Android（java）学习笔记57：PC and Phone 通信程序
1. 首先我写的程序代码如下：
1 package com.himi.udpsend; 2 3 import java.net.DatagramPacket; 4 import java.net.DatagramSocket; 5 import java.net.DatagramSocketImpl; 6 import java.net.InetAddress; 7 import java.net.SocketException; 8 import java.net.UnknownHostException; 9 10 import android.app.Activity; 11 import android.content.Context; 12 import android.net.wifi.WifiInfo; 13 import android.net.wifi.WifiManager; 14 import android.os.Bundle; 15 import android.util.Log; 16 import android.view.View; 17 import android.view.View.OnClickListener; 18 import android.widget.Button; 19 import android.widget.EditText; 20 21 22 public class MainActivity extends Activity implements OnClickListener { 23 private EditText edittext; 24 private Button btnsend; 25 private WifiInfo wifiInfo; //类Wifiinfo，WifiInfo获取各种Wifi各种信息 26 @Override 27 protected void onCreate(Bundle savedInstanceState) { 28 super.onCreate(savedInstanceState); 29 setContentView(R.layout.activity_main); 30 31 edittext = (EditText)findViewById(R.id.edit); 32 btnsend = (Button)findViewById(R.id.btnsend); 33 34 btnsend.setOnClickListener(this); 35 36 WifiManager wman = (WifiManager) getSystemService(Context.WIFI_SERVICE); 37 38 if(! wman.isWifiEnabled()) { 39 wman.setWifiEnabled(true); 40 } 41 Log.e("ip", intToIp(wifiInfo.getIpAddress()));//获得本机ip 42 43 } 44 45 46 public String intToIp(int i) { 47 return (i & 0xff) +"."+ 48 ((i>>8) & 0xff )+"."+ 49 ((i>>16) & 0xff )+"."+ 50 ((i>>24) & 0xff ); 51 52 } 53 54 55 56 57 public void onClick(View v) { 58 59 try { 60 // 创建发送端socket对象 61 DatagramSocket ds = new DatagramSocket(); 62 63 // 创建数据并打包 64 String edit_content = edittext.getText().toString(); 65 byte[] bys = edit_content.getBytes(); 66 int length = bys.length; 67 68 String ip = intToIp(wifiInfo.getIpAddress()); 69 InetAddress hostip = InetAddress.getByName(ip); 70 71 int port = 10086; 72 DatagramPacket dp = new DatagramPacket(bys, length, hostip, port); 73 74 //发送数据包 75 ds.send(dp); 76 //关闭套接字，回收资源 77 ds.close(); 78 79 } catch (Exception e) { 80 // TODO 自动生成的 catch 块 81 e.printStackTrace(); 82 } 83 84 85 } 86 87 88 89 90 }
2. 报错如下：
06-26 02:37:29.270: E/Trace(841): error opening trace file: No such file or directory (2) 06-26 02:37:30.400: D/AndroidRuntime(841): Shutting down VM 06-26 02:37:30.400: W/dalvikvm(841): threadid=1: thread exiting with uncaught exception (group=0x40a71930) 06-26 02:37:30.540: E/AndroidRuntime(841): FATAL EXCEPTION: main 06-26 02:37:30.540: E/AndroidRuntime(841): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.himi.udpsend/com.himi.udpsend.MainActivity}: java.lang.NullPointerException 06-26 02:37:30.540: E/AndroidRuntime(841): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180) 06-26 02:37:30.540: E/AndroidRuntime(841): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230) 06-26 02:37:30.540: E/AndroidRuntime(841): at android.app.ActivityThread.access$600(ActivityThread.java:141) 06-26 02:37:30.540: E/AndroidRuntime(841): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234) 06-26 02:37:30.540: E/AndroidRuntime(841): at android.os.Handler.dispatchMessage(Handler.java:99) 06-26 02:37:30.540: E/AndroidRuntime(841): at android.os.Looper.loop(Looper.java:137) 06-26 02:37:30.540: E/AndroidRuntime(841): at android.app.ActivityThread.main(ActivityThread.java:5041) 