Socket也被稱為“套接字”程式設計，它分為流式套接字和使用者資料套接字兩種，分別對應於網路傳輸控制中層中TCP和UDP協議。TCP協議是面向連線的協議，提供穩定的雙向通訊功能，TCP連線的建立需要經過”三次握手”才能實現，為了實現穩定的資料傳輸功能，其本身提供了超時重傳機制，具有很高的穩定性。UDP是無連線的，提供不穩定的單向通訊功能,當然UDP也可以實現雙向通訊功能，在效能上，UDP就有更好的效率，其缺點是不能保證資料一定傳輸正確，尤其是網路擁塞的情況下。
演示一個Socket聊天的程式：

服務端

package com.app.service;

import java.io.BufferedReader;
import
 
 java.io.BufferedWriter;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.io.UnsupportedEncodingException;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.Random;

import android.app.Service;
import
 
 android.content.Intent;
import android.os.IBinder;
import android.util.Log;

public class TcpService extends Service {
    private boolean mIsServiceDestoryed = false;
    private String[] mDefinedMessage = new String[] { "你好", "hello", "辛苦了" };
    private ServerSocket mServerSocket;

    @Override
 

    public IBinder onBind(Intent intent) {
        return null;
    }

    @Override
    public void onCreate() {
        super.onCreate();
        new Thread(new TcpThread()).start();
    }

    class TcpThread implements Runnable {

        @Override
        public void run() {
            try {
                //服務端在8688這個埠監聽
                mServerSocket = new ServerSocket(8688);
            } catch (IOException e) {
                e.printStackTrace();
                return;
            }

            while (!mIsServiceDestoryed) {
                try {
                    //一個阻塞的方法，當有客戶端Socket時會執行
                    final Socket socket = mServerSocket.accept();
                    new Thread() {
                        @Override
                        public void run() {
                            responseClient(socket);
                        }
                    }.start();
                } catch (IOException e) {
                    e.printStackTrace();
                }

            }

        }

    }
    //服務端相應客戶端
    private void responseClient(Socket socket) {
        try {
            //讀取客戶端發來的訊息
            BufferedReader in = new BufferedReader(new InputStreamReader(
                    socket.getInputStream()));
            //用於向客戶端傳送訊息
            PrintWriter out = new PrintWriter(new BufferedWriter(
                    new OutputStreamWriter(socket.getOutputStream())), true);
            out.println("歡迎來到聊天室!");
            while (!mIsServiceDestoryed) {
                String str = in.readLine();//通訊連線後,客戶端不發訊息，這個方法會一直阻塞
                Log.e("tag", "客戶端訊息：====" + str);
                if (str == null) {
                    break;
                }
                int i = new Random().nextInt(mDefinedMessage.length);
                String msg = mDefinedMessage[i];
                out.println(msg);//向客戶端傳送訊息
            }
            //客戶端連線斷開後，要關閉資料流
            in.close();
            out.close();
            socket.close();
        } catch (UnsupportedEncodingException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

    }

    @Override
    public void onDestroy() {
        mIsServiceDestoryed = true;//停止執行緒
        super.onDestroy();
    }
}

客戶端
先看佈局程式碼：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:background="#fff"
    android:orientation="vertical" >

    <EditText
        android:id="@+id/et_recore"
        android:layout_width="fill_parent"
        android:layout_height="200dip"
        android:gravity="top|left"
        android:text="聊天記錄：\n" >
    </EditText>

    <EditText
        android:id="@+id/et_send_content"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="輸入要傳送的內容" >
    </EditText>

    <Button
        android:id="@+id/btn_send"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="傳送"
        android:textColor="#000" />

</LinearLayout>

java程式碼：

package com.app.client;

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.io.UnsupportedEncodingException;
import java.net.Socket;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.os.SystemClock;
import android.text.TextUtils;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;

public class MainActivity extends Activity implements OnClickListener {
    private static final int MESSAGE_RECEIVE_NEW_MSG = 1;
    private static final int MESSAGE_SOCKET_CONNECTED = 2;
    private Button mButton_Send;
    private EditText mEditText01, mEditText02;
    private PrintWriter mPintWriter;
    private Socket mClientSocket;

    private Handler mHandler = new Handler() {

        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
            case MESSAGE_RECEIVE_NEW_MSG:
                mEditText01.setText(mEditText01.getText() + (String) msg.obj);
                break;
            case MESSAGE_SOCKET_CONNECTED:
                mButton_Send.setEnabled(true);
                mEditText02.setEnabled(true);
                break;

            default:
                break;
            }
        }
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mButton_Send = (Button) findViewById(R.id.btn_send);
        mEditText01 = (EditText) findViewById(R.id.et_recore);
        mEditText02 = (EditText) findViewById(R.id.et_send_content);
        mButton_Send.setEnabled(false);
        mButton_Send.setOnClickListener(this);
        //開啟服務
        Intent i = new Intent("com.app.service");
        startService(i);
        new Thread() {
            @Override
            public void run() {
                connectTCPServer();
            }
        }.start();
    }

    protected void connectTCPServer() {
        Socket socket = null;
        //while裡面使用了超時重連策略
        while (socket == null) {
            try {
                socket = new Socket("localhost", 8688);
                mClientSocket = socket;
                mPintWriter = new PrintWriter(new BufferedWriter(
                        new OutputStreamWriter(socket.getOutputStream())), true);
                mHandler.sendEmptyMessage(MESSAGE_SOCKET_CONNECTED);
            } catch (Exception e) {
                SystemClock.sleep(1000);
            }
        }
        try {
            //用於讀取服務端的訊息
            BufferedReader br = new BufferedReader(new InputStreamReader(
                    mClientSocket.getInputStream(), "utf-8"));
            while (!MainActivity.this.isFinishing()) {
                String msg = br.readLine();//讀取
                Log.e("tag", "服務端訊息：====" + msg);
                if (msg != null) {
                    String showeMsg = "server : " + msg + "\n";
                    mHandler.obtainMessage(MESSAGE_RECEIVE_NEW_MSG, showeMsg)
                            .sendToTarget();
                }
            }
            //關閉資源
            mPintWriter.close();
            br.close();
            socket.close();
        } catch (UnsupportedEncodingException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

    }

    @Override
    public void onClick(View v) {
        String msg = mEditText02.getText().toString();
        if (!TextUtils.isEmpty(msg) && mPintWriter != null) {
            mPintWriter.println(msg);
            mEditText02.setText("");
            String showMsg = "client : " + msg + "\n";
            mEditText01.setText(mEditText01.getText() + showMsg);
        }
    }

    @Override
    protected void onDestroy() {
        if (mClientSocket != null) {
            try {
                mClientSocket.shutdownInput();
                mClientSocket.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        super.onDestroy();
    }
}

客戶端和服務端聯網需要許可權：

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