源码学习-Tomcat-02-一个简单的Servlet容器

本文会尝试介绍怎么实现一个简单的响应静态资源请求，或者servlet请求的Servlet容器。

Web 服务器也称为超文本传输协议(HTTP)服务器，因为它使用 HTTP 来跟客户端进行通信的，这通常是个 web 浏览器。一个基于 java 的 web 服务器使用两个重要的类：java.net.Socket 和 java.net.ServerSocket，并通过 HTTP 消息进行通信。

一个全功能的 servlet 容器会为 servlet 的每个 HTTP 请求做下面一些工作：

• 当第一次调用 servlet 的时候，加载该 servlet 类并调用 servlet 的 init 方法(仅仅一次)。

• 对每次请求，构造一个 javax.servlet.ServletRequest 实例和一个javax.servlet.ServletResponse 实例。
• 调用 servlet 的 service 方法，同时传递 ServletRequest 和 ServletResponse 对象。
• 当 servlet 类被关闭的时候，调用 servlet 的 destroy 方法并卸载 servlet 类。

本文的 servlet 容器不是全功能的。因此，她不能运行什么除了非常简单的 servlet，而且也不调用 servlet 的 init 方法和 destroy 方法。相反它做了下面的事情：

• 等待 HTTP 请求。

• 构造一个 ServletRequest 对象和一个 ServletResponse 对象。
• 假如该请求需要一个静态资源的话，调用 StaticResourceProcessor 实例的 process 方法，同时传递 ServletRequest 和 ServletResponse 对象。
• 假如该请求需要一个 servlet 的话，加载 servlet 类并调用 servlet 的 service 方法，同时传递 ServletRequest 和 ServletResponse 对象。

本实现的Servlet容器类图如下：

本实现的Servlet容器时序图如下：

本实现的main函数在HttpServer1类中，

预备知识：

• HTTP  的请求格式：

方法—统一资源标识符(URI)—协议/版本

请求的头部

主体内容

比如：

GET /index.html HTTP/1.1
Accept: text/html, application/xhtml+xml, */*
Accept-Language: zh-CN
User-Agent: Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; WOW64; Trident/5.0) QQBrowser/8.2.4258.400
Accept-Encoding: gzip, deflate
Host: localhost:8080
Connection: Keep-Alive

name=shouhui

• jvm类加载机制：

操作：

访问指定静态页面：http://localhost:8080/index.html

访问指定Servlet类：http://localhost:8080/servlet/PrimitiveServlet

停止该Servlet容器的运行：http://localhost:8080/SHUTDOWN

参考文章：

百度百科，servlet： http://baike.baidu.com/link?url=-WtPi8Z5ygr6g1gGDmtM3ObRwVwhoR7h-7lmBR-KX8kO5GsrTIxHtvkrMG2TW4MjtRRjUGnTfbCT9Fy2y0v4Sq

Budi Kurniawan, Paul Deck. 深入剖析Tomcat (M). 北京: 机械工业出版社华章公司, 2011.

参考代码：

public class HttpServer1 {

    /**
     * WEB_ROOT is the directory where our HTML and other files reside. For this
     * package, WEB_ROOT is the "webroot" directory under the working directory.
     * The working directory is the location in the file system from where the
     * java command was invoked.
     */
    // shutdown command
    private static final String SHUTDOWN_COMMAND = "/SHUTDOWN";

    // the shutdown command received
    private boolean shutdown = false;

    public static void main(String[] args) {
        HttpServer1 server = new HttpServer1();
        server.await();
    }

    public void await() {
        ServerSocket serverSocket = null;
        int port = 8080;
        try {
            serverSocket = new ServerSocket(port, 1,
                    InetAddress.getByName("127.0.0.1"));
        } catch (IOException e) {
            e.printStackTrace();
            System.exit(1);
        }

        // Loop waiting for a request
        while (!shutdown) {
            Socket socket = null;
            InputStream input = null;
            OutputStream output = null;
            try {
                socket = serverSocket.accept();
                input = socket.getInputStream();
                output = socket.getOutputStream();

                // create Request object and parse
                Request request = new Request(input);
                request.parse();

                // create Response object
                Response response = new Response(output);
                response.setRequest(request);

                // check if this is a request for a servlet or a static resource
                // a request for a servlet begins with "/servlet/"
                if (request.getUri().startsWith("/servlet/")) {
                    ServletProcessor1 processor = new ServletProcessor1();
                    processor.process(request, response);
                } else {
                    StaticResourceProcessor processor = new StaticResourceProcessor();
                    processor.process(request, response);
                }

                // Close the socket
                socket.close();
                // check if the previous URI is a shutdown command
                shutdown = request.getUri().equals(SHUTDOWN_COMMAND);
            } catch (Exception e) {
                e.printStackTrace();
                System.exit(1);
            }
        }
    }
}

public class Request implements ServletRequest {

    private InputStream input;
    private String uri;

    public Request(InputStream input) {
        this.input = input;
    }

    public String getUri() {
        return uri;
    }

    private String parseUri(String requestString) {
        int index1, index2;
        index1 = requestString.indexOf(‘ ‘);
        if (index1 != -1) {
            index2 = requestString.indexOf(‘ ‘, index1 + 1);
            if (index2 > index1)
                return requestString.substring(index1 + 1, index2);
        }
        return null;
    }

    public void parse() {
        // Read a set of characters from the socket
        StringBuffer request = new StringBuffer(2048);
        int i;
        byte[] buffer = new byte[2048];
        try {
            i = input.read(buffer);
        } catch (IOException e) {
            e.printStackTrace();
            i = -1;
        }
        for (int j = 0; j < i; j++) {
            request.append((char) buffer[j]);
        }
        System.out.print(request.toString());
        uri = parseUri(request.toString());
    }

