一个简单的Servlet容器实现

　　上篇写了一个简单的Java web服务器实现，只能处理一些静态资源的请求，本篇文章实现的Servlet容器基于前面的服务器做了个小改造，增加了Servlet请求的处理。

程序执行步骤

1. 创建一个ServerSocket对象；
2. 调用ServerSocket对象的accept方法，等待连接，连接成功会返回一个Socket对象，否则一直阻塞等待；
3. 从Socket对象中获取InputStream和OutputStream字节流，这两个流分别对应request请求和response响应；
4. 处理请求：读取InputStream字节流信息，转成字符串形式，并解析，这里的解析比较简单，仅仅获取uri(统一资源标识符)信息;
5. 处理响应（分两种类型，静态资源请求响应或servlet请求响应）：如果是静态资源请求，则根据解析出来的uri信息，从WEB_ROOT目录中寻找请求的资源资源文件, 读取资源文件，并将其写入到OutputStream字节流中；如果是Servlet请求，则首先生成一个URLClassLoader类加载器，加载请求的servlet类，创建servlet对象，执行service方法（往OutputStream写入响应的数据）；
6. 关闭Socket对象；
7. 转到步骤2，继续等待连接请求；

代码实现：

添加依赖：

        <!-- https://mvnrepository.com/artifact/javax.servlet/servlet-api -->
        <dependency>
            <groupId>javax.servlet</groupId>
            <artifactId>servlet-api</artifactId>
            <version>2.3</version>
        </dependency>

服务器代码：

package ex02.pyrmont.first;

import java.net.Socket;
import java.net.ServerSocket;
import java.net.InetAddress;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.IOException;

import ex02.pyrmont.Request;
import ex02.pyrmont.Response;
import ex02.pyrmont.StaticResourceProcessor;

public class HttpServer1 {

    // 关闭服务命令
    private static final String SHUTDOWN_COMMAND = "/SHUTDOWN";

    public static void main(String[] args) {
        HttpServer1 server = new HttpServer1();
        //等待连接请求
        server.await();
    }

    public void await() {
        ServerSocket serverSocket = null;
        int port = 8080;
        try {
            //服务器套接字对象
            serverSocket = new ServerSocket(port, 1, InetAddress.getByName("127.0.0.1"));
        } catch (IOException e) {
            e.printStackTrace();
            System.exit(1);
        }

        // 循环等待请求
        while (true) {
            Socket socket = null;
            InputStream input = null;
            OutputStream output = null;
            try {
                //等待连接，连接成功后，返回一个Socket对象
                socket = serverSocket.accept();
                input = socket.getInputStream();
                output = socket.getOutputStream();

                // 创建Request对象并解析
                Request request = new Request(input);
                request.parse();
                // 检查是否是关闭服务命令
                if (request.getUri().equals(SHUTDOWN_COMMAND)) {
                    break;
                }

                // 创建 Response 对象
                Response response = new Response(output);
                response.setRequest(request);

                if (request.getUri().startsWith("/servlet/")) {
                    //请求uri以/servlet/开头，表示servlet请求
                    ServletProcessor1 processor = new ServletProcessor1();
                    processor.process(request, response);
                } else {
                    //静态资源请求
                    StaticResourceProcessor processor = new StaticResourceProcessor();
                    processor.process(request, response);
                }

                // 关闭 socket
                socket.close();

            } catch (Exception e) {
                e.printStackTrace();
                System.exit(1);
            }
        }
    }
}

常量类：

package ex02.pyrmont;

import java.io.File;

public class Constants {
    public static final String WEB_ROOT = System.getProperty("user.dir")
            + File.separator + "webroot";
    public static final String WEB_SERVLET_ROOT = System.getProperty("user.dir")
            + File.separator + "target" + File.separator + "classes";

}

Request：

package ex02.pyrmont;

import java.io.InputStream;
import java.io.IOException;
import java.io.BufferedReader;
import java.io.UnsupportedEncodingException;
import java.util.Enumeration;
import java.util.Locale;
import java.util.Map;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletInputStream;
import javax.servlet.ServletRequest;

public class Request implements ServletRequest {

    private InputStream input;
    private String uri;

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

    public String getUri() {
        return uri;
    }
    /**
     *
     * requestString形式如下：
     * GET /index.html HTTP/1.1
     * Host: localhost:8080
     * Connection: keep-alive
     * Cache-Control: max-age=0
     * ...
     * 该函数目的就是为了获取/index.html字符串
     */
    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;
    }
    //从InputStream中读取request信息，并从request中获取uri值
    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 {
    }

