Relaxed Binding 是 Spring Boot 中一个有趣的机制，它可以让开发人员更灵活地进行配置。但偶然的机会，我发现这个机制在 Spring Boot 1.5 与 Spring Boot 2.0 版本中的实现是不一样的。

Relaxed Binding机制

关于Relaxed Binding机制在Spring Boot的官方文档中有描述，链接如下：
24.7 Type-safe Configuration Properties
24.7.2 Relaxed Binding
这个Relaxed Binding机制的主要作用是，当配置文件（properties或yml）中的配置项值与带@ConfigurationProperties注解的配置类属性进行绑定时，可以进行相对宽松的绑定。
本文并不是针对@ConfigurationProperties与类型安全的属性配置进行解释，相关的说明请参考上面的链接。

Relax Binding示例

下面的例子有助于理解Relaxed Binding。
假设有一个带@ConfigurationProperties注解的属性类

@ConfigurationProperties(prefix="my")
public class MyProperties {
    private String firstName;
    public String getFirstName() {
        return this.firstName;
    }
    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }
}

在配置文件（properties文件）中，以下的写法都能将值正确注入到firstName这个属性中。

源码分析

出于好奇，我研究了一下SpringBoot的源码，发现在1.5版本与2.0版本中，Relaxed Binding 的具体实现是不一样的。
为了解释两个版本的不同之处，我首先在properties配置文件中做如下的配置。

my.first_Name=fonoisrev

Spring Boot 1.5版本的实现（以1.5.14.RELEASE版本为例）

属性与配置值的绑定逻辑始于ConfigurationPropertiesBindingPostProcessor类的postProcessBeforeInitialization函数。

public class ConfigurationPropertiesBindingPostProcessor implements BeanPostProcessor,
        BeanFactoryAware, EnvironmentAware, ApplicationContextAware, InitializingBean,
        DisposableBean, ApplicationListener<ContextRefreshedEvent>, PriorityOrdered {
...
    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName)
            throws BeansException {
        ...
        ConfigurationProperties annotation = AnnotationUtils
                .findAnnotation(bean.getClass(), ConfigurationProperties.class);
        if (annotation != null) {
            postProcessBeforeInitialization(bean, beanName, annotation);
        }
        ...
    }

    private void postProcessBeforeInitialization(Object bean, String beanName,
            ConfigurationProperties annotation) {
        Object target = bean;
        PropertiesConfigurationFactory<Object> factory = new PropertiesConfigurationFactory<Object>(
                target);
        factory.setPropertySources(this.propertySources);
        ...
        try {
            factory.bindPropertiesToTarget();
        }
        ...
    }
...
}

从以上代码可以看出，该类实现了BeanPostProcessor接口（BeanPostProcessor官方文档请点我）。BeanPostProcessor接口主要作用是允许自行实现Bean装配的逻辑，而postProcessBeforeInitialization函数则在执行Bean的初始化调用（如afterPropertiesSet）前被调用。

postProcessBeforeInitialization函数执行时，属性值绑定的工作被委派给了PropertiesConfigurationFactory<T>类。

public class PropertiesConfigurationFactory<T> implements FactoryBean<T>,
        ApplicationContextAware, MessageSourceAware, InitializingBean {
...
    public void bindPropertiesToTarget() throws BindException {
        ...
            doBindPropertiesToTarget();
        ...
    }
    private void doBindPropertiesToTarget() throws BindException {
        RelaxedDataBinder dataBinder = (this.targetName != null
                ? new RelaxedDataBinder(this.target, this.targetName)
                : new RelaxedDataBinder(this.target));
        ...
        Iterable<String> relaxedTargetNames = getRelaxedTargetNames();
        Set<String> names = getNames(relaxedTargetNames);
        PropertyValues propertyValues = getPropertySourcesPropertyValues(names,
                relaxedTargetNames);
        dataBinder.bind(propertyValues);
        ...
    }

    private PropertyValues getPropertySourcesPropertyValues(Set<String> names,
            Iterable<String> relaxedTargetNames) {
        PropertyNamePatternsMatcher includes = getPropertyNamePatternsMatcher(names,
                relaxedTargetNames);
        return new PropertySourcesPropertyValues(this.propertySources, names, includes,
                this.resolvePlaceholders);
    }
...
}

以上的代码其逻辑可以描述如下：

1. 借助RelaxedNames类，将注解@ConfigurationProperties(prefix="my")中的前缀“my”与MyProperties类的属性firstName可能存在的情况进行穷举（共28种情况，如下表）

