doc: update readme

README.md

@@ -78,6 +78,10 @@
 * [Spring MultiValueMap](/docs/Spring/clazz/Spring-MultiValueMap.md)
 * [Spring MethodOverride](/docs/Spring/clazz/Spring-MethodOverride.md)
 * [Spring BeanDefinitionReaderUtils](/docs/Spring/clazz/Spring-BeanDefinitionReaderUtils.md)
+* [Spring PropertyPlaceholderHelper](/docs/Spring/clazz/Spring-PropertyPlaceholderHelper.md)
+
+* [Spring PropertySource](/docs/Spring/clazz/PropertySource)
+* [Spring PlaceholderResolver](/docs/Spring/clazz/PlaceholderResolver)
 
 
 ### Spring5 新特性

docs/Spring/clazz/Spring-PropertyPlaceholderHelper.md

@@ -0,0 +1,154 @@
+# Spring PropertyPlaceholderHelper 
+
+
+
+- 类全路径: `org.springframework.util.PropertyPlaceholderHelper`
+
+
+
+
+
+
+
+## parseStringValue
+
+- `org.springframework.util.PropertyPlaceholderHelper#parseStringValue` 这个方法是主要方法
+
+
+
+```java
+protected String parseStringValue(
+      String value, PlaceholderResolver placeholderResolver, @Nullable Set<String> visitedPlaceholders) {
+
+   // 占位符所在位置
+   int startIndex = value.indexOf(this.placeholderPrefix);
+   if (startIndex == -1) {
+      return value;
+   }
+
+   // 返回值
+   StringBuilder result = new StringBuilder(value);
+   while (startIndex != -1) {
+      // 寻找结尾占位符
+      int endIndex = findPlaceholderEndIndex(result, startIndex);
+      if (endIndex != -1) {
+         // 返回值切分留下中间内容
+         String placeholder = result.substring(startIndex + this.placeholderPrefix.length(), endIndex);
+         String originalPlaceholder = placeholder;
+         if (visitedPlaceholders == null) {
+            visitedPlaceholders = new HashSet<>(4);
+         }
+         if (!visitedPlaceholders.add(originalPlaceholder)) {
+            throw new IllegalArgumentException(
+                  "Circular placeholder reference '" + originalPlaceholder + "' in property definitions");
+         }
+         // Recursive invocation, parsing placeholders contained in the placeholder key.
+         // 递归获取占位符内容
+         placeholder = parseStringValue(placeholder, placeholderResolver, visitedPlaceholders);
+         // Now obtain the value for the fully resolved key...
+         // 解析占位符内容获得真正的属性值
+         String propVal = placeholderResolver.resolvePlaceholder(placeholder);
+         if (propVal == null && this.valueSeparator != null) {
+            int separatorIndex = placeholder.indexOf(this.valueSeparator);
+            if (separatorIndex != -1) {
+               String actualPlaceholder = placeholder.substring(0, separatorIndex);
+               String defaultValue = placeholder.substring(separatorIndex + this.valueSeparator.length());
+               propVal = placeholderResolver.resolvePlaceholder(actualPlaceholder);
+               if (propVal == null) {
+                  propVal = defaultValue;
+               }
+            }
+         }
+         if (propVal != null) {
+            // Recursive invocation, parsing placeholders contained in the
+            // previously resolved placeholder value.
+            propVal = parseStringValue(propVal, placeholderResolver, visitedPlaceholders);
+            result.replace(startIndex, endIndex + this.placeholderSuffix.length(), propVal);
+            if (logger.isTraceEnabled()) {
+               logger.trace("Resolved placeholder '" + placeholder + "'");
+            }
+            startIndex = result.indexOf(this.placeholderPrefix, startIndex + propVal.length());
+         }
+         else if (this.ignoreUnresolvablePlaceholders) {
+            // Proceed with unprocessed value.
+            startIndex = result.indexOf(this.placeholderPrefix, endIndex + this.placeholderSuffix.length());
+         }
+         else {
+            throw new IllegalArgumentException("Could not resolve placeholder '" +
+                  placeholder + "'" + " in value \"" + value + "\"");
+         }
+         visitedPlaceholders.remove(originalPlaceholder);
+      }
+      else {
+         startIndex = -1;
+      }
+   }
+   return result.toString();
+}
+```
+
+
+
+在这里还需要关注一个接口
+
+- 占位符解析. 
+
+```java
+@FunctionalInterface
+public interface PlaceholderResolver {
+
+   /**
+    * Resolve the supplied placeholder name to the replacement value.
+    * @param placeholderName the name of the placeholder to resolve
+    * @return the replacement value, or {@code null} if no replacement is to be made
+    */
+   @Nullable
+   String resolvePlaceholder(String placeholderName);
+}
+```
+
+
+
+占位符解析请查看: [PlaceholderResolver](PlaceholderResolver)
+
+
+
+## findPlaceholderEndIndex
+
+- 寻找结尾占位符索引
+
+```java
+/**
+ * 寻找结尾占位符索引
+ */
+private int findPlaceholderEndIndex(CharSequence buf, int startIndex) {
+   int index = startIndex + this.placeholderPrefix.length();
+   int withinNestedPlaceholder = 0;
+   while (index < buf.length()) {
+      if (StringUtils.substringMatch(buf, index, this.placeholderSuffix)) {
+         if (withinNestedPlaceholder > 0) {
+            withinNestedPlaceholder--;
+            index = index + this.placeholderSuffix.length();
+         }
+         else {
+            return index;
+         }
+      }
+      else if (StringUtils.substringMatch(buf, index, this.simplePrefix)) {
+         withinNestedPlaceholder++;
+         index = index + this.simplePrefix.length();
+      }
+      else {
+         index++;
+      }
+   }
+   return -1;
+}
+```
+
+
+
+
+
+
+