Detailed explanation of the use of jetty

Introduction: Jetty is an open source, standards-based, feature-rich HTTP server and Web server implemented in Java. Containers are free for commercial use. The Jetty project was founded in 1995. There are already many successful products based on Jetty, such as Apache Geromino, JBoss, IBM Tivoli, Cisco SESM, etc. Jetty can be used as a traditional web server or as a dynamic content server, and Jetty can be easily embedded into Java applications.

Feature Introduction

Ease of use

Ease of use is the basic principle of Jetty design. Ease of use is mainly reflected in the following aspects:

1. Configure Jetty through XML or API;
2. Default The configuration can meet most needs;
3. Use Jetty Embedding into applications requires very little code;
Scalability

Web 2.0 using Ajax In applications where each connection needs to be maintained for a longer period of time, thread and memory consumption will increase dramatically. This makes us worry that the entire program will fall into a bottleneck due to a single component and affect the performance of the entire program. But there is Jetty:

1. Even when there are a large number of service requests, the performance of the system can be maintained in an acceptable state.
2.Use Continuation Mechanism to handle a large number of user requests and long connections.
In addition, Jetty has designed a very good interface, so in Jetty When a certain implementation of Jetty cannot meet the needs of users, users can easily modify some implementations of Jetty to make Jetty suitable for the needs of special applications.

Easy to embeddability

Jetty was designed as an excellent component from the beginning, which means that Jetty can be easily embedded into applications without the need for a program to use Jetty Make changes. To some extent, you can also think of Jetty as an embedded web server.

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Deploying Applications

Deploying your own applications to Jetty is very easy. Simple, first package the developed application into a WAR package and put it in Jetty's Webapps below the directory. Then use the following command to start the Jetty server: Java –jar start.jar, after starting the server. We can now access our application, Jetty The default port is 8080, and the name of the WAR is the Root Context of our application. For example, a typical URL That is: http://127.0.0.1:8080/sample/index.jsp.

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How to embed Jetty into a program

To embed Jetty into a program is Very simple, as shown in code 1: first we create a Server object and set the port to 8080, and then set the The object adds a default Handler. Then we use the configuration file jetty.xml to set up this server, and finally we use the method server.start() to Just start the Server. As can be seen from this code, Jetty is very suitable for being embedded into our application as a component. This is why Jetty a very important feature.

List 1. Code snippet

public class JettyServer {
    public static void main(String[] args) {
        Server server = new Server(8080);
        server.setHandler(new DefaultHandler());
        XmlConfiguration configuration = null;
        try {
            configuration = new XmlConfiguration(
                new FileInputStream("C:/development/Jetty/jetty-6.1.6rc0/etc/jetty.xml"));
        } catch (FileNotFoundException e1) {
            e1.printStackTrace();
        } catch (SAXException e1) {
            e1.printStackTrace();
        } catch (IOException e1) {
            e1.printStackTrace();
        }

        try {    
            configuration.configure(server);
            server.start();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

Next, let’s analyze how Jetty Server is started. First we notice the Server class. This class actually inherits HttpServer. When starting Jetty When using the server, that is to say, if you enter java -jar start.jar etc/jetty.xml from the command line in the Jetty root directory, note that there is a configuration file here jetty.xml is used as a running parameter. This parameter can also be other configuration files, which can be multiple XML configuration files. In fact, this configuration file is like when we use Struts struts-config.xml file, write the components needed to run Server, such as HttpServer in the previous section The component classes required for the configuration can be written in this configuration file. When Jetty Server is started according to the above method, main in the Server class will be called method. This entry method will first construct a Server class instance (in fact, it also constructs an HttpServer). During the process of creating the instance, the XmlConfiguration will be constructed. Class object to read the parameter configuration file, and then use the XmlConfiguration object generated by this configuration file to configure the Server. The configuration process actually uses Java Reflection mechanism, call the Server method and pass in the parameters written in the configuration file to add to this Server HttpListener, HttpContext, HttpHandler, and Web Application (corresponding to our Web application).

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Jetty’s Continuation mechanism

讨论 Jetty 的 Continuation 机制，首先需要提到 Ajax 技术，Ajax 技术是当前开发 Web 应用的非常热门的技术，也是 Web 2.0 的一个重要的组成部分。Ajax 技术中的一个核心对象是 XMLHttpRequest 对象，这个对象支持异步请求，所谓异步请求即是指当客户端发送一个请求到服务器的时候，客户端不必一直等待服务器的响应。这样就不会造成整个页面的刷新，给用户带来更好的体验。而当服务器端响应返回时，客户端利用一个 Javascript 函数对返回值进行处理，以更新页面上的部分元素的值。但很多时候这种异步事件只是在很小一部分的情况下才会发生，那么怎么保证一旦服务器端有了响应之后客户端马上就知道呢，我们有两种方法来解决这个问题，一是让浏览器每隔几秒请求服务器来获得更改，我们称之为轮询。二是服务器维持与浏览器的长时间的连接来传递数据，长连接的技术称之为 Comet。

