背景

在写代码的过程中需要用到一些自定义annotation，但是annotation中的String类型的value需要字符串常量，于是想是否可以将@Value所支持的表达式移植到自定义注解中。

实现

  1. Spring中的具体实现

org.springframework.beans.factory.support.DefaultListableBeanFactory#doResolveDependency

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Object value = getAutowireCandidateResolver().getSuggestedValue(descriptor);
if (value != null) {
    if (value instanceof String) {
        String strVal = resolveEmbeddedValue((String) value);
        BeanDefinition bd = (beanName != null && containsBean(beanName) ?
                getMergedBeanDefinition(beanName) : null);
        value = evaluateBeanDefinitionString(strVal, bd);
    }
    TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());
    try {
        return converter.convertIfNecessary(value, type, descriptor.getTypeDescriptor());
    } catch (UnsupportedOperationException ex) {
        // A custom TypeConverter which does not support TypeDescriptor resolution...
        return (descriptor.getField() != null ?
                converter.convertIfNecessary(value, type, descriptor.getField()) :
                converter.convertIfNecessary(value, type, descriptor.getMethodParameter()));
    }
}

以上是核心代码

  • resolveEmbeddedValue方法用于执行${}类型的表达式，如${spring.application.name}，可以实现将配置文件中的值注入到表达式中。
  • evaluateBeanDefinitionString方法用于执行#${}类型表达式，如#{T(System).currentTimeMillis()}、#{systemProperties['user.name']}，有一定的代码执行能力。
  1. 抽取所需要的代码
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public <T> T contextLoads(ConfigurableBeanFactory beanFactory, String expression, Class<T> resultType) {
    String s = beanFactory.resolveEmbeddedValue(expression);
    T result = null;
    if (Objects.nonNull(beanFactory.getBeanExpressionResolver())) {
        Object evaluateResult = beanFactory.getBeanExpressionResolver().evaluate(s,
                new BeanExpressionContext(beanFactory, null));
        TypeConverter converter = beanFactory.getTypeConverter();
        try {
            result = converter.convertIfNecessary(evaluateResult, resultType);
        } catch (UnsupportedOperationException ex) {
            // A custom TypeConverter which does not support TypeDescriptor resolution...
            log.error("UnsupportedOperationException", ex);
        }
    }
    return result;
}

使用样例

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@SpringBootTest
@Data
class ApplicationTests {
    private static final Logger log = LoggerFactory.getLogger(ApplicationTests.class);
    @Autowired
    private ConfigurableBeanFactory beanFactory;

    @Test
    void name() {
        Long currentTimeMillis = contextLoads(beanFactory, "#{T(System).currentTimeMillis()}", Long.class);
        System.out.println(currentTimeMillis);
    }

    public <T> T contextLoads(ConfigurableBeanFactory beanFactory, String expression, Class<T> resultType) {
        String s = beanFactory.resolveEmbeddedValue(expression);
        T result = null;
        if (Objects.nonNull(beanFactory.getBeanExpressionResolver())) {
            Object evaluateResult = beanFactory.getBeanExpressionResolver().evaluate(s,
                    new BeanExpressionContext(beanFactory, null));
            TypeConverter converter = beanFactory.getTypeConverter();
            try {
                result = converter.convertIfNecessary(evaluateResult, resultType);
            } catch (UnsupportedOperationException ex) {
                // A custom TypeConverter which does not support TypeDescriptor resolution...
                log.error("UnsupportedOperationException", ex);
            }
        }
        return result;
    }

}