1. 构造PropertySourcesPropertyValues对象从配置文件（如properties文件）中查找匹配的值，
2. 使用DataBinder操作PropertySourcesPropertyValues实现MyProperties类的setFirstName方法调用，完成绑定。

public class PropertySourcesPropertyValues implements PropertyValues {
...
    PropertySourcesPropertyValues(PropertySources propertySources,
            Collection<String> nonEnumerableFallbackNames,
            PropertyNamePatternsMatcher includes, boolean resolvePlaceholders) {
        ...
        PropertySourcesPropertyResolver resolver = new PropertySourcesPropertyResolver(
                propertySources);
        for (PropertySource<?> source : propertySources) {
            processPropertySource(source, resolver);
        }
    }

    private void processPropertySource(PropertySource<?> source,
            PropertySourcesPropertyResolver resolver) {
        ...
            processEnumerablePropertySource((EnumerablePropertySource<?>) source,
                    resolver, this.includes);
        ...
    }

    private void processEnumerablePropertySource(EnumerablePropertySource<?> source,
            PropertySourcesPropertyResolver resolver,
            PropertyNamePatternsMatcher includes) {
        if (source.getPropertyNames().length > 0) {
            for (String propertyName : source.getPropertyNames()) {
                if (includes.matches(propertyName)) {
                    Object value = getEnumerableProperty(source, resolver, propertyName);
                    putIfAbsent(propertyName, value, source);
                }
            }
        }
    }
...
}

从以上代码来看，整个匹配的过程其实是在PropertySourcesPropertyValues对象构造的过程中完成的。
更具体地说，是在DefaultPropertyNamePatternsMatcher的matches函数中完成字符串匹配的，如下代码。

class DefaultPropertyNamePatternsMatcher implements PropertyNamePatternsMatcher {
...
    public boolean matches(String propertyName) {
        char[] propertyNameChars = propertyName.toCharArray();
        boolean[] match = new boolean[this.names.length];
        boolean noneMatched = true;
        for (int i = 0; i < this.names.length; i++) {
            if (this.names[i].length() <= propertyNameChars.length) {
                match[i] = true;
                noneMatched = false;
            }
        }
        if (noneMatched) {
            return false;
        }
        for (int charIndex = 0; charIndex < propertyNameChars.length; charIndex++) {
            for (int nameIndex = 0; nameIndex < this.names.length; nameIndex++) {
                if (match[nameIndex]) {
                    match[nameIndex] = false;
                    if (charIndex < this.names[nameIndex].length()) {
                        if (isCharMatch(this.names[nameIndex].charAt(charIndex),
                                propertyNameChars[charIndex])) {
                            match[nameIndex] = true;
                            noneMatched = false;
                        }
                    }
                    else {
                        char charAfter = propertyNameChars[this.names[nameIndex]
                                .length()];
                        if (isDelimiter(charAfter)) {
                            match[nameIndex] = true;
                            noneMatched = false;
                        }
                    }
                }
            }
            if (noneMatched) {
                return false;
            }
        }
        for (int i = 0; i < match.length; i++) {
            if (match[i]) {
                return true;
            }
        }
        return false;
    }

    private boolean isCharMatch(char c1, char c2) {
        if (this.ignoreCase) {
            return Character.toLowerCase(c1) == Character.toLowerCase(c2);
        }
        return c1 == c2;
    }

    private boolean isDelimiter(char c) {
        for (char delimiter : this.delimiters) {
            if (c == delimiter) {
                return true;
            }
        }
        return false;
    }
}

如上代码，将字符串拆为单个字符进行比较，使用了双层循环匹配。

Spring Boot 2.0版本的实现（以2.0.4.RELEASE版本为例）

与1.5.14.RELEASE有很大区别。
属性与配置值的绑定逻辑依旧始于
ConfigurationPropertiesBindingPostProcessor 类的
postProcessBeforeInitialization 函数。
但 ConfigurationPropertiesBindingPostProcessor 类的定义与实现均发生了变化。先看代码。

public class ConfigurationPropertiesBindingPostProcessor implements BeanPostProcessor,
        PriorityOrdered, ApplicationContextAware, InitializingBean {
...
    private ConfigurationPropertiesBinder configurationPropertiesBinder;
...
    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName)
            throws BeansException {
        ConfigurationProperties annotation = getAnnotation(bean, beanName,
                ConfigurationProperties.class);
        if (annotation != null) {
            bind(bean, beanName, annotation);
        }
        return bean;
    }

    private void bind(Object bean, String beanName, ConfigurationProperties annotation) {
        ResolvableType type = getBeanType(bean, beanName);
        Validated validated = getAnnotation(bean, beanName, Validated.class);
        Annotation[] annotations = (validated != null)
                ? new Annotation[] { annotation, validated }
                : new Annotation[] { annotation };
        Bindable<?> target = Bindable.of(type).withExistingValue(bean)
                .withAnnotations(annotations);
        try {
            this.configurationPropertiesBinder.bind(target);
        }
        ...
    }
...
}

