Normally Servlets are initiated just once and web container simple spawns a new thread for every user request. Let's say if I create my own web container from scratch and instead of Threads, I simply create Servlets as Singleton. Will I be missing anything here? I guess, in this case, the singleton can only service one user request at a time and not multiple.
Normally Servlets are initiated just once and web container simple spawns a new thread for every user request.
The first statement is true, but the second actually not. Normally, threads are been created once during applications startup and kept in a thread pool. When a thread has finished its request-response processing job, it will be returned to the pool. That's also why using ThreadLocal in a servletcontainer must be taken with high care.
Let's say if I create my own web container from scratch and instead of Threads, I simply create Servlets as Singleton. Will I be missing anything here?
They does not necessarily need to follow the singleton pattern. Just create only one instance of them during application's startup and keep them in memory throughout application's lifetime and just let all threads access the same instance.
I guess, in this case, the singleton can only service one user request at a time and not multiple.
This is not true. This will only happen when you synchronize the access to the singleton's methods on an application-wide lock. For example by adding the synchronized modifier to the method of your servlet or a synchronized(this) in the manager's method who is delegating the requests to the servlets.
JavaEE used to have a mechanism for this - a marker interface called SingleThreadModel that your servlet could implement:
Ensures that servlets handle only one request at a time. This interface has no methods.
If a servlet implements this interface, you are guaranteed that no two threads will execute concurrently in the servlet's service method. The servlet container can make this guarantee by synchronizing access to a single instance of the servlet, or by maintaining a pool of servlet instances and dispatching each new request to a free servlet.
Note that SingleThreadModel does not solve all thread safety issues. For example, session attributes and static variables can still be accessed by multiple requests on multiple threads at the same time, even when SingleThreadModel servlets are used. It is recommended that a developer take other means to resolve those issues instead of implementing this interface, such as avoiding the usage of an instance variable or synchronizing the block of the code accessing those resources. This interface is deprecated in Servlet API version 2.4.
Containers could use this to instantiate a new servlet for each request, or maintain a pool of them, if they chose to.
This was deprecated in Servlet 2.4, for the reasons documented above. Those same reasons still apply to your question.
That's basically it.
I would question the motivations for creating your own container, with so many available for a wide range of purposes.
Related
I think that for every request , container will create 1 servlet instance and will spawn multiple threads for every request. But is it possible to create particular servlet instance for every request instead of creating threads.
This is not possible if the servlet container's implementation conforms to the Java servlet specification. (All major application servers do.) You can use class (static) variables and methods to maintain state between invocations but I don't see any compulsion to do this (aside from academic purposes).
#WebListener
public class AllRequestsWebListener implements ServletRequestListener {
#Inject HttpRequestProducer producer;
public void requestInitialized(ServletRequestEvent sre) {
producer.requestInitialized(sre);
}
}
...
#RequestScoped
public class HttpRequestProducer {
...
}
I don't know howto inject request-bean as method-parameter and therefore I can guess that it will work properly when Request-bean injection is threadLocal. Can someone explain me how it's implemented in a thread-safe manner?
What you have injected in your bean is a proxy representing the real deal. The proxy will always forward the invocation to the correct bean
Intuition based answer
I believe it is thread safe, as request scope is thread safe (session and above are not, as a user can open multiple browser sessions and use the same session ID)
I tested it, although it's empiric evidence, but the injected HttpRequestProducer gets a new instance each request.
Note that the requestInitialized and requestDestroyed can be (and in practice are) different threads, so I will investigate further if you intend to use the same injected object on both methods.
Specs backed answer
The hard part was to find hard evidence for this claim in the specs.
I looked into the CDI spec and couldn't quickly find conclusive evidence that a #RequestScoped object is thread safe (e.g. using thread local) however I assume that a #RequestScoped bean is using the same scope as the scoped beans in Java EE 5: (see here)
In there this clause is interesting:
Controlling Concurrent Access to Shared Resources In a multithreaded
server, it is possible for shared resources to be accessed
concurrently. In addition to scope object attributes, shared resources
include in-memory data (such as instance or class variables) and
external objects such as files, database connections, and network
connections.
