Context context = activity.getApplicationContext();
// 获取手机所有连接管理对象（包括对wi-fi,net等连接的管理）
ConnectivityManager connectivityManager = (ConnectivityManager)context.getSystemService(Context.CONNECTIVITY_SERVICE);

// 获取NetworkInfo对象
NetworkInfo[] networkInfo = connectivityManager.getAllNetworkInfo();

if (networkInfo[i].getState() == NetworkInfo.State.CONNECTED)
{
   return true;
}

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

public class ClassTestDemoActivity extends Activity
{
  @Override
  public void onCreate(Bundle savedInstanceState)
  {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    if (isNetworkAvailable(ClassTestDemoActivity.this))
    {
      Toast.makeText(getApplicationContext(), "当前有可用网络！", Toast.LENGTH_LONG).show();
    }
    else
    {
      Toast.makeText(getApplicationContext(), "当前没有可用网络！", Toast.LENGTH_LONG).show();
    }
  }
  /**
   * 检查当前网络是否可用
   * 
   * @param context
   * @return
   */
  public boolean isNetworkAvailable(Activity activity)
  {
    Context context = activity.getApplicationContext();
    // 获取手机所有连接管理对象（包括对wi-fi,net等连接的管理）
    ConnectivityManager connectivityManager = (ConnectivityManager) context.getSystemService(Context.CONNECTIVITY_SERVICE);
    if (connectivityManager == null)
    {
      return false;
    }
    else
    {
      // 获取NetworkInfo对象
      NetworkInfo[] networkInfo = connectivityManager.getAllNetworkInfo();
      if (networkInfo != null && networkInfo.length > 0)
      {
        for (int i = 0; i < networkInfo.length; i++)
        {
          System.out.println(i + "===状态===" + networkInfo[i].getState());
          System.out.println(i + "===类型===" + networkInfo[i].getTypeName());
          // 判断当前网络状态是否为连接状态
          if (networkInfo[i].getState() == NetworkInfo.State.CONNECTED)
          {
            return true;
          }
        }
      }
    }
    return false;
  }
}

 /**
   * 检查当前网络是否可用
   * 
   * @param context
   * @return
   */
  public boolean isNetworkAvailable(Activity activity)
  {
    Context context = activity.getApplicationContext();
    // 获取手机所有连接管理对象（包括对wi-fi,net等连接的管理）
    ConnectivityManager connectivityManager = (ConnectivityManager) context.getSystemService(Context.CONNECTIVITY_SERVICE);
    if (connectivityManager == null)
    {
      return false;
    }
    else
    {
      // 获取NetworkInfo对象
      NetworkInfo[] networkInfo = connectivityManager.getAllNetworkInfo();
      if (networkInfo != null && networkInfo.length > 0)
      {
        for (int i = 0; i < networkInfo.length; i++)
        {
          System.out.println(i + "===状态===" + networkInfo[i].getState());
          System.out.println(i + "===类型===" + networkInfo[i].getTypeName());
          // 判断当前网络状态是否为连接状态
          if (networkInfo[i].getState() == NetworkInfo.State.CONNECTED)
          {
            return true;
          }
        }
      }
    }
    return false;
  }

@Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);  
    setContentView(R.layout.activity_main);
    if(isNetworkAvailable(Maintivity.this){
 Toast.makeText(this, "当前有可用网络！", Toast.LENGTH_LONG).show();
 }else{
 Toast.makeText(this,"当前没有网络！",Toast.LENGTH_LONG).show();}
 }

<!-- 允许应用可以联网 -->
  <uses-permission android:name="android.permission.INTERNET" />
   <!--允许应用检查网络状况 -->
 <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"></uses-permission>