doc: Spring-SystemPropertyUtils.md

docs/Spring/clazz/Spring-SystemPropertyUtils.md

@@ -0,0 +1,90 @@
+# Spring SystemPropertyUtils
+- spring 中获取系统属性的工具类
+
+
+
+- 内部属性
+
+```java
+/**
+ *
+ * Prefix for system property placeholders: "${".
+ * 前缀占位符
+ * */
+public static final String PLACEHOLDER_PREFIX = "${";
+
+/**
+ *  Suffix for system property placeholders: "}".
+ *  后缀占位符
+ * */
+public static final String PLACEHOLDER_SUFFIX = "}";
+
+/**
+ * Value separator for system property placeholders: ":".
+ * 值分割符号
+ * */
+public static final String VALUE_SEPARATOR = ":";
+
+
+/**
+ * 占位符解析类
+ */
+private static final PropertyPlaceholderHelper strictHelper =
+      new PropertyPlaceholderHelper(PLACEHOLDER_PREFIX, PLACEHOLDER_SUFFIX, VALUE_SEPARATOR, false);
+
+/**
+ * 占位符解析类
+ */
+private static final PropertyPlaceholderHelper nonStrictHelper =
+      new PropertyPlaceholderHelper(PLACEHOLDER_PREFIX, PLACEHOLDER_SUFFIX, VALUE_SEPARATOR, true);
+```
+
+
+
+
+
+## resolvePlaceholders
+
+- 解析属性
+
+![SystemPropertyUtils-resolvePlaceholders.png](/images/spring/SystemPropertyUtils-resolvePlaceholders.png)
+
+
+
+时序图因为有递归所以看着有点长, 其核心方法最后会指向 PlaceholderResolver 
+
+通过 PlaceholderResolver 获取属性值
+
+在 `SystemPropertyUtils` 内部有 `PlaceholderResolver ` 实现
+
+- 最终通过下面的类来获取具体的属性值
+
+```java
+private static class SystemPropertyPlaceholderResolver implements PropertyPlaceholderHelper.PlaceholderResolver {
+
+   private final String text;
+
+   public SystemPropertyPlaceholderResolver(String text) {
+      this.text = text;
+   }
+
+   @Override
+   @Nullable
+   public String resolvePlaceholder(String placeholderName) {
+      try {
+         String propVal = System.getProperty(placeholderName);
+         if (propVal == null) {
+            // Fall back to searching the system environment.
+            // 获取系统属性
+            propVal = System.getenv(placeholderName);
+         }
+         return propVal;
+      }
+      catch (Throwable ex) {
+         System.err.println("Could not resolve placeholder '" + placeholderName + "' in [" +
+               this.text + "] as system property: " + ex);
+         return null;
+      }
+   }
+}
+```