堆栈内存溢出
  繁体   English   中英

cache2k 和 Generic T 不能很好地一起玩

[英]cache2k and Generic T do not play nice together

 原文  2018-09-27 00:09:16  9   1    java/ generics/ java-8/ cache2k

问题描述

在 cache2k 用户指南中， §2.3。 Cache Aside有一个关于 cache-aside 的代码示例和句子。

 Cache<String, String> routeToAirline = new Cache2kBuilder<String, String>() {} .name("routeToAirline") .build(); private String findFavoriteAirline(String origin, String destination) { // expensive operation to find the best airline for this route // for example, ask all friends... } public String lookupFavoirteAirline(String origin, String destination) { String route = origin + "-" + destination; String airline = routeToAirline.peek(route); if (airline == null) { airline = findFavoriteAirline(origin, destination); routeToAirline.put(route, airline); } return airline; }

上述模式称为缓存旁白。

我喜欢“太棒了......我喜欢缓存 Aside 模式”。 所以我尝试使用泛型而不是硬编码（值）类型来实现一个。

我想出了这个使用泛型。 我注入了“获取”物品的方法（供应商）。 完全可重用的代码！

package mypackage;

import java.util.function.Supplier;

import org.cache2k.Cache;
import org.cache2k.Cache2kBuilder;

//import mypackage.interfaces.IGenericCacheAside;

public class Cache2kGenericCacheAside<TEntity> ////implements IGenericCacheAside<TEntity> {

    public final String CacheKeyPrefix = "GenericCacheAsidePrefix";

    private volatile Cache<String, TEntity> theCache = null; /* static not allowed for TEntity */

    public TEntity GetCacheAsideItem(String uniqueIdentifier, long itemLifeMiliseconds,
            final Supplier<TEntity> valueFactory) {
        this.initiateCacheObject();
        String cacheKey = this.GetFullCacheKey(uniqueIdentifier);
        TEntity cachedOrFreshItem = this.GetFromCache(cacheKey, itemLifeMiliseconds, valueFactory);
        return cachedOrFreshItem;
    }

    public TEntity RemoveCacheAsideItem(String uniqueIdentifier) {
        TEntity removedItem = null;
        String cacheKey = this.GetFullCacheKey(uniqueIdentifier);
        if (this.theCache.containsKey(uniqueIdentifier)) {
            removedItem = this.theCache.peekAndRemove(cacheKey);
        }
        return removedItem;
    }

    private void initiateCacheObject(/* long duration, TimeUnit tu, long capacity */) {
        if (null == this.theCache) {

            theCache = new Cache2kBuilder<String, TEntity>() {
            }.name("myCache").eternal(true).build();

        }
    }



    private TEntity GetFromCache(String cacheKey, long millis, final Supplier<TEntity> valueFactory) {
        TEntity cachedOrFreshItem = theCache.peek(cacheKey);
        if (cachedOrFreshItem == null) {
            cachedOrFreshItem = valueFactory.get();
            theCache.put(cacheKey, cachedOrFreshItem);
            theCache.expireAt(cacheKey, millis);
        }

        return cachedOrFreshItem;
    }

    private String GetFullCacheKey(String uniqueIdentifier) {
        String returnValue = CacheKeyPrefix + uniqueIdentifier;
        return returnValue;
    }
}

我收到一个运行时错误：

java.lang.IllegalArgumentException：运行时类型不可用，得到：TEntity

我想我可能已经偶然发现了这个以前未知的类型擦除世界。

有没有办法实现这个 Generic-CacheAside 代码？ 这个隐藏的宝石（擦除）是可怕的。

<dependency>
    <groupId>org.cache2k</groupId>
    <artifactId>cache2k-api</artifactId>
    <version>1.2.0.Final</version>
</dependency>

附加：

这是一个例子。 基本上，任何时候我都需要缓存一个不受检索参数驱动的“东西”。 在下面的示例中，我正在缓存 SystemSetting(s)。 没有参数来驱动检索。

private static int NewedUpCounter = 0;
private static int CurrentRunCacheReads = 0;

private static void RunCacheAsideStuff() {

    /* example ONLY. use construction-injection for "real" code */
    ////IGenericCacheAside<Collection<SystemSetting>> igca = new Cache2kGenericCacheAside<Collection<SystemSetting>>();
    /* or */
    Cache2kGenericCacheAside<Collection<SystemSetting>> igca = new Cache2kGenericCacheAside<Collection<SystemSetting>>();

