Java是一種跨平臺性的編程語言，擁有著廣泛的應用。它的運用范圍涉及到網絡編程，以及和各種應用層協議的對接。下面將重點介紹如何讓Java和應用層協議相結合。

應用層協議：HTTP
public class URLConnectionReader {
public static void main(String[] args) throws Exception {
URL oracle = new URL("http://www.oracle.com/");
URLConnection yc = oracle.openConnection();
BufferedReader in = new BufferedReader(new InputStreamReader(
yc.getInputStream()));
String inputLine;
while ((inputLine = in.readLine()) != null) 
System.out.println(inputLine);
in.close();
}
}

在上述代碼中，用到了Java內置的URLConnection類和BufferedReader類，通過這兩個類可以構建一個與指定URL進行通信的URLConnection對象。然后，使用BufferedReader逐行讀取返回頁面的每一行內容，打印出來進行展示。

應用層協議：SMTP
public class SendEmail {
public static void main(String[] args) {
final String username = "myusername@gmail.com";
final String password = "mypassword";
String fromEmail = "myusername@gmail.com";
String toEmail = "receiveremail@gmail.com";
String subject = "Test";
String body = "This is a test email";
String smtpHost = "smtp.gmail.com";
int smtpPort = 587;
Properties props = new Properties();
props.put("mail.smtp.auth", "true");
props.put("mail.smtp.starttls.enable", "true");
props.put("mail.smtp.host", smtpHost);
props.put("mail.smtp.port", smtpPort);
Session session = Session.getInstance(props,
new javax.mail.Authenticator() {
protected PasswordAuthentication getPasswordAuthentication() {
return new PasswordAuthentication(username, password);
}
});
try {
Message message = new MimeMessage(session);
message.setFrom(new InternetAddress(fromEmail));
message.setRecipients(Message.RecipientType.TO,
InternetAddress.parse(toEmail));
message.setSubject(subject);
message.setText(body);
Transport.send(message);
System.out.println("Email sent!");
} catch (MessagingException e) {
throw new RuntimeException(e);
}
}
}

在上述代碼中，用到了JavaMail API提供的類，通過該庫可以實現發郵件功能。在這個例子里，需要設置以下內容：發送者的郵箱地址、密碼、以及SMTP服務器的地址和端口號。然后構建Session對象， 郵件的消息體通過MimeMessage類實現，并通過Transport類中的send方法實現郵件發送