大家很容易就能发现轮询方式的主要缺点是产生了大量的传输浪费。因为可能大部分向服务器的请求是无效的，也就是说客户端等待发生的事件没有发生，如果有大量的客户端的话，那么这种网络传输的浪费是非常厉害的。特别是对于服务器端很久才更新的应用程序来讲，比如邮件程序，这种浪费就更是巨大了。并且对 Server 端处理请求的能力也相应提高了要求。如果很长时间才向 Server 端发送一次请求的话，那么客户端就不能的得到及时的响应。

如果使用 Comet 技术的话，客户端和服务器端必须保持一个长连接，一般情况下，服务器端每一个 Servlet 都会独占一个线程，这样就会使得服务器端有很多线程同时存在，这在客户端非常多的情况下也会对服务器端的处理能力带来很大的挑战。

Jetty 利用 Java 语言的非堵塞 I/O 技术来处理并发的大量连接。 Jetty 有一个处理长连接的机制：一个被称为 Continuations 的特性。利用 Continuation 机制，Jetty 可以使得一个线程能够用来同时处理多个从客户端发送过来的异步请求，下面我们通过一个简化的聊天程序的服务器端的代码来演示不使用 Continuation 机制和使用 Continuation 的差别。

清单 2. Continuation 机制

public class ChatContinuation extends HttpServlet{

    public void doPost(HttpServletRequest request, HttpServletResponse response){
        postMessage(request, response);
    }

    private void postMessage(HttpServletRequest request, HttpServletResponse response)
    {
        HttpSession session = request.getSession(true);
        People people = (People)session.getAttribute(session.getId());
        if (!people.hasEvent())
        {
            Continuation continuation = 
                ContinuationSupport.getContinuation(request, this);
            people.setContinuation(continuation);
            continuation.suspend(1000);
        }
        people.setContinuation(null);
        people.sendEvent(response);
    }
}

大家注意到，首先获取一个 Continuation 对象，然后把它挂起 1 秒钟，直到超时或者中间被 resume 函数唤醒位置，这里需要解释的是，在调用完 suspend 函数之后，这个线程就可处理其他的请求了，这也就大大提高了程序的并发性，使得长连接能够获得非常好的扩展性。

如果我们不使用 Continuation 机制，那么程序就如 清单 3 所示：

清单 3. 不使用 Continuation 机制

public class Chat extends HttpServlet{
    public void doPost(HttpServletRequest request, HttpServletResponse response){
        postMessage(request, response);
    }

    private void postMessage(HttpServletRequest request, HttpServletResponse response)
    {
        HttpSession session = request.getSession(true);
        People people = (People)session.getAttribute(session.getId());
        while (!people.hasEvent())
        {
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        people.setContinuation(null);
        people.sendEvent(response);    
    }
}

大家注意到在等待事件发生的时间里，线程被挂起，直到所等待的事件发生为止，但在等待过程中，这个线程不能处理其他请求，这也就造成了在客户端非常多的情况下服务器的处理能力跟不上的情况。下面我们解释一下 Jetty 的 Continuation 的机制是如何工作的。

为了使用 Continuatins，Jetty 必须配置为使用它的 SelectChannelConnector 处理请求。这个 connector 构建在 java.nio API 之上，允许它维持每个连接开放而不用消耗一个线程。当使用 SelectChannelConnector 时，ContinuationSupport.getContinuation() 提供一个 SelectChannelConnector.RetryContinuation 实例（但是，您必须针对 Continuation 接口编程）。当在 RetryContinuation 上调用 suspend() 时，它抛出一个特殊的运行时异常 -- RetryRequest，该异常传播到 servlet 外并且回溯到 filter 链，最后被 SelectChannelConnector 捕获。但是不会发送一个异常响应给客户端，而是将请求维持在未决 Continuations 队列里，则 HTTP 连接保持开放。这样，用来服务请求的线程返回给 ThreadPool，然后又可以用来服务其他请求。暂停的请求停留在未决 Continuations 队列里直到指定的过期时间，或者在它的 Continuation 上调用 resume() 方法。当任何一个条件触发时，请求会重新提交给 servlet（通过 filter 链）。这样，整个请求被"重播"直到 RetryRequest 异常不再抛出，然后继续按正常情况执行。

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Jetty 的安全性

为了防止任何人都有权限去关闭一个已经开启的 Jetty 服务器， 我们可以通过在启动 Jetty 服务器的时候指定参数来进行控制，使得用户必须提供密码才能关闭 Jetty 服务器，启动 Jetty 服务器的命令如下所示：

java -DSTOP.PORT=8079 -DSTOP.KEY=mypassword -jar start.jar

这样，用户在停止 Jetty 服务器的时候，就必须提供密码“mypassword”。

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总结

Jetty 是一个非常方便使用的 Web 服务器，它的特点在于非常小，很容易嵌入到我们的应用程序当中，而且针对 Web 2.0 的 Ajax 技术进行了特别的优化，这也使得我们的使用 Ajax 的应用程序可以拥有更好的性能。

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