1、SpringSecurity：策略模式

这段代码采用了声明的：策略模式 strategy
源码如下：

public class SecurityContextHolder {

	public static final String MODE_THREADLOCAL = "MODE_THREADLOCAL";

	public static final String MODE_INHERITABLETHREADLOCAL = "MODE_INHERITABLETHREADLOCAL";

	public static final String MODE_GLOBAL = "MODE_GLOBAL";

	private static final String MODE_PRE_INITIALIZED = "MODE_PRE_INITIALIZED";

	public static final String SYSTEM_PROPERTY = "spring.security.strategy";

	private static String strategyName = System.getProperty(SYSTEM_PROPERTY);

	private static SecurityContextHolderStrategy strategy;

	private static int initializeCount = 0;

	static {
		initialize();
	}

	private static void initialize() {
		initializeStrategy();
		initializeCount++;
	}

	private static void initializeStrategy() {
		if (MODE_PRE_INITIALIZED.equals(strategyName)) {
			Assert.state(strategy != null, "When using " + MODE_PRE_INITIALIZED
					+ ", setContextHolderStrategy must be called with the fully constructed strategy");
			return;
		}
		if (!StringUtils.hasText(strategyName)) {
			// Set default
			strategyName = MODE_THREADLOCAL;
		}
		if (strategyName.equals(MODE_THREADLOCAL)) {
			strategy = new ThreadLocalSecurityContextHolderStrategy();
			return;
		}
		if (strategyName.equals(MODE_INHERITABLETHREADLOCAL)) {
			strategy = new InheritableThreadLocalSecurityContextHolderStrategy();
			return;
		}
		if (strategyName.equals(MODE_GLOBAL)) {
			strategy = new GlobalSecurityContextHolderStrategy();
			return;
		}
		// Try to load a custom strategy
		try {
			Class<?> clazz = Class.forName(strategyName);
			Constructor<?> customStrategy = clazz.getConstructor();
			strategy = (SecurityContextHolderStrategy) customStrategy.newInstance();
		}
		catch (Exception ex) {
			ReflectionUtils.handleReflectionException(ex);
		}
	}

	/**
	 * Explicitly clears the context value from the current thread.
	 */
	public static void clearContext() {
		strategy.clearContext();
	}

	/**
	 * Obtain the current <code>SecurityContext</code>.
	 * @return the security context (never <code>null</code>)
	 */
	public static SecurityContext getContext() {
		return strategy.getContext();
	}

	public static int getInitializeCount() {
		return initializeCount;
	}


	public static void setContext(SecurityContext context) {
		strategy.setContext(context);
	}

	public static void setStrategyName(String strategyName) {
		SecurityContextHolder.strategyName = strategyName;
		initialize();
	}

	public static void setContextHolderStrategy(SecurityContextHolderStrategy strategy) {
		Assert.notNull(strategy, "securityContextHolderStrategy cannot be null");
		SecurityContextHolder.strategyName = MODE_PRE_INITIALIZED;
		SecurityContextHolder.strategy = strategy;
		initialize();
	}

	/**
	 * Allows retrieval of the context strategy. See SEC-1188.
	 * @return the configured strategy for storing the security context.
	 */
	public static SecurityContextHolderStrategy getContextHolderStrategy() {
		return strategy;
	}

	/**
	 * Delegates the creation of a new, empty context to the configured strategy.
	 */
	public static SecurityContext createEmptyContext() {
		return strategy.createEmptyContext();
	}

	@Override
	public String toString() {
		return "SecurityContextHolder[strategy='" + strategy.getClass().getSimpleName() + "'; initializeCount="
				+ initializeCount + "]";
	}

}

2、策略模式作用

2.1 通过策略-程序员选择线程的方式

三种线程方式

	// 本地线程
	public static final String MODE_THREADLOCAL = "MODE_THREADLOCAL";
    // 本地线程，即子线程、子子。。。
	public static final String MODE_INHERITABLETHREADLOCAL = "MODE_INHERITABLETHREADLOCAL";
	// 全局（不推荐使用）
	public static final String MODE_GLOBAL = "MODE_GLOBAL";

2.2 获取springSecurity的 SecurityContext 上下文

通过指定某一个策略，决定实现类

2.3 在jvm中指定环境

默认为：ThreadLocal
如果不指定策略，则在子线程中获取authentication则空指针