doc: PlaceholderResolver

docs/Spring/clazz/PlaceholderResolver/Spring-PlaceholderResolver.md

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+# Spring PlaceholderResolver
+- 类全路径: `org.springframework.util.PropertyPlaceholderHelper.PlaceholderResolver`
+
+- 类作用将占位符中的内容替换成属性值.
+    - 假设现有属性表: user.dir = c:\home
+      传入参数 user.dir 会获得 c:\home 
+
+```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);
+	}
+
+```
+
+- 类图如下
+
+![PropertyPlaceholderConfigurerResolver](/images/spring/PropertyPlaceholderConfigurerResolver.png)

docs/Spring/clazz/PlaceholderResolver/Spring-PropertyPlaceholderConfigurerResolver.md

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+# Spring PropertyPlaceholderConfigurerResolver
+
+- 类全路径: `org.springframework.beans.factory.config.PropertyPlaceholderConfigurer.PropertyPlaceholderConfigurerResolver`
+
+
+- 这个类是从 Properties 中获取属性
+
+```java
+	private final class PropertyPlaceholderConfigurerResolver implements PlaceholderResolver {
+
+		private final Properties props;
+
+		private PropertyPlaceholderConfigurerResolver(Properties props) {
+			this.props = props;
+		}
+
+		@Override
+		@Nullable
+		public String resolvePlaceholder(String placeholderName) {
+			return PropertyPlaceholderConfigurer.this.resolvePlaceholder(placeholderName,
+					this.props, systemPropertiesMode);
+		}
+	}
+
+```
+
+- 详细方法如下
+
+```java
+
+	@Nullable
+	protected String resolvePlaceholder(String placeholder, Properties props, int systemPropertiesMode) {
+		String propVal = null;
+		if (systemPropertiesMode == SYSTEM_PROPERTIES_MODE_OVERRIDE) {
+			propVal = resolveSystemProperty(placeholder);
+		}
+		if (propVal == null) {
+			propVal = resolvePlaceholder(placeholder, props);
+		}
+		if (propVal == null && systemPropertiesMode == SYSTEM_PROPERTIES_MODE_FALLBACK) {
+			propVal = resolveSystemProperty(placeholder);
+		}
+		return propVal;
+	}
+
+```
+
+
+```java
+	@Nullable
+	protected String resolvePlaceholder(String placeholder, Properties props) {
+		return props.getProperty(placeholder);
+	}
+```
+
+
+```java
+	@Nullable
+	protected String resolveSystemProperty(String key) {
+		try {
+			String value = System.getProperty(key);
+			if (value == null && this.searchSystemEnvironment) {
+				value = System.getenv(key);
+			}
+			return value;
+		}
+		catch (Throwable ex) {
+			if (logger.isDebugEnabled()) {
+				logger.debug("Could not access system property '" + key + "': " + ex);
+			}
+			return null;
+		}
+	}
+
+```

docs/Spring/clazz/PlaceholderResolver/Spring-ServletContextPlaceholderResolver.md

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+# Spring ServletContextPlaceholderResolver
+
+
+- 类全路径: `org.springframework.web.util.ServletContextPropertyUtils.ServletContextPlaceholderResolver`
+
+```java
+	private static class ServletContextPlaceholderResolver
+			implements PropertyPlaceholderHelper.PlaceholderResolver {
+
+		private final String text;
+
+		private final ServletContext servletContext;
+
+		public ServletContextPlaceholderResolver(String text, ServletContext servletContext) {
+			this.text = text;
+			this.servletContext = servletContext;
+		}
+
+		@Override
+		@Nullable
+		public String resolvePlaceholder(String placeholderName) {
+			try {
+				// servlet 上下文获取
+				String propVal = this.servletContext.getInitParameter(placeholderName);
+				if (propVal == null) {
+					// Fall back to system properties.
+					propVal = System.getProperty(placeholderName);
+					if (propVal == null) {
+						// Fall back to searching the system environment.
+						propVal = System.getenv(placeholderName);
+					}
+				}
+				return propVal;
+			}
+			catch (Throwable ex) {
+				System.err.println("Could not resolve placeholder '" + placeholderName + "' in [" +
+						this.text + "] as ServletContext init-parameter or system property: " + ex);
+				return null;
+			}
+		}
+	}
+
+```

docs/Spring/clazz/PlaceholderResolver/Spring-SystemPropertyPlaceholderResolver.md

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+# Spring SystemPropertyPlaceholderResolver 
+
+- 类全路径: `org.springframework.util.SystemPropertyUtils.SystemPropertyPlaceholderResolver`
+
+
+```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;
+			}
+		}
+	}
+
+```