Why Servlet has init method?

Hello,
i am new bie to servlet. Can you tell me why servlet has init method to initialize?
Why do not we use constructor instead of init method to initialize? Does servlet allow constructor?
Why init method has SevletConfig as argument?
Please help me to get knowledge regarding this. Any URL for this(sun or anything), please send.
Thanks,
balachandar

http://www.unix.org.ua/orelly/java-ent/servlet/ch03_03.htm

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Why servlet has init() method insteadof constructor

Why servlet has init() method insteadof constructor

Depends completely on the servlet. If the servlet implements SingleThreadedModel, then multiple instances of the servlet will be created.
Reason: Most servlets work quite well with multiple threads, the bulk of the variables are declared locally within the service method. Sometimes, however, the servlet needs to be able to handle multiple threads even though it is not thread safe. In such as case the SingleThreadedModel is implemented to tell the container to use a new (or one from a pool) servlet. In this case the init method will be called each time.
[As an aside: there is of course one (not obscure, but not normal) scenario where you would call the constructor of a servlet directly - this would be where you are writing a container. 'Ah!' you say, 'I will leave that kind of madness to the boys and girls at Jakarta' - however, you may wish to write a container as a test frame for you servlet, which you should be doing.]

Why GenericServlet has init() with no arg method?

1)why GenericServlet has init() with no arg method also?.......when do we need this method, is there any such scenario?
2) why subclass of HttpServlet must override at least one DoXXX method and why we don't override service method? how does the server via "public service" method recognize to call the corresponding doXXX method like for get request call doGet() ?
Thanks in advance
Rahul

1) the javadocs explain exactly what the use of this method is. It's nothing more than a convenience method.
2) Technically you can override service() and ignore doGet() and doPost(). But that's just bad design, for all you know the service() method does more then just calling doGet() and doPost(). Do what the javadocs tell you, and they tell you to implement doGet() and/or doPost().
how does the server via "public service" method recognize to call the corresponding doXXX method like for get request call doGet()The answer is already in your question! The service() method checks the request method (get/post/put) and then calls the corresponding method in HttpServlet :s You can check this yourself, call HttpServletRequest.getMethod().

Why Servlets having Init() even though its having constructors

Hi friends , i ve one doubt,
in servlets all initilization part can be handled by Iinit() method,
my qn is each servlet is singleton oriented i.e, each has only one instance, it means only one time constructor will called, and init() also called only once, so instead of init() we can use constructor, in this case why sun people provide us init() method,
please give me soln.

-nevermind-
I've found something interesting in Google:
http://www.unix.org.ua/orelly/java-ent/servlet/ch03_03.htm
The init() method is typically used to perform servlet initialization--creating or loading objects that are used by the servlet in the handling of its requests. Why not use a constructor instead? Well, in JDK 1.0 (for which servlets were originally written), constructors for dynamically loaded Java classes (such as servlets) couldn't accept arguments. So, in order to provide a new servlet any information about itself and its environment, a server had to call a servlet's init() method and pass along an object that implements the ServletConfig interface. Also, Java doesn't allow interfaces to declare constructors. This means that the javax.servlet.Servlet interface cannot declare a constructor that accepts a ServletConfig parameter. It has to declare another method, like init(). It's still possible, of course, for you to define constructors for your servlets, but in the constructor you don't have access to the ServletConfig object or the ability to throw a ServletException.

