概述

Spring框架將某個屬性源抽象成了類PropertySource，又將多個屬性源PropertySource抽象為介面PropertySources。對某個PropertySource物件中屬性的解析，抽象成了介面PropertyResolver,而類PropertySourcesPropertyResolver則是Spring用於解析一個PropertySources物件中屬性的工具類。Spring應用Environment物件中對其PropertySources物件的屬性解析，就是通過這樣一個物件。

// Spring Environment 實現類的抽象基類 AbstractEnvironment程式碼片段
 

private final ConfigurablePropertyResolver propertyResolver =
			new PropertySourcesPropertyResolver(this.propertySources);

原始碼解析

package org.springframework.core.env;

import org.springframework.lang.Nullable;

/**
 * PropertyResolver implementation that resolves property values against
 * an underlying set of PropertySources.
 * 一個PropertyResolver實現類，用於解析一個PropertySources物件中的屬性源集合中的屬性。
 * 
 * 這裡PropertySourcesPropertyResolver繼承自基類AbstractPropertyResolver，
 * AbstractPropertyResolver提供了很多方法實現，不過這裡不做過多解析，而僅僅關注
 * 如何獲取一個屬性的值的邏輯。
 * 
 * @author Chris Beams
 * @author Juergen Hoeller
 * @since 3.1
 * @see PropertySource
 * @see PropertySources
 * @see AbstractEnvironment
 */
 

public class PropertySourcesPropertyResolver extends AbstractPropertyResolver {
	// PropertySources形式存在的屬性源集合，該工具的工作物件
	@Nullable
	private final PropertySources propertySources;


	/**
	 * Create a new resolver against the given property sources.
	 * @param propertySources the set of PropertySource objects to use
	 */
 

	public PropertySourcesPropertyResolver(@Nullable PropertySources propertySources) {
		this.propertySources = propertySources;
	}

	// 檢視是否包含某個指定名稱的屬性，判斷方法：
	// 1. 底層任何一個屬性源包含該屬性的話就認為是包含；
	// 2. 否則認為是不包含。
	@Override
	public boolean containsProperty(String key) {
		if (this.propertySources != null) {
			for (PropertySource<?> propertySource : this.propertySources) {
				if (propertySource.containsProperty(key)) {
					return true;
				}
			}
		}
		return false;
	}

	// 獲取某個指定名稱的屬性的值，如果該屬性不被包含的話，返回null
	// 不管該屬性的值是什麼型別，將它轉換成字串型別
	@Override
	@Nullable
	public String getProperty(String key) {
		return getProperty(key, String.class, true);
	}

	// 獲取某個指定名稱的屬性的值，並將其轉換成指定的型別，如果該屬性不被包含的話，返回null
	@Override
	@Nullable
	public <T> T getProperty(String key, Class<T> targetValueType) {
		return getProperty(key, targetValueType, true);
	}
	
	// 獲取某個指定名稱的屬性的值，如果該屬性不被包含的話，返回null
	// 不管該屬性的值是什麼型別，將它轉換成字串型別
	@Override
	@Nullable
	protected String getPropertyAsRawString(String key) {
		return getProperty(key, String.class, false);
	}
	// 獲取某個指定名稱的屬性的值，並將其轉換成指定的型別，如果該屬性不被包含的話，返回null
	// resolveNestedPlaceholders引數指示是否要解析屬性值中包含的佔位符
	@Nullable
	protected <T> T getProperty(String key, Class<T> targetValueType, boolean resolveNestedPlaceholders) {
		if (this.propertySources != null) {
			// 遍歷每個屬性源，如果發現目標屬性被某個屬性源包含，則獲取它的值並按要求做相應的處理然後返回處理
			// 後的值從這裡使用for迴圈的方式來看，可以將屬性源看作是一個List，索引較小的屬性源先被訪問，也就
			// 是說，索引較小的屬性源具有較高優先順序
			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) {
						// 解析值中的佔位符
						value = resolveNestedPlaceholders((String) value);
					}
					logKeyFound(key, propertySource, value);
					// 根據要求做相應的型別轉換然後返回轉換後的值
					return convertValueIfNecessary(value, targetValueType);
				}
			}
		}
		if (logger.isTraceEnabled()) {
			logger.trace("Could not find key '" + key + "' in any property source");
		}
		// 任何屬性源中都不包含該屬性，返回null
		return null;
	}

	/**
	 * Log the given key as found in the given PropertySource, resulting in
	 * the given value.
	 * The default implementation writes a debug log message with key and source.
	 * As of 4.3.3, this does not log the value anymore in order to avoid accidental
	 * logging of sensitive settings. Subclasses may override this method to change
	 * the log level and/or log message, including the property's value if desired.
	 * @param key the key found
	 * @param propertySource the PropertySource that the key has been found in
	 * @param value the corresponding value
	 * @since 4.3.1
	 */
	protected void logKeyFound(String key, PropertySource<?> propertySource, Object value) {
		if (logger.isDebugEnabled()) {
			logger.debug("Found key '" + key + "' in PropertySource '" + propertySource.getName() +
					"' with value of type " + value.getClass().getSimpleName());
		}
	}

}