"Original" https://www.toutiao.com/i6594316694657696264/
Parsing the Java servlet work processAbout Servlet
Package Gacl.servlet.study;import Java.io.ioexception;import Java.io.printwriter;import Javax.servlet.servletexception;import Javax.servlet.http.httpservlet;import Javax.servlet.http.httpservletrequest;import Javax.servlet.http.HttpServletResponse; public class ServletDemo1 extends HttpServlet {/** * The Doget method of the Servlets. <br> * This method is Calle D when the a form has its tag value method equals to get. * * @param request the request send by the client to the server * @param response the response send by the server to the C Lient * @throws servletexception If an error occurred * @throws IOException If an error occurred */public void doget (Http ServletRequest request, HttpServletResponse response) throws Servletexception, IOException {response.setcontenttype (" Text/html "); PrintWriter out = Response.getwriter (); Out.println ("<! DOCTYPE HTML PUBLIC "-//w3c//dtd HTML 4.01 transitional//en" > "); Out.println ("<HTML>"); Out.println ("
Servlet development attention to detail
<servlet>
<servlet-name>ServletDemo1</servlet-name>
<servlet-class>gacl.servlet.study.ServletDemo1</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>ServletDemo1</servlet-name>
<url-pattern>/*</url-pattern>
http://localhost:8080/Project Name/NVLKFMNDLMD
For some of the following mapping relationships:
Servlet1 Mapping to/abc/
Servlet2 Map to/
Servlet3 Mapping to/ABC
Servlet4 map to. Do
Problem:
When the request URL is "/abc/a.html", "/abc/" and "/" are matched, which Servlet responds
The servlet engine calls Servlet1.
When the request URL is "/ABC", both "/abc/" and "/ABC" match, which Servlet responds
The servlet engine calls Servlet3.
When the request URL is "/abc/a.do", both "/abc/" and ". Do" match, which Servlet responds
The servlet engine calls Servlet1.
When the request URL is "/a.do", the "/" and ". Do" matches, which Servlet responds
The servlet engine calls Servlet2.
When the request URL is "/xxx/yyy/a.do", the "/" and ". Do" matches, which Servlet responds
The servlet engine calls Servlet2.
The principle of matching is, "who looks more like who's looking?"
<servlet><servlet-name>invoker</servlet-name><servlet-class> Org.apache.catalina.servlets.invokerservlet</servlet-class><load-on-startup>1</load-on-startup ></servlet>
<!--Configure ServletDemo2 as the default servlet-->8 <servlet-mapping>9 <servlet-name>servletdemo2</ Servlet-name>10 <url-pattern>/</url-pattern>11 </servlet-mapping>
public void doget (HttpServletRequest request,httpservletresponse response) throws Servletexception, IOException {/*** Is there a thread-safety issue when multithreading concurrently accesses the code inside this method? * I variables are accessed concurrently by multiple threads, but there is no thread safety problem, because I is a local variable inside the Doget method, * when there are multiple threads accessing the Doget method concurrently, each thread has its own I variable, * each thread operates its own I variable, so there is no thread safety issue * Multithreading concurrent access to a method, if some resources (variables, collections, etc.) are defined within the method * then each thread has these things, so there is no thread safety issue */int I=1;i++;response.getwriter (). write (i); public void DoPost (HttpServletRequest request, httpservletresponse response) throws Servletexception, IOException { Doget (request, response);} By defining I as a global variable, there is a thread-safety problem when multiple threads concurrently access the variable i. Thread-safety problems exist only when multiple threads concurrently operate the same resource, so when writing a servlet, there is a thread-safety problem when accessing a resource (variables, collections, etc.) concurrently. So how do we solve this problem? First look at the following code: public class ServletDemo3 extends HttpServlet {int i=1;public void doget (HttpServletRequest request, HttpServletResponse response) throws Servletexception, IOException {/*** added synchronized, there is no thread safety problem when accessing I concurrently, * Why is there no thread safety problem after adding synchronized? * If there is a thread accessing the Servlet object now, it will first get the lock of the Servlet object * Wait until it finishes executing before returning the lock back to the Servlet object, because it first got the lock of the Servlet object, * So when there are other threads accessing this Servlet object,Since the lock has been taken away by the previous thread, the thread behind it can only wait for **/synchronized (this) {///in Java, each object has a lock, this is the Servlet object I++;try {thread.sleep ( 1000*4);} catch (Interruptedexception e) {e.printstacktrace ();} Response.getwriter (). Write (i+ "");}}5. This is now done by adding a lock to the Servlet object to ensure that only one thread accesses the resources in the Servlet object at any time, so there is no thread-safety issue.
This approach solves the thread-safety problem, but writing a servlet is no way to deal with thread safety, and if 9,999 people visit the servlet at the same time, then the 9,999 must queue in turn.
For servlet thread-safety issues, Sun is offering a solution: let the servlet implement a Singlethreadmodel interface, if a servlet implements the Singlethreadmodel interface, The servlet engine will then invoke its service method in single-threaded mode.
Looking at the API of Sevlet, you can see that there are no methods and constants defined in the Singlethreadmodel interface, in Java, an interface that does not define any methods and constants is called a labeled interface, and one of the most typical markup interfaces that is often seen is "Serializable" , this interface also does not define any methods and constants, what is the use of the markup interface in Java? The main function is to give an object a flag, tell the JVM, what this object can do, such as the implementation of the "Serializable" Interface class object can be serialized, there is a "cloneable" interface, This is also a markup interface, by default, the object in Java is not allowed to be cloned, like real life people, not allow cloning, but as long as the implementation of the "Cloneable" interface, then the object can be cloned.
Let the servlet implement the Singlethreadmodel interface, as long as the definition of the servlet class is added to implement the Singlethreadmodel interface declaration.
The Servlet,servlet engine, which implements the Singlethreadmodel interface, still supports multithreaded concurrent access to the servlet by producing multiple servlet instance objects. Each concurrent thread invokes a separate Servlet instance object.
Implementing the Singlethreadmodel interface does not really address the thread safety of the servlet, because the servlet engine creates multiple servlet instance objects. The real problem with multithreading security is that a Servlet instance object is called simultaneously by multiple threads. In fact, in servlet API 2.4, Singlethreadmodel has been marked as deprecated (obsolete).
"Go" Java Learning---Parsing the Java servlet work process
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