SpringBoot环境属性占位符解析和类型转换

前提

前面写过一篇关于Environment属性加载的源码分析和扩展，里面提到属性的占位符解析和类型转换是相对复杂的，这篇文章就是要分析和解读这两个复杂的问题。关于这两个问题，选用一个比较复杂的参数处理方法PropertySourcesPropertyResolver#getProperty，解析占位符的时候依赖到PropertySourcesPropertyResolver#getPropertyAsRawString：

protected String getPropertyAsRawString(String key) {
	return getProperty(key, String.class, false);
}

protected <T> T getProperty(String key, Class<T> targetValueType, boolean resolveNestedPlaceholders) {
	if (this.propertySources != null) {
		for (PropertySource<?> propertySource : this.propertySources) {
			if (logger.isTraceEnabled()) {
				logger.trace("Searching for key '" + key + "' in PropertySource '" +
							propertySource.getName() + "'");
			}
			Object value = propertySource.getProperty(key);
			if (value != null) {
				if (resolveNestedPlaceholders && value instanceof String) {
                    //解析带有占位符的属性
					value = resolveNestedPlaceholders((String) value);
				}
				logKeyFound(key, propertySource, value);
                //需要时转换属性的类型
				return convertValueIfNecessary(value, targetValueType);
			}
		}
	}
	if (logger.isDebugEnabled()) {
		logger.debug("Could not find key '" + key + "' in any property source");
	}
	return null;
}

属性占位符解析

属性占位符的解析方法是PropertySourcesPropertyResolver的父类AbstractPropertyResolver#resolveNestedPlaceholders：

protected String resolveNestedPlaceholders(String value) {
	return (this.ignoreUnresolvableNestedPlaceholders ?
		resolvePlaceholders(value) : resolveRequiredPlaceholders(value));
}

ignoreUnresolvableNestedPlaceholders属性默认为false，可以通过AbstractEnvironment#setIgnoreUnresolvableNestedPlaceholders(boolean ignoreUnresolvableNestedPlaceholders)设置，当此属性被设置为true，解析属性占位符失败的时候(并且没有为占位符配置默认值)不会抛出异常，返回属性原样字符串，否则会抛出IllegalArgumentException。我们这里只需要分析AbstractPropertyResolver#resolveRequiredPlaceholders：

//AbstractPropertyResolver中的属性：
//ignoreUnresolvableNestedPlaceholders=true情况下创建的PropertyPlaceholderHelper实例
@Nullable
private PropertyPlaceholderHelper nonStrictHelper;

//ignoreUnresolvableNestedPlaceholders=false情况下创建的PropertyPlaceholderHelper实例
@Nullable
private PropertyPlaceholderHelper strictHelper;

//是否忽略无法处理的属性占位符，这里是false，也就是遇到无法处理的属性占位符且没有默认值则抛出异常
private boolean ignoreUnresolvableNestedPlaceholders = false;

//属性占位符前缀，这里是"${"
private String placeholderPrefix = SystemPropertyUtils.PLACEHOLDER_PREFIX;

//属性占位符后缀，这里是"}"
private String placeholderSuffix = SystemPropertyUtils.PLACEHOLDER_SUFFIX;

//属性占位符解析失败的时候配置默认值的分隔符，这里是":"
@Nullable
private String valueSeparator = SystemPropertyUtils.VALUE_SEPARATOR;

public String resolveRequiredPlaceholders(String text) throws IllegalArgumentException {
	if (this.strictHelper == null) {
		this.strictHelper = createPlaceholderHelper(false);
	}
	return doResolvePlaceholders(text, this.strictHelper);
}

//创建一个新的PropertyPlaceholderHelper实例，这里ignoreUnresolvablePlaceholders为false
private PropertyPlaceholderHelper createPlaceholderHelper(boolean ignoreUnresolvablePlaceholders) {
	return new PropertyPlaceholderHelper(this.placeholderPrefix, this.placeholderSuffix, this.valueSeparator, ignoreUnresolvablePlaceholders);
}

//这里最终的解析工作委托到PropertyPlaceholderHelper#replacePlaceholders完成
private String doResolvePlaceholders(String text, PropertyPlaceholderHelper helper) {
	return helper.replacePlaceholders(text, this::getPropertyAsRawString);
}

最终只需要分析PropertyPlaceholderHelper#replacePlaceholders，这里需要重点注意：

• 注意到这里的第一个参数text就是属性值的源字符串，例如我们需要处理的属性为myProperties: ${server.port}-${spring.application.name}，这里的text就是${server.port}-${spring.application.name}。
• replacePlaceholders方法的第二个参数placeholderResolver，这里比较巧妙，这里的方法引用this::getPropertyAsRawString相当于下面的代码：

//PlaceholderResolver是一个函数式接口
@FunctionalInterface
public interface PlaceholderResolver {
  @Nullable
  String resolvePlaceholder(String placeholderName);
}
//this::getPropertyAsRawString相当于下面的代码
return new PlaceholderResolver(){

