Android Service服务源码相关分析

一、onStartCommand(Intent intent, int flags, int startId)

位置：/frameworks/base/core/java/android/app/Service.java

  /**
     * Called by the system every time a client explicitly starts the service by calling 
     * {@link android.content.Context#startService}, providing the arguments it supplied and a 
     * unique integer token representing the start request.  Do not call this method directly.
     * 
     * <p>For backwards compatibility, the default implementation calls
     * {@link #onStart} and returns either {@link #START_STICKY}
     * or {@link #START_STICKY_COMPATIBILITY}.
     * 
     * <p>If you need your application to run on platform versions prior to API
     * level 5, you can use the following model to handle the older {@link #onStart}
     * callback in that case.  The <code>handleCommand</code> method is implemented by
     * you as appropriate:
     * 
     * {@sample development/samples/ApiDemos/src/com/example/android/apis/app/ForegroundService.java
     *   start_compatibility}
     *
     * <p class="caution">Note that the system calls this on your
     * service's main thread.  A service's main thread is the same
     * thread where UI operations take place for Activities running in the
     * same process.  You should always avoid stalling the main
     * thread's event loop.  When doing long-running operations,
     * network calls, or heavy disk I/O, you should kick off a new
     * thread, or use {@link android.os.AsyncTask}.</p>
     *
     * @param intent The Intent supplied to {@link android.content.Context#startService}, 
     * as given.  This may be null if the service is being restarted after
     * its process has gone away, and it had previously returned anything
     * except {@link #START_STICKY_COMPATIBILITY}.
     * @param flags Additional data about this start request.  Currently either
     * 0, {@link #START_FLAG_REDELIVERY}, or {@link #START_FLAG_RETRY}.
     * @param startId A unique integer representing this specific request to 
     * start.  Use with {@link #stopSelfResult(int)}.
     * 
     * @return The return value indicates what semantics the system should
     * use for the service's current started state.  It may be one of the
     * constants associated with the {@link #START_CONTINUATION_MASK} bits.
     * 
     * @see #stopSelfResult(int)
     */
    public int onStartCommand(Intent intent, int flags, int startId) {
        onStart(intent, startId);
        return mStartCompatibility ? START_STICKY_COMPATIBILITY : START_STICKY;
    }

【START_STICKY】：如果service进程被kill掉，保留service的状态为开始状态，但不保留递送的intent对象。随后系统会尝试重新创建service，由于服务状态为开始状态，
所以创建服务后一定会调用onStartCommand(Intent,int,int)方法。如果在此期间没有任何启动命令被传递到service，那么参数Intent将为null。

【START_NOT_STICKY】：“非粘性的”。使用这个返回值时，如果在执行完onStartCommand后，服务被异常kill掉，系统不会自动重启该服务。

【START_REDELIVER_INTENT】：重传Intent。使用这个返回值时，如果在执行完onStartCommand后，服务被异常kill掉，系统会自动重启该服务，并将Intent的值传入。

【START_STICKY_COMPATIBILITY】：START_STICKY的兼容版本，但不保证服务被kill后一定能重启。