利用注解 + 反射消除重復(fù)代碼，妙！

1.1 案例場景

假設(shè)銀行提供了一些 API 接口，對參數(shù)的序列化有點特殊，不使用 JSON，而是需要我們把參數(shù)依次拼在一起構(gòu)成一個大字符串：

1）按照銀行提供的API文檔順序，將所有的參數(shù)構(gòu)成定長的數(shù)據(jù)，并且拼接在一起作為一整個字符串

2）因為每一種參數(shù)都有固定長度，未達(dá)到長度需要進(jìn)行填充處理

• 字符串類型參數(shù)不滿長度部分要以下劃線右填充，即字符串內(nèi)容靠左
• 數(shù)字類型的參數(shù)不滿長度部分以0左填充，即實際數(shù)字靠右
• 貨幣類型的表示需要把金額向下舍入2位到分，以分為單位，作為數(shù)字類型同樣進(jìn)行左填充
• 參數(shù)做MD5 操作作為簽名

1.2 初步代碼實現(xiàn)

public?class?BankService?{

????//創(chuàng)建用戶方法
????public?static?String?createUser(String?name,?String?identity,?String?mobile,?int?age)?throws?IOException?{
????????StringBuilder?stringBuilder?=?new?StringBuilder();
????????//字符串靠左，多余的地方填充_
????????stringBuilder.append(String.format("%-10s",?name).replace('?',?'_'));
????????//字符串靠左，多余的地方填充_
????????stringBuilder.append(String.format("%-18s",?identity).replace('?',?'_'));
????????//數(shù)字靠右，多余的地方用0填充
????????stringBuilder.append(String.format("%05d",?age));
????????//字符串靠左，多余的地方用_填充
????????stringBuilder.append(String.format("%-11s",?mobile).replace('?',?'_'));
????????//最后加上MD5作為簽名
????????stringBuilder.append(DigestUtils.md2Hex(stringBuilder.toString()));
????????return?Request.Post("http://localhost:45678/reflection/bank/createUser")
????????????????.bodyString(stringBuilder.toString(),?ContentType.APPLICATION_JSON)
????????????????.execute().returnContent().asString();
????}

????//支付方法
????public?static?String?pay(long?userId,?BigDecimal?amount)?throws?IOException?{
????????StringBuilder?stringBuilder?=?new?StringBuilder();
????????//數(shù)字靠右，多余的地方用0填充
????????stringBuilder.append(String.format("%020d",?userId));
????????//金額向下舍入2位到分，以分為單位，作為數(shù)字靠右，多余的地方用0填充
????????stringBuilder.append(String.format("%010d",?amount.setScale(2,?RoundingMode.DOWN).multiply(new?BigDecimal("100")).longValue()));
????????//最后加上MD5作為簽名
????????stringBuilder.append(DigestUtils.md2Hex(stringBuilder.toString()));
????????return?Request.Post("http://localhost:45678/reflection/bank/pay")
????????????????.bodyString(stringBuilder.toString(),?ContentType.APPLICATION_JSON)
????????????????.execute().returnContent().asString();
????}
}

這樣做能夠基本滿足需求，但是存在一些問題：

• 處理邏輯互相之間有重復(fù)，稍有不慎就會出現(xiàn)Bug
• 處理流程中字符串拼接、加簽和發(fā)請求的邏輯，在所有方法重復(fù)
• 實際方法的入?yún)⒌膮?shù)類型和順序，不一定和接口要求一致，容易出錯
• 代碼層面參數(shù)硬編碼，無法清晰進(jìn)行核對

1.3 使用接口和反射優(yōu)化代碼

1.3.1 實現(xiàn)定義了所有接口參數(shù)的POJO類

@Data
public?class?CreateUserAPI?{
????private?String?name;
????private?String?identity;
????private?String?mobile;
????private?int?age;
}

1.3.2 定義注解本身

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Inherited
public?@interface?BankAPI?{
????String?desc()?default?"";
????String?url()?default?"";
}


@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
@Documented
@Inherited
public?@interface?BankAPIField?{
????int?order()?default?-1;
????int?length()?default?-1;
????String?type()?default?"";
}