The servlet weblogic.servlet.AsyncInitServlet init method failed

I installed WLS 9.2.2 recently with my app being on 8.1.5 for a long time.
I chose to create a new domain instead of upgrading my existing one.
I use the eclipse 3.2.2 with the WL plugin.
When I start the server and login to the console not using eclipse it works fine.
Trying to start the server and login to the console gives a internal server error.
Any idea what could be wrong? I bolded the parts where I have looked into.
Here is the console output:
<Nov 19, 2008 3:13:02 PM CST> <Notice> <WebLogicServer> <BEA-000395> <Following extensions directory contents added to the end of the classpath:
C:\bea92\weblogic92\platform\lib\p13n\p13n-schemas.jar;C:\bea92\weblogic92\platform\lib\p13n\p13n_common.jar;*C:\bea92\weblogic92\platform\lib\p13n\p13n_system.jar*;C:\bea92\weblogic92\platform\lib\wlp\netuix_common.jar;C:\bea92\weblogic92\platform\lib\wlp\netuix_schemas.jar;C:\bea92\weblogic92\platform\lib\wlp\netuix_system.jar;C:\bea92\weblogic92\platform\lib\wlp\wsrp-common.jar>
<Nov 19, 2008 3:13:03 PM CST> <Info> <WebLogicServer> <BEA-000377> <Starting WebLogic Server with Java HotSpot(TM) Client VM Version 1.5.0_10-b03 from Sun Microsystems Inc.>
<Nov 19, 2008 3:13:05 PM CST> <Info> <Management> <BEA-141107> <Version: WebLogic Server 9.2 MP2 Mon Jun 25 01:32:01 EDT 2007 952826 >
<Nov 19, 2008 3:13:10 PM CST> <Info> <WebLogicServer> <BEA-000215> <Loaded License : C:\bea92\license.bea>
<Nov 19, 2008 3:13:10 PM CST> <Notice> <WebLogicServer> <BEA-000365> <Server state changed to STARTING>
<Nov 19, 2008 3:13:10 PM CST> <Info> <WorkManager> <BEA-002900> <Initializing self-tuning thread pool>
<Nov 19, 2008 3:13:11 PM CST> <Notice> <Log Management> <BEA-170019> <The server log file C:\bea92\user_projects\domains\domain\servers\AdminServer\logs\AdminServer.log is opened. All server side log events will be written to this file.>
<Nov 19, 2008 3:13:15 PM CST> <Notice> <Security> <BEA-090082> <Security initializing using security realm myrealm.>
<Nov 19, 2008 3:13:22 PM CST> <Notice> <WebLogicServer> <BEA-000365> <Server state changed to STANDBY>
<Nov 19, 2008 3:13:22 PM CST> <Notice> <WebLogicServer> <BEA-000365> <Server state changed to STARTING>
<Nov 19, 2008 3:13:30 PM CST> <Error> <HTTP> <BEA-101359> <The servlet weblogic.servlet.AsyncInitServlet init method failed while it was run in the background. The exception was: java.lang.NoClassDefFoundError: com/bea/p13n/management/ApplicationFilePoller$Handler.
java.lang.NoClassDefFoundError: com/bea/p13n/management/ApplicationFilePoller$Handler
     at java.lang.ClassLoader.defineClass1(Native Method)
     at java.lang.ClassLoader.defineClass(ClassLoader.java:620)
     at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:124)
     at java.net.URLClassLoader.defineClass(URLClassLoader.java:260)
     at java.net.URLClassLoader.access$100(URLClassLoader.java:56)
     Truncated. see log file for complete stacktrace
>
<Nov 19, 2008 3:13:37 PM CST> <Notice> <Log Management> <BEA-170027> <The server initialized the domain log broadcaster successfully. Log messages will now be broadcasted to the domain log.>
<Nov 19, 2008 3:13:37 PM CST> <Notice> <WebLogicServer> <BEA-000365> <Server state changed to ADMIN>
<Nov 19, 2008 3:13:37 PM CST> <Notice> <WebLogicServer> <BEA-000365> <Server state changed to RESUMING>
<Nov 19, 2008 3:13:39 PM CST> <Notice> <Server> <BEA-002613> <Channel "Default" is now listening on 127.0.0.1:7005 for protocols iiop, t3, ldap, http.>
<Nov 19, 2008 3:13:39 PM CST> <Notice> <WebLogicServer> <BEA-000331> <Started WebLogic Admin Server "AdminServer" for domain "domain" running in Development Mode>
<Nov 19, 2008 3:13:40 PM CST> <Notice> <WebLogicServer> <BEA-000365> <Server state changed to RUNNING>
<Nov 19, 2008 3:13:40 PM CST> <Notice> <WebLogicServer> <BEA-000360> <Server started in RUNNING mode>
<Nov 19, 2008 3:22:04 PM CST> <Error> <HTTP> <BEA-101020> <[weblogic.servlet.internal.WebAppServletContext@d6c6a0 - appName: 'consoleapp', name: 'console', context-path: '/console'] Servlet failed with Exception
java.lang.NullPointerException
     at weblogic.servlet.AsyncInitServlet.service(AsyncInitServlet.java:124)
     at weblogic.servlet.internal.StubSecurityHelper$ServletServiceAction.run(StubSecurityHelper.java:227)
     at weblogic.servlet.internal.StubSecurityHelper.invokeServlet(StubSecurityHelper.java:125)
     at weblogic.servlet.internal.ServletStubImpl.execute(ServletStubImpl.java:283)
     at weblogic.servlet.internal.TailFilter.doFilter(TailFilter.java:26)
     Truncated. see log file for complete stacktrace
Edited by: user5384782 on Nov 19, 2008 1:42 PM
Edited by: user5384782 on Nov 19, 2008 3:59 PM

