API 32 源码看多线程创建流程

参考资料 https://blog.csdn.net/MarketAndTechnology/article/details/130772687

1、public final class Looper

源码：这是一个实现Looper线程的典型例子，使用 prepare 和 loop 来创建一个最初的处理程序与Looper进行通信。

  *  class LooperThread extends Thread {
  *      public Handler mHandler;
  *
  *      public void run() {
  *          Looper.prepare();
  *
  *          mHandler = new Handler(Looper.myLooper()) {
  *              public void handleMessage(Message msg) {
  *                  // process incoming messages here
  *              }
  *          };
  *
  *          Looper.loop();
  *      }
  *  }

1. 先从 Looper.prepare()开始。

    /** 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));
    }

2. 然后是 new Looper(quitAllowed)。

    private Looper(boolean quitAllowed) {
        mQueue = new MessageQueue(quitAllowed);
        mThread = Thread.currentThread();
    }

3. 可以看到 Looper.prepare()最后就是给两个成员变量（mQueue和mThread）赋值，继续看 Looper.loop()的实现。

    /**
     * Run the message queue in this thread. Be sure to call
     * {@link #quit()} to end the loop.
     */
    @SuppressWarnings("AndroidFrameworkBinderIdentity")
    public static void loop() {
        final Looper me = myLooper();
        if (me == null) {
            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
        }
        if (me.mInLoop) {
            Slog.w(TAG, "Loop again would have the queued messages be executed"
                    + " before this one completed.");
        }

        me.mInLoop = true;

        // 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);

        me.mSlowDeliveryDetected = false;

        for (;;) {
            if (!loopOnce(me, ident, thresholdOverride)) {
                return;
            }
        }
    }

4. 最后获取到当前Looper ，然后循环检查 loopOnce(me, ident, thresholdOverride)，结果为 false才停止。

    /**
     * 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();
    }

    /**
     * Poll and deliver single message, return true if the outer loop should continue.
     */
    @SuppressWarnings("AndroidFrameworkBinderIdentity")
    private static boolean loopOnce(final Looper me,
            final long ident, final int thresholdOverride) {
        Message msg = me.mQueue.next(); // might block
        if (msg == null) {
            // No message indicates that the message queue is quitting.
            return false;
        }
        ......  省略部分观察者、日志之类的操作
        long origWorkSource = ThreadLocalWorkSource.setUid(msg.workSourceUid);
        try {
            msg.target.dispatchMessage(msg);
            if (observer != null) {
                observer.messageDispatched(token, msg);
            }
            dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
        } catch (Exception exception) {
            if (observer != null) {
                observer.dispatchingThrewException(token, msg, exception);
            }
            throw exception;
        } finally {
            ThreadLocalWorkSource.restore(origWorkSource);
            if (traceTag != 0) {
                Trace.traceEnd(traceTag);
            }
        }
        ......  继续省略日志打印
        // 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();

        return true;
    }

6. 以上代码获取到当前队列的 Message，然后执行msg.target.dispatchMessage(msg)，接下来跳到 Message源码。

2、public final class Message implements Parcelable

1. 接下来跳到 Handler。

    @UnsupportedAppUsage
    /*package*/ Handler target;

3、public class Handler

1. 看 dispatchMessage怎么实现。

    /**
     * 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);
        }
    }

2. 先看 handleCallback，具体就是 Message的成员变量 Runnable执行 run()。

    private static void handleCallback(Message message) {
        message.callback.run();
    }

    @UnsupportedAppUsage
    /*package*/ Runnable callback;

3. 再看 handleMessage，这里已经回调给创建者实现线程任务了。

    /**
     * Callback interface you can use when instantiating a Handler to avoid
     * having to implement your own subclass of Handler.
     */
    public interface Callback {
        /**
         * @param msg A {@link android.os.Message Message} object
         * @return True if no further handling is desired
         */
        boolean handleMessage(@NonNull Message msg);
    }
    
    /**
     * Subclasses must implement this to receive messages.
     */
    public void handleMessage(@NonNull Message msg) {
    }

4、另一种线程，在 Activity 调用 runOnUiThread

1. 先看 Activity 源码

    /**
     * Runs the specified action on the UI thread. If the current thread is the UI
     * thread, then the action is executed immediately. If the current thread is
     * not the UI thread, the action is posted to the event queue of the UI thread.
     *
     * @param action the action to run on the UI thread
     */
    public final void runOnUiThread(Runnable action) {
        if (Thread.currentThread() != mUiThread) {
            mHandler.post(action);
        } else {
            action.run();
        }
    }

