简介
ThreadLocal能够为当前线程提供存储和读取变量的能力，提供一个静态方法，从而能够让若干模块解耦；也为多线程并发提供一个思路，在ThreadLocal中为当前储存变量，只为当前线程所用，让多线程之间不互相干扰。
本文简单介绍ThreadLocal，列举一些常见框架中的使用场景，从而对它有更好的认识。
ThreadLocal API
ThreadLocal常用的有三个方法set, get, remove,下面用一小段代码来看看这三个方法的使用。
public class ThreadLocalTest {
final static ThreadLocal threadLocal = new ThreadLocal<>();
@Test
public void testThreadLocal() {
final String valueBeforeSet = threadLocal.get();
log.info("value before set: {}", valueBeforeSet);
threadLocal.set("test");
final String valueAfterSet = threadLocal.get();
log.info("value after set: {}", valueAfterSet);
threadLocal.remove();
final String valueAfterRemove = threadLocal.get();
log.info("value after remove: {}", valueAfterRemove);
}
}
输出：
2018-05-21 20:43:41,390{GMT} INFO value before set: null
2018-05-21 20:43:41,396{GMT} INFO value after set: test
2018-05-21 20:43:41,396{GMT} INFO value after remove: null
上面的代码中可以清楚看到get和set方法的使用，同时remove也是非常重要的，因为线程池的原因，如果不执行remove操作，这个线程在下次被重复使用的时候，存储在ThreadLocal中的值仍可使用。
在框架中的使用
Spring Security中的使用
如果你使用过Spring Security,那么对SecurityContextHolder肯定不陌生：
SecurityContext securityContext = SecurityContextHolder.getContext();
Authentication authentication = securityContext.getAuthentication();
上面这段代码经常用来获取当前认证过的用户相关信息，而这个方法之所以能够工作，其中之一就用的ThreadLocal。
详细代码可以可见github,这里摘取一些通过ThreadLocal来存储认证信息片段。
public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain)
throws IOException, ServletException {
HttpServletRequest request = (HttpServletRequest) req;
HttpServletResponse response = (HttpServletResponse) res;
if (request.getAttribute(FILTER_APPLIED) != null) {
// ensure that filter is only applied once per request
chain.doFilter(request, response);
return;
}
final boolean debug = logger.isDebugEnabled();
request.setAttribute(FILTER_APPLIED, Boolean.TRUE);
if (forceEagerSessionCreation) {
HttpSession session = request.getSession();
if (debug && session.isNew()) {
logger.debug("Eagerly created session: " + session.getId());
}
}
HttpRequestResponseHolder holder = new HttpRequestResponseHolder(request, response);
// 下面摘取HttpSessionSecurityContextRepository代码来看看这个从哪里获取
SecurityContext contextBeforeChainExecution = repo.loadContext(holder);
try {
// 调用set方法存储
SecurityContextHolder.setContext(contextBeforeChainExecution);
// 继续调用其他filter
chain.doFilter(holder.getRequest(), holder.getResponse());
} finally {
SecurityContext contextAfterChainExecution = SecurityContextHolder.getContext();
// Crucial removal of SecurityContextHolder contents - do this before anything else.
// finally方法中一定要remove，防止线程被重复使用，变量仍在
SecurityContextHolder.clearContext();
repo.saveContext(contextAfterChainExecution, holder.getRequest(), holder.getResponse());
request.removeAttribute(FILTER_APPLIED);
if (debug) {
logger.debug("SecurityContextHolder now cleared, as request processing completed");
}
}
}
下面在看看上面如何获取SecurityContext,这里摘取HttpSessionSecurityContextRepository源码片段,
public SecurityContext loadContext(HttpRequestResponseHolder requestResponseHolder) {
// 从requestResponseHolder中获取request和response
HttpServletRequest request = requestResponseHolder.getRequest();
HttpServletResponse response = requestResponseHolder.getResponse();
HttpSession httpSession = request.getSession(false);
// 下面private方法看看如何从session里面获取context
SecurityContext context = readSecurityContextFromSession(httpSession);
if (context == null) {
if (logger.isDebugEnabled()) {
logger.debug("No SecurityContext was available from the HttpSession: " + httpSession +". " +
"A new one will be created.");
}
context = generateNewContext();
}
SaveToSessionResponseWrapper wrappedResponse = new SaveToSessionResponseWrapper(response, request, httpSession != null, context);
requestResponseHolder.setResponse(wrappedResponse);
if(isServlet3) {
requestResponseHolder.setRequest(new Servlet3SaveToSessionRequestWrapper(request, wrappedResponse));
}
return context;
}
/**
*
* @param httpSession the session obtained from the request.
