java框架：Spring Boot@Value使用注解支持类似@ConfigurationProperties的功能

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;
}

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) {
}

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();
}

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) {   
}

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()));
    }
}

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();
            }
        });
    }
}

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,\

最终效果

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

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