    for (int i = 0; i < 20; i++) {

        /* in the below, it shows how the "time to keep in the cache" might change over time */
        int cacheMilliseconds = 2500 + (500 * i);
        System.out.println(String.format("        cacheMilliseconds=%1s",
                cacheMilliseconds));            
        Collection<SystemSetting> cacheAsideSettings = igca.GetCacheAsideItem("myuniqueIdentifier", cacheMilliseconds,
                TimeUnit.MILLISECONDS, App::CreateDummySystemSettings);

        if (null != cacheAsideSettings) {
            System.out.println("--------------");
            System.out.println(String.format("        CurrentRunCacheReads=%1s",
                    ++CurrentRunCacheReads));                   
            System.out.println(String.format("        Cached Collection<SystemSetting> Read .. size=%1s",
                    cacheAsideSettings.size()));
            for (SystemSetting sett : cacheAsideSettings) {
                System.out.println(String.format(
                        "cacheAsideSettings !! SystemSetting.Key=%1s, SystemSetting.Value = %2s, i = %3s , Time= %4s",
                        sett.getSystemSettingKey(), sett.getSettingValue(), i, LocalDateTime.now()));
            }
            System.out.println("--------------");
        }

        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            throw new RuntimeException(e);
        }
    }

    System.out.println(String.format("NewedUpCounter=%1s",
            NewedUpCounter));

}

private static Collection<SystemSetting> CreateDummySystemSettings() {

    NewedUpCounter++;
    CurrentRunCacheReads=0;

    long LOWER_RANGE = 10000; // assign lower range value
    long UPPER_RANGE = 20000; // assign upper range value
    Random random = new Random();

    long randomValue = LOWER_RANGE + (long) (random.nextDouble() * (UPPER_RANGE - LOWER_RANGE));

    Collection<SystemSetting> returnItems = new ArrayList<>();
    for (int i = 101; i < 104; i++) {
        SystemSetting newSetting = new SystemSetting();
        newSetting.setSystemSettingKey(i);
        newSetting.setSettingValue(String.format("ValueOf%1s*", randomValue));
        returnItems.add(newSetting);
    }

    System.out.println(String.format("NEW Collection<SystemSetting> CREATED !! size=%1s  ************************************************", returnItems.size()));

