常用系统服务:
后台Service在系统启动时被SystemServer开启：
1.MountService:监听是否有SD卡安装及移除
2.ClipboardService:提供剪切板功能
3.PackageManagerService:提供软件包的安装移除及查看
4.电量,网络连接状态等等

使用的时候通过getSystemService()的方法来实现:
这是一个Activity的一个方法,通过传入name->object>服务对象

demo:注意添加权限

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

activity_main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MainActivity" >

    <Button
        android:id="@+id/network"
        android:onClick="doClick"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_alignParentRight="true"
        android:layout_alignParentTop="true"
        android:text="判断网络是否连接" />

    <Button
        android:id="@+id/enableOrDisable_WIFI"
        android:layout_width="wrap_content"
         android:onClick="doClick"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/network"
        android:layout_alignRight="@+id/network"
        android:layout_below="@+id/network"
        android:text="开关WIFI" />

    <Button
        android:id="@+id/getvoice"
         android:onClick="doClick"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/enableOrDisable_WIFI"
        android:layout_alignRight="@+id/enableOrDisable_WIFI"
        android:layout_below="@+id/enableOrDisable_WIFI"
        android:text="获取系统的音量" />

    <Button
        android:id="@+id/getPackagename"
                 android:onClick="doClick"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/getvoice"
        android:layout_alignRight="@+id/getvoice"
        android:layout_below="@+id/getvoice"
        android:text="获取当前进程包名" />

</RelativeLayout>

MainActivity.java

import android.app.Activity;
import android.app.ActivityManager;
import android.content.Context;
import android.media.AudioManager;
import android.net.ConnectivityManager;
import android.net.NetworkInfo;
import android.net.wifi.WifiManager;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.widget.Toast;

public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        LayoutInflater inflater = (LayoutInflater) MainActivity.this
                .getSystemService(LAYOUT_INFLATER_SERVICE);
        View view = inflater.inflate(R.layout.activity_main, null);
        setContentView(view);
    }

    public void doClick(View v) {
        switch (v.getId()) {
        case R.id.network:
            if (isNetWorkConnected(MainActivity.this)==true) {
                Toast.makeText(MainActivity.this, "网络已经打开", Toast.LENGTH_SHORT).show();
            }else {
                Toast.makeText(MainActivity.this, "网络未连接", Toast.LENGTH_SHORT).show();
            }
            break;
        case R.id.enableOrDisable_WIFI:
        WifiManager wifiManager =   (WifiManager) MainActivity.this.getSystemService(WIFI_SERVICE);
            if (wifiManager.isWifiEnabled()) {
                wifiManager.setWifiEnabled(false);
                Toast.makeText(MainActivity.this, "WIFI已经关闭", Toast.LENGTH_SHORT).show();
            }else {
                wifiManager.setWifiEnabled(true);
                Toast.makeText(MainActivity.this, "WIFI已经打开", Toast.LENGTH_SHORT).show();
            }
            
        break;
        case R.id.getvoice:
             AudioManager mAudioManager= (AudioManager) MainActivity.this.getSystemService(AUDIO_SERVICE);
             int max = mAudioManager.getStreamMaxVolume(AudioManager.STREAM_SYSTEM);
             int current = mAudioManager.getStreamVolume(AudioManager.STREAM_RING);
             Toast.makeText(MainActivity.this, "系统的最大音量为："+max+",当前音量是："+current, Toast.LENGTH_SHORT).show();
             break;
        case R.id.getPackagename:
            ActivityManager activityManager = (ActivityManager) MainActivity.this.getSystemService(ACTIVITY_SERVICE);
            String packageName = activityManager.getRunningTasks(1).get(0).topActivity.getPackageName();
            Toast.makeText(MainActivity.this, "当前运行的Activity包名："+packageName, Toast.LENGTH_SHORT).show();
            break;
        }
    }

    public boolean isNetWorkConnected(Context context) {
        if (context != null) {
            ConnectivityManager mConnectivityManager = (ConnectivityManager) context
                    .getSystemService(CONNECTIVITY_SERVICE);
            NetworkInfo mNetWorkInfo = mConnectivityManager
                    .getActiveNetworkInfo();
            if (mNetWorkInfo != null) {
                return mNetWorkInfo.isAvailable();
            }
        }
        return false;
    }
}

演示效果：

sys.gif