循循渐进的GenericServlet

在做web开发时，创建自己的servlet时，通常需要实现javax.servlet.Servlet接口，并覆写Servlet所有的方法，例如：

import java.io.IOException;

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

public class DemoServlet implements Servlet{

public void init(ServletConfig config) throws ServletException {
// TODO Auto-generated method stub

}

public ServletConfig getServletConfig() {
// TODO Auto-generated method stub
return null;
}

public void service(ServletRequest req, ServletResponse res)
throws ServletException, IOException {
// TODO Auto-generated method stub

}

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

public void destroy() {
// TODO Auto-generated method stub

}
}

但是，如果需要自定义很多个servlet，而每个servlet都必须覆写javax.servlet.Servlet的五个方法，况且有些方法只是简单的覆盖了一下，根本没用实际用到，这样在开发servlet时就显得很繁琐，是不是可以简化开发呢？于是想到，定义一个GenericServlet类，这个类实现javax.servlet.Servlet接口，而自定义的servlet自需要继承抽象类即可。由于每个自定义的servlet需要处理service(ServletRequest
, ServletResponse)的逻辑不一样，所以就让GenericServlet为抽象类，把service(ServletRequest , ServletResponse)抽象出来。如下：

import java.io.IOException;

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

public abstract class GenericServlet implements Servlet {

public void init(ServletConfig config) throws ServletException {
// TODO Auto-generated method stub

}

public ServletConfig getServletConfig() {
// TODO Auto-generated method stub
return null;
}

public abstract void service(ServletRequest req, ServletResponse res)
throws ServletException, IOException;

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

public void destroy() {
// TODO Auto-generated method stub

}

}

而对于刚才DemoServlet，便可以如下定义：

import java.io.IOException;

import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

public class DemoServlet extends GenericServlet {

@Override
public void service(ServletRequest req, ServletResponse res)
throws ServletException, IOException {
// TODO Auto-generated method stub
System.out.println("DemoServlet 的 service() 逻辑处理。");
}

}

但是可能自定的DemoServlet在实例化后需要额外做一些自己的init。于是需要这样做：

import java.io.IOException;

import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

public class DemoServlet extends GenericServlet {

@Override
public void init(ServletConfig config) throws ServletException {
// TODO Auto-generated method stub
System.out.println("DemoServlet 的 init() 逻辑处理。");
}

@Override
public void service(ServletRequest req, ServletResponse res)
throws ServletException, IOException {
// TODO Auto-generated method stub
System.out.println("DemoServlet 的 service() 逻辑处理。");
}

}

现在又有新的功能，需要DemoServlet能获取ServletConfig的参数信息，类似javax.servlet. ServletConfig接口的getServletName()、getServletContext()等方法,于是采用这样的方式：

import java.io.IOException;
import java.util.Enumeration;

import javax.servlet.ServletConfig;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

public class DemoServlet extends GenericServlet {
private ServletConfig config;

@Override
public void init(ServletConfig config) throws ServletException {
// TODO Auto-generated method stub
this.config = config;
System.out.println("DemoServlet 的 init() 逻辑处理。");
}

@Override
public void service(ServletRequest req, ServletResponse res)
throws ServletException, IOException {
// TODO Auto-generated method stub
System.out.println("DemoServlet 的 service() 逻辑处理。");
}

public String getInitParameter(String name) {
return config.getInitParameter(name);
}

public Enumeration getInitParameterNames() {
return config.getInitParameterNames();
}

public ServletContext getServletContext() {
return config.getServletContext();
}

public String getServletName() {
return config.getServletName();
}
}

更显然一点：

public class DemoServlet extends GenericServlet implements ServletConfig{

就是覆写了ServletConfig的方法，那么如果自定义很多servlet，开发又显得复杂起来了，于是想到让GenericServlet也实现ServletConfig接口，与是就有了如下的改进：

import java.io.IOException;
import java.util.Enumeration;

import javax.servlet.Servlet;
import javax.servlet.ServletConfig;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

public abstract class GenericServlet implements Servlet, ServletConfig {

private ServletConfig config;

public void init(ServletConfig config) throws ServletException {
// TODO Auto-generated method stub
this.config = config;
}

public ServletConfig getServletConfig() {
// TODO Auto-generated method stub
return this.config;
}

public abstract void service(ServletRequest req, ServletResponse res)
throws ServletException, IOException;

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

public void destroy() {
// TODO Auto-generated method stub

}

public String getServletName() {
// TODO Auto-generated method stub
return getServletConfig().getServletName();
}

public ServletContext getServletContext() {
// TODO Auto-generated method stub
return getServletConfig().getServletContext();
}

public String getInitParameter(String name) {
// TODO Auto-generated method stub
return getServletConfig().getInitParameter(name);
}

public Enumeration getInitParameterNames() {
// TODO Auto-generated method stub
return getServletConfig().getInitParameterNames();
}

}

这样，就ok了。但是作为GenericServlet的子类DemoServlet，如果要覆写init(ServletConfig config)，必须要这样做：

@Override
public void init(ServletConfig config) throws ServletException {
// TODO Auto-generated method stub
super.init(config);
System.out.println("DemoServlet 的 init() 逻辑处理。");
}

不然，GenericServlet就无法缓存config，就会出NullPointerException。这样，又会让编程人员在编码时需要额外注意这一个坑，有木有好的解决方法呢？答案是肯定的。

针对GenericServlet采取这样的方式：

public void init(ServletConfig config) throws ServletException {
// TODO Auto-generated method stub
this.config = config;
init();
}

public void init(){

}

如果子类需要做自己的init，只需要覆写init()就可以:

@Override
public void init() {
// TODO Auto-generated method stub
System.out.println("DemoServlet 的 init() 逻辑处理。");
}

只是一个值得学习的设计思路。

下面附上javax.servlet.GenericServlet的源码，以便好好琢磨琢磨：

package javax.servlet;

import java.io.IOException;
import java.util.Enumeration;

public abstract class GenericServlet implements Servlet, ServletConfig, java.io.Serializable {

private transient ServletConfig config;

public GenericServlet() { }

public void destroy() {
}

public String getInitParameter(String name) {
return getServletConfig().getInitParameter(name);
}

public Enumeration getInitParameterNames() {
return getServletConfig().getInitParameterNames();
}

public ServletConfig getServletConfig() {
return config;
}

public ServletContext getServletContext() {
return 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 msg) {
getServletContext().log(getServletName() + ": "+ msg);
}

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

public abstract void service(ServletRequest req, ServletResponse res)
throws ServletException, IOException;

public String getServletName() {
return config.getServletName();
}
}