public final class SpringProperties {
	// 存放spring系統(tǒng)屬性的配置文件
	private static final String PROPERTIES_RESOURCE_LOCATION = "spring.properties";
	// 使用Java的Properties保存spring的系統(tǒng)屬性。Properties類繼承了HashTable，可以理解為一個(gè)map
	public static Properties localProperties = new Properties();
	public static Properties getLocalProperties() {
		return localProperties;
	}
	static {
		try {
			// 獲取SpringProperties類的類加載器
			ClassLoader cl = SpringProperties.class.getClassLoader();
			// 使用類加載器將spring.properties配置文件解析成URL
			URL url = (cl != null ? cl.getResource(PROPERTIES_RESOURCE_LOCATION) :
					ClassLoader.getSystemResource(PROPERTIES_RESOURCE_LOCATION));
			// 如果獲取到spring.properties配置文件，會(huì)讀取到配置文件中的屬性并設(shè)置到spring的系統(tǒng)屬性中
			if (url != null) {
				// 將配置文件加載到輸入流中
				try (InputStream is = url.openStream()) {
					// 加載配置文件的鍵值對(duì)，添加到spring的系統(tǒng)屬性中
					localProperties.load(is);
				}
			}
		}
		catch (IOException ex) {
			System.err.println("Could not load 'spring.properties' file from local classpath: " + ex);
		}
	}
	private SpringProperties() {
	}
	/**
	 * Programmatically set a local property, overriding an entry in the
	 * {@code spring.properties} file (if any).
	 * @param key the property key
	 * @param value the associated property value, or {@code null} to reset it
	 */
	public static void setProperty(String key, @Nullable String value) {
		if (value != null) {
			localProperties.setProperty(key, value);
		}
		else {
			localProperties.remove(key);
		}
	}
	/**
	 * 檢索給定鍵的屬性值，首先檢查本地Spring屬性，然后返回到j(luò)vm級(jí)別的系統(tǒng)屬性。
	 * @Nullable 注解用在方法上，表示該方法可以返回空
	 *
	 * Retrieve the property value for the given key, checking local Spring
	 * properties first and falling back to JVM-level system properties.
	 * @param key the property key
	 * @return the associated property value, or {@code null} if none found
	 */
	@Nullable
	public static String getProperty(String key) {
		// 獲取spring的配置文件，可以通過(guò)spring.properties配置文件設(shè)置spring屬性
		String value = localProperties.getProperty(key);
		// 如果沒(méi)從spring的系統(tǒng)屬性中獲取到指定鍵的屬性，會(huì)從Java的系統(tǒng)屬性中獲取
		if (value == null) {
			try {
				// 從Java的系統(tǒng)屬性中獲取指定鍵的屬性值
				value = System.getProperty(key);
			}
			catch (Throwable ex) {
				System.err.println("Could not retrieve system property '" + key + "': " + ex);
			}
		}
		return value;
	}
	/**
	 * Programmatically set a local flag to "true", overriding an
	 * entry in the {@code spring.properties} file (if any).
	 * @param key the property key
	 */
	public static void setFlag(String key) {
		localProperties.put(key, Boolean.TRUE.toString());
	}
	/**
	 * 檢索給定屬性鍵的標(biāo)志。
	 *
	 * Retrieve the flag for the given property key.
	 * @param key the property key
	 * @return {@code true} if the property is set to "true",
	 * {@code} false otherwise
	 */
	public static boolean getFlag(String key) {
		return Boolean.parseBoolean(getProperty(key));
	}
}

spring.spel.ignore=true

/**
 * Boolean flag controlled by a {@code spring.spel.ignore} system property that instructs Spring to
 * ignore SpEL, i.e. to not initialize the SpEL infrastructure.
 * 由 {@code spring.spel.ignore} 系統(tǒng)屬性控制的布爾標(biāo)志，指示 Spring 忽略 SpEL，即不初始化 SpEL 基礎(chǔ)結(jié)構(gòu)。
 * 默認(rèn)值為 "false".
 */
// 靜態(tài)變量，加載該類的時(shí)候就會(huì)加載靜態(tài)變量。
// SpringProperties類中有靜態(tài)代碼塊。會(huì)通過(guò)類加載器的getResource獲取URL，參數(shù)為spring.properties
private static final boolean shouldIgnoreSpel = SpringProperties.getFlag("spring.spel.ignore");

static {
	try {
		// 獲取SpringProperties類的類加載器
		ClassLoader cl = SpringProperties.class.getClassLoader();
		// 使用類加載器將spring.properties配置文件解析成URL
		URL url = (cl != null ? cl.getResource(PROPERTIES_RESOURCE_LOCATION) :
				ClassLoader.getSystemResource(PROPERTIES_RESOURCE_LOCATION));
		// 如果獲取到spring.properties配置文件，會(huì)讀取到配置文件中的屬性并設(shè)置到spring的系統(tǒng)屬性中
		if (url != null) {
			// 將配置文件加載到輸入流中
			try (InputStream is = url.openStream()) {
				// 加載配置文件的鍵值對(duì)，添加到spring的系統(tǒng)屬性中
				localProperties.load(is);
			}
		}
	}
	catch (IOException ex) {
		System.err.println("Could not load 'spring.properties' file from local classpath: " + ex);
	}
}

spring.spel.ignore=true

// 靜態(tài)變量，加載該類的時(shí)候就會(huì)加載靜態(tài)變量。
// SpringProperties類中有靜態(tài)代碼塊。會(huì)通過(guò)類加載器的getResource獲取URL，參數(shù)為spring.properties
private static final boolean shouldIgnoreSpel = SpringProperties.getFlag("spring.spel.ignore");

static {
	try {
		// 獲取SpringProperties類的類加載器
		ClassLoader cl = SpringProperties.class.getClassLoader();
		// 使用類加載器將spring.properties配置文件解析成URL
		URL url = (cl != null ? cl.getResource(PROPERTIES_RESOURCE_LOCATION) :
				ClassLoader.getSystemResource(PROPERTIES_RESOURCE_LOCATION));
		// 如果獲取到spring.properties配置文件，會(huì)讀取到配置文件中的屬性并設(shè)置到spring的系統(tǒng)屬性中
		if (url != null) {
			// 將配置文件加載到輸入流中
			try (InputStream is = url.openStream()) {
				// 加載配置文件的鍵值對(duì)，添加到spring的系統(tǒng)屬性中
				localProperties.load(is);
			}
		}
	}
	catch (IOException ex) {
		System.err.println("Could not load 'spring.properties' file from local classpath: " + ex);
	}
}

public class ProjectApplication {
	private ProjectApplication() {}
	public static void main(String[] args) {
		ClassPathXmlApplicationContext classPathXmlApplicationContext = new ClassPathXmlApplicationContext();
		classPathXmlApplicationContext.setConfigLocation("classpath*:/*.xml");
		classPathXmlApplicationContext.refresh();
		Properties springProperties = SpringProperties.getLocalProperties();
		for (Object springPropertyKey : springProperties.keySet()) {
			System.out.println(springPropertyKey + " = " + springProperties.getProperty((String) springPropertyKey));
		}
	}
}

user = User{name='#{user.name}', age=21}
user2 = User{name='南宮仆射', age=22}
spring.spel.ignore = true