    return returnItems;
}

和输出。 显示项目/缓存时间的示例可以更改。

        cacheMilliseconds=2500
NEW Collection<SystemSetting> CREATED !! size=3  ************************************************
--------------
        CurrentRunCacheReads=1
        Cached Collection<SystemSetting> Read .. size=3
cacheAsideSettings !! SystemSetting.Key=101, SystemSetting.Value = ValueOf14000*, i =   0 , Time= 2018-09-28T13:04:14.043
cacheAsideSettings !! SystemSetting.Key=102, SystemSetting.Value = ValueOf14000*, i =   0 , Time= 2018-09-28T13:04:14.052
cacheAsideSettings !! SystemSetting.Key=103, SystemSetting.Value = ValueOf14000*, i =   0 , Time= 2018-09-28T13:04:14.052
--------------
        cacheMilliseconds=3000
--------------
        CurrentRunCacheReads=2
        Cached Collection<SystemSetting> Read .. size=3
cacheAsideSettings !! SystemSetting.Key=101, SystemSetting.Value = ValueOf14000*, i =   1 , Time= 2018-09-28T13:04:15.052
cacheAsideSettings !! SystemSetting.Key=102, SystemSetting.Value = ValueOf14000*, i =   1 , Time= 2018-09-28T13:04:15.053
cacheAsideSettings !! SystemSetting.Key=103, SystemSetting.Value = ValueOf14000*, i =   1 , Time= 2018-09-28T13:04:15.053
--------------
        cacheMilliseconds=3500
--------------
        CurrentRunCacheReads=3
        Cached Collection<SystemSetting> Read .. size=3
cacheAsideSettings !! SystemSetting.Key=101, SystemSetting.Value = ValueOf14000*, i =   2 , Time= 2018-09-28T13:04:16.054
cacheAsideSettings !! SystemSetting.Key=102, SystemSetting.Value = ValueOf14000*, i =   2 , Time= 2018-09-28T13:04:16.054
cacheAsideSettings !! SystemSetting.Key=103, SystemSetting.Value = ValueOf14000*, i =   2 , Time= 2018-09-28T13:04:16.054
--------------
        cacheMilliseconds=4000
NEW Collection<SystemSetting> CREATED !! size=3  ************************************************
--------------
        CurrentRunCacheReads=1
        Cached Collection<SystemSetting> Read .. size=3
cacheAsideSettings !! SystemSetting.Key=101, SystemSetting.Value = ValueOf17155*, i =   3 , Time= 2018-09-28T13:04:17.055
cacheAsideSettings !! SystemSetting.Key=102, SystemSetting.Value = ValueOf17155*, i =   3 , Time= 2018-09-28T13:04:17.055
cacheAsideSettings !! SystemSetting.Key=103, SystemSetting.Value = ValueOf17155*, i =   3 , Time= 2018-09-28T13:04:17.055
--------------
        cacheMilliseconds=4500
--------------
        CurrentRunCacheReads=2
        Cached Collection<SystemSetting> Read .. size=3
cacheAsideSettings !! SystemSetting.Key=101, SystemSetting.Value = ValueOf17155*, i =   4 , Time= 2018-09-28T13:04:18.056
cacheAsideSettings !! SystemSetting.Key=102, SystemSetting.Value = ValueOf17155*, i =   4 , Time= 2018-09-28T13:04:18.056
cacheAsideSettings !! SystemSetting.Key=103, SystemSetting.Value = ValueOf17155*, i =   4 , Time= 2018-09-28T13:04:18.056
--------------
        cacheMilliseconds=5000
--------------
        CurrentRunCacheReads=3
        Cached Collection<SystemSetting> Read .. size=3
cacheAsideSettings !! SystemSetting.Key=101, SystemSetting.Value = ValueOf17155*, i =   5 , Time= 2018-09-28T13:04:19.057
cacheAsideSettings !! SystemSetting.Key=102, SystemSetting.Value = ValueOf17155*, i =   5 , Time= 2018-09-28T13:04:19.058
cacheAsideSettings !! SystemSetting.Key=103, SystemSetting.Value = ValueOf17155*, i =   5 , Time= 2018-09-28T13:04:19.058
--------------
        cacheMilliseconds=5500
--------------
        CurrentRunCacheReads=4
        Cached Collection<SystemSetting> Read .. size=3
cacheAsideSettings !! SystemSetting.Key=101, SystemSetting.Value = ValueOf17155*, i =   6 , Time= 2018-09-28T13:04:20.058
cacheAsideSettings !! SystemSetting.Key=102, SystemSetting.Value = ValueOf17155*, i =   6 , Time= 2018-09-28T13:04:20.058
cacheAsideSettings !! SystemSetting.Key=103, SystemSetting.Value = ValueOf17155*, i =   6 , Time= 2018-09-28T13:04:20.058
--------------
        cacheMilliseconds=6000
NEW Collection<SystemSetting> CREATED !! size=3  ************************************************
--------------
        CurrentRunCacheReads=1
        Cached Collection<SystemSetting> Read .. size=3
cacheAsideSettings !! SystemSetting.Key=101, SystemSetting.Value = ValueOf17444*, i =   7 , Time= 2018-09-28T13:04:21.059
cacheAsideSettings !! SystemSetting.Key=102, SystemSetting.Value = ValueOf17444*, i =   7 , Time= 2018-09-28T13:04:21.060
cacheAsideSettings !! SystemSetting.Key=103, SystemSetting.Value = ValueOf17444*, i =   7 , Time= 2018-09-28T13:04:21.060
--------------
        cacheMilliseconds=6500
--------------
        CurrentRunCacheReads=2
        Cached Collection<SystemSetting> Read .. size=3
cacheAsideSettings !! SystemSetting.Key=101, SystemSetting.Value = ValueOf17444*, i =   8 , Time= 2018-09-28T13:04:22.060
cacheAsideSettings !! SystemSetting.Key=102, SystemSetting.Value = ValueOf17444*, i =   8 , Time= 2018-09-28T13:04:22.060
cacheAsideSettings !! SystemSetting.Key=103, SystemSetting.Value = ValueOf17444*, i =   8 , Time= 2018-09-28T13:04:22.060
--------------
        cacheMilliseconds=7000
--------------
        CurrentRunCacheReads=3
        Cached Collection<SystemSetting> Read .. size=3
cacheAsideSettings !! SystemSetting.Key=101, SystemSetting.Value = ValueOf17444*, i =   9 , Time= 2018-09-28T13:04:23.060
cacheAsideSettings !! SystemSetting.Key=102, SystemSetting.Value = ValueOf17444*, i =   9 , Time= 2018-09-28T13:04:23.060