从以上代码可以看出，该类依旧实现了BeanPostProcessor接口，但调用postProcessBeforeInitialization函数，属性值绑定的工作则被委派给了ConfigurationPropertiesBinder类，调用了bind函数。

class ConfigurationPropertiesBinder {
...
    public void bind(Bindable<?> target) {
        ConfigurationProperties annotation = target
                .getAnnotation(ConfigurationProperties.class);
        Assert.state(annotation != null,
                () -> "Missing @ConfigurationProperties on " + target);
        List<Validator> validators = getValidators(target);
        BindHandler bindHandler = getBindHandler(annotation, validators);
        getBinder().bind(annotation.prefix(), target, bindHandler);
    }

    private Binder getBinder() {
        if (this.binder == null) {
            this.binder = new Binder(getConfigurationPropertySources(),
                    getPropertySourcesPlaceholdersResolver(), getConversionService(),
                    getPropertyEditorInitializer());
        }
        return this.binder;
    }
...
}

ConfigurationPropertiesBinder类并不是一个public的类。实际上这相当于ConfigurationPropertiesBindingPostProcessor的一个内部静态类，表面上负责处理@ConfigurationProperties注解的绑定任务。
但从代码中可以看出，具体的工作委派给另一个Binder类的对象。
Binder类是SpringBoot 2.0版本后加入的类，其负责处理对象与多个ConfigurationPropertySource之间的绑定。

public class Binder {
...
    private static final List<BeanBinder> BEAN_BINDERS;
    static {
        List<BeanBinder> binders = new ArrayList<>();
        binders.add(new JavaBeanBinder());
        BEAN_BINDERS = Collections.unmodifiableList(binders);
    }
...
    public <T> BindResult<T> bind(String name, Bindable<T> target, BindHandler handler) {
        return bind(ConfigurationPropertyName.of(name), target, handler);
    }

    public <T> BindResult<T> bind(ConfigurationPropertyName name, Bindable<T> target,
            BindHandler handler) {
        ...
        Context context = new Context();
        T bound = bind(name, target, handler, context, false);
        return BindResult.of(bound);
    }

    protected final <T> T bind(ConfigurationPropertyName name, Bindable<T> target,
            BindHandler handler, Context context, boolean allowRecursiveBinding) {
        ...
        try {
            ...
            Object bound = bindObject(name, target, handler, context,
                    allowRecursiveBinding);
            return handleBindResult(name, target, handler, context, bound);
        }
        ...
    }

    private <T> Object bindObject(ConfigurationPropertyName name, Bindable<T> target,
            BindHandler handler, Context context, boolean allowRecursiveBinding) {
        ConfigurationProperty property = findProperty(name, context);
        if (property == null && containsNoDescendantOf(context.streamSources(), name)) {
            return null;
        }
        AggregateBinder<?> aggregateBinder = getAggregateBinder(target, context);
        if (aggregateBinder != null) {
            return bindAggregate(name, target, handler, context, aggregateBinder);
        }
        if (property != null) {
            try {
                return bindProperty(target, context, property);
            }
            catch (ConverterNotFoundException ex) {
                // We might still be able to bind it as a bean
                Object bean = bindBean(name, target, handler, context,
                        allowRecursiveBinding);
                if (bean != null) {
                    return bean;
                }
                throw ex;
            }
        }
        return bindBean(name, target, handler, context, allowRecursiveBinding);
    }

    private Object bindBean(ConfigurationPropertyName name, Bindable<?> target,
            BindHandler handler, Context context, boolean allowRecursiveBinding) {
        ...
        BeanPropertyBinder propertyBinder = (propertyName, propertyTarget) -> bind(
                name.append(propertyName), propertyTarget, handler, context, false);
        Class<?> type = target.getType().resolve(Object.class);
        ...
        return context.withBean(type, () -> {
            Stream<?> boundBeans = BEAN_BINDERS.stream()
                    .map((b) -> b.bind(name, target, context, propertyBinder));
            return boundBeans.filter(Objects::nonNull).findFirst().orElse(null);
        });
    }
...
    private ConfigurationProperty findProperty(ConfigurationPropertyName name,
            Context context) {
        ...
        return context.streamSources()
                .map((source) -> source.getConfigurationProperty(name))
                .filter(Objects::nonNull).findFirst().orElse(null);
    }
}

