Servlet技术（上）
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Servlet API 文档地址
Servlet API主要由两个Java包组成：javax.servlet和javax.servlet.http
其中javax.servlet包中定义了Servlet接口
servlet接口
GenericServlet抽象类
HttpServlet抽象类
ServletRequest接口
HttpServletRequest接口
ServletResponse接口
HttpServletResponse接口
ServletConfig接口
ServletContext接口
Servlet接口
供Servlet容器调用的方法:
init(ServletConfig config) ：初始化Servlet对象，容器创建好Servlet对象后，就调用该方法
service(ServletRequest req,ServletResponse res) ：负责响应客户的请求，为客户提供相应的服务。容器接收到客户端要访问的特定Servlet对象的请求时，就会调用改方法
destory() ：负责释放Servlet对象占用的资源，当Servlet对象结束生命周期时，容器会调用此方法
供Java Web应用中的程序代码访问的方法：
getServletConfig() : 返回一个ServletConfig对象，改对象包含了Servlet的初始化参数信息
getServletInfo() : 返回一个字符串，该字符串中包含了Servlet的创建者、版本和版权等信息
//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//
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();
}
GenericServlet抽象类 （运用了装饰设计模式）
该类提供了Servlet接口的通用实现，还实现了ServletConfig接口和Serializable接口，并且该抽象类有一个ServletConfig类型的私有实例变量config,从而使得GenericServlet抽象类与ServletConfig对象关联
GenericServlet类没有实现service（）方法，service（）方法是该类中唯一的抽象方法，GenericServlet类的子类必须实现该方法。
GenericSeervlet类实现类ServletConfig接口中的所有方法，因此GenericServlet及其子类可以直接调用在ServletConfig接口中定义的getServletContext()、getInitParameter()、getInitParameterNames()等方法
//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//
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();
}
}
HttpServlet抽象类
该类是GenericServlet抽象类的子类，该类为Servlet接口提供了与HTTP协议相关的通用实现，因此在开发基于HTTP协议的Java Web应用的时候，自定义的Servlet类一般都继承该类而不是GenericServlet类
HttpServlet类实现类service()方法，在该service(ServletRequest req,ServletResponse resp)方法中，将参数ServletRequest和ServletResponse转换成HttpServletRequset和HttpRequestResponse, 然后调用重载的service(HttpServletRequest req,HttpServletResponse resp)方法。
在service(HttpServletRequest req,HttpServletResponse resp)方法中，根据req获取客户端的请求方法（GET、POST、PUT、DELETE等），对于不同的请求方法，调用相应的doGet、doPost等方法，req和resp两个参数也会传给doGet或者doPost方法。
//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//
package javax.servlet.http;
import java.io.IOException;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.text.MessageFormat;
import java.util.Enumeration;
import java.util.ResourceBundle;
import javax.servlet.DispatcherType;
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 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 {
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) {