cacheAsideSettings !! SystemSetting.Key=103, SystemSetting.Value = ValueOf17444*, i =   9 , Time= 2018-09-28T13:04:23.060
--------------
        cacheMilliseconds=7500
--------------
        CurrentRunCacheReads=4
        Cached Collection<SystemSetting> Read .. size=3
cacheAsideSettings !! SystemSetting.Key=101, SystemSetting.Value = ValueOf17444*, i =  10 , Time= 2018-09-28T13:04:24.060
cacheAsideSettings !! SystemSetting.Key=102, SystemSetting.Value = ValueOf17444*, i =  10 , Time= 2018-09-28T13:04:24.061
cacheAsideSettings !! SystemSetting.Key=103, SystemSetting.Value = ValueOf17444*, i =  10 , Time= 2018-09-28T13:04:24.061
--------------
        cacheMilliseconds=8000
--------------
        CurrentRunCacheReads=5
        Cached Collection<SystemSetting> Read .. size=3
cacheAsideSettings !! SystemSetting.Key=101, SystemSetting.Value = ValueOf17444*, i =  11 , Time= 2018-09-28T13:04:25.061
cacheAsideSettings !! SystemSetting.Key=102, SystemSetting.Value = ValueOf17444*, i =  11 , Time= 2018-09-28T13:04:25.061
cacheAsideSettings !! SystemSetting.Key=103, SystemSetting.Value = ValueOf17444*, i =  11 , Time= 2018-09-28T13:04:25.061
--------------
        cacheMilliseconds=8500
--------------
        CurrentRunCacheReads=6
        Cached Collection<SystemSetting> Read .. size=3
cacheAsideSettings !! SystemSetting.Key=101, SystemSetting.Value = ValueOf17444*, i =  12 , Time= 2018-09-28T13:04:26.063
cacheAsideSettings !! SystemSetting.Key=102, SystemSetting.Value = ValueOf17444*, i =  12 , Time= 2018-09-28T13:04:26.063
cacheAsideSettings !! SystemSetting.Key=103, SystemSetting.Value = ValueOf17444*, i =  12 , Time= 2018-09-28T13:04:26.064
--------------
        cacheMilliseconds=9000
NEW Collection<SystemSetting> CREATED !! size=3  ************************************************
--------------
        CurrentRunCacheReads=1
        Cached Collection<SystemSetting> Read .. size=3
cacheAsideSettings !! SystemSetting.Key=101, SystemSetting.Value = ValueOf14680*, i =  13 , Time= 2018-09-28T13:04:27.065
cacheAsideSettings !! SystemSetting.Key=102, SystemSetting.Value = ValueOf14680*, i =  13 , Time= 2018-09-28T13:04:27.065
cacheAsideSettings !! SystemSetting.Key=103, SystemSetting.Value = ValueOf14680*, i =  13 , Time= 2018-09-28T13:04:27.066
--------------
        cacheMilliseconds=9500
--------------
        CurrentRunCacheReads=2
        Cached Collection<SystemSetting> Read .. size=3
cacheAsideSettings !! SystemSetting.Key=101, SystemSetting.Value = ValueOf14680*, i =  14 , Time= 2018-09-28T13:04:28.066
cacheAsideSettings !! SystemSetting.Key=102, SystemSetting.Value = ValueOf14680*, i =  14 , Time= 2018-09-28T13:04:28.066
cacheAsideSettings !! SystemSetting.Key=103, SystemSetting.Value = ValueOf14680*, i =  14 , Time= 2018-09-28T13:04:28.066
--------------
        cacheMilliseconds=10000
--------------
        CurrentRunCacheReads=3
        Cached Collection<SystemSetting> Read .. size=3
cacheAsideSettings !! SystemSetting.Key=101, SystemSetting.Value = ValueOf14680*, i =  15 , Time= 2018-09-28T13:04:29.067
cacheAsideSettings !! SystemSetting.Key=102, SystemSetting.Value = ValueOf14680*, i =  15 , Time= 2018-09-28T13:04:29.067
cacheAsideSettings !! SystemSetting.Key=103, SystemSetting.Value = ValueOf14680*, i =  15 , Time= 2018-09-28T13:04:29.067
--------------
        cacheMilliseconds=10500
--------------
        CurrentRunCacheReads=4
        Cached Collection<SystemSetting> Read .. size=3
cacheAsideSettings !! SystemSetting.Key=101, SystemSetting.Value = ValueOf14680*, i =  16 , Time= 2018-09-28T13:04:30.068
cacheAsideSettings !! SystemSetting.Key=102, SystemSetting.Value = ValueOf14680*, i =  16 , Time= 2018-09-28T13:04:30.068
cacheAsideSettings !! SystemSetting.Key=103, SystemSetting.Value = ValueOf14680*, i =  16 , Time= 2018-09-28T13:04:30.068
--------------
        cacheMilliseconds=11000
--------------
        CurrentRunCacheReads=5
        Cached Collection<SystemSetting> Read .. size=3
cacheAsideSettings !! SystemSetting.Key=101, SystemSetting.Value = ValueOf14680*, i =  17 , Time= 2018-09-28T13:04:31.068
cacheAsideSettings !! SystemSetting.Key=102, SystemSetting.Value = ValueOf14680*, i =  17 , Time= 2018-09-28T13:04:31.068
cacheAsideSettings !! SystemSetting.Key=103, SystemSetting.Value = ValueOf14680*, i =  17 , Time= 2018-09-28T13:04:31.068
--------------
        cacheMilliseconds=11500
--------------
        CurrentRunCacheReads=6
        Cached Collection<SystemSetting> Read .. size=3
cacheAsideSettings !! SystemSetting.Key=101, SystemSetting.Value = ValueOf14680*, i =  18 , Time= 2018-09-28T13:04:32.068
cacheAsideSettings !! SystemSetting.Key=102, SystemSetting.Value = ValueOf14680*, i =  18 , Time= 2018-09-28T13:04:32.068
cacheAsideSettings !! SystemSetting.Key=103, SystemSetting.Value = ValueOf14680*, i =  18 , Time= 2018-09-28T13:04:32.068
--------------
        cacheMilliseconds=12000
--------------
        CurrentRunCacheReads=7
        Cached Collection<SystemSetting> Read .. size=3
cacheAsideSettings !! SystemSetting.Key=101, SystemSetting.Value = ValueOf14680*, i =  19 , Time= 2018-09-28T13:04:33.069
cacheAsideSettings !! SystemSetting.Key=102, SystemSetting.Value = ValueOf14680*, i =  19 , Time= 2018-09-28T13:04:33.069
cacheAsideSettings !! SystemSetting.Key=103, SystemSetting.Value = ValueOf14680*, i =  19 , Time= 2018-09-28T13:04:33.069
--------------
NewedUpCounter=4 /* this last one not accurate since its "bailing out */