Thanks for your replies adrian and david.
The only thing I see different is that eclipse has different arguments when calling the jvm at the top.
startWebLogic.cmd:
C:\bea92\JDK150~1\bin\java -client -Xms256m -Xmx512m -XX:CompileThreshold=8000
-XX:PermSize=48m -XX:MaxPermSize=128m -Xverify:none -da -Dplatform.home=C:\b
ea92\WEBLOG~1 -Dwls.home=C:\bea92\WEBLOG~1\server -Dwli.home=C:\bea92\WEBLOG~1\i
ntegration -Dweblogic.management.discover=true -Dwlw.iterativeDev= -Dwlw.testC
onsole= -Dwlw.logErrorsToConsole= -Dweblogic.ext.dirs=C:\bea92\patch_weblogic922
\profiles\default\sysext_manifest_classpath -Dweblogic.Name=AdminServer -Djava.s
ecurity.policy=C:\bea92\WEBLOG~1\server\lib\weblogic.policy weblogic.Server
eclipse:
-hotspot -Xms32m -Xmx200m -Dweblogic.ProductionModeEnabled=
Eclipse may be calling the cmd I dont know.
I will try upgrading my eclipse like adrian said, but in the meantime I appreciate any further advice/tips.

How servlet's init method is overridden ?

Here is a sample code to override servlet's init method.
public class BookDBServlet ... {
        private BookstoreDB books;
        public void init(ServletConfig config) throws ServletException {
            // Store the ServletConfig object and log the initialization
            super.init(config);
            // Load the database to prepare for requests
            books = new BookstoreDB();
    }I dont get why super.init(config); is called.
Comment says +// Store the ServletConfig object and log the initialization+
what does "Store" mean here ? Where it is stored ?
how it is logged ( log4j / commons -logging ? ) and where (which file) it is logged ?
Can we remove this line without any damage ?

There are several calls you can make to the servlet object which are simply delegated to the ServletConfig object, most notably getServletContext(). The default action of the init(SerletConfig config) method is to store the config in a field which is defined in GenericServlet, so that these calls can work. If you don't do this then calling getServletContext() at some later time will give you a NullPointerException.
I think the reference to logging is that, if you use the built in servlet logging system, rather than log4j or java.util.loging then this depends on a valid servlet context, which in turn, depends on a valid servlet config reference.

Urgent....How can i redirect to my jsp page from servlet in init() method..

How can i redirect to my jsp page from servlet in init() method..Becoz that servlet is calling while server startsup..so im writing some piece of code in init() method..after that i want to redirect to some jsp page ...is it possible?
using RequestDispatcher..its not possible..becoz
RequestDispatcher rd = sc.getRequestDispatcher("goto.jsp");
rd.foward(req,res);
Here the request and response are null objects..
So mi question can frame as how can i get request/response in servlet's init method()..