}

Response：

package ex02.pyrmont;

import java.io.OutputStream;
import java.io.IOException;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.File;
import java.io.PrintWriter;
import java.util.Locale;
import javax.servlet.ServletResponse;
import javax.servlet.ServletOutputStream;

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;
    }

    //将web文件写入到OutputStream字节流中
    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) {
    }
}

静态资源请求处理：

package ex02.pyrmont;

import java.io.IOException;

public class StaticResourceProcessor {

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

Servlet请求处理：

package ex02.pyrmont.first;

import java.net.URL;
import java.net.URLClassLoader;
import java.net.URLStreamHandler;
import java.io.IOException;

import javax.servlet.Servlet;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

import ex02.pyrmont.Constants;
import ex02.pyrmont.Request;
import ex02.pyrmont.Response;

public class ServletProcessor1 {

    public void process(Request request, Response response) {

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

        //类加载器，用于从指定JAR文件或目录加载类
        URLClassLoader loader = null;
        try {
            URLStreamHandler streamHandler = null;
            //创建类加载器
            loader = new URLClassLoader(new URL[]{new URL(null, "file:" + Constants.WEB_SERVLET_ROOT, streamHandler)});
        } catch (IOException e) {
            System.out.println(e.toString());
        }

        Class<?> myClass = null;
        try {
            //加载对应的servlet类
            myClass = loader.loadClass(servletName);
        } catch (ClassNotFoundException e) {
            System.out.println(e.toString());
        }

        Servlet servlet = null;

        try {
            //生产servlet实例
            servlet = (Servlet) myClass.newInstance();
            //执行ervlet的service方法
            servlet.service((ServletRequest) request,(ServletResponse) response);
        } catch (Exception e) {
            System.out.println(e.toString());
        } catch (Throwable e) {
            System.out.println(e.toString());
        }

    }
}

Servlet类：

import javax.servlet.*;
import java.io.IOException;
import java.io.PrintWriter;

public class PrimitiveServlet implements Servlet {

    public void init(ServletConfig config) throws ServletException {
        System.out.println("init");
    }

    public void service(ServletRequest request, ServletResponse response)
            throws ServletException, IOException {
        System.out.println("from service");
        PrintWriter out = response.getWriter();
        out.println("Hello. Roses are red.");
        out.print("Violets are blue.");
    }

    public void destroy() {
        System.out.println("destroy");
    }

    public String getServletInfo() {
        return null;
    }

    public ServletConfig getServletConfig() {
        return null;
    }

}

结果测试：

静态资源请求：

servlet请求（因为只是第一个字符串被刷新到浏览器，所以你不能看到第二个字符串Violets are blue。我们将在后续完善该容器）：

改进

　　前面实现的Servlet容器有一个严重的问题，用户在servlet里可以直接将ServletRequest、ServletResponse向下转 型为Request和Response类型，并直接调用其内部的public方法，这是一个不好的设计，改进方法是给Request、Response 增加外观类，这样，用户只能访问外观类里定义的public方法。

Request外观类

package ex02.pyrmont.second;

import java.io.IOException;
import java.io.BufferedReader;
import java.io.UnsupportedEncodingException;
import java.util.Enumeration;
import java.util.Locale;
import java.util.Map;

import javax.servlet.RequestDispatcher;
import javax.servlet.ServletInputStream;
import javax.servlet.ServletRequest;

import ex02.pyrmont.Request;

public class RequestFacade implements ServletRequest {

    private ServletRequest request = null;

    public RequestFacade(Request request) {
        this.request = request;
    }

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

    public Enumeration<?> getAttributeNames() {
        return request.getAttributeNames();
    }

    @SuppressWarnings("deprecation")
    public String getRealPath(String path) {
        return request.getRealPath(path);
    }

    public RequestDispatcher getRequestDispatcher(String path) {
        return request.getRequestDispatcher(path);
    }

    public boolean isSecure() {
        return request.isSecure();
    }

    public String getCharacterEncoding() {
        return request.getCharacterEncoding();
    }

    public int getContentLength() {
        return request.getContentLength();
    }

    public String getContentType() {
        return request.getContentType();
    }

    public ServletInputStream getInputStream() throws IOException {
        return request.getInputStream();
    }

    public Locale getLocale() {
        return request.getLocale();
    }

    public Enumeration<?> getLocales() {
        return request.getLocales();
    }

    public String getParameter(String name) {
        return request.getParameter(name);
    }

    public Map<?, ?> getParameterMap() {
        return request.getParameterMap();
    }

    public Enumeration<?> getParameterNames() {
        return request.getParameterNames();
    }

