Groovy 设计模式 -- 保镖模式

Bouncer Pattern

http://groovy-lang.org/design-patterns.html#_bouncer_pattern

保镖模式主要负责对函数的输入参数的合法性检查， 如果遇到非法输出，则停止函数后续执行。

groovy提供了 assert 机制， 语言级别内置功能。

The Bouncer Pattern describes usage of a method whose sole purpose is to either throw an exception (when particular conditions hold) or do nothing. Such methods are often used to defensively guard pre-conditions of a method.

When writing utility methods, you should always guard against faulty input arguments. When writing internal methods, you may be able to ensure that certain pre-conditions always hold by having sufficient unit tests in place. Under such circumstances, you may reduce the desirability to have guards on your methods.

Groovy differs from other languages in that you frequently use the assert method within your methods rather than having a large number of utility checker methods or classes.

例子

void doStuff(String name, Object value) {
    assert name != null, 'name should not be null'
    assert value != null, 'value should not be null'
    // do stuff
}

或者， 合法性检查， 检测出非法性， 主动 抛出异常。

class NumberChecker {
    static final String NUMBER_PATTERN = "\\d+(\\.\\d+(E-?\\d+)?)?"
    static isNumber(str) {
        if (!str ==~ NUMBER_PATTERN) {
            throw new IllegalArgumentException("Argument '$str' must be a number")
        }
    }
    static isNotZero(number) {
        if (number == 0) {
            throw new IllegalArgumentException('Argument must not be 0')
        }
    }
}

def stringDivide(String dividendStr, String divisorStr) {
    NumberChecker.isNumber(dividendStr)
    NumberChecker.isNumber(divisorStr)
    def dividend = dividendStr.toDouble()
    def divisor = divisorStr.toDouble()
    NumberChecker.isNotZero(divisor)
    dividend / divisor
}

println stringDivide('1.2E2', '3.0')
// => 40.0