MG>maybe.. see below ________________________________ From: Zahid Rahman <zahidr1...@gmail.com> Sent: Friday, December 13, 2019 5:09 PM To: Struts Users Mailing List <user@struts.apache.org> Subject: Re: Java Singleton , Framework Design Patterns
Hi, >?am interested to know which topic prompts your question ? As you can see from the log file (below) the spring framework creates and destroys a singleton of a bean. So the singleton pattern is used by the container. I was also informed that the singleton is used to create a login screen. That also appears to be the case from the page http://www.blackwasp.co.uk/Singleton.aspx. The login screen I will be using is the login.java and login.jsp in the struts2 "blank app" in "struts-examples-master". The struts framework also helps me with duplicate form submission. So do I need to use the singleton pattern ? MG>here is ted husted struts-blank login.java (un-edited) /* * $Id: Login.java 471756 2006-11-06 15:01:43Z husted $ * * Licensed to the Apache Software Foundation (ASF) under one * or more contributor license agreements. See the NOTICE file * distributed with this work for additional information * regarding copyright ownership. The ASF licenses this file * to you under the Apache License, Version 2.0 (the * "License"); you may not use this file except in compliance * with the License. You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, * software distributed under the License is distributed on an * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY * KIND, either express or implied. See the License for the * specific language governing permissions and limitations * under the License. */ package example; public class Login extends ExampleSupport { public String execute() throws Exception { if (isInvalid(getUsername())) return INPUT; if (isInvalid(getPassword())) return INPUT; return SUCCESS; } private boolean isInvalid(String value) { return (value == null || value.length() == 0); } private String username; public String getUsername() { return username; } public void setUsername(String username) { this.username = username; } private String password; public String getPassword() { return password; } public void setPassword(String password) { this.password = password; } } //no singleton yet as there is no public static ClassName getInstance() //with private constructor pointing to getInstance() //BUT singleton is used to instantate and set default ObjectFactory for XmlConfigurationProvider //as seen here in ConfigurationTestBase.java package com.opensymphony.xwork2.config.providers; public abstract class ConfigurationTestBase extends com.opensymphone.xwork2.XWorkTestCase { protected com.opensymphony.xwork2.config.ConfigurationProvider buildConfigurationProvider(final String filename) { configuration = new com.opensymphony.xwork2.config.providers.MockConfiguration(); ((com.opensymphony.xwork2.config.providers.MockConfiguration)configuration).selfRegister(); container = configuration.getContainer(); XmlConfigurationProvider prov = new XmlConfigurationProvider(filename, true); prov.setObjectFactory(container.getInstance(com.opensymphony.xwork2.ObjectFactory.class)); //but the provider does not use Spring Object Factory which we can remedy here prov.setObjectFactory(container.getInstance(com.opensymphony.xwork2.spring.SpringObjectFactory.class)); //as you mention we must follow springs example of declaring singleton bean in ./WEB-INF/<application>context.xml <?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN//EN" "http://www.springframework.org/dtd/spring-beans.dtd";> <beans default-autowire="byName"> <bean id="default" class="com.opensymphony.xwork2.spring.SpringObjectFactory" singleton="true" /> //where struts specifies default <Spring>ObjectFactory thru ./classes/org/apache/struts2/default.properties ### if specified, the default object factory can be overridden here ### Note: short-hand notation is supported in some cases, such as "spring" ### Alternatively, you can provide a com.opensymphony.xwork2.ObjectFactory subclass name here struts.objectFactory=com.opensymphony.xwork2.spring.SpringObjectFactory.class //the line struts.objectFactory=com.opensymphony.xwork2.spring.SpringObjectFactory.class in default.properties should be sufficient //to route all instances of ObjectFactory to SpringObjectFactory does this help? 2019-12-13 21:01:06,882 [main] [DEBUG] o.s.b.f.s.DefaultListableBeanFactory - Pre-instantiating singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory@6f1d30d: defining beans [numberGenerator]; root of factory hierarchy 2019-12-13 21:01:06,882 [main] [DEBUG] o.s.b.f.s.DefaultListableBeanFactory - Creating shared instance of singleton bean 'numberGenerator' 2019-12-13 21:01:06,882 [main] [DEBUG] o.s.b.f.s.DefaultListableBeanFactory - Creating instance of bean 'numberGenerator' 2019-12-13 21:01:06,895 [main] [DEBUG] o.s.b.f.s.DefaultListableBeanFactory - Eagerly caching bean 'numberGenerator' to allow for resolving potential circular references 2019-12-13 21:01:06,896 [main] [DEBUG] o.s.b.f.s.DefaultListableBeanFactory - Finished creating instance of bean 