Hi guys
did any one get a solution for this issue. calling a jsp in the startup of the servlet, i mean in the startup servlet. I do have a same req like i need to call a JSP which does some data reterival and calculations and i am putting the results in the cache. so in the jsp there in no output of HTML. when i use the URLConnection i am getting a error as below.
java.net.SocketException: Unexpected end of file from server
at sun.net.www.http.HttpClient.parseHTTPHeader(HttpClient.java:707)
at sun.net.www.http.HttpClient.parseHTTP(HttpClient.java:612)
at sun.net.www.http.HttpClient.parseHTTPHeader(HttpClient.java:705)
at sun.net.www.http.HttpClient.parseHTTP(HttpClient.java:612)
at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLCon
nection.java:519)
at com.toysrus.fns.alphablox.Startup.callJSP(Unknown Source)
at com.toysrus.fns.alphablox.Startup.init(Unknown Source)
at org.apache.tomcat.core.ServletWrapper.doInit(ServletWrapper.java:317)
so plz do let me know how to call a jsp in the start up of a servlet.
Thanks
Vidya

LDAP Context Initialization Problem in a Servlet's Init method

Hi
I have a servlet where I am using the InitialDirContext to Initialize context to a LDAP Server. I have created the context as a static variable. I am initializing the context in the init method and am using the same reference in the service method of the servlet. But if the context fails (due to naming exception and time out) in the init method all my requests in the service method will fail. Is there any way to validate the reference before using the same in the service method.
Also what is the best approach to initialize the context in a servlet. Whether to initialize in the service method or the init method.
I would appreciate if anyone can answer the above 2 queries.
Thanks in advace,
Ashish

If you want to use threadsafe objects in doPost/doGet there are two common ways to do this.
First, create the object in doPost/doGet. This may get expensive depending on the object, but it is 'easy' code and works.
The second option is to create a pool of objects in the servlet's init() method and to just have doPost/doGet get an object from the pool, use it and return it when done. This take a little more work to code, but the overhead of creating objects is almost eliminated. Look at jakarta/apache commons pool for a quick way to do this.
If you create on object in init() and use it in doPost/doGet, any time you get two request for the same servlet at the same time you will have synchronization issues that will usually cause problems. There is one instance of each Servlet, so all request for that servlet go through the single instance. Anything in doPost/doGet must be threadsafe.

Printing in servlet's init() method

i am trying to print out some information in init() method, but i can not create a 'response' object of ServletResponse to print with out.prinln() method for displaying these information in browser. please help me for working it out. i have given code below.
public void init(ServletConfig config) throws ServletException {
        //response.setContentType("text/html;charset=UTF-8");
        //PrintWriter out = res.getWriter();
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        while(parameters.hasMoreElements()) {
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            System.out.println("Parameter name: " + parameter);
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You don't 'create' a response object, the container does it and passes it to your code.
You don't service requests in the init method, it's for initialization. You only setup stuff that you want to use later there.
And in any case, do you even see 'response' declared anywhere?
Move your code to a doGet() or doPost() method.
People on the forum help others voluntarily, it's not their job.
Help them help you.
Learn how to ask questions first: http://www.catb.org/~esr/faqs/smart-questions.html
----------------------------------------------------------------

