Javax.servlet.singlethreadmodel interface

The difficult thing can be to understand that Java passes "objects as references" passed by value. Two important types of Out Of Memory Error are often encountered Long Term Solution: Increasing the Start/Max Heap size or changing Garbage Collection options may not always be a long term solution for your Out Of Memory Error problem.

This can certainly get confusing and I would recommend reading this article from an expert: read this interesting thread with example on Stack Overflow : Java Pass By Ref or Value This Error is thrown when the Java Virtual Machine cannot allocate an object because it is out of memory, and no more memory could be made available by the garbage collector. Best approach is to understand the memory needs of your program and ensure it uses memory wisely and does not have leaks.

Servlets are very important topic of Java EE and all of the web applications framework such as Spring and Struts are built on top of it. Here I am providing a list of 50 servlet interview questions with answers to help you tackle most of the interview questions related to servlets and web applications in java.

Javax.servlet.singlethreadmodel interface

As a senior and matured Java Programmer you must know the answers to these questions to demonstrate basic understanding of Java language and depth of knowledge.Date instance must be 'normalized' by setting the hours, minutes, seconds, and milliseconds to zero. Whereas a typical interface specifies methods that an implementing class must support, a marker interface does not do so.The mere presence of such an interface indicates specific behavior on the part of the implementing class.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.

Javax.servlet.singlethreadmodel interface

The only difference between experienced and inexperienced software developers is that the experienced ones realize when they're making a mistake.The servlet programmer should implement Single Thread Model interface to ensure that servlet can handle only one request at a time. This interface is currently deprecated since Servlet API 2.4 because it doesn't solves all the thread-safety issues such as static variable and session attributes can be accessed by multiple threads at the same time even if we have implemented the Single Thread Model interface. 摘 要:介绍了Servlet多线程机制,通过一个实例并结合Java 的内存模型说明引起Servlet线程不安全的原因,给出了保证Servlet线程安全的三种解决方案,并说明三种方案在实际开发中的取舍。 Servlet/JSP技术和ASP、PHP等相比,由于其多线程运行而具有很高的执行效率。由于Servlet/JSP默认是以多线程模式执行的,所 以,在编写代码时需要非常细致地考虑多线程的安全性问题。然而,很多人编写Servlet/JSP程序时并没有注意到多线程安全性的问题,这往往造成编写 的程序在少量用户访问时没有任何问题,而在并发用户上升到一定值时,就会经常出现一些莫明其妙的问题。 Servlet的多线程机制 Servlet体系结构是建立在Java多线程机制之上的,它的生命周期是由Web容器负责的。当客户端第一次请求某个Servlet时,Servlet容器将会根据web.xml配置文件实例化这个Servlet类。当有新的客户端请求该Servlet时,一般不会再实例化该Servlet类,也就是有多个线程在使用这个实例。Servlet容器会自动使用线程池等技术来支持系统的运行,如图1所示。 这样,当两个或多个线程同时访问同一个Servlet时,可能会发生多个线程同时访问同一资源的情况,数据可能会变得不一致。所以在用Servlet构建的Web应用时如果不注意线程安全的问题,会使所写的Servlet程序有难以发现的错误。 Servlet的线程安全问题 Servlet的线程安全问题主要是由于实例变量使用不当而引起的,这里以一个现实的例子来说明。 该Servlet中定义了一个实例变量output,在service方法将其赋值为用户的输出。当一个用户访问该Servlet时,程序会正常的运行,但当多个用户并发访问时,就可能会出现其它用户的信息显示在另外一些用户的浏览器上的问题。这是一个严重的问题。为了突出并发问题,便于测试、观察,我们在回显用户信息时执行了一个延时的操作。假设已在web.xml配置文件中注册了该Servlet,现有两个用户a和b同时访问该Servlet(可以启动两个IE浏览器,或者在两台机器上同时访问),即同时在浏览器中输入: a: Test/Concurrent Test? Username=b 如果用户b比用户a回车的时间稍慢一点,将得到如图2所示的输出: 图2 a用户和b用户的浏览器输出 从图2中可以看到,Web服务器启动了两个线程分别处理来自用户a和用户b的请求,但是在用户a的浏览器上却得到一个空白的屏幕,用户a的信息显示在用户b的浏览器上。该Servlet存在线程不安全问题。下面我们就从分析该实例的内存模型入手,观察不同时刻实例变量output的值来分析使该Servlet线程不安全的原因。 Java的内存模型JMM(Java Memory Model)JMM主要是为了规定了线程和内存之间的一些关系。根据JMM,我们可以将论文中所讨论的Servlet实例的内存模型抽象为图3所示的模型。 下面根据图3所示的内存模型,来分析当用户a和b的线程(简称为a线程、b线程)并发执行时,Servlet实例中所涉及变量的变化情况及线程的执行情况,如图4所示。 从图4中可以清楚的看到,由于b线程对实例变量output的修改覆盖了a线程对实例变量output的修改,从而导致了用户a的信息显示在了用户b的浏览器上。如果在a线程执行输出语句时,b线程对output的修改还没有刷新到主存,那么将不会出现图2所示的输出结果,因此这只是一种偶然现象,但这更增加了程序潜在的危险性。 设计线程安全的Servlet 通过上面的分析,我们知道了 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.The Java Spec says that everything in Java is pass-by-value.

There is no such thing as "pass-by-reference" in Java.

A milliseconds value represents the number of milliseconds that have passed since January 1, 1970 .000 GMT. It provides a way to associate metadata with a class where the language does not have explicit support for such metadata.

To conform with the definition of SQL DATE, the millisecond values wrapped inside a To use this pattern, a class implements a marker interface, and code that interact with instances of that class test for the existence of the interface.

The competition for getting a Java job is getting fierce, and its really important that you be well prepared for you dream interview.

We highly recommend you to also refer a good java book to get in depth knowledge on each of the below mentioned topic.

Javax.servlet.singlethreadmodel interface-55

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