'numberGenerator' 2019-12-13 21:01:06,897 [main] [DEBUG] o.s.c.s.ClassPathXmlApplicationContext - Unable to locate LifecycleProcessor with name 'lifecycleProcessor': using default [org.springframework.context.support.DefaultLifecycleProcessor@d967f908] 2019-12-13 21:01:06,897 [main] [DEBUG] o.s.b.f.s.DefaultListableBeanFactory - Returning cached instance of singleton bean 'lifecycleProcessor' 2019-12-13 21:01:06,900 [main] [DEBUG] o.s.c.e.PropertySourcesPropertyResolver - Could not find key 'spring.liveBeansView.mbeanDomain' in any property source 2019-12-13 21:01:06,910 [main] [DEBUG] o.s.b.f.s.DefaultListableBeanFactory - Returning cached instance of singleton bean 'numberGenerator' 2019-12-13 21:01:06,910 [main] [INFO ] academy.learnprogramming.Main - number = 0 2019-12-13 21:01:06,912 [main] [INFO ] o.s.c.s.ClassPathXmlApplicationContext - Closing org.springframework.context.support.ClassPathXmlApplicationContext@3e92e10: startup date [Fri Dec 13 21:01:06 GMT 2019]; root of context hierarchy 2019-12-13 21:01:06,912 [main] [DEBUG] o.s.b.f.s.DefaultListableBeanFactory - Returning cached instance of singleton bean 'lifecycleProcessor' 2019-12-13 21:01:06,912 [main] [DEBUG] o.s.b.f.s.DefaultListableBeanFactory - Destroying singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory@6f1d30d: defining beans [numberGenerator]; root of factory hierarchy On Fri, 13 Dec 2019 at 01:33, Martin Gainty <mgai...@hotmail.com> wrote: > Singleton: > i prefer thread-safe implementations and generally eschew singletons > (unless extreme political pressure dictates otherwise) > https://www.geeksforgeeks.org/singleton-design-pattern/ > [https://www.geeksforgeeks.org/wp-content/uploads/gfg_200X200.png]< > https://www.geeksforgeeks.org/singleton-design-pattern/> > Singleton Design Pattern | Implementation - GeeksforGeeks< > https://www.geeksforgeeks.org/singleton-design-pattern/> > The singleton pattern is one of the simplest design patterns. Sometimes we > need to have only one instance of our class for example a single DB > connection shared by multiple objects as creating a separate DB connection > for every object may be costly. Similarly, there can be a single > configuration ... > www.geeksforgeeks.org<http://www.geeksforgeeks.org> > > Design Patterns: > have a look at this discussion from stackoverflow on Visitor Pattern vs > Command Pattern for lambda expresssions > https://stackoverflow.com/questions/2186931/java-pass-method-as-parameter > [ > https://cdn.sstatic.net/Sites/stackoverflow/img/apple-touch-i...@2.png?v=73d79a89bded > ]< > https://stackoverflow.com/questions/2186931/java-pass-method-as-parameter> > interface - Java Pass Method as Parameter - Stack Overflow< > https://stackoverflow.com/questions/2186931/java-pass-method-as-parameter> > In Java 8, you can now pass a method more easily using Lambda Expressions > and Method References. First, some background: a functional interface is an > interface that has one and only one abstract method, although it can > contain any number of default methods (new in Java 8) and static methods. A > lambda expression can quickly implement the abstract method, without all > the unnecessary syntax ... > stackoverflow.com > although this discussion is more specific on when to implement Visitor > Pattern vs when to use Command Pattern > > https://stackoverflow.com/questions/2857880/command-pattern-vs-visitor-pattern > [ > https://cdn.sstatic.net/Sites/stackoverflow/img/apple-touch-i...@2.png?v=73d79a89bded > ]< > https://stackoverflow.com/questions/2857880/command-pattern-vs-visitor-pattern > > > Command Pattern vs. Visitor Pattern - Stack Overflow< > https://stackoverflow.com/questions/2857880/command-pattern-vs-visitor-pattern > > > Each pattern has it's own pros, cons and use cases. You can use Command > pattern to . Decouple the invoker & receiver of command . Implement > callback mechanism. Implement undo and redo functionality. Maintain a > history of commands. Use Visitor pattern in below scenarios:. Similar > operations have to be performed on objects of different types grouped in a > structure ; You need to execute many ... > stackoverflow.com > > ?am interested to know which topic prompts your question? > > /br/ > Martin > ________________________________ > From: Lukasz Lenart <lukaszlen...@apache.org> > Sent: Thursday, December 12, 2019 1:56 AM > To: Struts Users Mailing List <user@struts.apache.org> > Subject: Re: Java Singleton , Framework Design Patterns > > czw., 12 gru 2019 o 06:13 Zahid Rahman <zahidr1...@gmail.com> napisał(a): > > So my point is I have not been able to find accurate information , if > some > > one could furnish me a Java language specification or recommend a book > > which accurately describes these I would be grateful. > > Start with Gang of Four > http://www.blackwasp.co.uk/gofpatterns.aspx > > > Regards > -- > Łukasz > + 48 606 323 122 http://www.lenart.org.pl/ > > --------------------------------------------------------------------- > To unsubscribe, e-mail: user-unsubscr...@struts.apache.org > For additional commands, e-mail: user-h...@struts.apache.org > >
