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request.setAttribute("name",name); is not working when project is run in 2nd time [duplicate]

Suppose, I have a webserver which holds numerous servlets. For information passing among those servlets I am setting session and instance variables.
Now, if 2 or more users send request to this server then what happens to the session variables?
Will they all be common for all the users or they will be different for each user?
If they are different, then how was the server able to differentiate between different users?
One more similar question, if there are n users accessing a particular servlet, then this servlet gets instantiated only the first time the first user accessed it or does it get instantiated for all the users separately?
In other words, what happens to the instance variables?

ServletContext
When the servlet container (like Apache Tomcat) starts up, it will deploy and load all its web applications. When a web application is loaded, the servlet container creates the ServletContext once and keeps it in the server's memory. The web app's web.xml and all of included web-fragment.xml files is parsed, and each <servlet>, <filter> and <listener> found (or each class annotated with #WebServlet, #WebFilter and #WebListener respectively) will be instantiated once and be kept in the server's memory as well, registred via the ServletContext. For each instantiated filter, its init() method is invoked with a new FilterConfig argument which in turn contains the involved ServletContext.
When a Servlet has a <servlet><load-on-startup> or #WebServlet(loadOnStartup) value greater than 0, then its init() method is also invoked during startup with a new ServletConfig argument which in turn contains the involved ServletContext. Those servlets are initialized in the same order specified by that value (1 is 1st, 2 is 2nd, etc). If the same value is specified for more than one servlet, then each of those servlets is loaded in the same order as they appear in the web.xml, web-fragment.xml, or #WebServlet classloading. In the event the "load-on-startup" value is absent, the init() method will be invoked whenever the HTTP request hits that servlet for the very first time.
When the servlet container is finished with all of the above described initialization steps, then the ServletContextListener#contextInitialized() will be invoked with a ServletContextEvent argument which in turn contains the involved ServletContext. This will allow the developer the opportunity to programmatically register yet another Servlet, Filter or Listener.
When the servlet container shuts down, it unloads all web applications, invokes the destroy() method of all its initialized servlets and filters, and all Servlet, Filter and Listener instances registered via the ServletContext are trashed. Finally the ServletContextListener#contextDestroyed() will be invoked and the ServletContext itself will be trashed.
HttpServletRequest and HttpServletResponse
The servlet container is attached to a web server that listens for HTTP requests on a certain port number (port 8080 is usually used during development and port 80 in production). When a client (e.g. user with a web browser, or programmatically using URLConnection) sends an HTTP request, the servlet container creates new HttpServletRequest and HttpServletResponse objects and passes them through any defined Filter in the chain and, eventually, the Servlet instance.
In the case of filters, the doFilter() method is invoked. When the servlet container's code calls chain.doFilter(request, response), the request and response continue on to the next filter, or hit the servlet if there are no remaining filters.
In the case of servlets, the service() method is invoked. By default, this method determines which one of the doXxx() methods to invoke based off of request.getMethod(). If the determined method is absent from the servlet, then an HTTP 405 error is returned in the response.
The request object provides access to all of the information about the HTTP request, such as its URL, headers, query string and body. The response object provides the ability to control and send the HTTP response the way you want by, for instance, allowing you to set the headers and the body (usually with generated HTML content from a JSP file). When the HTTP response is committed and finished, both the request and response objects are recycled and made available for reuse.
