簡介
ThreadLocal能夠?yàn)楫?dāng)前線程提供存儲和讀取變量的能力瓦灶，提供一個靜態(tài)方法烁登，從而能夠讓若干模塊解耦奶陈；也為多線程并發(fā)提供一個思路捧挺，在ThreadLocal中為當(dāng)前儲存變量，只為當(dāng)前線程所用尿瞭，讓多線程之間不互相干擾。
本文簡單介紹ThreadLocal翅睛，列舉一些常見框架中的使用場景声搁，從而對它有更好的認(rèn)識捕发。
ThreadLocal API
ThreadLocal常用的有三個方法set, get, remove,下面用一小段代碼來看看這三個方法的使用疏旨。
public class ThreadLocalTest {
final static ThreadLocal<String> 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);
}
}
輸出：
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
上面的代碼中可以清楚看到get和set方法的使用檐涝，同時remove也是非常重要的，因?yàn)榫€程池的原因法挨，如果不執(zhí)行remove操作谁榜，這個線程在下次被重復(fù)使用的時候凡纳，存儲在ThreadLocal中的值仍可使用。
在框架中的使用
Spring Security中的使用
如果你使用過Spring Security,那么對SecurityContextHolder肯定不陌生：
SecurityContext securityContext = SecurityContextHolder.getContext();
Authentication authentication = securityContext.getAuthentication();
上面這段代碼經(jīng)常用來獲取當(dāng)前認(rèn)證過的用戶相關(guān)信息荐糜，而這個方法之所以能夠工作巷怜，其中之一就用的ThreadLocal。
詳細(xì)代碼可以可見github,這里摘取一些通過ThreadLocal來存儲認(rèn)證信息片段暴氏。
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代碼來看看這個從哪里獲取
SecurityContext contextBeforeChainExecution = repo.loadContext(holder);
try {
// 調(diào)用set方法存儲
SecurityContextHolder.setContext(contextBeforeChainExecution);
// 繼續(xù)調(diào)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，防止線程被重復(fù)使用关带，變量仍在
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獲取SecurityContext,這里摘取HttpSessionSecurityContextRepository源碼片段,
public SecurityContext loadContext(HttpRequestResponseHolder requestResponseHolder) {
// 從requestResponseHolder中獲取request和response
HttpServletRequest request = requestResponseHolder.getRequest();
HttpServletResponse response = requestResponseHolder.getResponse();
HttpSession httpSession = request.getSession(false);
// 下面private方法看看如何從session里面獲取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.
// 原來session里面存儲著一個特殊的屬性key為SPRING_SECURITY_CONTEXT研儒，這個就是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;
}
上面代碼我們就看出了ThreadLocalSecurityContextHolderStrategy是如何工作的了。
Spring MVC中的使用
如果你在Spring MVC中啟用過過org.springframework.web.context.request.RequestContextListener好芭，那么對RequestContextHolder肯定不陌生，通過RequestContextHolder提供的靜態(tài)方法招狸，可以獲取當(dāng)前request對象邻薯。
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
}
上述方法能夠生效只要看看RequestContextListener源碼就很好理解了
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());
//存儲
RequestContextHolder.setRequestAttributes(attributes);
}
public void requestDestroyed(ServletRequestEvent requestEvent) {
//銷毀
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();
}
}
多數(shù)據(jù)源切換
在開發(fā)中可能會遇到有多個數(shù)據(jù)源的情況厕诡，例如兩個Datasource之間的切換累榜，配合Spring的AOP和AbstractRoutingDataSource灵嫌，在加上改變ThreadLocal中的值來完成數(shù)據(jù)源的切換。
實(shí)現(xiàn)AbstractRoutingDataSource時猖凛，實(shí)現(xiàn)determineCurrentLookupKey方法來返回ThreadLocal中的值绪穆，通過其他方法
去改變ThreadLocal中的值，例如對某些方法的切面, 自定義注解等等玖院。
log4j中的使用
如果想在·log4j·中打印一些公共的變量。
例如TrackingId表示每一個請求难菌，可以在filter中先通過org.apache.log4j.MDC.put(String key, Object o)插入一個trackingId的key(例如TACKING_ID)和value,然后配置log4j的格式%X{TACKING_ID}即可在日志中實(shí)現(xiàn)插入一個值，即使為null也沒有問題耍共。