Concurrent access can arise in several situations:
Multiple web components accessing objects stored in the web context.
Multiple web components accessing objects stored in a session.
Multiple threads within a web component accessing instance variables.
A web container will typically create a thread to handle each request.
If you want to ensure that a servlet instance handles only one request
at a time, a servlet can implement the SingleThreadModel interface. If
a servlet implements this interface, you are guaranteed that no two
threads will execute concurrently in the servlet’s service method. A
web container can implement this guarantee by synchronizing access to
a single instance of the servlet, or by maintaining a pool of web
component instances and dispatching each new request to a free
instance. This interface does not prevent synchronization problems
that result from web components accessing shared resources such as
static class variables or external objects. In addition, the Servlet
2.4 specification deprecates the SingleThreadModel interface.
So in theory, it seems that the object itself is going to have one instance per request thread, however I couldn't find any hard evidence that this is supported.
I have a WCF service with ServiceBehavior(InstanceContextMode = InstanceContextMode.Single, ConcurrencyMode = ConcurrencyMode.Multiple). I want to use ThreadStatic variable to srore data.
I start worrying about is it possible two parallel requests for the same or different operationContracts get handled by the same thread serverside, because if this happens my ThreadStatic variable will get overriden.(I.e. something like the thread changing between HttpHandlers and HttpModules in ASP.NET)
I made a spike service with the same ServiceBehaviour and maxConcurrentCalls="2". After that a wcf client called the service with 50 parallel requests and my worry did not occur. However this is not a 100% proof.
Thank in advance!
Irrespective of the ConcurrencyMode, a ThreadStatic value will persist when your request terminates and the thread is returned to the thread pool. The same thread can be reused for a subsequent request, which will therefore be able to see your ThreadStatic value.
Obviously this won't be true for two concurrent requests, because by definition they will be executed on different threads.
From comments:
Also by definition MSDN says: 'The service instance is multi-threaded. No synchronization guarantees are made. Because other threads can change your service object at any time, you must handle synchronization and state consistency at all times.' So it is not so obvious:)
This means that a single instance of your service class can be accessed concurrently by multiple requests. So you would need to handle synchronization for any accesses to instance members of the service class.
However ThreadStatic members are by definition only used by one thread (and hence one request) at a time, so don't need synchronization.
The direct answer to your question is Joe's answer.
However you mention in the comments you are using an ambient design pattern. That pattern is already implemented in WCF as the OperationContext and is specifically designed to be extensible. I highly recommend using OperationContext over any custom thread storage.
See Where to store data for current WCF call? Is ThreadStatic safe?
I wanted to add to Joe's answer here because I would recommend that you use some sort of correlation for your requests if you're needing to store state. The threading model will become very convoluted and unreliable in production.
Further, now imagine you have two IIS servers hosting this service and a hardware or software load balancer forward facing so that you can consume it. To ensure that the correct state is gathered you'll need correlation because you never know which server the service will be started on. In the post below I mocked up a simplified version of how that might work. One thing to keep in mind is that the SessionState would need to be kept in a shared location to all instances of the service, an AppFabric Cache server for example.
Global Variable between two WCF Methods
I am new to server side coding and JSP/servlets. I have a code which has 3 classes. 1st is a Serv class inherited from java httpservlet. In this i have doPost() method implemented. In doPost() i use object of the 2nd class ResourceClass. ResourceClass is a singleton class. Hence essentially to use any method is do something like ResourceClass.getInstance().readResource();
Now readResource furthur uses Java Native access library to read a resource from disk. Now my question is Since as i understand if 1000 clients connect to my server(Apache Tomcat) for each new request i will have a new servlet serving the request. But all these servlets will essentially use the same singleton object. Hence will this reading be thread safe.
I do not change any internal state. So i think it wont effect my output hence the whole stuff is Idempotent. But will all these requests be queued making the singleton class object a bottleneck. Or will each servlet has its own copy.
