JSR 354定義了一套新的Java貨幣API，計(jì)劃會(huì)在Java 9中正式引入。本文中我們將來(lái)看一下它的參考實(shí)現(xiàn)：JavaMoney的當(dāng)前進(jìn)展。

　　正如我在之前那篇Java 8新的日期時(shí)間API一文中那樣，本文主要也是通過(guò)一些代碼來(lái)演示下新的API的用法 。

　　在開(kāi)始之前，我想先用一段話來(lái)簡(jiǎn)短地總結(jié)一下規(guī)范定義的這套新的API的用意何在：

對(duì)許多應(yīng)用而言貨幣價(jià)值都是一個(gè)關(guān)鍵的特性，但JDK對(duì)此卻幾乎沒(méi)有任何支持。嚴(yán)格來(lái)講，現(xiàn)有的java.util.Currency類只是代表了當(dāng)前ISO 4217貨幣的一個(gè)數(shù)據(jù)結(jié)構(gòu)，但并沒(méi)有關(guān)聯(lián)的值或者自定義貨幣。JDK對(duì)貨幣的運(yùn)算及轉(zhuǎn)換也沒(méi)有內(nèi)建的支持，更別說(shuō)有一個(gè)能夠代表貨幣值的標(biāo)準(zhǔn)類型了。

　　如果你用的是Maven的話，只需把下面的引用添加到工里面便能夠體驗(yàn)下該參考實(shí)現(xiàn)的當(dāng)前功能了：

<dependency>
  <groupId>org.javamoney</groupId>
  <artifactId>moneta</artifactId>
  <version>0.9</version>
</dependency>

　　規(guī)范中提到的類及接口都在javax.money.*包下面。

　　我們先從核心的兩個(gè)接口CurrencyUnit與MonetaryAmount開(kāi)始講起。

　　CurrencyUnit及MonetaryAmount

　　CurrencyUnit代表的是貨幣。它有點(diǎn)類似于現(xiàn)在的java.util.Currency類，不同之處在于它支持自定義的實(shí)現(xiàn)。從規(guī)范的定義來(lái)看，java.util.Currency也是可以實(shí)現(xiàn)該接口的。CurrencyUnit的實(shí)例可以通過(guò)MonetaryCurrencies工廠來(lái)獲�。�

// 根據(jù)貨幣代碼來(lái)獲取貨幣單位 CurrencyUnit euro = MonetaryCurrencies.getCurrency("EUR");
    CurrencyUnit usDollar = MonetaryCurrencies.getCurrency("USD"); // 根據(jù)國(guó)家及地區(qū)來(lái)獲取貨幣單位
    CurrencyUnit yen = MonetaryCurrencies.getCurrency(Locale.JAPAN); CurrencyUnit
    canadianDollar = MonetaryCurrencies.getCurrency(Locale.CANADA);

　　MontetaryAmount代表的是某種貨幣的具體金額。通常它都會(huì)與某個(gè)CurrencyUnit綁定。

　　MontetaryAmount和CurrencyUnit一樣，也是一個(gè)能支持多種實(shí)現(xiàn)的接口。

　　CurrencyUnit與MontetaryAmount的實(shí)現(xiàn)必須是不可變，線程安全且可比較的。

/ get MonetaryAmount from CurrencyUnit
CurrencyUnit euro = MonetaryCurrencies.getCurrency("EUR");
MonetaryAmount fiveEuro = Money.of(5, euro);
 
// get MonetaryAmount from currency code
MonetaryAmount tenUsDollar = Money.of(10, "USD");
 
// FastMoney is an alternative MonetaryAmount factory that focuses on performance
MonetaryAmount sevenEuro = FastMoney.of(7, euro);

　　Money與FastMoney是JavaMoney庫(kù)中MonetaryAmount的兩種實(shí)現(xiàn)。Money是默認(rèn)實(shí)現(xiàn)，它使用BigDecimal來(lái)存儲(chǔ)金額。FastMoney是可選的另一個(gè)實(shí)現(xiàn)，它用long類型來(lái)存儲(chǔ)金額。根據(jù)文檔來(lái)看，F(xiàn)astMoney上的操作要比Money的快10到15倍左右。然而，F(xiàn)astMoney的金額大小與精度都受限于long類型。

　　注意了，這里的Money和FastMoney都是具體的實(shí)現(xiàn)類（它們?cè)趏rg.javamoney.moneta.*包下面，而不是javax.money.*）。如果你不希望指定具體類型的話，可以通過(guò)MonetaryAmountFactory來(lái)生成一個(gè)MonetaryAmount的實(shí)例:

