当我想打开ftp连接时，应用程序崩溃了

Question

这是logcat的错误：

FATAL EXCEPTION: main
android.os.NetworkOnMainThreadException
at android.os.StrictMode$AndroidBlockGuardPolicy.onNetwork(StrictMode.java:1117)
at java.net.InetAddress.lookupHostByName(InetAddress.java:385)
at java.net.InetAddress.getAllByNameImpl(InetAddress.java:236)
at java.net.InetAddress.getByName(InetAddress.java:289)
at com.liwona_3.MainActivity$1.onClick(MainActivity.java:40)
at android.view.View.performClick(View.java:4084)
at android.view.View$PerformClick.run(View.java:16966)
at android.os.Handler.handleCallback(Handler.java:615)
at android.os.Handler.dispatchMessage(Handler.java:92)
at android.os.Looper.loop(Looper.java:137)
at android.app.ActivityThread.main(ActivityThread.java:4745)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:511)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
at dalvik.system.NativeStart.main(Native Method)

这里只有我的代码，在onClick（）

            FTPClient ftpClient = new FTPClient();
            try {
                ftpClient.connect("server", 21);
                ftpClient.login("username", "pass");
                ftpClient.logout();
                ftpClient.disconnect();
            } catch (IOException e) {
            }

我已经添加了互联网访问的权限......我做错了什么？