Also if i change the Resource state then in that case will it be thread safe.
First of all, you won't have a new servlet for each request. The same, unique instance of servlet will be used to concurrently handle all the requests. The servlet is also a singleton: the web container instantiates only one instance.
You say that the requests to your ResourceClass singleton will be queued. They won't, unless you mark the method as synchronized or use some other locking mechanism. If you don't, then the threads will invoke your singleton method concurrently.
Whether it's thread-safe or not is impossible to say without seeing the code of your singleton and the code of the JNI library. The fact that it's read-only is a sign that it could be thread-safe, but it's not guaranteed.
In a Java EE server, you only have 1 instance of each servlet.
On the other hand, each http request is processed by the server in its own thread.
There is one instance of ResourceClass because it's a singleton so you will have a bottleneck if the readResource() method is synchronized.
This question already has answers here:
How do servlets work? Instantiation, sessions, shared variables and multithreading
(8 answers)
Closed 6 years ago.
I need to know why the servlets are not thread safe ? And whats the reason that in Struts 2.0 framework controller servlet is thread safe ?
I need to know why the servlets are not thread safe ?
Servlet instances are inherently not thread safe because of the multi threaded nature of the Java programming language in general. The Java Virtual Machine supports executing the same code by multiple threads. This is a great performance benefit on machines which have multiple processors. This also allows the same code to be executed by multiple concurrent users without blocking each other.
Imagine a server with 4 processors wherein a normal servlet can handle 1000 requests per second. If that servlet were threadsafe, then the web application would act like as if it runs on a server with 1 processor wherein the servlet can handle only 250 requests per second (okay, it's not exactly like that, but you got the idea).
If you encounter threadsafety issues when using servlets, then it is your fault, not Java's nor Servlet's fault. You'd need to fix the servlet code as such that request or session scoped data is never assigned as an instance variable of the servlet. For an in-depth explanation, see also How do servlets work? Instantiation, sessions, shared variables and multithreading.
And whats the reason that in Struts 2.0 framework controller servlet is thread safe ?
It is not thread safe. You're confusing the Struts dispatcher servlet filter with Struts actions. The struts actions are re-created on every single request. So every single request has its own instance of the request scoped Struts action. The Struts dispatcher servlet filter does not store them as its own instance variable. Instead, it stores it as an attribute of the HttpServletRequest.
Servlets are normal java classes and thus are NOT Thread Safe.
But that said, Java classes are Thread safe if you do not have instance variables. Only instance variables need to synchronize. (Instance variable are variables declared in the class and not in within its methods.
Variables declared in the methods are thread safe as each thread creates it own Program Stack and function variables are allocated in the stack. This means that variable in a methods are created for each thread, hence does not have any thread sync issues associated.
Method variables are thread-safe, class variables are not.
There is a single instance of a servlet per servlet mapping; all instance properties are shared between all requests. Access to those properties must take that in to account.
Struts 2 actions (not "controller servlet", they're neither servlets nor controllers) are instantiated per-request. Action properties will be accessed only by a single request's thread.
Servlets are normally multi-threaded.
Servlet containers usually manage concurrent requests by creating a new Java thread for each request. The new thread is given an object reference to the requested servlet, which issues the response through the same thread. This is why it is important to design for concurrency when you write a servlet, because multiple requests may be handled by the same servlet instance.
The way that servlet containers handle servlet requests is implementation dependent; they may use a single servlet, they may use servlet pooling, it depends on the vendor's system architecture.
Struts 2 Action objects are instantiated for each request, so there are no thread-safety issues.
Servlet is not thread safe but we can make it as a thread safe by implementing that servlet class to SingleThreadModel
like the given below class definition but again the performance problem will be there so better option would be use synchronized portion
public class SurveyServlet extends HttpServlet
implements SingleThreadModel
{
servlet code here..
...
}
Servlet is not thread-safe by itself. You can make it thread-safe by making the service method synchronized.
you need to implement SingleThreadInterface to make it thread-safe.