MonetaryAmount specAmount = MonetaryAmounts.getDefaultAmountFactory()
                .setNumber(123.45) .setCurrency("USD") .create();

　　當(dāng)且僅當(dāng)實(shí)現(xiàn)類，貨幣單位，以及數(shù)值全部相等時(shí)才認(rèn)為這兩個(gè)MontetaryAmount實(shí)例是相等的。

MonetaryAmount oneEuro = Money.of(1, MonetaryCurrencies.getCurrency("EUR"));
boolean isEqual = oneEuro.equals(Money.of(1, "EUR")); // true
boolean isEqualFast = oneEuro.equals(FastMoney.of(1, "EUR")); // false

　　MonetaryAmount內(nèi)包含豐富的方法，可以用來(lái)獲取具體的貨幣，金額，精度等等：

MonetaryAmount monetaryAmount = Money.of(123.45, euro);
CurrencyUnit currency = monetaryAmount.getCurrency();
NumberValue numberValue = monetaryAmount.getNumber();
 
int intValue = numberValue.intValue(); // 123
double doubleValue = numberValue.doubleValue(); // 123.45
long fractionDenominator = numberValue.getAmountFractionDenominator(); // 100
long fractionNumerator = numberValue.getAmountFractionNumerator(); // 45
int precision = numberValue.getPrecision(); // 5
 
// NumberValue extends java.lang.Number. 
// So we assign numberValue to a variable of type Number
Number number = numberValue;

　　MonetaryAmount的使用

　　可以在MonetaryAmount上進(jìn)行算術(shù)運(yùn)算： 

MonetaryAmount twelveEuro = fiveEuro.add(sevenEuro); // "EUR 12"
MonetaryAmount twoEuro = sevenEuro.subtract(fiveEuro); // "EUR 2"
MonetaryAmount sevenPointFiveEuro = fiveEuro.multiply(1.5); // "EUR 7.5"
 
// MonetaryAmount can have a negative NumberValue
MonetaryAmount minusTwoEuro = fiveEuro.subtract(sevenEuro); // "EUR -2"
 
// some useful utility methods
boolean greaterThan = sevenEuro.isGreaterThan(fiveEuro); // true
boolean positive = sevenEuro.isPositive(); // true
boolean zero = sevenEuro.isZero(); // false
 
// Note that MonetaryAmounts need to have the same CurrencyUnit to do mathematical operations
// this fails with: javax.money.MonetaryException: Currency mismatch: EUR/USD
fiveEuro.add(tenUsDollar);

　　舍入操作是金額換算里面非常重要的一部分。MonetaryAmount可以使用舍入操作符來(lái)進(jìn)行四舍五入：

CurrencyUnit usd = MonetaryCurrencies.getCurrency("USD");
MonetaryAmount dollars = Money.of(12.34567, usd);
MonetaryOperator roundingOperator = MonetaryRoundings.getRounding(usd);
MonetaryAmount roundedDollars = dollars.with(roundingOperator); // USD 12.35

　　這里12.3456美金就會(huì)按當(dāng)前貨幣默認(rèn)的舍入規(guī)則來(lái)進(jìn)行換算。

　　在操作MonetaryAmount集合時(shí)，有許多實(shí)用的工具方法可以用來(lái)進(jìn)行過(guò)濾，排序以及分組。這些方法還可以與Java 8的流API一起配套使用。

　　看一下下面這個(gè)集合：

List<MonetaryAmount> amounts = new ArrayList<>();
amounts.add(Money.of(2, "EUR"));
amounts.add(Money.of(42, "USD"));
amounts.add(Money.of(7, "USD"));
amounts.add(Money.of(13.37, "JPY"));
amounts.add(Money.of(18, "USD"));

　　我們可以根據(jù)CurrencyUnit來(lái)進(jìn)行金額過(guò)濾： 

CurrencyUnit yen = MonetaryCurrencies.getCurrency("JPY");
CurrencyUnit dollar = MonetaryCurrencies.getCurrency("USD");
// 根據(jù)貨幣過(guò)濾，只返回美金
// result is [USD 18, USD 7, USD 42]
List<MonetaryAmount> onlyDollar = amounts.stream()
    .filter(MonetaryFunctions.isCurrency(dollar))
    .collect(Collectors.toList());
 
