#实现原理
Android系统启动时会发送一个ACTION_BOOT_COMPLETED的广播，android.intent.action.BOOT_COMPLETED

#代码
新建一个空项目

package com.example.autorun;

import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.TextView;

public class MainActivity extends ActionBarActivity {

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    
    TextView tv = new TextView(this);
    tv.setText("Hello Android");
    setContentView(R.layout.activity_main);
  }

  @Override
  public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
  }

  @Override
  public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();
    if (id == R.id.action_settings) {
      return true;
    }
    return super.onOptionsItemSelected(item);
  }
}

新建一个类，从BroadcastReceiver继承

package com.example.autorun;

import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;

public class AutoRun extends BroadcastReceiver {
  private final String ACTION = "android.intent.action.BOOT_COMPLETED";

  @Override
  public void onReceive(Context context, Intent intent) {
    if (intent.getAction().equals(ACTION)) ;
    {
      Intent in = new Intent(context, MainActivity.class);
      in.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
      //或者用in.setFlags，暂时不知道有什么区别
      context.startActivity(in);

    }

  }
}

配置文件：

<uses-permission android:name="android.permission.RECEIVE_BOOT_COMPLETED"></uses-permission>
    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name=".MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

        <!-- 向系统注册receiver -->
        <receiver android:name="com.example.autorun.AutoRun" >
            <intent-filter>
                <action android:name="android.intent.action.BOOT_COMPLETED" />
            </intent-filter>
        </receiver>
    </application>