Java Development @ PostConstruct and @ PreConstruct annotations, postconstruct annotations

Java Development @ PostConstruct and @ PreConstruct annotations, postconstruct annotations

Starting from the Java EE5 specification, Servlet adds two annotations (Annotation) that affect the Servlet lifecycle: @ PostConstruct and @ PreConstruct. These two annotations are used to modify a non-static void () method, and this method cannot throw an exception declaration.

Usage, for example:

1 @ PostConstruct // Method 12 public void someMethod () {3... 4} 5 6 public @ PostConstruct void someMethod () {// Method 27... 8}

1. @ PostConstruct description

The method modified by @ PostConstruct will run when the server loads the Servlet and will only be called once by the server, similar to the Serclet inti () method. The method modified by @ PostConstruct runs before the init () method after the constructor.

2. @ PreConstruct description

The method modified by @ PreConstruct runs when the server detaches the Servlet and is called only once by the server, similar to the destroy () method of Servlet. The method modified by @ PreConstruct will run after the destroy () method before the Servlet is completely uninstalled. (For details, see the following program practices)

3. Program practice

Web. xml

1 <! -- @ PostConstruct and @ PreDestroy annotation --> 2 <servlet> 3 <servlet-name> AnnotationServlet </servlet-name> 4 <servlet-class> com. servlet. annotationServlet </servlet-class> 5 </servlet> 6 <servlet-mapping> 7 <servlet-name> AnnotationServlet </servlet-name> 8 <url-pattern>/servlet/ annotationServlet </url-pattern> 9 </servlet-mapping>

AnnotationServlet

1 package com. servlet; 2 3 import java. io. IOException; 4 import java. io. printWriter; 5 import java. SQL. time; 6 import java. text. simpleDateFormat; 7 import java. util. date; 8 9 import javax. annotation. postConstruct; 10 import javax. annotation. preDestroy; 11 import javax. servlet. servletException; 12 import javax. servlet. http. httpServlet; 13 import javax. servlet. http. httpServletRequest; 14 import javax. servlet. http. httpServletResponse; 15 16 public class AnnotationServlet extends HttpServlet {17 SimpleDateFormat df = new SimpleDateFormat ("HH: mm: ss. SSS "); // set the date format, accurate to 18 ms 19 public AnnotationServlet () {20 System. out. println ("time:" + df. format (new Date () + "execute the constructor... "); 21} 22 23 public void destroy () {24 this. log ("time:" + df. format (new Date () + "execute the destroy () method... "); 25 // super. destroy (); // Just puts "destroy" string in log26 // Put your code here27} 28 29 @ PostConstruct30 public void someMethod () {31 // this. log ("execute @ PostConstruct modified someMethod () method... "); // Note: 32 System errors will occur. out. println ("time:" + df. format (new Date () + "execute the someMethod () method modified by @ PostConstruct... "); 33} 34 35 @ PreDestroy36 public void otherMethod () {37 System. out. println ("time:" + df. format (new Date () + "execute the otherMethod () method modified by @ PreDestroy... "); 38} 39 40 public void doGet (HttpServletRequest request, HttpServletResponse response) 41 throws ServletException, IOException {42 this. log ("time:" + df. format (new Date () + "execute the doGet () method... "); 43} 44 45 public void init () throws ServletException {46 // Put your code here47 this. log ("time:" + df. format (new Date () + "execute the init () method... "); 48} 49 50 protected void service (HttpServletRequest request, HttpServletResponse response) 51 throws ServletException, IOException {52 this. log ("time:" + df. format (new Date () + "run the service () method... "); 53 super. service (request, response); 54} 55 56}

Running result:

4. Notes

Note how much will affect the server startup speed. When the server is started, it will traverse all the class files under the WEB-INF/classes of the Web application and all jar files under the WEB-INF/lib to check which classes use the annotation. If no annotation is used in the program, you can set the metadatacomplete attribute of <web-app> to true in web. xml to disable the routine check during server startup.

Servers that support annotation must support Servlet2.5 and later specifications, so Tomcat must be 6.0.X or later.