// 根據(jù)貨幣過(guò)濾，只返回美金和日元
// [USD 18, USD 7, JPY 13.37, USD 42]
List<MonetaryAmount> onlyDollarAndYen = amounts.stream()
    .filter(MonetaryFunctions.isCurrency(dollar, yen))
    .collect(Collectors.toList());

　　我們還可以過(guò)濾出大于或小于某個(gè)閾值的金額：

MonetaryAmount tenDollar = Money.of(10, dollar);
 
// [USD 42, USD 18]
List<MonetaryAmount> greaterThanTenDollar = amounts.stream()
    .filter(MonetaryFunctions.isCurrency(dollar))
    .filter(MonetaryFunctions.isGreaterThan(tenDollar))
    .collect(Collectors.toList());

　　排序也是類似的：

// Sorting dollar values by number value
// [USD 7, USD 18, USD 42]
List<MonetaryAmount> sortedByAmount = onlyDollar.stream()
    .sorted(MonetaryFunctions.sortNumber())
    .collect(Collectors.toList());
 
// Sorting by CurrencyUnit
// [EUR 2, JPY 13.37, USD 42, USD 7, USD 18]
List<MonetaryAmount> sortedByCurrencyUnit = amounts.stream()
    .sorted(MonetaryFunctions.sortCurrencyUnit())
    .collect(Collectors.toList());

　　還有分組操作：

// 按貨幣單位進(jìn)行分組
// {USD=[USD 42, USD 7, USD 18], EUR=[EUR 2], JPY=[JPY 13.37]}
Map<CurrencyUnit, List<MonetaryAmount>> groupedByCurrency = amounts.stream()
    .collect(MonetaryFunctions.groupByCurrencyUnit());
 
// 分組并進(jìn)行匯總
Map<CurrencyUnit, MonetarySummaryStatistics> summary = amounts.stream()
    .collect(MonetaryFunctions.groupBySummarizingMonetary()).get();
 
// get summary for CurrencyUnit USD
MonetarySummaryStatistics dollarSummary = summary.get(dollar);
MonetaryAmount average = dollarSummary.getAverage(); // "USD 22.333333333333333333.."
MonetaryAmount min = dollarSummary.getMin(); // "USD 7"
MonetaryAmount max = dollarSummary.getMax(); // "USD 42"
MonetaryAmount sum = dollarSummary.getSum(); // "USD 67"
long count = dollarSummary.getCount(); // 3

　　MonetaryFunctions還提供了歸約函數(shù)，可以用來(lái)獲取最大值，最小值，以及求和：

List<MonetaryAmount> amounts = new ArrayList<>();
amounts.add(Money.of(10, "EUR"));
amounts.add(Money.of(7.5, "EUR"));
amounts.add(Money.of(12, "EUR"));
 
Optional<MonetaryAmount> max = amounts.stream().reduce(MonetaryFunctions.max()); // "EUR 7.5"
Optional<MonetaryAmount> min = amounts.stream().reduce(MonetaryFunctions.min()); // "EUR 12"
Optional<MonetaryAmount> sum = amounts.stream().reduce(MonetaryFunctions.sum()); //

　　自定義的MonetaryAmount操作

　　MonetaryAmount還提供了一個(gè)非常友好的擴(kuò)展點(diǎn)叫作MonetaryOperator。MonetaryOperator是一個(gè)函數(shù)式接口，它接收一個(gè)MonetaryAmount入?yún)⒉⒎祷匾粋�(gè)新的MonetaryAmount對(duì)象。

// A monetary operator that returns 10% of the input MonetaryAmount
// Implemented using Java 8 Lambdas
MonetaryOperator tenPercentOperator = (MonetaryAmount amount) -> {
  BigDecimal baseAmount = amount.getNumber().numberValue(BigDecimal.class);
  BigDecimal tenPercent = baseAmount.multiply(new BigDecimal("0.1"));
  return Money.of(tenPercent, amount.getCurrency());
};
 
MonetaryAmount dollars = Money.of(12.34567, "USD");
 
// apply tenPercentOperator to MonetaryAmount
MonetaryAmount tenPercentDollars = dollars.with(tenPercentOperator); // USD 1.234567

　　標(biāo)準(zhǔn)的API特性都是通過(guò)MonetaryOperator的接口來(lái)實(shí)現(xiàn)的。比方說(shuō)，前面看到的舍入操作就是以MonetaryOperator接口的形式來(lái)提供的。