2. 再跳到 Handler 看 mHandler.post(action)，最后跳到 MessageQueue 执行 queue.enqueueMessage(msg, uptimeMillis)。

    /**
     * Causes the Runnable r to be added to the message queue.
     * The runnable will be run on the thread to which this handler is 
     * attached. 
     *  
     * @param r The Runnable that will be executed.
     * 
     * @return Returns true if the Runnable was successfully placed in to the 
     *         message queue.  Returns false on failure, usually because the
     *         looper processing the message queue is exiting.
     */
    public final boolean post(@NonNull Runnable r) {
       return  sendMessageDelayed(getPostMessage(r), 0);
    }

    /**
     * Enqueue a message into the message queue after all pending messages
     * before (current time + delayMillis). You will receive it in
     * {@link #handleMessage}, in the thread attached to this handler.
     *  
     * @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 final boolean sendMessageDelayed(@NonNull Message msg, long delayMillis) {
        if (delayMillis < 0) {
            delayMillis = 0;
        }
        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
    }

    /**
     * Enqueue a message into the message queue after all pending messages
     * before the absolute time (in milliseconds) uptimeMillis.
     * The time-base is {@link android.os.SystemClock#uptimeMillis}.
     * 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);
    }

5、public final class MessageQueue

1. 继续看 enqueueMessage ，就是最后进行 Message具体的等待阻塞执行等一系列操作了。

    boolean enqueueMessage(Message msg, long when) {
        if (msg.target == null) {
            throw new IllegalArgumentException("Message must have a target.");
        }

        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;
            Message p = mMessages;
            boolean needWake;
            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;
                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;
    }

6、public class ThreadLocal

1. 在 Looper.prepare 会执行 sThreadLocal.set(new Looper(quitAllowed))；
   而创建 Hander 最终调用两种函数；

    @UnsupportedAppUsage
    public Handler(@NonNull Looper looper, @Nullable Callback callback, boolean async) {
        mLooper = looper;
        mQueue = looper.mQueue;
        mCallback = callback;
        mAsynchronous = async;
    }
    
    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();
        if (mLooper == null) {
            throw new RuntimeException(
                "Can't create handler inside thread " + Thread.currentThread()
                        + " that has not called Looper.prepare()");
        }
        mQueue = mLooper.mQueue;
        mCallback = callback;
        mAsynchronous = async;
    }

2. 不指定 Looper 的情况下会调用 mLooper = Looper.myLooper()，函数返回sThreadLocal.get()；最终通过 ThreadLocal 实现 Hander 与 Looper 绑定。

    /**
     * Returns the value in the current thread's copy of this
     * thread-local variable.  If the variable has no value for the
     * current thread, it is first initialized to the value returned
     * by an invocation of the {@link #initialValue} method.
     *
     * @return the current thread's value of this thread-local
     */
    public T get() {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null) {
                @SuppressWarnings("unchecked")
                T result = (T)e.value;
                return result;
            }
        }
        return setInitialValue();
    }

7、public class Thread implements Runnable

1. 所有public Thread() 方法都跳转到 init() 方法，然后跳到 init2() 实现，最后执行 ThreadLocal.createInheritedMap。

    private void init(ThreadGroup g, Runnable target, String name,
                      long stackSize, AccessControlContext acc) {
        if (name == null) {
            throw new NullPointerException("name cannot be null");
        }

        this.name = name;

        Thread parent = currentThread();
        // Android-removed: SecurityManager stubbed out on Android.
        // SecurityManager security = System.getSecurityManager();
        if (g == null) {
            // Android-changed: SecurityManager stubbed out on Android.
            /*
            /* Determine if it's an applet or not *

            /* If there is a security manager, ask the security manager
               what to do. *
            if (security != null) {
                g = security.getThreadGroup();
            }

            /* If the security doesn't have a strong opinion of the matter
               use the parent thread group. *
            if (g == null) {
            */
                g = parent.getThreadGroup();
            // }
        }

        // Android-removed: SecurityManager stubbed out on Android.
        /*
        /* checkAccess regardless of whether or not threadgroup is
           explicitly passed in. *
        g.checkAccess();

