java - 无法理解 volatile 字段的工作原理以及它们在共享值时如何与多个线程一起工作

Question

我正在开发一个程序，需要从多个线程中的选项文件加载相同的信息。因此，我基于抽象模型对每个文件做了一个简单的类表示:(下面显示的是其中许多类的 1 个示例)

package OptionManager;

import java.util.Properties;

public class EngineOptions extends AbstractOptions{
    //values
    private static String debugEnabled;
    private static String debugAvgLoadtime;
    private static String showShaderUsed;
    private static String mainLanguage;

    //keys
    public static final String DEBUGENABLED_KEY = "debugEnabled";
    public static final String DEBUGAVGLOADTIME_KEY = "debugAvgLoadtime";
    public static final String SHOWSHADERUSED_KEY = "showShaderUsed";
    public static final String MAINLANGUAGE_KEY = "mainLanguage";

    public static String getProperty(String key) {
        return properties.getProperty(key);
    }

    public static void loadFromFile(String filename) {      
        OptionReader loader = new OptionReader(filename);
        //load properties
        debugEnabled = loader.getProperty(DEBUGENABLED_KEY);
        debugAvgLoadtime = loader.getProperty(DEBUGAVGLOADTIME_KEY);
        showShaderUsed = loader.getProperty(SHOWSHADERUSED_KEY);
        mainLanguage = loader.getProperty(MAINLANGUAGE_KEY);

        properties.put(DEBUGENABLED_KEY, debugEnabled);
        properties.put(DEBUGAVGLOADTIME_KEY, debugAvgLoadtime);
        properties.put(SHOWSHADERUSED_KEY, showShaderUsed);
        properties.put(MAINLANGUAGE_KEY, mainLanguage);
    }
}

这是它使用的抽象类:

package OptionManager;

import java.util.Properties;

public abstract class AbstractOptions {
    protected static volatile Properties properties;

    public static void setupProperties() {
        properties = new Properties();
    }

    public static void setupProperties(Properties properties) {}

    public static void setProperty(String key, String value) {
        if(properties.getProperty(key) == null) {
            //throw exception.
        }
        properties.setProperty(key, value);
    }

    public static String getProperty(String key) {
        System.out.println(properties);
        return properties.getProperty(key);
    }

    //public static void loadFromFile(String filename) {}
}

我能够在主线程(JVM 为了启动程序而创建的线程)中加载选项文件，并在该线程中使用该系统来获取我想要的所有选项。事实上，作为测试，每次访问给出以下输出的选项时，我都会打印 EngineOptions 类中的整个选项列表(当获取 mainLanguage 选项时):

{mainLanguage=language_IT, debugAvgLoadtime=1, debugEnabled=1, showShaderUsed=1} language_IT

但是，当我尝试访问另一个线程中的相同选项(由主线程创建并在打印上面的输出后启动)时，我得到:

{} null

这让我相信对于每个线程，我的静态字段的值不是共享的。所以我找到了this answer建议使用“ volatile ”来解决问题。然而这并没有奏效。

另一个选择是在每个线程中加载选项文件，但我不想这样做，因为我必须为每个线程多次加载选项文件的原因是我首先创建这个系统的原因。

如何解决此问题并在多个线程上共享选项的值？

编辑:

如何创建新选项列表:

OptionHandler.addOptionFile(OptionHandler.ENGINE_OPTION_ID, new EngineOptions(), "EngineOptions");
OptionHandler.loadOptionListFromFile(OptionHandler.ENGINE_OPTION_ID, OptionHandler.ENGINE_OPTION_TYPE);

我如何从线程调用选项:

String currentLang = OptionHandler.getProperty(EngineOptions.MAINLANGUAGE_KEY, OptionHandler.ENGINE_OPTION_ID);
        System.out.println(currentLang);

编辑2:

package OptionManager;

import java.util.HashMap;
import java.util.Map;

public class OptionHandler {
    private static HashMap<Integer, AbstractOptions> optionList;
    private static HashMap<Integer, String> optionFilename;

    //OptionFIleID's (Starting from 101 to 199)
    public static final int GRAPHIC_OPTION_ID = 101;
    public static final int ENGINE_OPTION_ID = 102;
    public static final int CURRENT_LANGUAGE_ID = 103;

    public static final int GRAPHIC_OPTION_TYPE = 201;
    public static final int ENGINE_OPTION_TYPE = 202;
    public static final int CURRENT_LANGUAGE_TYPE = 203;

    public static void setupOptions() {
        optionList = new HashMap<Integer, AbstractOptions>();
        optionFilename = new HashMap<Integer, String>();
    }

    public static void addOptionFile(int id, AbstractOptions options, String filename) {
        options.setupProperties();
        optionList.put(id, options);
        optionFilename.put(id, filename);
    }

    public static String getProperty(String optionKey, int optionFileID) {
        return optionList.get(optionFileID).getProperty(optionKey);
    }

    public static void loadOptionListFromFile(int id, int type) {
        System.out.println(optionFilename.get(id));
        if(type == GRAPHIC_OPTION_TYPE)
            GraphicOptions.loadFromFile(optionFilename.get(id));
        if(type == ENGINE_OPTION_TYPE)
            EngineOptions.loadFromFile(optionFilename.get(id));
        if(type == CURRENT_LANGUAGE_TYPE)
            CurrentLanguage.loadFromFile(optionFilename.get(id));
    }
}

