如何使@Value注解支持类似@ConfigurationProperties的功能（@Value支持对象类型）？

左手的ㄟ右手 2023-09-26 15:42 47阅读 0赞

真实业务场景（不希望配置类注册为Bean 或 不希望声明@ConfigurationProperties）

假设某一个jar包内封装了DataSourceProperties

@Configuration
    @ConfigurationProperties(
        prefix = "my.datasource"
    )
    @Data
    public class DataSourceProperties {
        private List<String> suffix;
        private List<DataSourceDetailProperties> db;
    }
    复制代码

在jar包的Configuration中，某个@Bean的构造过程中引用了这个DataSourceProperties

public JdbcTemplate buildJdbcTemplate(DataSourceProperties dataSourceProperties) {
    }
    复制代码

在某个业务场景中，同时存在两个DataSourceProperties 会造成一个问题，注入的时候会提示有多个候选的bean 但是没法去修改Jar包中的内容

自己重复写一个DataSourceProperties 不是很优雅

这时候引出了一个需求，DataSourceProperties不希望注册为Bean，但是能够从配置文件读取构建对象

解决方案一

使用`org.springframework.boot.context.properties.bind.Binder` 从配置文件构建配置对象

@Bean
    public JdbcTemplate buildJdbcTemplate(Environment environment) {
         Binder binder = Binder.get(environment);
         DataSourceProperties
                    properties1 = binder.bind("my.datasource1", Bindable.of(DataSourceProperties.class)).get(),
                    properties2 = binder.bind("my.datasource2", Bindable.of(DataSourceProperties.class)).get();
    }
    复制代码

> binder.bind("xxx", Bindable.of(type)).get() 似乎是重复的编码方式？

解决方案二

使`@Value`注解能够支持自动调用这段代码 `binder.bind("xxx", Bindable.of(type)).get()` 例如

@Bean
    public JdbcTemplate buildJdbcTemplate(@Value("my.datasource1") DataSourceProperties properties1,
                                          @Value("my.datasource2") DataSourceProperties properties2) {   
    }
    复制代码

`org.springframework.beans.factory.support.DefaultListableBeanFactory#doResolveDependency` 最后会交由converter处理

Class<?> type = descriptor.getDependencyType();
    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()));
        }
    }
    复制代码

项目启动时，添加String to Object的转换器，支持`@Value` 并且 "bind:"开头（防止影响`@Value`原有功能）

package com.nuonuo.accounting.guiding.support.spring;
    
    import org.springframework.beans.factory.annotation.Value;
    import org.springframework.boot.context.properties.bind.Bindable;
    import org.springframework.boot.context.properties.bind.Binder;
    import org.springframework.boot.convert.ApplicationConversionService;
    import org.springframework.context.ApplicationContextInitializer;
    import org.springframework.context.ConfigurableApplicationContext;
    import org.springframework.core.convert.TypeDescriptor;
    import org.springframework.core.convert.converter.ConditionalGenericConverter;
    
    import java.util.Set;
    
    import static java.util.Collections.singleton;
    
    /**
     * @author uhfun
     */
    public class ValuePropertiesBindableAnnotationSupport implements ApplicationContextInitializer<ConfigurableApplicationContext> {
    
        private static final String PREFIX = "bind:";
    
        @Override
        public void initialize(ConfigurableApplicationContext context) {
            Binder binder = Binder.get(context.getEnvironment());
            ((ApplicationConversionService) context.getBeanFactory().getConversionService()).addConverter(new ConditionalGenericConverter() {
                @Override
                public boolean matches(TypeDescriptor sourceType, TypeDescriptor targetType) {
                    Value value = targetType.getAnnotation(Value.class);
                    return value != null && value.value().startsWith(PREFIX);
                }
    
                @Override
                public Set<ConvertiblePair> getConvertibleTypes() {
                    return singleton(new ConvertiblePair(String.class, Object.class));
                }
    
                @Override
                public Object convert(Object source, TypeDescriptor sourceType, TypeDescriptor targetType) {
                    Value value = targetType.getAnnotation(Value.class);
                    Class<?> type = targetType.getType();
                    assert value != null;
                    return binder.bind(value.value().replace(PREFIX, ""), Bindable.of(type)).get();
                }
            });
        }
    }
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转换后代码执行 `binder.bind(value.value().replace(PREFIX, ""), Bindable.of(type)).get();` 目的就达成了

在`META-INF/spring.factories`中添加注册的Bean

# ApplicationContextInitializer
    org.springframework.context.ApplicationContextInitializer=\
    com.nuonuo.accounting.guiding.support.spring.ValuePropertiesBindableAnnotationSupport,\
    复制代码

最终效果

@Bean
    public JdbcTemplate buildJdbcTemplate(@Value("bind:my.datasource1") DataSourceProperties properties1,
                                          @Value("bind:my.datasource2") DataSourceProperties properties2) {   
    }