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();
    }
}