ThreadLocal的原理与应用

MySql作为一般公司最常用的关系型数据库，需要按照一定的规范来进行设计和运用，才能优雅的进行后续的开发。本文通过对阿里云mysql设计规范进行理解，来详细阐述各个规范的相关场景。

Spring请求上下文Holder

管理请求上下文的Request参数。

public abstract class RequestContextHolder  {

	private static final boolean jsfPresent =
			ClassUtils.isPresent("javax.faces.context.FacesContext", RequestContextHolder.class.getClassLoader());

	private static final ThreadLocal<RequestAttributes> requestAttributesHolder =
			new NamedThreadLocal<>("Request attributes");

	private static final ThreadLocal<RequestAttributes> inheritableRequestAttributesHolder =
			new NamedInheritableThreadLocal<>("Request context");
  
  ...
}

Spring事务管理器

由于事务执行的原子性，事务回滚的connection对象必须和执行sql时的sql一致

public abstract class TransactionSynchronizationManager {

	private static final Log logger = LogFactory.getLog(TransactionSynchronizationManager.class);

	private static final ThreadLocal<Map<Object, Object>> resources =
			new NamedThreadLocal<>("Transactional resources");

	private static final ThreadLocal<Set<TransactionSynchronization>> synchronizations =
			new NamedThreadLocal<>("Transaction synchronizations");

	private static final ThreadLocal<String> currentTransactionName =
			new NamedThreadLocal<>("Current transaction name");

	private static final ThreadLocal<Boolean> currentTransactionReadOnly =
			new NamedThreadLocal<>("Current transaction read-only status");

	private static final ThreadLocal<Integer> currentTransactionIsolationLevel =
			new NamedThreadLocal<>("Current transaction isolation level");

	private static final ThreadLocal<Boolean> actualTransactionActive =
			new NamedThreadLocal<>("Actual transaction active");
  
  ...
}

Spring 安全上下文

执行安全策略时，安全信息上下文SecurityContext为线程私有的。

final class ThreadLocalSecurityContextHolderStrategy implements
		SecurityContextHolderStrategy {
  
	private static final ThreadLocal<SecurityContext> contextHolder = new ThreadLocal<>();

	public void clearContext() {
		contextHolder.remove();
	}

	public SecurityContext getContext() {
		SecurityContext ctx = contextHolder.get();

		if (ctx == null) {
			ctx = createEmptyContext();
			contextHolder.set(ctx);
		}

		return ctx;
	}

	public void setContext(SecurityContext context) {
		Assert.notNull(context, "Only non-null SecurityContext instances are permitted");
		contextHolder.set(context);
	}

	public SecurityContext createEmptyContext() {
		return new SecurityContextImpl();
	}
}

Q.E.D.