HttpSession
When a client visits the webapp for the first time and/or the HttpSession is obtained for the first time via request.getSession(), the servlet container creates a new HttpSession object, generates a long and unique ID (which you can get by session.getId()), and stores it in the server's memory. The servlet container also sets a Cookie in the Set-Cookie header of the HTTP response with JSESSIONID as its name and the unique session ID as its value.
As per the HTTP cookie specification (a contract any decent web browser and web server must adhere to), the client (the web browser) is required to send this cookie back in subsequent requests in the Cookie header for as long as the cookie is valid (i.e. the unique ID must refer to an unexpired session and the domain and path are correct). Using your browser's built-in HTTP traffic monitor, you can verify that the cookie is valid (press F12 in Chrome / Firefox 23+ / IE9+, and check the Net/Network tab). The servlet container will check the Cookie header of every incoming HTTP request for the presence of the cookie with the name JSESSIONID and use its value (the session ID) to get the associated HttpSession from server's memory.
The HttpSession stays alive until it has been idle (i.e. not used in a request) for more than the timeout value specified in <session-timeout>, a setting in web.xml. The timeout value defaults to 30 minutes. So, when the client doesn't visit the web app for longer than the time specified, the servlet container trashes the session. Every subsequent request, even with the cookie specified, will not have access to the same session anymore; the servlet container will create a new session.
On the client side, the session cookie stays alive for as long as the browser instance is running. So, if the client closes the browser instance (all tabs/windows), then the session is trashed on the client's side. In a new browser instance, the cookie associated with the session wouldn't exist, so it would no longer be sent. This causes an entirely new HttpSession to be created, with an entirely new session cookie being used.
In a nutshell
The ServletContext lives for as long as the web app lives. It is shared among all requests in all sessions.
The HttpSession lives for as long as the client is interacting with the web app with the same browser instance, and the session hasn't timed out at the server side. It is shared among all requests in the same session.
The HttpServletRequest and HttpServletResponse live from the time the servlet receives an HTTP request from the client, until the complete response (the web page) has arrived. It is not shared elsewhere.
All Servlet, Filter and Listener instances live as long as the web app lives. They are shared among all requests in all sessions.
Any attribute that is defined in ServletContext, HttpServletRequest and HttpSession will live as long as the object in question lives. The object itself represents the "scope" in bean management frameworks such as JSF, CDI, Spring, etc. Those frameworks store their scoped beans as an attribute of its closest matching scope.
Thread Safety
That said, your major concern is possibly thread safety. You should now know that servlets and filters are shared among all requests. That's the nice thing about Java, it's multithreaded and different threads (read: HTTP requests) can make use of the same instance. It would otherwise be too expensive to recreate, init() and destroy() them for every single request.
You should also realize that you should never assign any request or session scoped data as an instance variable of a servlet or filter. It will be shared among all other requests in other sessions. That's not thread-safe! The below example illustrates this:
public class ExampleServlet extends HttpServlet {
private Object thisIsNOTThreadSafe;
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
Object thisIsThreadSafe;
thisIsNOTThreadSafe = request.getParameter("foo"); // BAD!! Shared among all requests!
thisIsThreadSafe = request.getParameter("foo"); // OK, this is thread safe.
}
}
See also:
What is the difference between JSF, Servlet and JSP?
Best option for Session management in Java
Difference between / and /* in servlet mapping url pattern
doGet and doPost in Servlets
Servlet seems to handle multiple concurrent browser requests synchronously
Why Servlets are not thread Safe?

