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社区首页 >问答首页 >cache2k和泛型T不能一起玩得很好

问cache2k和泛型T不能一起玩得很好
EN

Stack Overflow用户
提问于 2018-09-27 00:09:16
回答 1查看 646关注 0票数 2

在cache2k用户指南中，第2.3条.缓存暂存有一个关于缓存的代码示例和句子。

Cache routeToAirline =新Cache2kBuilder() {} .name("routeToAirline") .build()；私有字符串findFavoriteAirline(字符串来源，字符串目的地){ //昂贵的操作，以找到这条航线的最佳航线//例如，询问所有朋友.}公共字符串lookupFavoirteAirline(字符串来源，字符串目的地){字符串路由=原产地+“+目的地；字符串航空公司=routeToAirline.peek(路由)；if (航空公司== null) {航空公司=findFavoriteAirline(原产地，目的地)；routeToAirline.put(航线，航空公司)；}返回航空公司；} 上面的模式称为缓存。

我就像“伟大的.我喜欢缓存旁边的模式”。因此，我尝试使用泛型来实现一个，而不是硬编码(值)类型。

我是用仿制药想出来的。然后注入“获取”项目(供应商)的方法。完全可重用的代码！

代码语言：javascript
复制
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;
    }
}

我得到一个运行时错误：

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

我想我可能是跌跌撞撞地走进了这个以前未知的类型擦除世界。

有任何方法来实现这个通用的-CacheAside代码吗？这个隐藏的宝石(擦除)是可怕的。

代码语言：javascript
复制
<dependency>
    <groupId>org.cache2k</groupId>
    <artifactId>cache2k-api</artifactId>
    <version>1.2.0.Final</version>
</dependency>

附录：

下面是一个例子。基本上，任何时候我都需要缓存一个不是由参数驱动的“东西”来进行检索。在下面的示例中，我正在缓存SystemSetting。没有参数来驱动检索。

代码语言：javascript
复制
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/缓存时间的示例可以更改。

代码语言：javascript
复制
        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 */
java-8
cache2k
java
generics
EN

回答 1

Stack Overflow用户

发布于 2018-10-26 23:48:09

匿名类创建表达式new Cache2kBuilder<String, String>() {}用作“超级类型令牌”，用于在运行时表示参数化类型。他们这样做的方式是创建一个子类(通常是匿名类)，它扩展了一个参数化类型，其中类型参数是在编译时固定的具体类型(硬编码)。因为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，并且可以通过反射在运行时检索这些信息。

请注意，可以在运行时从类文件中检索的正是编译时硬编码的内容。这就是为什么new Cache2kBuilder<String, TEntity>() {}不能工作的原因--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。

如果类在编译时无法修复，并且只能在运行时知道，Cache2k提供了一种不同的构造Cache2kBuilder的方法：

代码语言：javascript
复制
theCache = Cache2kBuilder.of(String.class, entityClass)
  .name("myCache").eternal(true).build();

其中entityClass是一个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，而不仅仅是实体的Supplier。

票数 4
EN
页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：

https://stackoverflow.com/questions/52527614

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