    /* implementation of the ServletRequest */
    public Object getAttribute(String attribute) {
        return null;
    }

    public Enumeration getAttributeNames() {
        return null;
    }

    public String getRealPath(String path) {
        return null;
    }

    public RequestDispatcher getRequestDispatcher(String path) {
        return null;
    }

    public boolean isSecure() {
        return false;
    }

    public String getCharacterEncoding() {
        return null;
    }

    public int getContentLength() {
        return 0;
    }

    public String getContentType() {
        return null;
    }

    public ServletInputStream getInputStream() throws IOException {
        return null;
    }

    public Locale getLocale() {
        return null;
    }

    public Enumeration getLocales() {
        return null;
    }

    public String getParameter(String name) {
        return null;
    }

    public Map getParameterMap() {
        return null;
    }

    public Enumeration getParameterNames() {
        return null;
    }

    public String[] getParameterValues(String parameter) {
        return null;
    }

    public String getProtocol() {
        return null;
    }

    public BufferedReader getReader() throws IOException {
        return null;
    }

    public String getRemoteAddr() {
        return null;
    }

    public String getRemoteHost() {
        return null;
    }

    public String getScheme() {
        return null;
    }

    public String getServerName() {
        return null;
    }

    public int getServerPort() {
        return 0;
    }

    public void removeAttribute(String attribute) {
    }

    public void setAttribute(String key, Object value) {
    }

    public void setCharacterEncoding(String encoding)
            throws UnsupportedEncodingException {
    }

    @Override
    public int getRemotePort() {
        // TODO Auto-generated method stub
        return 0;
    }

    @Override
    public String getLocalName() {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public String getLocalAddr() {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public int getLocalPort() {
        // TODO Auto-generated method stub
        return 0;
    }

}

public class Response implements ServletResponse {

    private static final int BUFFER_SIZE = 1024;
    Request request;
    OutputStream output;
    PrintWriter writer;

    public Response(OutputStream output) {
        this.output = output;
    }

    public void setRequest(Request request) {
        this.request = request;
    }

    /* This method is used to serve a static page */
    public void sendStaticResource() throws IOException {
        byte[] bytes = new byte[BUFFER_SIZE];
        FileInputStream fis = null;
        try {
            /* request.getUri has been replaced by request.getRequestURI */
            File file = new File(Constants.WEB_ROOT, request.getUri());
            fis = new FileInputStream(file);
            /*
             * HTTP Response = Status-Line(( general-header | response-header |
             * entity-header ) CRLF) CRLF [ message-body ] Status-Line =
             * HTTP-Version SP Status-Code SP Reason-Phrase CRLF
             */
            int ch = fis.read(bytes, 0, BUFFER_SIZE);
            while (ch != -1) {
                output.write(bytes, 0, ch);
                ch = fis.read(bytes, 0, BUFFER_SIZE);
            }
        } catch (FileNotFoundException e) {
            String errorMessage = "HTTP/1.1 404 File Not Found\r\n"
                    + "Content-Type: text/html\r\n" + "Content-Length: 23\r\n"
                    + "\r\n" + "<h1>File Not Found</h1>";
            output.write(errorMessage.getBytes());
        } finally {
            if (fis != null)
                fis.close();
        }
    }

    /** implementation of ServletResponse */
    public void flushBuffer() throws IOException {
    }

    public int getBufferSize() {
        return 0;
    }

    public String getCharacterEncoding() {
        return null;
    }

    public Locale getLocale() {
        return null;
    }

    public ServletOutputStream getOutputStream() throws IOException {
        return null;
    }

    public PrintWriter getWriter() throws IOException {
        // autoflush is true, println() will flush,
        // but print() will not.
        writer = new PrintWriter(output, true);
        return writer;
    }

    public boolean isCommitted() {
        return false;
    }

    public void reset() {
    }

    public void resetBuffer() {
    }

    public void setBufferSize(int size) {
    }

    public void setContentLength(int length) {
    }

    public void setContentType(String type) {
    }

    public void setLocale(Locale locale) {
    }

    @Override
    public String getContentType() {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public void setCharacterEncoding(String charset) {
        // TODO Auto-generated method stub

    }
}

public class ServletProcessor1 {

    public void process(Request request, Response response) {

        String uri = request.getUri();
        String servletName = uri.substring(uri.lastIndexOf("/") + 1);
        URLClassLoader loader = null;

        try {
            // create a URLClassLoader
            URL[] urls = new URL[1];
            URLStreamHandler streamHandler = null;
            File classPath = new File(Constants.WEB_ROOT);
            // the forming of repository is taken from the createClassLoader
            // method in
            // org.apache.catalina.startup.ClassLoaderFactory
            String repository = (new URL("file", null,
                    classPath.getCanonicalPath() + File.separator)).toString();
            // the code for forming the URL is taken from the addRepository
            // method in
            // org.apache.catalina.loader.StandardClassLoader class.
            urls[0] = new URL(null, repository, streamHandler);
            loader = new URLClassLoader(urls);
        } catch (IOException e) {
            System.out.println(e.toString());
        }
        Class myClass = null;
        try {
            myClass = loader.loadClass(servletName);
        } catch (ClassNotFoundException e) {
            System.out.println(e.toString());
        }

        Servlet servlet = null;

        try {
            servlet = (Servlet) myClass.newInstance();
            servlet.service((ServletRequest) request,
                    (ServletResponse) response);
        } catch (Exception e) {
            System.out.println(e.toString());
        } catch (Throwable e) {
            System.out.println(e.toString());
        }

    }
}

public class StaticResourceProcessor {

    public void process(Request request, Response response) {
        try {
            response.sendStaticResource();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}