Sessions
In short: the web server issues a unique identifier to each visitor on his first visit. The visitor must bring back that ID for him to be recognised next time around. This identifier also allows the server to properly segregate objects owned by one session against that of another.
Servlet Instantiation
If load-on-startup is false:
If load-on-startup is true:
Once he's on the service mode and on the groove, the same servlet will work on the requests from all other clients.
Why isn't it a good idea to have one instance per client? Think about this: Will you hire one pizza guy for every order that came? Do that and you'd be out of business in no time.
It comes with a small risk though. Remember: this single guy holds all the order information in his pocket: so if you're not cautious about thread safety on servlets, he may end up giving the wrong order to a certain client.

Session in Java servlets is the same as session in other languages such as PHP. It is unique to the user. The server can keep track of it in different ways such as cookies, url rewriting etc. This Java doc article explains it in the context of Java servlets and indicates that exactly how session is maintained is an implementation detail left to the designers of the server. The specification only stipulates that it must be maintained as unique to a user across multiple connections to the server. Check out this article from Oracle for more information about both of your questions.
Edit There is an excellent tutorial here on how to work with session inside of servlets. And here is a chapter from Sun about Java Servlets, what they are and how to use them. Between those two articles, you should be able to answer all of your questions.

When the servlet container (like Apache Tomcat) starts up, it will read from the web.xml file (only one per application) if anything goes wrong or shows up an error at container side console, otherwise, it will deploy and load all web applications by using web.xml (so named it as deployment descriptor).
During instantiation phase of the servlet, servlet instance is ready but it cannot serve the client request because it is missing with two pieces of information:
1: context information
2: initial configuration information
Servlet engine creates servletConfig interface object encapsulating the above missing information into it
servlet engine calls init() of the servlet by supplying servletConfig object references as an argument. Once init() is completely executed servlet is ready to serve the client request.
Q) In the lifetime of servlet how many times instantiation and initialization happens ??
A)only once (for every client request a new thread is created)
only one instance of the servlet serves any number of the client request ie, after serving one client request server does not die. It waits for other client requests ie what CGI (for every client request a new process is created) limitation is overcome with the servlet (internally servlet engine creates the thread).
Q)How session concept works?
A)whenever getSession() is called on HttpServletRequest object
Step 1: request object is evaluated for incoming session ID.
Step 2: if ID not available a brand new HttpSession object is created and its corresponding session ID is generated (ie of HashTable) session ID is stored into httpservlet response object and the reference of HttpSession object is returned to the servlet (doGet/doPost).
Step 3: if ID available brand new session object is not created session ID is picked up from the request object search is made in the collection of sessions by using session ID as the key.
Once the search is successful session ID is stored into HttpServletResponse and the existing session object references are returned to the doGet() or doPost() of UserDefineservlet.
Note:
1)when control leaves from servlet code to client don't forget that session object is being held by servlet container ie, the servlet engine
2)multithreading is left to servlet developers people for implementing ie., handle the multiple requests of client nothing to bother about multithread code
Inshort form:
A servlet is created when the application starts (it is deployed on the servlet container) or when it is first accessed (depending on the load-on-startup setting)
when the servlet is instantiated, the init() method of the servlet is called
then the servlet (its one and only instance) handles all requests (its service() method being called by multiple threads). That's why it is not advisable to have any synchronization in it, and you should avoid instance variables of the servlet
when the application is undeployed (the servlet container stops), the destroy() method is called.

Sessions - what Chris Thompson said.
Instantiation - a servlet is instantiated when the container receives the first request mapped to the servlet (unless the servlet is configured to load on startup with the <load-on-startup> element in web.xml). The same instance is used to serve subsequent requests.

The Servlet Specification JSR-315 clearly defines the web container behavior in the service (and doGet, doPost, doPut etc.) methods (2.3.3.1 Multithreading Issues, Page 9):
A servlet container may send concurrent requests through the service
method of the servlet. To handle the requests, the Servlet Developer
must make adequate provisions for concurrent processing with multiple
threads in the service method.
Although it is not recommended, an alternative for the Developer is to
implement the SingleThreadModel interface which requires the container
to guarantee that there is only one request thread at a time in the
service method. A servlet container may satisfy this requirement by
serializing requests on a servlet, or by maintaining a pool of servlet
instances. If the servlet is part of a Web application that has been
marked as distributable, the container may maintain a pool of servlet
instances in each JVM that the application is distributed across.
For servlets not implementing the SingleThreadModel interface, if the
service method (or methods such as doGet or doPost which are
dispatched to the service method of the HttpServlet abstract class)
has been defined with the synchronized keyword, the servlet container
cannot use the instance pool approach, but must serialize requests
through it. It is strongly recommended that Developers not synchronize
the service method (or methods dispatched to it) in these
circumstances because of detrimental effects on performance

No. Servlets are not Thread safe
This is allows accessing more than one threads at a time
if u want to make it Servlet as Thread safe ., U can go for
Implement SingleThreadInterface(i)
which is a blank Interface there is no
methods
or we can go for synchronize methods
we can make whole service method as synchronized by using synchronized
keyword in front of method
Example::
public Synchronized class service(ServletRequest request,ServletResponse response)throws ServletException,IOException
or we can the put block of the code in the Synchronized block
Example::
Synchronized(Object)
{
----Instructions-----
}
I feel that Synchronized block is better than making the whole method
Synchronized

