(四) 发送E-mail
为了发送E-mail，必须建立一个到端口25(即SMTP端口)的套接字连接。简单邮件传输协议用于描述E-mail消息的格式。
可以连接到任何一个提供SMTP服务的服务器，不过需要确定服务器是否愿意接受请求。SMTP服务器通常总是愿意发送来自任何人的E-mail，但是由于垃圾邮件泛滥，大多数服务器都内置有检查功能，它们只接受那些来自它们所信任的用户或IP地址段的请求。
一旦连接到服务器，就可以发送一个邮件报头(采用SMTP格式)。紧随其后是邮件消息。
操作的详细过程:
1.打开一个到达主机的套接字:
Socket socket = new Socket("mail.mailserver.com", 25); PrintWriter out = new PrintWriter(socket.getOutputStream());?
2.发送以下信息到输出流
??? HELO sending host
??? MAIL FROM: <sender e-mail address>
??? RCPT TO: <recipient e-mail address>
??? DATA
??? mail message
??? (any number of lines)
??? .
??? QUIT
e.g.
OutputStream outStream = socket.getOutputStream(); PrintWriter out = new PrintWriter(new OutputStreamWriter(outStream, "utf-8"), true); String hostName = InetAddress.getLocalHost().getHostName(); out.print("HELO " + hostName + "\r\n"); out.print("MAIL FROM: <" + from.getText() + ">\r\n"); out.print("RCPT TO: <" + to.getText() + ">\r\n"); out.print("DATA\r\n"); out.print(("the mail message...\n").replaceAll("\n", "\r\n")); out.print(".\r\n"); out.print("QUIT\r\n"));?
3.SMTP规范(RFC 821)规定，每行都要以\r再紧跟一个\n来结尾。
4.有些SMTP服务器并不检查信息的真实性，可以随意填写任何喜欢使用的发件人名字。
5.现在大多数SMTP服务器都会检查守信IP，其他服务器将使用"SMTP之前先POP"规范，即要求在发送任何消息之前先下载你的E-mail，即要求提供密码。
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DEMO
import java.awt.EventQueue;import javax.swing.JFrame;public class MailTest {public static void main(String[] args) {EventQueue.invokeLater(new Runnable(){public void run(){JFrame frame = new MailTestFrame();frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);frame.setVisible(true);}});}}?
/** * 该程序会因为受限于SMTP服务器检测IP地址的原因，导致失败 */import java.awt.GridBagLayout;import java.awt.event.ActionEvent;import java.awt.event.ActionListener;import java.io.IOException;import java.io.InputStream;import java.io.OutputStream;import java.io.OutputStreamWriter;import java.io.PrintWriter;import java.net.InetAddress;import java.net.InetSocketAddress;import java.net.Socket;import java.util.Scanner;import javax.swing.JButton;import javax.swing.JFrame;import javax.swing.JLabel;import javax.swing.JPanel;import javax.swing.JScrollPane;import javax.swing.JTextArea;import javax.swing.JTextField;import javax.swing.SwingWorker;public class MailTestFrame extends JFrame {private Scanner in;private PrintWriter out;private JTextField from;private JTextField to;private JTextField smtpServer;private JTextArea message;private JTextArea comm;public static final int DEFAULT_WIDTH = 300;public static final int DEFAULT_HEIGHT = 300;public MailTestFrame(){setSize(DEFAULT_WIDTH, DEFAULT_HEIGHT);setTitle("MailTest");setLayout(new GridBagLayout());add(new JLabel("From:"), new GBC(0,0).setFill(GBC.HORIZONTAL));from = new JTextField(20);add(from, new GBC(1, 0).setFill(GBC.HORIZONTAL).setWeight(100, 0));add(new JLabel("TO:"), new GBC(0, 1).setFill(GBC.HORIZONTAL));to = new JTextField(20);add(to, new GBC(1, 1).setFill(GBC.HORIZONTAL).setWeight(100, 0));add(new JLabel("SMTP server:"), new GBC(0, 2).setFill(GBC.HORIZONTAL));smtpServer = new JTextField(20);add(smtpServer, new GBC(1, 2).setFill(GBC.HORIZONTAL).setWeight(100, 0));message = new JTextArea();add(new JScrollPane(message), new GBC(0, 3, 2, 1).setFill(GBC.BOTH).setWeight(100, 100));comm = new JTextArea();add(new JScrollPane(comm), new GBC(0, 4, 2, 1).setFill(GBC.BOTH).setWeight(100, 100));JPanel buttonPanel = new JPanel();add(buttonPanel, new GBC(0, 5, 2, 1));JButton sendButton = new JButton("Send");buttonPanel.add(sendButton);sendButton.addActionListener(new ActionListener(){@Overridepublic void actionPerformed(ActionEvent e) {new SwingWorker<Void, Void>(){protected Void doInBackground() throws Exception{comm.setText("");sendMail();return null;}}.execute();}});}public void sendMail(){try{//Socket socket = new Socket(smtpServer.getText(), 25);Socket socket = new Socket();socket.connect(new InetSocketAddress(smtpServer.getText(), 25), 2000);try{InputStream inStream = socket.getInputStream();OutputStream outStream = socket.getOutputStream();in = new Scanner(inStream);out = new PrintWriter(new OutputStreamWriter(outStream, "utf-8"), true);String hostName = InetAddress.getLocalHost().getHostName();receive();send("HELO " + hostName);receive();send("MAIL FROM: <" + from.getText() + ">");receive();send("RCPT TO: <" + to.getText() + ">");receive();send("DATA");receive();send(message.getText());send(".");receive();}finally{socket.close();}}catch(IOException e){comm.append("Error: " + e);}}private void send(String str) throws IOException{comm.append(str);comm.append("\n");out.print(str.replaceAll("\n", "\r\n"));out.flush();}private void receive() {String line = in.nextLine();comm.append(line);comm.append("\n");}}?import java.awt.GridBagConstraints;import java.awt.Insets;public class GBC extends GridBagConstraints{public GBC(int gridx, int gridy){this.gridx = gridx;this.gridy = gridy;}public GBC(int gridx, int gridy, int gridwidth, int gridheight){this.gridx = gridx;this.gridy = gridy;this.gridwidth = gridwidth;this.gridheight = gridheight;}public GBC setAnchor(int anchor){this.anchor = anchor;return this;}public GBC setFill(int fill){this.fill = fill;return this;}public GBC setWeight(double weightx, double weighty){this.weightx = weightx;this.weighty = weighty;return this;}public GBC setInsets(int distance){this.insets = new Insets(distance, distance, distance, distance);return this;}public GBC setInsets(int top, int left, int bottom, int right){this.insets = new Insets(top, left, bottom, right);return this;}public GBC setIpad(int ipadx, int ipady){this.ipadx = ipadx;this.ipady = ipady;return this;}}??