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

    public String getProtocol() {
        return request.getProtocol();
    }

    public BufferedReader getReader() throws IOException {
        return request.getReader();
    }

    public String getRemoteAddr() {
        return request.getRemoteAddr();
    }

    public String getRemoteHost() {
        return request.getRemoteHost();
    }

    public String getScheme() {
        return request.getScheme();
    }

    public String getServerName() {
        return request.getServerName();
    }

    public int getServerPort() {
        return request.getServerPort();
    }

    public void removeAttribute(String attribute) {
        request.removeAttribute(attribute);
    }

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

    public void setCharacterEncoding(String encoding)
            throws UnsupportedEncodingException {
        request.setCharacterEncoding(encoding);
    }

}

Response外观类

package ex02.pyrmont.second;

import java.io.IOException;
import java.io.PrintWriter;
import java.util.Locale;

import javax.servlet.ServletResponse;
import javax.servlet.ServletOutputStream;

import ex02.pyrmont.Response;

public class ResponseFacade implements ServletResponse {

    private ServletResponse response;

    public ResponseFacade(Response response) {
        this.response = response;
    }

    public void flushBuffer() throws IOException {
        response.flushBuffer();
    }

    public int getBufferSize() {
        return response.getBufferSize();
    }

    public String getCharacterEncoding() {
        return response.getCharacterEncoding();
    }

    public Locale getLocale() {
        return response.getLocale();
    }

    public ServletOutputStream getOutputStream() throws IOException {
        return response.getOutputStream();
    }

    public PrintWriter getWriter() throws IOException {
        return response.getWriter();
    }

    public boolean isCommitted() {
        return response.isCommitted();
    }

    public void reset() {
        response.reset();
    }

    public void resetBuffer() {
        response.resetBuffer();
    }

    public void setBufferSize(int size) {
        response.setBufferSize(size);
    }

    public void setContentLength(int length) {
        response.setContentLength(length);
    }

    public void setContentType(String type) {
        response.setContentType(type);
    }

    public void setLocale(Locale locale) {
        response.setLocale(locale);
    }

}

处理Servlet请求类：

package ex02.pyrmont.second;

import java.net.URL;
import java.net.URLClassLoader;
import java.net.URLStreamHandler;
import java.io.IOException;

import javax.servlet.Servlet;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

import ex02.pyrmont.Constants;
import ex02.pyrmont.Request;
import ex02.pyrmont.Response;

public class ServletProcessor2 {

    public void process(Request request, Response response) {

        String uri = request.getUri();
        String servletName = uri.substring(uri.lastIndexOf("/") + 1);
        // 类加载器，用于从指定JAR文件或目录加载类
        URLClassLoader loader = null;
        try {
            URLStreamHandler streamHandler = null;
            // 创建类加载器
            loader = new URLClassLoader(new URL[] { new URL(null, "file:"
                    + Constants.WEB_SERVLET_ROOT, streamHandler) });
        } catch (IOException e) {
            System.out.println(e.toString());
        }

        Class<?> myClass = null;
        try {
            // 加载对应的servlet类
            myClass = loader.loadClass(servletName);
        } catch (ClassNotFoundException e) {
            System.out.println(e.toString());
        }

        Servlet servlet = null;
        //给request、response增加外观类，安全性考虑，防止用户在servlet里直接将ServletRequest、ServletResponse向下转型为Request和Response类型，
        //并直接调用其内部的public方法，因为RequestFacade、ResponseFacade里不会有parse、sendStaticResource等方法；
        RequestFacade requestFacade = new RequestFacade(request);
        ResponseFacade responseFacade = new ResponseFacade(response);
        try {
            servlet = (Servlet) myClass.newInstance();
            servlet.service((ServletRequest) requestFacade, (ServletResponse) responseFacade);
        } catch (Exception e) {
            System.out.println(e.toString());
        } catch (Throwable e) {
            System.out.println(e.toString());
        }

    }
}

其它代码与前面实现的Servlet容器基本一致。

验证程序，分别请求静态资源和Servlet，发现结果与前面实现的容器一致；

参考资料：《深入剖析Tomcat》