As is clear from above explanations, by implementing the SingleThreadModel, a servlet can be assured thread-safety by the servlet container. The container implementation can do this in 2 ways:
1) Serializing requests (queuing) to a single instance - this is similar to a servlet NOT implementing SingleThreadModel BUT synchronizing the service/ doXXX methods; OR
2) Creating a pool of instances - which's a better option and a trade-off between the boot-up/initialization effort/time of the servlet as against the restrictive parameters (memory/ CPU time) of the environment hosting the servlet.

javax.servlet.ServletException: AS-WEB-CORE-00089 exception In servlets

I have a servlet called Document to which I want to redirect all requests starting with Document something like
http://localhost:8080/CollabEdit/Document/abcdwcklsclds
should be redirected to servlet Document.
So, I used an annotation like this :
#WebServlet("/Document/*")
However, for some unidentified reason, it gives an exception that says:
javax.servlet.ServletException: AS-WEB-CORE-00089
This exception is in Document.java is thrown as soon as I call
request.getRequestDispatched("main.html").forward(request, response) .
otherwise no exception.
however, With same request in other servlets, main.html gets called just fine.

A forward and a redirect are not the same thing. A redirect allows you to go to any URL, eventually on another server or with another protocol, because you ask the client to query that URL.
In a forward you ask the servlet container to pass the control to another servlet of the same application (inside same context). As you use a relative path you are actually requesting what is the servlet for : http://host.do.main/appname/Document/main.html because the relative URL is added at the end of the address of current page (it could even be .../Document/.../main.html) !
And you declared that any page under /Document should be served by the Document servlet ... so the infinite loop ...
You can fix it two ways :
if you really need to use relative paths (it is inherently dangerous but you might have reasons to do it), try ../main.html if called from /Document or ../../main.html if called from /Document/something
use an absolute path :
contextPath = request.getContextPath();
request.getRequestDispatcher(contextPath + "/main.html").forward(request, response)

The Filter looks right.
Are you using Servlets 3.0 and no older version? In older versions you would have to edit the web.xml. And in 3.0 it would have to look like in this Post
Does Document extend HttpServlet?
Could you show us some more of your Code, maybe then it would be easier to see what's going wrong.
Greetings

Where does getRequestDispatcher("path") look?

Using embedded tomcat, this code:
System.out.println("getServletPath: " + request.getServletPath());
System.out.println("getServletContext: " + request.getServletContext().getContextPath());
System.out.println("getServerName: " + request.getServerName());
System.out.println("getServerPort: " + request.getServerPort());
Prints out:
getServletPath: /example
getServletContext:
getServerName: localhost
getServerPort: 9090
Does that mean that:
request.getRequestDispatcher("/example/read.jsp").forward(request, response);
Will look at this URL to forward(request, response) to the JSP:
http://localhost:9090/example/read.jsp?
Is there a way to print out what absolute URL getRequestDispatcher("relativePath") is addressing?