1.3.3 反射配合注解實現(xiàn)動態(tài)的接口參數(shù)組裝

private?static?String?remoteCall(AbstractAPI?api)?throws?IOException?{
????//從BankAPI注解獲取請求地址
????BankAPI?bankAPI?=?api.getClass().getAnnotation(BankAPI.class);
????bankAPI.url();
????StringBuilder?stringBuilder?=?new?StringBuilder();
????Arrays.stream(api.getClass().getDeclaredFields())?//獲得所有字段
????????????.filter(field?->?field.isAnnotationPresent(BankAPIField.class))?//查找標(biāo)記了注解的字段
????????????.sorted(Comparator.comparingInt(a?->?a.getAnnotation(BankAPIField.class).order()))?//根據(jù)注解中的order對字段排序
????????????.peek(field?->?field.setAccessible(true))?//設(shè)置可以訪問私有字段
????????????.forEach(field?->?{
????????????????//獲得注解
????????????????BankAPIField?bankAPIField?=?field.getAnnotation(BankAPIField.class);
????????????????Object?value?=?"";
????????????????try?{
????????????????????//反射獲取字段值
????????????????????value?=?field.get(api);
????????????????}?catch?(IllegalAccessException?e)?{
????????????????????e.printStackTrace();
????????????????}
????????????????//根據(jù)字段類型以正確的填充方式格式化字符串
????????????????switch?(bankAPIField.type())?{
????????????????????case?"S":?{
????????????????????????stringBuilder.append(String.format("%-"?+?bankAPIField.length()?+?"s",?value.toString()).replace('?',?'_'));
????????????????????????break;
????????????????????}
????????????????????case?"N":?{
????????????????????????stringBuilder.append(String.format("%"?+?bankAPIField.length()?+?"s",?value.toString()).replace('?',?'0'));
????????????????????????break;
????????????????????}
????????????????????case?"M":?{
????????????????????????if?(!(value?instanceof?BigDecimal))
????????????????????????????throw?new?RuntimeException(String.format("{}?的?{}?必須是BigDecimal",?api,?field));
????????????????????????stringBuilder.append(String.format("%0"?+?bankAPIField.length()?+?"d",?((BigDecimal)?value).setScale(2,?RoundingMode.DOWN).multiply(new?BigDecimal("100")).longValue()));
????????????????????????break;
????????????????????}
????????????????????default:
????????????????????????break;
????????????????}
????????????});
????//簽名邏輯
???stringBuilder.append(DigestUtils.md2Hex(stringBuilder.toString()));
????String?param?=?stringBuilder.toString();
????long?begin?=?System.currentTimeMillis();
????//發(fā)請求
????String?result?=?Request.Post("http://localhost:45678/reflection"?+?bankAPI.url())
????????????.bodyString(param,?ContentType.APPLICATION_JSON)
????????????.execute().returnContent().asString();
????log.info("調(diào)用銀行API?{}?url:{}?參數(shù):{}?耗時:{}ms",?bankAPI.desc(),?bankAPI.url(),?param,?System.currentTimeMillis()?-?begin);
????return?result;
}

通過反射來動態(tài)獲得class的信息，并在runtime的時候完成組裝過程。

這樣做的好處是開發(fā)的時候會方便直觀很多，然后將邏輯與細(xì)節(jié)隱藏起來，并且集中放到了一個方法當(dāng)中，減少了重復(fù)，以及維護當(dāng)中bug的出現(xiàn)。

1.3.4 在代碼中的應(yīng)用

@BankAPI(url?=?"/bank/createUser",?desc?=?"創(chuàng)建用戶接口")
@Data
public?class?CreateUserAPI?extends?AbstractAPI?{
????@BankAPIField(order?=?1,?type?=?"S",?length?=?10)
????private?String?name;
????@BankAPIField(order?=?2,?type?=?"S",?length?=?18)
????private?String?identity;
????@BankAPIField(order?=?4,?type?=?"S",?length?=?11)?//注意這里的order需要按照API表格中的順序
????private?String?mobile;
????@BankAPIField(order?=?3,?type?=?"N",?length?=?5)
????private?int?age;
}



@BankAPI(url?=?"/bank/pay",?desc?=?"支付接口")
@Data
public?class?PayAPI?extends?AbstractAPI?{
????@BankAPIField(order?=?1,?type?=?"N",?length?=?20)
????private?long?userId;
????@BankAPIField(order?=?2,?type?=?"M",?length?=?10)
????private?BigDecimal?amount;
}