06-26 02:37:30.540: E/AndroidRuntime(841): at java.lang.reflect.Method.invokeNative(Native Method) 06-26 02:37:30.540: E/AndroidRuntime(841): at java.lang.reflect.Method.invoke(Method.java:511) 06-26 02:37:30.540: E/AndroidRuntime(841): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793) 06-26 02:37:30.540: E/AndroidRuntime(841): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560) 06-26 02:37:30.540: E/AndroidRuntime(841): at dalvik.system.NativeStart.main(Native Method) 06-26 02:37:30.540: E/AndroidRuntime(841): Caused by: java.lang.NullPointerException 06-26 02:37:30.540: E/AndroidRuntime(841): at com.himi.udpsend.MainActivity.onCreate(MainActivity.java:41) 06-26 02:37:30.540: E/AndroidRuntime(841): at android.app.Activity.performCreate(Activity.java:5104) 06-26 02:37:30.540: E/AndroidRuntime(841): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080) 06-26 02:37:30.540: E/AndroidRuntime(841): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144) 06-26 02:37:30.540: E/AndroidRuntime(841): ... 11 more 06-26 02:37:34.910: I/Process(841): Sending signal. PID: 841 SIG: 9
原因：空指针异常，这时就要查看自己的程序中是否存在空指针。
4. 解决办法是：
1 package com.himi.udpsend; 2 3 import java.net.DatagramPacket; 4 import java.net.DatagramSocket; 5 import java.net.DatagramSocketImpl; 6 import java.net.InetAddress; 7 import java.net.SocketException; 8 import java.net.UnknownHostException; 9 10 import android.app.Activity; 11 import android.content.Context; 12 import android.net.wifi.WifiInfo; 13 import android.net.wifi.WifiManager; 14 import android.os.Bundle; 15 import android.util.Log; 16 import android.view.View; 17 import android.view.View.OnClickListener; 18 import android.widget.Button; 19 import android.widget.EditText; 20 21 22 public class MainActivity extends Activity implements OnClickListener { 23 private EditText edittext; 24 private Button btnsend; 25 private WifiInfo wifiInfo; 26 @Override 27 protected void onCreate(Bundle savedInstanceState) { 28 super.onCreate(savedInstanceState); 29 setContentView(R.layout.activity_main); 30 31 edittext = (EditText)findViewById(R.id.edit); 32 btnsend = (Button)findViewById(R.id.btnsend); 33 34 btnsend.setOnClickListener(this); 35 36 WifiManager wman = (WifiManager) getSystemService(Context.WIFI_SERVICE); 37 38 if(! wman.isWifiEnabled()) { 39 wman.setWifiEnabled(true); 40 }//没有正确使用Log（） 41 //Log.e("ip", intToIp(wifiInfo.getIpAddress()));//获得本机ip 42 43 } 44 45 46 public String intToIp(int i) { 47 return (i & 0xff) +"."+ 48 ((i>>8) & 0xff )+"."+ 49 ((i>>16) & 0xff )+"."+ 50 ((i>>24) & 0xff ); 51 52 } 53 54 55 56 57 public void onClick(View v) { 58 59 try { 60 // 创建发送端socket对象 61 DatagramSocket ds = new DatagramSocket(); 62 63 // 创建数据并打包 64 String edit_content = edittext.getText().toString(); 65 byte[] bys = edit_content.getBytes(); 66 int length = bys.length; 67 68 String ip = intToIp(wifiInfo.getIpAddress()); 69 InetAddress hostip = InetAddress.getByName(ip); 70 71 int port = 10086; 72 DatagramPacket dp = new DatagramPacket(bys, length, hostip, port); 73 74 //发送数据包 75 ds.send(dp); 76 //关闭套接字，回收资源 77 ds.close(); 78 79 } catch (Exception e) { 80 // TODO 自动生成的 catch 块 81 e.printStackTrace(); 82 } 83 84 85 } 86 87 88 89 90 }
附加：
DatagramSocket：套接字用来UDP通信
DatagramPacket：用来包装UDP通信数据包
InetAddress：是Java对IP地址的封装