　　匯率

　　貨幣兌換率可以通過(guò)ExchangeRateProvider來(lái)獲取。JavaMoney自帶了多個(gè)不同的ExchangeRateProvider的實(shí)現(xiàn)。其中最重要的兩個(gè)是ECBCurrentRateProvider與 IMFRateProvider。

　　ECBCurrentRateProvider查詢的是歐洲中央銀行(European Central Bank,ECB)的數(shù)據(jù)而IMFRateProvider查詢的是國(guó)際貨幣基金組織（International Monetary Fund，IMF）的匯率。

// get the default ExchangeRateProvider (CompoundRateProvider)
ExchangeRateProvider exchangeRateProvider = MonetaryConversions.getExchangeRateProvider();
 
// get the names of the default provider chain
// [IDENT, ECB, IMF, ECB-HIST]
List<String> defaultProviderChain = MonetaryConversions.getDefaultProviderChain();
 
// get a specific ExchangeRateProvider (here ECB)
ExchangeRateProvider ecbExchangeRateProvider = MonetaryConversions.getExchangeRateProvider("ECB");

　　如果沒(méi)有指定ExchangeRateProvider的話返回的就是CompoundRateProvider。CompoundRateProvider會(huì)將匯率轉(zhuǎn)換請(qǐng)求委派給一個(gè)ExchangeRateProvider鏈并將第一個(gè)返回準(zhǔn)確結(jié)果的提供商的數(shù)據(jù)返回。

// get the exchange rate from euro to us dollar
ExchangeRate rate = exchangeRateProvider.getExchangeRate("EUR", "USD");
 
NumberValue factor = rate.getFactor(); // 1.2537 (at time writing)
CurrencyUnit baseCurrency = rate.getBaseCurrency(); // EUR
CurrencyUnit targetCurrency = rate.getCurrency(); // USD

　　貨幣轉(zhuǎn)換

　　不同貨幣間的轉(zhuǎn)換可以通過(guò)ExchangeRateProvider返回的CurrencyConversions來(lái)完成。

// get the CurrencyConversion from the default provider chain
CurrencyConversion dollarConversion = MonetaryConversions.getConversion("USD");
 
// get the CurrencyConversion from a specific provider
CurrencyConversion ecbDollarConversion = ecbExchangeRateProvider.getCurrencyConversion("USD");
 
MonetaryAmount tenEuro = Money.of(10, "EUR");
 
// convert 10 euro to us dollar 
MonetaryAmount inDollar = tenEuro.with(dollarConversion); // "USD 12.537" (at the time writing)

　　請(qǐng)注意CurrencyConversion也實(shí)現(xiàn)了MonetaryOperator接口。正如其它操作一樣，它也能通過(guò)MonetaryAmount.with()方法來(lái)調(diào)用。

　　格式化及解析

　　MonetaryAmount可以通過(guò)MonetaryAmountFormat來(lái)與字符串進(jìn)行解析/格式化。

// formatting by locale specific formats
MonetaryAmountFormat germanFormat = MonetaryFormats.getAmountFormat(Locale.GERMANY);
MonetaryAmountFormat usFormat = MonetaryFormats.getAmountFormat(Locale.CANADA);
 
MonetaryAmount amount = Money.of(12345.67, "USD");
 
String usFormatted = usFormat.format(amount); // "USD12,345.67"
String germanFormatted = germanFormat.format(amount); // 12.345,67 USD
 
// A MonetaryAmountFormat can also be used to parse MonetaryAmounts from strings
MonetaryAmount parsed = germanFormat.parse("12,4 USD");

　　可以通過(guò)AmountFormatQueryBuilder來(lái)生成自定義的格式。

// Creating a custom MonetaryAmountFormat
MonetaryAmountFormat customFormat = MonetaryFormats.getAmountFormat(
    AmountFormatQueryBuilder.of(Locale.US)
        .set(CurrencyStyle.NAME)
        .set("pattern", "00,00,00,00.00 ¤")
        .build());
 
// results in "00,01,23,45.67 US Dollar"
String formatted = customFormat.format(amount);

　　注意，這里的¤符號(hào)在模式串中是作為貨幣的占位符。

　　總結(jié)

　　新的貨幣API這里已經(jīng)介紹得差不多了。并且目前它的實(shí)現(xiàn)也已經(jīng)相對(duì)穩(wěn)定了（但還需要多補(bǔ)充些文檔）。期待能在Java 9中看到這套新的接口!

　　上述示例可在Github中下載到。

　　原文出處：Michael Scharhag

標(biāo)簽： isp 安全 代碼

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