Spring Boot环境属性占位符解析及类型转换详解

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。

来源：https://www.cnblogs.com/throwable/p/9417827.html