMs, long slowDeliveryThresholdMs) { mSlowDispatchThresholdMs = slowDispatchThresholdMs; mSlowDeliveryThresholdMs = slowDeliveryThresholdMs; } /** * Quits the looper. * <p> * Causes the {@link #loop} method to terminate without processing any * more messages in the message queue. * </p><p> * Any attempt to post messages to the queue after the looper is asked to quit will fail. * For example, the {@link Handler#sendMessage(Message)} method will return false. * </p><p class="note"> * Using this method may be unsafe because some messages may not be delivered * before the looper terminates. Consider using {@link #quitSafely} instead to ensure * that all pending work is completed in an orderly manner. * </p> * * @see #quitSafely */ public void quit() { mQueue.quit(false); } /** * Quits the looper safely. * <p> * Causes the {@link #loop} method to terminate as soon as all remaining messages * in the message queue that are already due to be delivered have been handled. * However pending delayed messages with due times in the future will not be * delivered before the loop terminates. * </p><p> * Any attempt to post messages to the queue after the looper is asked to quit will fail. * For example, the {@link Handler#sendMessage(Message)} method will return false. * </p> */ public void quitSafely() { mQueue.quit(true); } /** * Gets the Thread associated with this Looper. * * @return The looper's thread. */ public @NonNull Thread getThread() { return mThread; } /** * Gets this looper's message queue. * * @return The looper's message queue. */ public @NonNull MessageQueue getQueue() { return mQueue; } /** * Dumps the state of the looper for debugging purposes. * * @param pw A printer to receive the contents of the dump. * @param prefix A prefix to prepend to each line which is printed. */ public void dump(@NonNull Printer pw, @NonNull String prefix) { pw.println(prefix + toString()); mQueue.dump(pw, prefix + " ", null); } /** * Dumps the state of the looper for debugging purposes. * * @param pw A printer to receive the contents of the dump. * @param prefix A prefix to prepend to each line which is printed. * @param handler Only dump messages for this Handler. * @hide */ public void dump(@NonNull Printer pw, @NonNull String prefix, Handler handler) { pw.println(prefix + toString()); mQueue.dump(pw, prefix + " ", handler); } /** @hide */ public void writeToProto(ProtoOutputStream proto, long fieldId) { final long looperToken = proto.start(fieldId); proto.write(LooperProto.THREAD_NAME, mThread.getName()); proto.write(LooperProto.THREAD_ID, mThread.getId()); if (mQueue != null) { mQueue.writeToProto(proto, LooperProto.QUEUE); } proto.end(looperToken); } @Override public String toString() { return "Looper (" + mThread.getName() + ", tid " + mThread.getId() + ") {" + Integer.toHexString(System.identityHashCode(this)) + "}"; } }