如下代码，Binder类中实现了一个比较复杂的递归调用。

1. ConfigurationPropertiesBinder 调用Binder类的bind函数时，参数通过层层转换，来到bindObject函数中。
2. bindObject函数中，通过bindAggregate，bindProperty与bindBean等私有方法逐步推导绑定，bindBean是最后一步。
3. bindBean函数通过定义BeanPropertyBinder的lambda表达式，允许bean绑定过程递归调用bindObject函数。

实际上，bindObject函数中findProperty函数调用是从properties文件中查找匹配项的关键，根据lambda表达式的执行结果，properties配置文件的配置项对应的是SpringIterableConfigurationPropertySource类型，因此调用的是其getConfigurationProperty函数，关键代码如下。

class SpringIterableConfigurationPropertySource extends SpringConfigurationPropertySource
        implements IterableConfigurationPropertySource {
...
    @Override
    public ConfigurationProperty getConfigurationProperty(
            ConfigurationPropertyName name) {
        ConfigurationProperty configurationProperty = super.getConfigurationProperty(
                name);
        if (configurationProperty == null) {
            configurationProperty = find(getPropertyMappings(getCache()), name);
        }
        return configurationProperty;
    }

    protected final ConfigurationProperty find(PropertyMapping[] mappings,
            ConfigurationPropertyName name) {
        for (PropertyMapping candidate : mappings) {
            if (candidate.isApplicable(name)) {
                ConfigurationProperty result = find(candidate);
                if (result != null) {
                    return result;
                }
            }
        }
        return null;
    }
...
}

最终，isApplicable函数中判断properties文件中的配置项（my.first_Name）与MyProperties类中的属性（firstName）是否匹配，其字符串比较过程使用的是ConfigurationPropertyName类重写的equals方法。

public final class ConfigurationPropertyName
        implements Comparable<ConfigurationPropertyName> {
    @Override
    public boolean equals(Object obj) {
        ...
        ConfigurationPropertyName other = (ConfigurationPropertyName) obj;
        ...
        for (int i = 0; i < this.elements.length; i++) {
            if (!elementEquals(this.elements[i], other.elements[i])) {
                return false;
            }
        }
        return true;
    }

    private boolean elementEquals(CharSequence e1, CharSequence e2) {
        int l1 = e1.length();
        int l2 = e2.length();
        boolean indexed1 = isIndexed(e1);
        int offset1 = indexed1 ? 1 : 0;
        boolean indexed2 = isIndexed(e2);
        int offset2 = indexed2 ? 1 : 0;
        int i1 = offset1;
        int i2 = offset2;
        while (i1 < l1 - offset1) {
            if (i2 >= l2 - offset2) {
                return false;
            }
            char ch1 = indexed1 ? e1.charAt(i1) : Character.toLowerCase(e1.charAt(i1));
            char ch2 = indexed2 ? e2.charAt(i2) : Character.toLowerCase(e2.charAt(i2));
            if (ch1 == '-' || ch1 == '_') {
                i1++;
            }
            else if (ch2 == '-' || ch2 == '_') {
                i2++;
            }
            else if (ch1 != ch2) {
                return false;
            }
            else {
                i1++;
                i2++;
            }
        }
        while (i2 < l2 - offset2) {
            char ch = e2.charAt(i2++);
            if (ch != '-' && ch != '_') {
                return false;
            }
        }
        return true;
    }
}

两者的异同

总结一下两个版本实现的不同。

题外话：为什么我发现了这两个版本实现的不一致？

前一两个月，我写了一个自动识别消息的框架，在消息中模糊查找可能存在的关键信息，用了1.5版本的RelaxedNames类，虽然这个类在官方文档上没有提到，但是挺好用的。
然而最近，我想把框架迁移到2.0版本上，结果遇到了编译错误。因此特别研究了一下，发现这些类都不见了。
幸好只是小问题，我把RelaxedNames源码拷贝过来一份，就解决了问题，但这也给我提了个醒，大版本升级还是要仔细阅读下代码，避免留下一些坑。
另外也从一个侧面反映出，Spring Boot的2.0版本相比于1.5确实做了不小的更改，有空再好好琢磨下。