Why does SunOne 6.1 call servlet init() method twice on startup

Hi,
SunOne6.1 sp 5 is running my servlet's init() method twice when I put load-on-startup in the web-xml file. Is this a known problem and is there a work around as no amount of synchronising seems to work.
I'm running 2.3.dtd for the default_web.xml file and my entries look like this :-
<servlet>
<servlet-name>RelatisLogin</servlet-name>
<servlet-class>com.MyServlet</servlet-class>
<load-on-startup>7</load-on-startup>
</servlet>
This init method only contains a basic check at the moment :-
public void init()
System.out.println("init.....");
If I take <load-on-startup> away and run the server then call the servlet via the url it only runs init once.
I've tried synchronizing the block but this doesn't work.
The synchonization code I used looks like this:-
public void init(ServletConfig config) throws ServletException
synchronized( AnotherClass.staticString )
if ( AnotherClass.staticString.length() > 0 ) return;
System.out.println( AnotherClass.staticString.length() );
AnotherClass.staticString = "init has been called";
try
int i = 0;
i = i/i;
} catch(Exception e)
e.printStackTrace();
The declaration of "AnotherClass.staticString" is :-
public static String staticString = "";
The output of "System.out.println( AnotherClass.staticString.length() );" is 0 and displayed twice.
I deliberately cause a divide by zero error to get a stack trace so that i can determine what called init(), the stack trace is printed twice and is always :-
java.lang.ArithmeticException: / by zero
at com.MyServlet.init(MyServlet.java:47)
at org.apache.catalina.core.StandardWrapper.loadServlet(StandardWrapper.java:921)
at org.apache.catalina.core.StandardWrapper.load(StandardWrapper.java:813)
at org.apache.catalina.core.StandardContext.loadOnStartup(StandardContext.java:3478)
at org.apache.catalina.core.StandardContext.start(StandardContext.java:3760)
at com.iplanet.ias.web.WebModule.start(WebModule.java:251)
at org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1133)
at org.apache.catalina.core.StandardHost.start(StandardHost.java:652)
at org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1133)
at org.apache.catalina.core.StandardEngine.start(StandardEngine.java:355)
at org.apache.catalina.startup.Embedded.start(Embedded.java:995)
at com.iplanet.ias.web.WebContainer.start(WebContainer.java:431)
at com.iplanet.ias.web.WebContainer.startInstance(WebContainer.java:500)
at com.iplanet.ias.server.J2EERunner.confPostInit(J2EERunner.java:161)
It looks like SunOne starts then calls init() and then restarts and calls init() again.
The same code and same web.xml entries work fine in Tomcat 5, init() only every gets called once.
Any ideas? Can I post this as a bug?
Thanks in advance for any help.
Chris.

Sorry to have not replied to this thread earlier.....forgot all about it.
The reason init was being called twice by IPanet (Sun One) is IPanet comes with a default context called "Search" already in server.xml file.
This context was loading all the classes on my class path and this causing "init" to run. Then my context entry also loaded all the files in the class path.
Quite why Sun include this "Search" context is beyond me......annoying. If you take it out the the problem goes away.
Basically if you leave this in then your running to web servers (JVM's) your one and Sun's stupid "Search" one. As they are separate JVM's no amount of synchronisation would ever fix it.

Servlet init() method called multiple times

We have a Servlet-based application which does lengthy initialization.
We've noticed that when multiple requests arrive,
but the Servlet's init() method has not returned yet
for the first request, the application server calls init() multiple times.
You can even see this with the Fortune sample
Servlet. Starting the app server and sending 20 or 30
simultaneous requests causes multiple Fortune Servlet init messages to appear in the kjs log.
Why is this? We do not implement SingleThreadModel.
Doesn't the J2EE 1.2 spec say init will be called
only once?
How do we prevent it?
A Startup class which inits our
Servlet?
Or a synchronized block within our init method, with some checks of static member variables
so that the initialization work only happens once?
Or is there some kregedit magic or deployment descriptor change I could make?
Again, it's not a SingleThreadModel issue, so don't
point me there.
This is iAS 6.0 SP4, on Solaris, by the way.

Hi
Yes,you are right. In iAS6.0 SP4 When multiple requests arrive,and the Servlet's init() method has not returned yet for the first request, the application server calls init()multiple times. This problem is resolved in iAS 6.5.
There is no kregedit magic that will solve this problem.
Right now the workaround is any of the solution you proposed. Or use iAS6.5. I have tested it with fortune.
Please download it and check with your application.
you can find free download at http://developer.iplanet.com/appserver/testdrive/testdrive_65.html
Thanks

Init() method is executing twice

Hi,
I have a servlet & implemented the init() method. But, the init() method is executing twice.
Also, the destroy() method is executing twice.
Any input guys.
Thanks

JAXMServlet most certainly IS an HttpServlet - the javadocs that I see say it extends HttpServlet. It won't be very useful in a SOAP implementation if it's not.
I thought the SingleThreadModel interface. Why? I thought that was a bad practice that was discouraged. What writable data members does this "test" servlet have that require such careful treatment? If your servlet implements doGet and doPost methods, with only local variables, it'll be thread-safe enough.
I follow the Sun Java coding standards, even for dash-off classes. "test" should be capitalized. Is there no more descriptive name you can think of?
I'm not aware of a problem with Tomcat, and I haven't checked the bug list. But I'd put my money on your code being the problem here. Do you call super.init() in your servlet's init() method? Maybe you're seeing the superclass init() being called?
%

