Servlet 源码分析

我就是我 2022-05-21 08:43 144阅读 0赞

Servlet的结构图如下：

![70][]

1.Servlet接口源代码如下：

//javax.servlet.Servlet
    public interface Servlet {
        void init(ServletConfig var1) throws ServletException;    //在容器启动时被容器调用,只会调用一次
    
        ServletConfig getServletConfig();                         //用于获取ServletConfig
    
        void service(ServletRequest var1, ServletResponse var2) throws ServletException, IOException;     //    用于具体处理一些请求
    
        String getServletInfo();                                   //用于获取一些Servlet相关信息，如作者，版权等
    
        void destroy();                                            //用于在Servlet销毁时释放一些资源，一般指关闭服务器，也只会调用一次
    }

1.1 init方法调用时，会接收到一个ServletConfig(就是Servlet的配置)参数进去，我们再web.xml中定义的Servlet通过init-param标签配置的参数就是通过ServletConfig来保存的。

2.ServletConfig接口源代码

// javax.servlet;
    
    public interface ServletConfig {
        String getServletName();                //用于获取Servlet的名字，就是在Web.xml中定义的Servlet-name
    
        ServletContext getServletContext();      //返回的是这个应用本身
    
        String getInitParameter(String var1);    //用于获取init-param配置的参数
    
        Enumeration<String> getInitParameterNames();    //用于获取配置的所有init-param名字的集合
    }

结论：ServletConfig是Servlet级别的，ServletContext是应用级（Application）别的。

3.GenericServlet抽象类

主要做了三件事：

1》实现了ServletConfig接口，我们可以直接调ServletConfig里面的方法，

2》提供无参的init方法

3》提供log方法，一个记录日志，一个记录异常

源码：

package javax.servlet;
    
    import java.io.IOException;
    import java.io.Serializable;
    import java.util.Enumeration;
    
    public abstract class GenericServlet implements Servlet, ServletConfig, Serializable {
        private static final long serialVersionUID = 1L;
        private transient ServletConfig config;
    
        public GenericServlet() {
        }
    
        public void destroy() {
        }
    
        public String getInitParameter(String name) {
            return this.getServletConfig().getInitParameter(name);
        }
    
        public Enumeration<String> getInitParameterNames() {
            return this.getServletConfig().getInitParameterNames();
        }
    
        public ServletConfig getServletConfig() {
            return this.config;
        }
    
        public ServletContext getServletContext() {
            return this.getServletConfig().getServletContext();        //这里可以直接调用ServletContext,
        }
    
        public String getServletInfo() {
            return "";
        }
    
        public void init(ServletConfig config) throws ServletException {
            this.config = config;
            this.init();
        }
    
        public void init() throws ServletException {            //新增的无参init方法
        }
    
        public void log(String msg) {
            this.getServletContext().log(this.getServletName() + ": " + msg);
        }
    
        public void log(String message, Throwable t) {
            this.getServletContext().log(this.getServletName() + ": " + message, t);
        }
    
        public abstract void service(ServletRequest var1, ServletResponse var2) throws ServletException, IOException;
    
        public String getServletName() {
            return this.config.getServletName();
        }
    }

4.HttpServlet类

HttpServlet类是用Http协议实现的Servlet的基类，SpringMVC中的DispatcherServlet就是继承了HttpServlet，我们主要关心HttpServlet怎么处理请求，所以主要讲解下service方法。

在Service中将ServletRequest和ServletResponse转换为了HttpServletRequest和HttpServletResponse,然后根据不同的请求路由到不同的页面。

