两种类型：标准广播、有序广播
标准广播：完全异步执行的广播。效率高，无法被截断。
有序广播：同步执行的，优先级高的先执行，并且可截断。

在MainActivity中添加：

class NetworkChangeReceiver extends BroadcastReceiver{
    @Override
    public void onReceive(Context context, Intent intent) {
        Toast.makeText(context, "网络变化了", Toast.LENGTH_SHORT).show();
    }
}

然后在onCreate()中加入：

intentFilter = new IntentFilter();
intentFilter.addAction("android.net.conn.CONNECTIVITY_CHANGE");
networkChangeReceiver = new NetworkChangeReceiver();
registerReceiver(networkChangeReceiver, intentFilter);

onDestroy()中加入：

unregisterReceiver(networkChangeReceiver);

判断网络是否可用
首先添加访问系统网络状态的权限：

AndroidManifest.xml

...
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
...

修改NetworkChangeReceiver：

MainActivity.java

public class MainActivity extends AppCompatActivity {
    ...
    class NetworkChangeReceiver extends BroadcastReceiver{
        @Override
        public void onReceive(Context context, Intent intent) {
            ConnectivityManager connectivityManager = (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);
            NetworkInfo networkInfo = connectivityManager.getActiveNetworkInfo();
            if (networkInfo != null && networkInfo.isAvailable()){
                Toast.makeText(context, "网络可用", Toast.LENGTH_SHORT).show();
            } else {
                Toast.makeText(context, "网络不可用", Toast.LENGTH_SHORT).show();
            }
        }
    }
}

右键com.exampel.broadcasttest包 ⇒ 新建 ⇒ Other ⇒ Broadcast Receiver，命名为BootCompleteReceiver。

BootCompleteReceiver.java

public class BootCompleteReceiver extends BroadcastReceiver {
    @Override
    public void onReceive(Context context, Intent intent) {
        Toast.makeText(context, "开机启动完成", Toast.LENGTH_SHORT).show();
    }
}

然后需要在AndroidManifest.xml中注册，这一步刚才Android Studio已经自动帮我们完成了。
可以看到在<application>标签内新增的标签<receiver>。
另外需要再添加监听系统开机广播的权限，android.permission.RECEIVE_BOOT_COHPLETED，还要在<receiver>标签中添加<intent-filter>内容，最后添加的内容如下：

AndroidManifest.xml

...
 
    <uses-permission android:name="android.permission.RECEIVE_BOOT_COHPLETED" />
...
        <receiver
            android:name=".BootCompleteReceiver"
            android:enabled="true"
            android:exported="true">
            <intent-filter>
                <action android:name="android.intent.action.BOOT_COMPLETED" />
            </intent-filter>
        </receiver>
...

备注：开机启动了程序，但是没有提示，后续查看为什么。

新建MyBroadcastReceiver类。

MyBroadcastReceiver.java

public class MyBroadcastReceiver extends BroadcastReceiver {
 
    @Override
    public void onReceive(Context context, Intent intent) {
        Toast.makeText(context, "received in MyBroadcastReceiver", Toast.LENGTH_SHORT).show();
    }
}

修改AndroidManifest.xml：

AndroidManifest.xml

        ...
        <receiver
            android:name=".MyBroadcastReceiver"
            android:enabled="true"
            android:exported="true">
            <intent-filter>
                <action android:name="com.example.broadcasttest.MY_BROADCAST" />
            </intent-filter>
        </receiver>
        ...