Java web基础知识

why8112023-09-24 17:27

Servlet

Servlet是sun公司开发的动态web技术
sun在API中提供了一个接口叫做 Servlet ，一个简单的Servlet 程序只需要完成两个步骤

  • 编写一个实现了Servlet接口的类

  • 把这个Java部署到web服务器中

一般来说把实现了Servlet接口的java程序叫做，Servlet

初步认识Servlet

Servlet接口sun公司有两个默认的实现抽象类：HttpServlet，GenericServlet

看看Serlvet接口是怎么在这两个类中实现的

Servlet接口

package javax.servlet;

import java.io.IOException;

public interface Servlet {
    void init(ServletConfig var1) throws ServletException;

    ServletConfig getServletConfig();

    void service(ServletRequest var1, ServletResponse var2) throws ServletException, IOException;

    String getServletInfo();

    void destroy();
}

这个接口里有五个抽象方法

  • init(ServletConfig var1)
  • getServletConfig()
  • service(ServletRequest var1, ServletResponse var2)
  • getServletInfo()
  • destroy()

这五个方法建立起了Servlet的基本框架：初始化、获取配置、服务功能、获取状态信息、销毁

也体现了servlet的生命周期

  • Servlet 初始化后调用 init () 方法。
  • Servlet 调用 service() 方法来处理客户端的请求。
  • Servlet 销毁前调用 destroy() 方法。
  • 最后，Servlet 是由 JVM 的垃圾回收器进行垃圾回收的。

其中比较值得观察的是service方法

GenericServlet

java 复制代码 
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();
    }

    public String getServletInfo() {
        return "";
    }

    public void init(ServletConfig config) throws ServletException {
        this.config = config;
        this.init();
    }

    public void init() throws ServletException {
    }

    public void log(String message) {
        this.getServletContext().log(this.getServletName() + ": " + message);
    }

    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();
    }
}

还是那五个抽象方法

  • init(ServletConfig var1)
  • getServletConfig()
  • service(ServletRequest var1, ServletResponse var2)
  • getServletInfo()
  • destroy()

可以看到对于service方法这个类并没有做任何的改变，只是对初始化和状态信息获取做出了改变

HttpServlet

java 复制代码 
package javax.servlet.http;

import java.io.IOException;
import java.lang.reflect.Method;
import java.text.MessageFormat;
import java.util.Enumeration;
import java.util.ResourceBundle;
import javax.servlet.GenericServlet;
import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

public abstract class HttpServlet extends GenericServlet {
    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 ResourceBundle lStrings = ResourceBundle.getBundle("javax.servlet.http.LocalStrings");

    public HttpServlet() {
    }

    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String protocol = req.getProtocol();
        String msg = lStrings.getString("http.method_get_not_supported");
        if (protocol.endsWith("1.1")) {
            resp.sendError(405, msg);
        } else {
            resp.sendError(400, msg);
        }

    }

    protected long getLastModified(HttpServletRequest req) {
        return -1L;
    }

    protected void doHead(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        NoBodyResponse response = new NoBodyResponse(resp);
        this.doGet(req, response);
        response.setContentLength();
    }

    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 {
        String protocol = req.getProtocol();
        String msg = lStrings.getString("http.method_put_not_supported");
        if (protocol.endsWith("1.1")) {
            resp.sendError(405, msg);
        } else {
            resp.sendError(400, msg);
        }

    }

    protected void doDelete(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String protocol = req.getProtocol();
        String msg = lStrings.getString("http.method_delete_not_supported");
        if (protocol.endsWith("1.1")) {
            resp.sendError(405, msg);
        } else {
            resp.sendError(400, msg);
        }

    }

    private Method[] getAllDeclaredMethods(Class<? extends HttpServlet> c) {
        Class<?> clazz = c;

        Method[] allMethods;
        for(allMethods = null; !clazz.equals(HttpServlet.class); clazz = clazz.getSuperclass()) {
            Method[] thisMethods = clazz.getDeclaredMethods();
            if (allMethods != null && allMethods.length > 0) {
                Method[] subClassMethods = allMethods;
                allMethods = new Method[thisMethods.length + allMethods.length];
                System.arraycopy(thisMethods, 0, allMethods, 0, thisMethods.length);
                System.arraycopy(subClassMethods, 0, allMethods, thisMethods.length, subClassMethods.length);
            } else {
                allMethods = thisMethods;
            }
        }

        return allMethods != null ? allMethods : new Method[0];
    }

    protected void doOptions(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        Method[] methods = this.getAllDeclaredMethods(this.getClass());
        boolean ALLOW_GET = false;
        boolean ALLOW_HEAD = false;
        boolean ALLOW_POST = false;
        boolean ALLOW_PUT = false;
        boolean ALLOW_DELETE = false;
        boolean ALLOW_TRACE = true;
        boolean ALLOW_OPTIONS = true;

        for(int i = 0; i < methods.length; ++i) {
            String methodName = methods[i].getName();
            if (methodName.equals("doGet")) {
                ALLOW_GET = true;
                ALLOW_HEAD = true;
            } else if (methodName.equals("doPost")) {
                ALLOW_POST = true;
            } else if (methodName.equals("doPut")) {
                ALLOW_PUT = true;
            } else if (methodName.equals("doDelete")) {
                ALLOW_DELETE = true;
            }
        }