    @Override
    String resolvePlaceholder(String placeholderName){
        //这里调用到的是PropertySourcesPropertyResolver#getPropertyAsRawString，有点绕
        return getPropertyAsRawString(placeholderName);
    }
}

接着看PropertyPlaceholderHelper#replacePlaceholders的源码：

//基础属性
//占位符前缀，默认是"${"
private final String placeholderPrefix;
//占位符后缀，默认是"}"
private final String placeholderSuffix;
//简单的占位符前缀，默认是"{"，主要用于处理嵌套的占位符如${xxxxx.{yyyyy}}
private final String simplePrefix;

//默认值分隔符号，默认是":"
@Nullable
private final String valueSeparator;
//替换属性占位符
public String replacePlaceholders(String value, PlaceholderResolver placeholderResolver) {
	Assert.notNull(value, "'value' must not be null");
	return parseStringValue(value, placeholderResolver, new HashSet<>());
}

//递归解析带占位符的属性为字符串
protected String parseStringValue(
		String value, PlaceholderResolver placeholderResolver, Set<String> visitedPlaceholders) {
	StringBuilder result = new StringBuilder(value);
	int startIndex = value.indexOf(this.placeholderPrefix);
	while (startIndex != -1) {
        //搜索第一个占位符后缀的索引
		int endIndex = findPlaceholderEndIndex(result, startIndex);
		if (endIndex != -1) {
            //提取第一个占位符中的原始字符串，如${server.port}->server.port
			String placeholder = result.substring(startIndex + this.placeholderPrefix.length(), endIndex);
			String originalPlaceholder = placeholder;
            //判重
			if (!visitedPlaceholders.add(originalPlaceholder)) {
				throw new IllegalArgumentException(
						"Circular placeholder reference '" + originalPlaceholder + "' in property definitions");
			}
			// Recursive invocation, parsing placeholders contained in the placeholder key.
            // 递归调用，实际上就是解析嵌套的占位符，因为提取的原始字符串有可能还有一层或者多层占位符
			placeholder = parseStringValue(placeholder, placeholderResolver, visitedPlaceholders);
			// Now obtain the value for the fully resolved key...
            // 递归调用完毕后，可以确定得到的字符串一定是不带占位符，这个时候调用getPropertyAsRawString获取key对应的字符串值
			String propVal = placeholderResolver.resolvePlaceholder(placeholder);
            // 如果字符串值为null，则进行默认值的解析，因为默认值有可能也使用了占位符，如${server.port:${server.port-2:8080}}
			if (propVal == null && this.valueSeparator != null) {
				int separatorIndex = placeholder.indexOf(this.valueSeparator);
				if (separatorIndex != -1) {
					String actualPlaceholder = placeholder.substring(0, separatorIndex);
                    // 提取默认值的字符串
					String defaultValue = placeholder.substring(separatorIndex + this.valueSeparator.length());
                    // 这里是把默认值的表达式做一次解析，解析到null，则直接赋值为defaultValue
					propVal = placeholderResolver.resolvePlaceholder(actualPlaceholder);
					if (propVal == null) {
						propVal = defaultValue;
					}
				}
			}
            // 上一步解析出来的值不为null，但是它有可能是一个带占位符的值，所以后面对值进行递归解析
			if (propVal != null) {
				// Recursive invocation, parsing placeholders contained in the
				// previously resolved placeholder value.
				propVal = parseStringValue(propVal, placeholderResolver, visitedPlaceholders);
                // 这一步很重要，替换掉第一个被解析完毕的占位符属性，例如${server.port}-${spring.application.name} -> 9090--${spring.application.name}
				result.replace(startIndex, endIndex + this.placeholderSuffix.length(), propVal);
				if (logger.isTraceEnabled()) {
					logger.trace("Resolved placeholder '" + placeholder + "'");
				}
                // 重置startIndex为下一个需要解析的占位符前缀的索引，可能为-1，说明解析结束
				startIndex = result.indexOf(this.placeholderPrefix, startIndex + propVal.length());
			}
			else if (this.ignoreUnresolvablePlaceholders) {
                // 如果propVal为null并且ignoreUnresolvablePlaceholders设置为true，直接返回当前的占位符之间的原始字符串尾的索引，也就是跳过解析
				// Proceed with unprocessed value.
				startIndex = result.indexOf(this.placeholderPrefix, endIndex + this.placeholderSuffix.length());
			}
			else {
                // 如果propVal为null并且ignoreUnresolvablePlaceholders设置为false，抛出异常
				throw new IllegalArgumentException("Could not resolve placeholder '" +
							placeholder + "'" + " in value \"" + value + "\"");
			}
            // 递归结束移除判重集合中的元素
			visitedPlaceholders.remove(originalPlaceholder);
		}
		else {
            // endIndex = -1说明解析结束
			startIndex = -1;
		}
	}
	return result.toString();
}

