PropertySourcesPropertyResolver#getPropertyAsRawString：

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

protected  T getProperty(String key, Class 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;
}

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

//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);
}

//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);
 }
}  

//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 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;
}

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

protected  T convertValueIfNecessary(Object value, @Nullable Class 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);
}

public  T convert(@Nullable Object source, Class 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);
}