关于spring:如何使Value注解支持类似ConfigurationProperties的功能Value支持对象类型

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该文章为原创（转载请注明出处）：如何使 @Value 注解反对相似 @ConfigurationProperties 的性能？– 简书 (jianshu.com)

假如某一个 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 的结构过程中援用了这个 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，注入的时候会提醒有多个候选的 bean
然而没法去批改 Jar 包中的内容

本人反复写一个 DataSourceProperties 不是很优雅

这时候引出了一个需要，DataSourceProperties 不心愿注册为 Bean，然而可能从配置文件读取构建对象

应用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()
仿佛是反复的编码方式？

使 @Value 注解可能反对主动调用这段代码 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
最初会交由 converter 解决

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()));
    }
}

我的项目启动时，增加 String to Object 的转换器，反对 @Value 并且 “bind:” 结尾（避免影响@Value 原有性能）

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(); 目标就达成了

在 META-INF/spring.factories 中增加注册的 Bean

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

最终成果

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