Answer 1

这是一个线程同步问题，如果能保证主线程中的加载选项 Action 先于其他线程中的读取 Action 发生，那么使用volatile就没有问题。如果不能，最好使用线程同步 API，例如 CountDownLatch。这是我对您的代码的修改，重点是 OptionHandler 类，我的输出是正确的。

import java.util.Properties;

public abstract class AbstractOptions {
    protected volatile Properties properties;

    public void setupProperties() {
        properties = new Properties();
    }

    public void setupProperties(Properties properties) {}

    public void setProperty(String key, String value) {
        if(properties.getProperty(key) == null) {
            //throw exception.
        }
        properties.setProperty(key, value);
    }

    public String getProperty(String key) {
        System.out.println(properties);
        return properties.getProperty(key);
    }

    public abstract void loadFromFile(String filename);
}


public class EngineOptions extends AbstractOptions{
    //values
    private String debugEnabled;
    private String debugAvgLoadtime;
    private String showShaderUsed;
    private String mainLanguage;

    //keys
    public static final String DEBUGENABLED_KEY = "debugEnabled";
    public static final String DEBUGAVGLOADTIME_KEY = "debugAvgLoadtime";
    public static final String SHOWSHADERUSED_KEY = "showShaderUsed";
    public static final String MAINLANGUAGE_KEY = "mainLanguage";

    //public String getProperty(String key) {
    //return properties.getProperty(key);
    //}

    public void loadFromFile(String filename) {
        //OptionReader loader = new OptionReader(filename);
        //load properties
        debugEnabled = "language_IT";//loader.getProperty(DEBUGENABLED_KEY);
        debugAvgLoadtime = "1";//loader.getProperty(DEBUGAVGLOADTIME_KEY);
        showShaderUsed = "1";//loader.getProperty(SHOWSHADERUSED_KEY);
        mainLanguage = "1";//loader.getProperty(MAINLANGUAGE_KEY);

        properties.put(DEBUGENABLED_KEY, debugEnabled);
        properties.put(DEBUGAVGLOADTIME_KEY, debugAvgLoadtime);
        properties.put(SHOWSHADERUSED_KEY, showShaderUsed);
        properties.put(MAINLANGUAGE_KEY, mainLanguage);
    }
}

import java.util.HashMap;
import java.util.concurrent.CountDownLatch;

public class OptionHandler {
    private static HashMap<Integer, AbstractOptions> optionList;
    private static HashMap<Integer, String> optionFilename;
    private static CountDownLatch sync;

    static {
        setupOptions();
    }

    //OptionFIleID's (Starting from 101 to 199)
    public static final int GRAPHIC_OPTION_ID = 101;
    public static final int ENGINE_OPTION_ID = 102;
    public static final int CURRENT_LANGUAGE_ID = 103;

    public static final int GRAPHIC_OPTION_TYPE = 201;
    public static final int ENGINE_OPTION_TYPE = 202;
    public static final int CURRENT_LANGUAGE_TYPE = 203;

    public static void setupOptions() {
        optionList = new HashMap<Integer, AbstractOptions>();
        optionFilename = new HashMap<Integer, String>();
        //initialize
        sync = new CountDownLatch(1);
    }

    public static void addOptionFile(int id, AbstractOptions options, String filename) {
        options.setupProperties();
        optionList.put(id, options);
        optionFilename.put(id, filename);
    }

    public static String getProperty(String optionKey, int optionFileID) {
        try {
            //await when the property is not ready yet
            sync.await();
        } catch (InterruptedException e) {
            //log("thread was interrupted")
            Thread.currentThread().interrupt();
        }
        return optionList.get(optionFileID).getProperty(optionKey);
    }

    public static void loadOptionListFromFile(int id, int type) {
        System.out.println(optionFilename.get(id));
//        if(type == GRAPHIC_OPTION_TYPE)
//            GraphicOptions.loadFromFile(optionFilename.get(id));
        if(type == ENGINE_OPTION_TYPE)
            optionList.get(id).loadFromFile(optionFilename.get(id));
//        if(type == CURRENT_LANGUAGE_TYPE)
//            CurrentLanguage.loadFromFile(optionFilename.get(id));
        //Notify other threads that the property is ready
        sync.countDown();
    }

    public static void main(String[] args) throws InterruptedException {
        new Thread(() -> {
            String currentLang = 
                  OptionHandler.getProperty(EngineOptions.MAINLANGUAGE_KEY, 
                  OptionHandler.ENGINE_OPTION_ID);
            System.out.println(currentLang);
        }).start();
        Thread.sleep(3000);
        OptionHandler.addOptionFile(OptionHandler.ENGINE_OPTION_ID, new EngineOptions(), "EngineOptions");
        OptionHandler.loadOptionListFromFile(OptionHandler.ENGINE_OPTION_ID, OptionHandler.ENGINE_OPTION_TYPE);
    }
}