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面试题:线程安全的单例模式

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作者 正文
   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
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 * 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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