The Servlet Specification explains this
The getRequestDispatcher method takes a String argument describing a
path within the scope of the ServletContext. This path must be
relative to the root of the ServletContext and begin with a ‘/’, or
be empty. The method uses the path to look up a servlet, using the
servlet path matching rules in Chapter 12, “Mapping Requests to
Servlets”, wraps it with a RequestDispatcher object, and returns the
resulting object. If no servlet can be resolved based on the given
path, a RequestDispatcher is provided that returns the content for
that path.
Those rules are the following
The container will try to find an exact match of the path of the request to the path of the servlet. A successful match selects the
servlet.
The container will recursively try to match the longest path-prefix. This is done by stepping down the path tree a directory
at a time, using the ’/’ character as a path separator. The longest
match determines the servlet selected.
If the last segment in the URL path contains an extension (e.g. .jsp), the servlet container will try to match a servlet that handles
requests for the extension. An extension is defined as the part of
the last segment after the last ’.’ character.
If neither of the previous three rules result in a servlet match, the container will attempt to serve content appropriate for the
resource requested. If a "default" servlet is defined for the
application, it will be used. Many containers provide an implicit
default servlet for serving content.
You ask
Does that mean that:
request.getRequestDispatcher("/example/display.jsp").forward(request,
response); Will look at this URL to forward(request, response) to the
JSP:
http://localhost:9090/example/display.jsp?
No, it doesn't send an HTTP request, so the path has nothing to do with a URI. It's more of an internal path that the Servlet container will try to match with its various url-mappings for Servlets.
You also ask
Is there a way to print out what absolute URL getRequestDispatcher("relativePath") is addressing?
No. And it isn't exactly an absolute URL. It's a path that can be handled by some resource in the web application context.
After your edit, you addWebapp to your Tomcat instance.
tomcat.addWebapp(null, "/view2/example2", new File("src/com/example/view/example").getAbsolutePath());
You then send a request to
/view2/example2/read.jsp
I'm going to assume that read.jsp is in
src/com/example/view/example/
I believe it's in the publicly accessible part of the web application and therefore the Servlet container can render it and respond with it.
You've also added a webapp with addContext which seems to be similar to addWebapp
context = tomcat.addContext("", base.getAbsolutePath());
and added servlet mappings to this context.
Tomcat.addServlet(context, "example", new ExampleController());
context.addServletMapping("/example/*", "example");
I was wrong about the /example/* not being able to handle /example.
When you send a request to
/example
since the context path is "", the Context above will be used and the mapping will match the ExampleController registered above. Your Servlet code will execute and reach
request.getRequestDispatcher("/view2/example2/read.jsp").forward(request, response);
Note the javadoc of ServletRequest#getRequestDispatcher(String)
The pathname specified may be relative, although it cannot extend
outside the current servlet context.
In other words, this Servlet, ExampleController was registered in the ServletContext mapped to the context path "", ie. root. The path /view2/example2/read.jsp is referring to another context. Since this context doesn't have a mapping for it, it responds with 404.
You can get a reference to another web applications in a different context. You have to use ServletContext#getContext(String). For example
ServletContext otherContext = request.getServletContext().getContext("/view2/example2");
Now that you have the ServletContext, you can get a RequestDispatcher for a resource in that context.
otherContext.getRequestDispatcher("/read.jsp").forward(request, response);
since ServletContext#getRequestDispatcher(String) states
The pathname must begin with a / and is interpreted as relative to the current context root.
Final Answer:
getRequestDispatcher("path") will look at the directory set in the addWebapp method when referencing a JSP file. If a blank page or NullPointerException is displayed, ensure that you have done the following:
Remove all the addWebapp definitions.
Run addContext then addWebApp like this so they both point to ROOT:
File base = new File("src/com/example/view");
context = tomcat.addContext("", base.getAbsolutePath());
tomcat.addWebapp(null, "/", base.getAbsolutePath());
In the servlet point to the jsp using request.getRequestDispatcher("/example/read.jsp").forward(request, response); provided that the directory /example exists in "src/com/example/view".

Make all requests to mysite.com/user/specified/path run the same JSP

I want to allow users to create groups in my application and access them via URL. For example, if you made a group called "sweethatclub," you could access it at http://mysite.com/sweethatclub. Of course, the same code will run for /sweethatclub and /drillteam and even /students/yearbook
I'm running in a Java servlet environment, and can't quite get the paths to align for this. I can write a filter that intercepts all requests and adds information to the request by parsing the URL, but then I want to run the code of an index.jsp. I don't want to map index.jsp to all URLs, because, for example, /images/smiley.jpg still needs to respond the with appropriate file instead of index.jsp.
Is there a way to send all requests to a servlet, unless the request is matched by a plain-old file? Or, is there some other way to accomplish what I want here?
Please let me know if I need to supply more information. I'm new to this environment.

The URL patterns in the web.xml are not supposed to be smart enough to figure out target URL's nature. If you can tolerate it, the easiest way would be to place all the user specified paths under a a well known root... someplace separate from the static files. So you end up with user specified paths like http://mysite.com/sites/sweethatclub.
Alternatively, you can move all your static content under http://mysite.com/static/, and set up the servlet mappings or filters to treat anything starting with 'static' different from the dynamic URL space.