源码

package javax.servlet.http;
    
    public abstract class HttpServlet extends GenericServlet {
        private static final long serialVersionUID = 1L;
        private static final String METHOD_DELETE = "DELETE";
        private static final String METHOD_HEAD = "HEAD";
        private static final String METHOD_GET = "GET";
        private static final String METHOD_OPTIONS = "OPTIONS";
        private static final String METHOD_POST = "POST";
        private static final String METHOD_PUT = "PUT";
        private static final String METHOD_TRACE = "TRACE";
        private static final String HEADER_IFMODSINCE = "If-Modified-Since";
        private static final String HEADER_LASTMOD = "Last-Modified";
        private static final String LSTRING_FILE = "javax.servlet.http.LocalStrings";
        private static final ResourceBundle lStrings = ResourceBundle.getBundle("javax.servlet.http.LocalStrings");
    
        public HttpServlet() {
        }
    
        protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
          .......
        }
    
        protected long getLastModified(HttpServletRequest req) {
            return -1L;
        }
    
        protected void doHead(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
             ......
        }
    
        protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
            String protocol = req.getProtocol();
            String msg = lStrings.getString("http.method_post_not_supported");
            if (protocol.endsWith("1.1")) {
                resp.sendError(405, msg);
            } else {
                resp.sendError(400, msg);
            }
    
        }
    
        protected void doPut(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
         ......
    
        }
    
        protected void doDelete(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
           ......
    
        }
    
        private static Method[] getAllDeclaredMethods(Class<?> c) {
            if (c.equals(HttpServlet.class)) {
                return null;
            } else {
                Method[] parentMethods = getAllDeclaredMethods(c.getSuperclass());
                Method[] thisMethods = c.getDeclaredMethods();
                if (parentMethods != null && parentMethods.length > 0) {
                    Method[] allMethods = new Method[parentMethods.length + thisMethods.length];
                    System.arraycopy(parentMethods, 0, allMethods, 0, parentMethods.length);
                    System.arraycopy(thisMethods, 0, allMethods, parentMethods.length, thisMethods.length);
                    thisMethods = allMethods;
                }
    
                return thisMethods;
            }
        }
    
        protected void doOptions(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
          ........
        }
    
        protected void doTrace(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
            .......
        }
        //主要是这个方法，根据不同的请求类型路由到不同的方法中
        protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
            String method = req.getMethod();
            long lastModified;
            if (method.equals("GET")) {
                lastModified = this.getLastModified(req);
                if (lastModified == -1L) {
                    this.doGet(req, resp);
                } else {
                    long ifModifiedSince;
                    try {
                        ifModifiedSince = req.getDateHeader("If-Modified-Since");
                    } catch (IllegalArgumentException var9) {
                        ifModifiedSince = -1L;
                    }
    
                    if (ifModifiedSince < lastModified / 1000L * 1000L) {
                        this.maybeSetLastModified(resp, lastModified);
                        this.doGet(req, resp);
                    } else {
                        resp.setStatus(304);
                    }
                }
            } else if (method.equals("HEAD")) {
                lastModified = this.getLastModified(req);
                this.maybeSetLastModified(resp, lastModified);
                this.doHead(req, resp);
            } else if (method.equals("POST")) {
                this.doPost(req, resp);
            } else if (method.equals("PUT")) {
                this.doPut(req, resp);
            } else if (method.equals("DELETE")) {
                this.doDelete(req, resp);
            } else if (method.equals("OPTIONS")) {
                this.doOptions(req, resp);
            } else if (method.equals("TRACE")) {
                this.doTrace(req, resp);
            } else {
                String errMsg = lStrings.getString("http.method_not_implemented");
                Object[] errArgs = new Object[]{method};
                errMsg = MessageFormat.format(errMsg, errArgs);
                resp.sendError(501, errMsg);
            }
    
        }
    
        private void maybeSetLastModified(HttpServletResponse resp, long lastModified) {
           .....
        }

`    //转换ServletRequest和ServletReponse`

public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException {
            HttpServletRequest request;
            HttpServletResponse response;
            try {
                request = (HttpServletRequest)req;
                response = (HttpServletResponse)res;
            } catch (ClassCastException var6) {
                throw new ServletException("non-HTTP request or response");
            }
    
            this.service(request, response);
        }
    }

[70]: /images/20220521/dcdcba08c7ab44449c5be8e30efda92b.png