Three implementation methods of Servlet

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Preface

Servlet yes JavaWeb Of One of the three components , It's a dynamic resource .Servlet The role of is to process requests , The server will deliver the received request to Servlet To deal with it , stay Servlet We usually need ：

• Receive request data ;

• Processing requests ;

• Complete the response .

For example, the client issues a login request , Or output registration request , These requests should be made by Servlet To complete the process ！Servlet We need to write it ourselves , Every Servlet Must be realized javax.servlet.Servlet Interface .

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Tips ： The following is the main body of this article , The following cases can be used for reference

One 、Servlet Life cycle

Servlet The default creation time : Create on the first request For the first time
init(): Execute on server startup
service ： The service method , The server will execute when the update is turned on .
destroy： The method of destruction , Only execute once when the server is shut down normally .
getServletConfig(): obtain servlet Configuration information .
getServletInfo()： obtain servlet Some basic information of , For example, author , Copyright and so on

• Be careful ： There is only one in memory servlet object , But it can be referenced by multiple classes , There are security issues , So try not to define member variables in its methods .

Two 、 Realization Servlet Three ways ？

Realization Servlet There are three ways ：

• Realization javax.servlet.Servlet Interface ;

• Inherit javax.servlet.GenericServlet class ;

• Inherit javax.servlet.http.HttpServlet class ;

Usually we will inherit HttpServlet Class to complete our Servlet, But learning Servlet Also from the javax.servlet.Servlet The interface begins to learn .

public interface Servlet {
     
    public void  init(ServletConfig config)  throws  ServletException; 
    public ServletConfig getServletConfig();       
    public void  service(ServletRequest req, ServletResponse res) throws      ServletException, IOException;      
    public  String getServletInfo();       
    public  void   destroy();
}

3、 ... and 、GenericServlet

summary :GenericServlet yes Servlet Implementation class of interface , We can do it through inheritance GenericServlet To write your own Servlet. Here is GenericServlet Source code of class ：

public abstract class GenericServlet implements Servlet, ServletConfig,    java.io.Serializable {
           
    private  static  final long serialVersionUID = 1L;       
    private  transient ServletConfig config;       
    public   GenericServlet() {
    } 
    @Override
    public void destroy() {
    }  
    @Override       
    public String getInitParameter(String name) {
             
        return getServletConfig().getInitParameter(name);  
    }  
    @Override       
    public Enumeration<String> getInitParameterNames() {
             
        return getServletConfig().getInitParameterNames();  
    }  
    @Override       
    public ServletConfig getServletConfig() {
             
        return config;  
    }  
    @Override       
    public ServletContext getServletContext() {
             
        return getServletConfig().getServletContext();  
    }
    @Override       
    public String getServletInfo() {
             
        return      "";  
    }  
    @Override       
    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);  
    }  
    @Override       
    public abstract void service(ServletRequest req, ServletResponse res) throws ServletException, IOException; 
    @Override       
    public String getServletName() {
             
        return config.getServletName();  
    }
}

• Realized Servlet Of init(ServletConfig) Method , Put parameters config Assigned to members of this class config, Then call the class's own nonparametric init() Method .

• init() The method is GenericServlet Own way , Not from Servlet Inherited from . When we customize Servlet when , If you want to finish the initialization, don't repeat init(ServletConfig) The method , Instead, we should rewrite init() Method . Because in GenericServlet Medium init(ServletConfig) Method ServletConfig object , If you overwrite save ServletConfig Code for , Then you can't use ServletConfig 了 .

• stay GenericServlet in , Defined a ServletConfig config Instance variables , And in init(ServletConfig) Method ServletConfig Assigned to the instance variable . Then instance variables are used in many methods of this class config.

3.1GenericServlet Medium init()

• If subclasses cover GenericServlet Of init(StringConfig) Method , that this.config=config This statement will be overwritten , in other words GenericServlet Instance variable of config The value of is null, Then all dependencies config Can't use any of the methods . If you really want to complete some initialization operations , Then cover it GenericServlet Provided init() Method , It has no parameters init() Method , It will be init(ServletConfig) Method is called .

3.2 Realized ServletConfig Interface

• GenericServlet It's also achieved ServletConfig Interface , So you can call getInitParameter()、getServletContext() etc. ServletConfig Methods .

2.3.3 Realized ServletConfig Interface

Four 、HttpServlet

summary :HttpServlet Class is GenericServlet Subclasses of , It provides the right HTTP Special support requested , So usually we will inherit HttpServlet To complete the custom Servlet.

4.1

HttpServlet Class provides service(HttpServletRequest,HttpServletResponse) Method , The method is HttpServlet Own way , Not from Servlet inherited . stay HttpServlet Of service(ServletRequest,ServletResponse) The method will put ServletRequest and ServletResponse Strong conversion HttpServletRequest and HttpServletResponse, And then call service(HttpServletRequest,HttpServletResponse) Method , This shows that subclasses can cover service(HttpServletRequest,HttpServletResponse) The method can , There is no need to force the request and response objects .

​ In fact, subclasses do not need to cover service(HttpServletRequest,HttpServletResponse) Method , because HttpServlet There is another step to simplify the operation , We'll talk about that .

4.2 Implementation code

public abstract class HttpServlet extends GenericServlet {
           
    protected void service(HttpServletRequest req, HttpServletResponse resp)       throws ServletException, IOException {
     
        //... Code ellipsis 
    }  
    @Override       
    public void service(ServletRequest req, ServletResponse res) throws      ServletException, IOException {
         
        HttpServletRequest  request;    
        HttpServletResponse response;          
        try{
    
            // Strong go 
            request = (HttpServletRequest) req;      
            response = (HttpServletResponse) res;    
        }catch(ClassCastException e) {
               
            throw new ServletException("non-HTTP request or response");    
        }
        // call service(HttpServletRequest,HttpServletResponse) Method 
        service(request, response);
    }
    // Other methods omitted ……
}

4.3 doGet() and doPost()

stay HttpServlet Of service(HttpServletRequest,HttpServletResponse) Method will judge whether the current request is valid GET still POST, If it is GET request , Then I will call this class doGet() Method , If it is POST The request will be called doPost() Method , This means that we can cover doGet() or doPost() The method can .

public class AServlet extends  HttpServlet {
    	     
    public void doGet(HttpServletRequest request, HttpServletResponse response)		throws ServletException, IOException {
    	
        System.out.println("hello doGet()...");	
    }
}

 public class BServlet extends HttpServlet {
    	     
     public void doPost(HttpServletRequest request, HttpServletResponse response)	throws ServletException, IOException {
    	
         System.out.println("hello doPost()...");	
     }
 }