If you are in a Unix invironment, you could just create all the "group sites" as virtual directories that just point to your default one.

Map the servlet on a specific URL pattern
<url-pattern>/groups/*</url-pattern>
Put all static content in a common folder, e.g. /static and fix all URLs in the pages to point to that URL instead.
Create a filter which is mapped on
<url-pattern>/*</url-pattern>
and does the following job in doFilter() method
String uri = ((HttpServletRequest) request).getRequestURI();
if (uri.startsWith("/static/")) {
chain.doFilter(request, response); // Goes to default servlet.
} else {
request.getRequestDispatcher("/groups" + uri).forward(request, response);
}
No, this does not end up with /groups in browser address bar URL. It's fully transparent. You can if necessary make "/static" and/or "/groups" an <init-param> of the filter so that it's externally configureable.

How can I share a variable or object between two or more Servlets?

I would like to know if there is some way to share a variable or an object between two or more Servlets, I mean some "standard" way. I suppose that this is not a good practice but is a easier way to build a prototype.
I don't know if it depends on the technologies used, but I'll use Tomcat 5.5
I want to share a Vector of objects of a simple class (just public attributes, strings, ints, etc). My intention is to have a static data like in a DB, obviously it will be lost when the Tomcat is stopped. (it's just for Testing)

I think what you're looking for here is request, session or application data.
In a servlet you can add an object as an attribute to the request object, session object or servlet context object:
protected void doGet(HttpServletRequest request, HttpServletResponse response) {
String shared = "shared";
request.setAttribute("sharedId", shared); // add to request
request.getSession().setAttribute("sharedId", shared); // add to session
this.getServletConfig().getServletContext().setAttribute("sharedId", shared); // add to application context
request.getRequestDispatcher("/URLofOtherServlet").forward(request, response);
}
If you put it in the request object it will be available to the servlet that is forwarded to until the request is finished:
request.getAttribute("sharedId");
If you put it in the session it will be available to all the servlets going forward but the value will be tied to the user:
request.getSession().getAttribute("sharedId");
Until the session expires based on inactivity from the user.
Is reset by you:
request.getSession().invalidate();
Or one servlet removes it from scope:
request.getSession().removeAttribute("sharedId");
If you put it in the servlet context it will be available while the application is running:
this.getServletConfig().getServletContext().getAttribute("sharedId");
Until you remove it:
this.getServletConfig().getServletContext().removeAttribute("sharedId");

Put it in one of the 3 different scopes.
request - lasts life of request
session - lasts life of user's session
application - lasts until applciation is shut down
You can access all of these scopes via the HttpServletRequest variable that is passed in to the methods that extend from the HttpServlet class

Depends on the scope of the intended use of the data.
If the data is only used on a per-user basis, like user login info, page hit count, etc. use the session object
(httpServletRequest.getSession().get/setAttribute(String [,Object]))
If it is the same data across multiple users (total web page hits, worker threads, etc) use the ServletContext attributes. servlet.getServletCongfig().getServletContext().get/setAttribute(String [,Object])). This will only work within the same war file/web applicaiton. Note that this data is not persisted across restarts either.

Another option, share data betwheen contexts...
share-data-between-servlets-on-tomcat
<Context path="/myApp1" docBase="myApp1" crossContext="true"/>
<Context path="/myApp2" docBase="myApp2" crossContext="true"/>
On myApp1:
ServletContext sc = getServletContext();
sc.setAttribute("attribute", "value");
On myApp2:
ServletContext sc = getServletContext("/myApp1");
String anwser = (String)sc.getAttribute("attribute");

Couldn't you just put the object in the HttpSession and then refer to it by its attribute name in each of the servlets?
e.g:
getSession().setAttribute("thing", object);
...then in another servlet:
Object obj = getSession.getAttribute("thing");

Here's how I do this with Jetty.
https://stackoverflow.com/a/46968645/1287091
Uses the server context, where a singleton is written to during startup of an embedded Jetty server and shared among all webapps for the life of the server. Can also be used to share objects/data between webapps assuming there is only one writer to the context - otherwise you need to be mindful of concurrency.