锁定老帖子 主题:面试题:线程安全的单例模式
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发表时间:2009-12-01
public class ResourceFactory { private static class ResourceHolder { public static Resource resource = new Resource(); } public static Resource getResource() { return ResourceFactory.ResourceHolder.resource; } static class Resource { } } java concurrency in practice中建议的方式 |
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发表时间:2009-12-01
可以查一下java5以后的volatile关键字
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发表时间:2009-12-01
这样的问题,在闫宏的《java与模式》一书中已经讲得非常清晰和透彻了,楼主还有楼上的许多位同学还是应该多看看书的.....
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发表时间:2009-12-01
凤舞凰扬 写道 这样的问题,在闫宏的《java与模式》一书中已经讲得非常清晰和透彻了,楼主还有楼上的许多位同学还是应该多看看书的.....
有何高见,很想听你分析分析。 |
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发表时间:2009-12-01
其实这是两个问题,1.什么时候使用单例,单例有什么问题。2. 单例的写法。
1. 单例会导致这个类永远都不会被GC,如果过度使用代理会导致停留在堆区的类超级多。所以说,单例也要慎用。 2. 写法,满天飞。饱汉,饿汉,holder,enum都能实现。都有自己的说法。 |
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发表时间:2009-12-01
晨星★~雨泪 写道 凤舞凰扬 写道 这样的问题,在闫宏的《java与模式》一书中已经讲得非常清晰和透彻了,楼主还有楼上的许多位同学还是应该多看看书的.....
有何高见,很想听你分析分析。 java 不支持二次检查 生成单例 ,楼主的例子一次检查就更不用说了。 |
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发表时间:2009-12-01
macro.son 写道 为何不考虑用ThreadLocal?
这个才是正路~~ |
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发表时间:2009-12-01
最后修改:2009-12-01
看看tomcat里面sun公司的程序员实现的多例模式,
由于java语言的特性,双重检查锁是不行的,所以 看了它的实现,感觉只要在get的时候加上synchronized关键 字就可以了吧。 /* * The Apache Software License, Version 1.1 * * Copyright (c) 1999 The Apache Software Foundation. All rights * reserved. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions * are met: * * 1. Redistributions of source code must retain the above copyright * notice, this list of conditions and the following disclaimer. * * 2. Redistributions in binary form must reproduce the above copyright * notice, this list of conditions and the following disclaimer in * the documentation and/or other materials provided with the * distribution. * * 3. The end-user documentation included with the redistribution, if * any, must include the following acknowlegement: * "This product includes software developed by the * Apache Software Foundation (http://www.apache.org/)." * Alternately, this acknowlegement may appear in the software itself, * if and wherever such third-party acknowlegements normally appear. * * 4. The names "The Jakarta Project", "Tomcat", and "Apache Software * Foundation" must not be used to endorse or promote products derived * from this software without prior written permission. For written * permission, please contact apache@apache.org. * * 5. Products derived from this software may not be called "Apache" * nor may "Apache" appear in their names without prior written * permission of the Apache Group. * * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESSED OR IMPLIED * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE * DISCLAIMED. IN NO EVENT SHALL THE APACHE SOFTWARE FOUNDATION OR * ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF * USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF * SUCH DAMAGE. * ==================================================================== * * This software consists of voluntary contributions made by many * individuals on behalf of the Apache Software Foundation. For more * information on the Apache Software Foundation, please see * <http://www.apache.org/>. * * [Additional notices, if required by prior licensing conditions] * */ package org.apache.catalina.util; import java.text.MessageFormat; import java.util.Hashtable; import java.util.MissingResourceException; import java.util.ResourceBundle; /** * An internationalization / localization helper class which reduces * the bother of handling ResourceBundles and takes care of the * common cases of message formating which otherwise require the * creation of Object arrays and such. * * <p>The StringManager operates on a package basis. One StringManager * per package can be created and accessed via the getManager method * call. * * <p>The StringManager will look for a ResourceBundle named by * the package name given plus the suffix of "LocalStrings". In * practice, this means that the localized information will be contained * in a LocalStrings.properties file located in the package * directory of the classpath. * * <p>Please see the documentation for java.util.ResourceBundle for * more information. * * @author James Duncan Davidson [duncan@eng.sun.com] * @author James Todd [gonzo@eng.sun.com] */ public class StringManager { /** * The ResourceBundle for this StringManager. */ private ResourceBundle bundle; /** * Creates a new