1 个解决方案

解决方案1
 3  2018-10-26 23:48:09

匿名类创建表达式new Cache2kBuilder<String, String>() {}用作“超类型标记”以在运行时表示参数化类型。 他们这样做的方法是创建一个扩展参数化类型的子类（通常是匿名类），其中类型参数是在编译时固定（硬编码）的具体类型。 由于类的超类是类声明的一部分，所以它与类文件的声明部分中的泛型信息一起存储，并且可以在运行时通过反射检索这些信息。

请注意，在运行时可以从类文件中检索到的内容正是在编译时硬编码的内容。 这就是new Cache2kBuilder<String, TEntity>() {}不起作用的原因——在编译时源代码中硬编码的是有一个名为TEntity的类型变量，而不是一个具体的类。

Cache2k 提供了一种不同的方法来构造Cache2kBuilder如果该类不能在编译时修复，并且只能在运行时知道：

theCache = Cache2kBuilder.of(String.class, entityClass)
  .name("myCache").eternal(true).build();

其中entityClass是Class<TEntity> ，它是运行时实体类的类对象。 因此，您还必须传入实体的Class对象，而不仅仅是它的Supplier 。

暂无
暂无

声明:本站的技术帖子网页，遵循CC BY-SA 4.0协议，如果您需要转载，请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.

相关问题 cache2k 是持久缓存吗？ 如何将cache2k与休眠集成 一次重新加载所有 cache2k 条目 cache2k cachesource使用mysql作为后端 如何在cache2k中定义持久性存储？ 如何访问cache2k的内部状态 如何使用 cache2k 对过滤器进行单元测试 cache2k put（）是否更新到期时间 Cache2K auto refreshAhead 没有按预期工作 我如何让Groovy和JAXB一起玩得很好
相关标签
 
粤ICP备18138465号  © 2020-2024 STACKOOM.COM