*/
private SecurityContext readSecurityContextFromSession(HttpSession httpSession) {
final boolean debug = logger.isDebugEnabled();
if (httpSession == null) {
if (debug) {
logger.debug("No HttpSession currently exists");
}
return null;
}
// Session exists, so try to obtain a context from it.
// 原来session里面存储着一个特殊的属性key为SPRING_SECURITY_CONTEXT，这个就是context
Object contextFromSession = httpSession.getAttribute(springSecurityContextKey);
if (contextFromSession == null) {
if (debug) {
logger.debug("HttpSession returned null object for SPRING_SECURITY_CONTEXT");
}
return null;
}
// We now have the security context object from the session.
if (!(contextFromSession instanceof SecurityContext)) {
if (logger.isWarnEnabled()) {
logger.warn(springSecurityContextKey + " did not contain a SecurityContext but contained: '"
+ contextFromSession + "'; are you improperly modifying the HttpSession directly "
+ "(you should always use SecurityContextHolder) or using the HttpSession attribute "
+ "reserved for this class?");
}
return null;
}
if (debug) {
logger.debug("Obtained a valid SecurityContext from " + springSecurityContextKey + ": '" + contextFromSession + "'");
}
// Everything OK. The only non-null return from this method.
return (SecurityContext) contextFromSession;
}
上面代码我们就看出了ThreadLocalSecurityContextHolderStrategy是如何工作的了。
Spring MVC中的使用
如果你在Spring MVC中启用过过org.springframework.web.context.request.RequestContextListener，那么对RequestContextHolder肯定不陌生，通过RequestContextHolder提供的静态方法，可以获取当前request对象。
public static HttpServletRequest getRequest() {
ServletRequestAttributes attr = (ServletRequestAttributes) RequestContextHolder.currentRequestAttributes();
return attr.getRequest();
}
public static HttpSession getSession() {
ServletRequestAttributes attr = (ServletRequestAttributes) RequestContextHolder.currentRequestAttributes();
return attr.getRequest().getSession(false); // true == allow create
}
上述方法能够生效只要看看RequestContextListener源码就很好理解了
public void requestInitialized(ServletRequestEvent requestEvent) {
if (!(requestEvent.getServletRequest() instanceof HttpServletRequest)) {
throw new IllegalArgumentException(
"Request is not an HttpServletRequest: " + requestEvent.getServletRequest());
}
HttpServletRequest request = (HttpServletRequest) requestEvent.getServletRequest();
ServletRequestAttributes attributes = new ServletRequestAttributes(request);
request.setAttribute(REQUEST_ATTRIBUTES_ATTRIBUTE, attributes);
LocaleContextHolder.setLocale(request.getLocale());
//存储
RequestContextHolder.setRequestAttributes(attributes);
}
public void requestDestroyed(ServletRequestEvent requestEvent) {
//销毁
ServletRequestAttributes attributes =
(ServletRequestAttributes) requestEvent.getServletRequest().getAttribute(REQUEST_ATTRIBUTES_ATTRIBUTE);
ServletRequestAttributes threadAttributes =
(ServletRequestAttributes) RequestContextHolder.getRequestAttributes();
if (threadAttributes != null) {
// We're assumably within the original request thread...
if (attributes == null) {
attributes = threadAttributes;
}
LocaleContextHolder.resetLocaleContext();
RequestContextHolder.resetRequestAttributes();
}
if (attributes != null) {
attributes.requestCompleted();
}
}
多数据源切换
在开发中可能会遇到有多个数据源的情况，例如两个Datasource之间的切换，配合Spring的AOP和AbstractRoutingDataSource，在加上改变ThreadLocal中的值来完成数据源的切换。
实现AbstractRoutingDataSource时，实现determineCurrentLookupKey方法来返回ThreadLocal中的值，通过其他方法
去改变ThreadLocal中的值，例如对某些方法的切面, 自定义注解等等。
log4j中的使用
如果想在·log4j·中打印一些公共的变量。
例如TrackingId表示每一个请求，可以在filter中先通过org.apache.log4j.MDC.put(String key, Object o)插入一个trackingId的key(例如TACKING_ID)和value,然后配置log4j的格式%X{TACKING_ID}即可在日志中实现插入一个值，即使为null也没有问题。