        StringBuilder allow = new StringBuilder();
        if (ALLOW_GET) {
            allow.append("GET");
        }

        if (ALLOW_HEAD) {
            if (allow.length() > 0) {
                allow.append(", ");
            }

            allow.append("HEAD");
        }

        if (ALLOW_POST) {
            if (allow.length() > 0) {
                allow.append(", ");
            }

            allow.append("POST");
        }

        if (ALLOW_PUT) {
            if (allow.length() > 0) {
                allow.append(", ");
            }

            allow.append("PUT");
        }

        if (ALLOW_DELETE) {
            if (allow.length() > 0) {
                allow.append(", ");
            }

            allow.append("DELETE");
        }

        if (ALLOW_TRACE) {
            if (allow.length() > 0) {
                allow.append(", ");
            }

            allow.append("TRACE");
        }

        if (ALLOW_OPTIONS) {
            if (allow.length() > 0) {
                allow.append(", ");
            }

            allow.append("OPTIONS");
        }

        resp.setHeader("Allow", allow.toString());
    }

    protected void doTrace(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String CRLF = "\r\n";
        StringBuilder buffer = (new StringBuilder("TRACE ")).append(req.getRequestURI()).append(" ").append(req.getProtocol());
        Enumeration reqHeaderEnum = req.getHeaderNames();

        while(reqHeaderEnum.hasMoreElements()) {
            String headerName = (String)reqHeaderEnum.nextElement();
            buffer.append(CRLF).append(headerName).append(": ").append(req.getHeader(headerName));
        }

        buffer.append(CRLF);
        int responseLength = buffer.length();
        resp.setContentType("message/http");
        resp.setContentLength(responseLength);
        ServletOutputStream out = resp.getOutputStream();
        out.print(buffer.toString());
    }

    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 = req.getDateHeader("If-Modified-Since");
                if (ifModifiedSince < lastModified) {
                    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) {
        if (!resp.containsHeader("Last-Modified")) {
            if (lastModified >= 0L) {
                resp.setDateHeader("Last-Modified", lastModified);
            }

        }
    }

    public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException {
        if (req instanceof HttpServletRequest && res instanceof HttpServletResponse) {
            HttpServletRequest request = (HttpServletRequest)req;
            HttpServletResponse response = (HttpServletResponse)res;
            this.service(request, response);
        } else {
            throw new ServletException("non-HTTP request or response");
        }
    }
}

可以看到这层的类实现了service方法的处理，每次服务器接收到一个 Servlet 请求时，服务器会产生一个新的线程并调用服务。service() 方法检查 HTTP 请求类型（GET、POST、PUT、DELETE 等），并在适当的时候调用 doGet、doPost、doPut，doDelete 等方法。

但是它默认处理并不是我们所需要的，所以我们自己写的java类就是要重写这些方法。

现在，这个简单的Servlet基础使用链就明了了

RMI

RMI (Java Remote Method Invocation) Java 远程方法调用，是一种允许一个 JVM 上的 object 调用另一个 JVM 上 object 方法的机制

⼀个RMI Server分为三部分：

  • ⼀个继承了 java.rmi.Remote 的接⼝，其中定义我们要远程调⽤的函数
  • ⼀个实现了此接⼝的类
  • ⼀个主类，⽤来创建Registry，并将上⾯的类实例化后绑定到⼀个地址

来分析一下这个RMI设计的意图

一个RMI服务器的三个部分：一个接口、两个类，我理解的它们的作用如下：

  • 接口：作为远程方法调用的调用目标
  • 实现接口的类：用来承载接口方法

为什么要有这样一个类？
有类就可以序列化，序列化之后就可以把一个难以传输的目标变成一个便于传输的流

  • 一个主类：作为Registry存在。

打个比方，它就像一个车站的售票处一样，你来坐车你就要先去它哪里找他买票，你有了对应的票才能找到、乘坐对应的车。
在RMI整体流程它也差不多扮演这个角色，客户端去向Registry请求某个方法，Registry返回一个stub给客户端，客户端在通过stub种的信息找到对应的地址端口建立TCP连接

RMI简单流程

一个简单的demo

服务端

java 复制代码 
package rmilearn;

import java.net.MalformedURLException;
import java.rmi.Naming;
import java.rmi.RemoteException;
import java.rmi.registry.LocateRegistry;
import java.rmi.server.UnicastRemoteObject;

public class Learn01 {
    public static void main(String[] args) throws RemoteException, MalformedURLException {
        HelloRmi rmi = new HelloRmi();
        LocateRegistry.createRegistry(1099);
        Naming.rebind("rmi://127.0.0.1:1099/hello",rmi);
    }
}

class HelloRmi extends  UnicastRemoteObject implements Hello {
    protected HelloRmi() throws RemoteException {
        super();
    }

    @Override
    public String HelloRm() throws RemoteException {
       return "hello rmi";
    }
}
--------------------------------------------------------------------------
package rmilearn;

import java.rmi.Remote;
import java.rmi.RemoteException;

public interface Hello extends Remote {
    String HelloRm() throws RemoteException;
}

客户端

java 复制代码 
public class client {

    public static void main(String[] args) throws RemoteException, NotBoundException {
      Registry re = LocateRegistry.getRegistry("localhost",1099);
      Hello h = (Hello) re.lookup("hello");
      String s = h.HelloRm();
      System.out.println(s);
    }
}
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