Empty init() methods

I am currently working on and cleaning up (old) Java servlet code that I inherited from another collegue. In a lot of the servlets, I find empty init() methods like:
public void init() {
}As a novice to Java servlets, I have just been reading the First Edition of Jason Hunter's 'Java Servlet Programming' book. There, on page 59 it is stated that every servlet's init() method should call super.init(config) as its first action. I was wondering if I could simply remove the empty init() methods that I encounter in the servlets... If i remove them, the init() method of the superclass will get called... and the behaviour of the superclass init() might be slightly different than an emtpy init() method... isn't it? But I wonder if it will make a big deal for my application...
Are there any possible side effects to expect here? What could probably happen if remove the empty init() methods and consequently start using the init() method from the superclass?

OK. Yesterday, I cleaned up all empty init() methods, now it's time for cleaning up all unnecessary init(ServletConfig cfg) methods. In the servlet code that i inherited from a collegue, I find a lot of servlets containing the following:
    public void init(ServletConfig cfg) throws ServletException {
        super.init(cfg);
    }In the meanwhile, i know that we are using Tomcat 5.5 as a container, so I looked into the Tomcat HttpServlet implementation and I found no init(ServletConfig cfg). I then looked at GenericServlet where i found:
    public void init(ServletConfig config) throws ServletException {
        this.config = config;
        this.init();
    }and since the init() method also doesn't do much, i think i can safely remove all
    public void init(ServletConfig cfg) throws ServletException {
        super.init(cfg);
    }methods in our own servlets... since what they do is actually just the same as the init(ServletConfig cfg) method from GenericServlet.
Can somebody who is more experienced in Java Servlets than me confirm?
Thanks!
Bart

Init() method: private vs. public

I have a question:
What is the difference in defining your init() method in a servlet as private vs. public since it is automatically executed when the class is instantiated?
thanks

cbreneman,
For future reference,
public void foo() {
    method vailable to all
protected void foo() {
    method vailable to pacakge
private void Foo() {
    method vailable to this
}You can do this:
public String bar;
protected String bar;
private String bar;You cannot do this:
private void Foo() {
    public String bar = "Oops!";
}For an Applet, init() is inherited when you extend the Applet class. It must be overidden if you have work to do before your subclass becomes active. It is defined by the superclass as public and void. If you were to create an init() method that was private and returned a String, you might (the compiler will stop you if you try to define init() twice) have a different method, but you would not have overidden public void init(). If you tried something like private void init() the compiler would stop you:
init() in FooBar cannot override init() in java.applet.Applet; attempting to assign weaker access privileges; was public
private void init() {
^
1 error
Make sense?
With servlets, the init() method is there for slightly different reasons and to be honest, I am not sure that you inherit it? I believe it acts as a special constructor? I don't write many servlets, so I'll shut up and let someone who does give a better answer.
Hope this helps.

Why Do We Have To Call super.init(config); in init() method of servlet?

Hi, everyone..
I wonder why we call super.init(config) in init method of servlet... If i dont call it ; when i try to get servletcontext in service method it throws java.lang.NullPointerException...when we call super.init() , what is happening behind the scene? If anybody has a technical explanation for my question , i will be very pleased...
THX FOR YOUR FUTURE REPLIES IN ADVANCE....

I am sorry about the uppercases and i dont want to seem smart on java forums... Anyway, m8 this is the thing that i know... i meant; for instance when we override doGet or doPost method ; we dont need to override init method; but the server loads the servlet and we can get the context of the servlet in these methods easily by calling getServletContext() method; however when we want to call service method implicitly by jndi, servlet needs to be loaded and init method must call its parent...(i also write down in web.xml <load-on-startup>.... for that servlet).
thx for your replies in advance....

Why Servlet has init method?

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