StringManager for a given package. This is a * private method and all access to it is arbitrated by the * static getManager method call so that only one StringManager * per package will be created. * * @param packageName Name of package to create StringManager for. */ private StringManager(String packageName) { String bundleName = packageName + ".LocalStrings"; bundle = ResourceBundle.getBundle(bundleName); } /** * Get a string from the underlying resource bundle. * * @param key */ public String getString(String key) { if (key == null) { String msg = "key is null"; throw new NullPointerException(msg); } String str = null; try { str = bundle.getString(key); } catch (MissingResourceException mre) { str = "Cannot find message associated with key '" + key + "'"; } return str; } /** * Get a string from the underlying resource bundle and format * it with the given set of arguments. * * @param key * @param args */ public String getString(String key, Object[] args) { String iString = null; String value = getString(key); // this check for the runtime exception is some pre 1.1.6 // VM's don't do an automatic toString() on the passed in // objects and barf out try { // ensure the arguments are not null so pre 1.2 VM's don't barf Object nonNullArgs[] = args; for (int i=0; i<args.length; i++) { if (args[i] == null) { if (nonNullArgs==args) nonNullArgs=(Object[])args.clone(); nonNullArgs[i] = "null"; } } iString = MessageFormat.format(value, nonNullArgs); } catch (IllegalArgumentException iae) { StringBuffer buf = new StringBuffer(); buf.append(value); for (int i = 0; i < args.length; i++) { buf.append(" arg[" + i + "]=" + args[i]); } iString = buf.toString(); } return iString; } /** * Get a string from the underlying resource bundle and format it * with the given object argument. This argument can of course be * a String object. * * @param key * @param arg */ public String getString(String key, Object arg) { Object[] args = new Object[] {arg}; return getString(key, args); } /** * Get a string from the underlying resource bundle and format it * with the given object arguments. These arguments can of course * be String objects. * * @param key * @param arg1 * @param arg2 */ public String getString(String key, Object arg1, Object arg2) { Object[] args = new Object[] {arg1, arg2}; return getString(key, args); } /** * Get a string from the underlying resource bundle and format it * with the given object arguments. These arguments can of course * be String objects. * * @param key * @param arg1 * @param arg2 * @param arg3 */ public String getString(String key, Object arg1, Object arg2, Object arg3) { Object[] args = new Object[] {arg1, arg2, arg3}; return getString(key, args); } /** * Get a string from the underlying resource bundle and format it * with the given object arguments. These arguments can of course * be String objects. * * @param key * @param arg1 * @param arg2 * @param arg3 * @param arg4 */ public String getString(String key, Object arg1, Object arg2, Object arg3, Object arg4) { Object[] args = new Object[] {arg1, arg2, arg3, arg4}; return getString(key, args); } // -------------------------------------------------------------- // STATIC SUPPORT METHODS // -------------------------------------------------------------- private static Hashtable managers = new Hashtable(); /** * Get the StringManager for a particular package. If a manager for * a package already exists, it will be reused, else a new * StringManager will be created and returned. * * @param packageName */ public synchronized static StringManager getManager(String packageName) { StringManager mgr = (StringManager)managers.get(packageName); if (mgr == null) { mgr = new StringManager(packageName); managers.put(packageName, mgr); } return mgr; } } |
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发表时间:2009-12-01
Singleton模式分两种,“懒汉”和“恶汉” 恶汉模式也就是前面很多人提到的
public class Singleton{ private static Singleton instance = new Singleton(); private Singleton(){ //私有化构造函数. }; public static Singleton getInstance(){ return instance; } } 由于实例在类加载时就已经创建,所以不存在线程同步问题,如果该Singleton初始化时不需要很大的开销(比如io操作/数据库连接)之类的,通常用这种办法就可以了. 某些情况下我们希望实例只有被第一次用到的时候才创建,那么这时候就使用“懒汉”模式,初学者通常会采用LZ的写法,正如面试官所说,这样会有线程同步问题,而导致多个实例被创建。如果在 getInstance()方法前面加上synchronize,又会大大降低系统性能。其实我们需要考虑的是,究竟要同步哪里? 我们只需要同步new Singleton()这个部分,保证只有一个实例被创建出来即可,而无须同步整个getInstance()方法,那么就可以考虑这样做 public static Singleton getInstance(){ if(instance == null){ synchronize{ if(instance == null){ instance = new Singleton(); } } } } |
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发表时间:2009-12-01
最后修改:2009-12-01
额,,楼上正解。
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