        /*
         * Do we have the required permissions?
         *
        if (security != null) {
            if (isCCLOverridden(getClass())) {
                security.checkPermission(SUBCLASS_IMPLEMENTATION_PERMISSION);
            }
        }
        */

        g.addUnstarted();

        this.group = g;
        this.daemon = parent.isDaemon();
        this.priority = parent.getPriority();
        // Android-changed: Moved into init2(Thread) helper method.
        /*
        if (security == null || isCCLOverridden(parent.getClass()))
            this.contextClassLoader = parent.getContextClassLoader();
        else
            this.contextClassLoader = parent.contextClassLoader;
        this.inheritedAccessControlContext =
                acc != null ? acc : AccessController.getContext();
        */
        this.target = target;
        // Android-removed: The priority parameter is unchecked on Android.
        // It is unclear why this is not being done (b/80180276).
        // setPriority(priority);
        // Android-changed: Moved into init2(Thread) helper method.
        // if (parent.inheritableThreadLocals != null)
        //     this.inheritableThreadLocals =
        //         ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
        init2(parent);

        /* Stash the specified stack size in case the VM cares */
        this.stackSize = stackSize;

        /* Set thread ID */
        tid = nextThreadID();
    }

    private void init2(Thread parent) {
        this.contextClassLoader = parent.getContextClassLoader();
        this.inheritedAccessControlContext = AccessController.getContext();
        if (parent.inheritableThreadLocals != null) {
            this.inheritableThreadLocals = ThreadLocal.createInheritedMap(
                    parent.inheritableThreadLocals);
        }
    }

2. 跳到 ThreadLocal 类，创建了 map 存储 thread 信息

    /**
     * Factory method to create map of inherited thread locals.
     * Designed to be called only from Thread constructor.
     *
     * @param  parentMap the map associated with parent thread
     * @return a map containing the parent's inheritable bindings
     */
    static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
        return new ThreadLocalMap(parentMap);
    }

        /**
         * Construct a new map including all Inheritable ThreadLocals
         * from given parent map. Called only by createInheritedMap.
         *
         * @param parentMap the map associated with parent thread.
         */
        private ThreadLocalMap(ThreadLocalMap parentMap) {
            Entry[] parentTable = parentMap.table;
            int len = parentTable.length;
            setThreshold(len);
            table = new Entry[len];

            for (int j = 0; j < len; j++) {
                Entry e = parentTable[j];
                if (e != null) {
                    @SuppressWarnings("unchecked")
                    ThreadLocal<Object> key = (ThreadLocal<Object>) e.get();
                    if (key != null) {
                        Object value = key.childValue(e.value);
                        Entry c = new Entry(key, value);
                        int h = key.threadLocalHashCode & (len - 1);
                        while (table[h] != null)
                            h = nextIndex(h, len);
                        table[h] = c;
                        size++;
                    }
                }
            }
        }

3. 看线程启动 Thread.start()，最后到 C 层实现。

    public synchronized void start() {
        /**
         * This method is not invoked for the main method thread or "system"
         * group threads created/set up by the VM. Any new functionality added
         * to this method in the future may have to also be added to the VM.
         *
         * A zero status value corresponds to state "NEW".
         */
        // Android-changed: Replace unused threadStatus field with started field.
        // The threadStatus field is unused on Android.
        if (started)
            throw new IllegalThreadStateException();

        /* Notify the group that this thread is about to be started
         * so that it can be added to the group's list of threads
         * and the group's unstarted count can be decremented. */
        group.add(this);

        // Android-changed: Use field instead of local variable.
        // It is necessary to remember the state of this across calls to this method so that it
        // can throw an IllegalThreadStateException if this method is called on an already
        // started thread.
        started = false;
        try {
            // Android-changed: Use Android specific nativeCreate() method to create/start thread.
            // start0();
            nativeCreate(this, stackSize, daemon);
            started = true;
        } finally {
            try {
                if (!started) {
                    group.threadStartFailed(this);
                }
            } catch (Throwable ignore) {
                /* do nothing. If start0 threw a Throwable then
                  it will be passed up the call stack */
            }
        }
    }

    private native static void nativeCreate(Thread t, long stackSize, boolean daemon);

8、nativeCreate 最终创建线程方法