//基于传入的起始索引，搜索第一个占位符后缀的索引，兼容嵌套的占位符
private int findPlaceholderEndIndex(CharSequence buf, int startIndex) {
    //这里index实际上就是实际需要解析的属性的第一个字符，如${server.port}，这里index指向s
	int index = startIndex + this.placeholderPrefix.length();
	int withinNestedPlaceholder = 0;
	while (index < buf.length()) {
        //index指向"}"，说明有可能到达占位符尾部或者嵌套占位符尾部
		if (StringUtils.substringMatch(buf, index, this.placeholderSuffix)) {
            //存在嵌套占位符，则返回字符串中占位符后缀的索引值
			if (withinNestedPlaceholder > 0) {
				withinNestedPlaceholder--;
				index = index + this.placeholderSuffix.length();
			}
			else {
                //不存在嵌套占位符，直接返回占位符尾部索引
				return index;
			}
		}
        //index指向"{"，记录嵌套占位符个数withinNestedPlaceholder加1，index更新为嵌套属性的第一个字符的索引
		else if (StringUtils.substringMatch(buf, index, this.simplePrefix)) {
			withinNestedPlaceholder++;
			index = index + this.simplePrefix.length();
		}
		else {
            //index不是"{"或者"}"，则进行自增
			index++;
		}
	}
    //这里说明解析索引已经超出了原字符串
	return -1;
}

//StringUtils#substringMatch，此方法会检查原始字符串str的index位置开始是否和子字符串substring完全匹配
public static boolean substringMatch(CharSequence str, int index, CharSequence substring) {
	if (index + substring.length() > str.length()) {
		return false;
	}
	for (int i = 0; i < substring.length(); i++) {
		if (str.charAt(index + i) != substring.charAt(i)) {
			return false;
		}
	}
	return true;
}

上面的过程相对比较复杂，因为用到了递归，我们举个实际的例子说明一下整个解析过程，例如我们使用了四个属性项，我们的目标是获取server.desc的值：

application.name=spring
server.port=9090
spring.application.name=${application.name}
server.desc=${server.port-${spring.application.name}}:${description:"hello"}

属性类型转换

在上一步解析属性占位符完毕之后，得到的是属性字符串值，可以把字符串转换为指定的类型，此功能由AbstractPropertyResolver#convertValueIfNecessary完成：

protected <T> T convertValueIfNecessary(Object value, @Nullable Class<T> targetType) {
	if (targetType == null) {
		return (T) value;
	}
	ConversionService conversionServiceToUse = this.conversionService;
	if (conversionServiceToUse == null) {
		// Avoid initialization of shared DefaultConversionService if
		// no standard type conversion is needed in the first place...
        // 这里一般只有字符串类型才会命中
		if (ClassUtils.isAssignableValue(targetType, value)) {
			return (T) value;
		}
		conversionServiceToUse = DefaultConversionService.getSharedInstance();
	}
	return conversionServiceToUse.convert(value, targetType);
}

实际上转换的逻辑是委托到DefaultConversionService的父类方法GenericConversionService#convert：

public <T> T convert(@Nullable Object source, Class<T> targetType) {
	Assert.notNull(targetType, "Target type to convert to cannot be null");
	return (T) convert(source, TypeDescriptor.forObject(source), TypeDescriptor.valueOf(targetType));
}

public Object convert(@Nullable Object source, @Nullable TypeDescriptor sourceType, TypeDescriptor targetType) {
	Assert.notNull(targetType, "Target type to convert to cannot be null");
	if (sourceType == null) {
		Assert.isTrue(source == null, "Source must be [null] if source type == [null]");
		return handleResult(null, targetType, convertNullSource(null, targetType));
	}
	if (source != null && !sourceType.getObjectType().isInstance(source)) {
		throw new IllegalArgumentException("Source to convert from must be an instance of [" +
					sourceType + "]; instead it was a [" + source.getClass().getName() + "]");
	}
    // 从缓存中获取GenericConverter实例，其实这一步相对复杂，匹配两个类型的时候，会解析整个类的层次进行对比
	GenericConverter converter = getConverter(sourceType, targetType);
	if (converter != null) {
        // 实际上就是调用转换方法
		Object result = ConversionUtils.invokeConverter(converter, source, sourceType, targetType);
        // 断言最终结果和指定类型是否匹配并且返回
		return handleResult(sourceType, targetType, result);
	}
	return handleConverterNotFound(source, sourceType, targetType);
}

上面所有的可用的GenericConverter的实例可以在DefaultConversionService的addDefaultConverters中看到，默认添加的转换器实例已经超过20个，有些情况下如果无法满足需求可以添加自定义的转换器，实现GenericConverter接口添加进去即可。

小结

SpringBoot在抽象整个类型转换器方面做的比较好，在SpringMVC应用中，采用的是org.springframework.boot.autoconfigure.web.format.WebConversionService，兼容了Converter、Formatter、ConversionService等转换器类型并且对外提供一套统一的转换方法。

（本文完）