示例1：普通共享对象的问题
importjava.util.ArrayList;importjava.util.List;classSharedObject{privateintcounter=0;publicvoidincrement(){counter++;}publicintgetCounter(){returncounter;}}classThreadLocalNotWorkingExample{privatestaticThreadLocal<SharedObject>threadLocal=ThreadLocal.withInitial(()->newSharedObject());publicvoidupdateCounter(){SharedObjectshared=threadLocal.get();shared.increment();System.out.println(Thread.currentThread().getName()+" - Counter: "+shared.getCounter());}publicstaticvoidmain(String[]args)throwsInterruptedException{ThreadLocalNotWorkingExampleexample1=newThreadLocalNotWorkingExample();ThreadLocalNotWorkingExampleexample2=newThreadLocalNotWorkingExample();// 同一个线程内，不同实例共享同一个 SharedObjectThreadthread=newThread(()->{// 两个不同对象的方法调用example1.updateCounter();// 输出 1example2.updateCounter();// 输出 2 - 问题：共享了同一个 SharedObject 实例},"Thread-1");thread.start();thread.join();}}示例2：多个对象间共享状态导致的竞态条件
importjava.util.concurrent.ExecutorService;importjava.util.concurrent.Executors;classOrderService{privatestaticThreadLocal<List<String>>orderThreadLocal=ThreadLocal.withInitial(ArrayList::new);publicvoidaddOrder(StringorderId){List<String>orders=orderThreadLocal.get();orders.add(orderId);System.out.println(Thread.currentThread().getName()+" 添加订单: "+orderId+"，当前订单数: "+orders.size());}publicvoidprocessOrders(){List<String>orders=orderThreadLocal.get();// 模拟处理订单for(Stringorder:orders){System.out.println(Thread.currentThread().getName()+" 处理订单: "+order);}}}classPaymentService{privatestaticThreadLocal<List<String>>paymentThreadLocal=ThreadLocal.withInitial(ArrayList::new);publicvoidaddPayment(StringpaymentId){List<String>payments=paymentThreadLocal.get();payments.add(paymentId);System.out.println(Thread.currentThread().getName()+" 添加支付: "+paymentId+"，当前支付数: "+payments.size());}}classProblemExample{privatestaticThreadLocal<SharedState>threadLocal=ThreadLocal.withInitial(SharedState::new);staticclassSharedState{List<String>allOperations=newArrayList<>();intoperationCount=0;publicsynchronizedvoidaddOperation(Stringoperation){allOperations.add(operation);operationCount++;}}publicvoiddoOperation1(){SharedStatestate=threadLocal.get();state.addOperation("Operation1");System.out.println("Operation count: "+state.operationCount);}publicvoiddoOperation2(){SharedStatestate=threadLocal.get();state.addOperation("Operation2");System.out.println("Operation count: "+state.operationCount);}publicstaticvoidmain(String[]args){ProblemExampleobj1=newProblemExample();ProblemExampleobj2=newProblemExample();// 使用线程池，线程会被复用ExecutorServiceexecutor=Executors.newFixedThreadPool(2);for(inti=0;i<5;i++){inttaskId=i;executor.submit(()->{System.out.println("\n=== 任务 "+taskId+"，线程: "+Thread.currentThread().getName()+" ===");// 同一个线程内，不同对象共享同一个 ThreadLocal 的 SharedStateobj1.doOperation1();obj2.doOperation2();// 这里会操作同一个 SharedState// 问题：线程复用时，ThreadLocal 里的数据没有清理SharedStatestate=threadLocal.get();System.out.println("总共操作数: "+state.operationCount+"，操作列表: "+state.allOperations);});}executor.shutdown();}}示例3：解决方案 - 显式传递共享状态
importjava.util.concurrent.ConcurrentHashMap;// 方案1：使用显式参数传递classOrderContext{privatefinalStringthreadId;privatefinalList<String>orders=newArrayList<>();privatefinalList<String>payments=newArrayList<>();publicOrderContext(StringthreadId){this.threadId=threadId;}publicsynchronizedvoidaddOrder(Stringorder){orders.add(order);}publicsynchronizedvoidaddPayment(Stringpayment){payments.add(payment);}publicvoidprocess(){System.out.println("处理线程 "+threadId+" 的订单和支付");System.out.println("订单: "+orders);System.out.println("支付: "+payments);}}classOrderProcessor{// 显式传递上下文，而不是使用 ThreadLocalpublicvoidprocessOrder(OrderContextcontext,StringorderId){context.addOrder(orderId);}}classPaymentProcessor{publicvoidprocessPayment(OrderContextcontext,StringpaymentId){context.addPayment(paymentId);}}// 方案2：使用请求级别的上下文classRequestContext{privatestaticfinalThreadLocal<RequestContext>contextThreadLocal=newThreadLocal<>();privatefinalStringrequestId;privatefinalMap<String,Object>attributes=newConcurrentHashMap<>();privateRequestContext(StringrequestId){this.requestId=requestId;}publicstaticvoidcreateContext(StringrequestId){contextThreadLocal.set(newRequestContext(requestId));}publicstaticRequestContextgetCurrentContext(){returncontextThreadLocal.get();}publicstaticvoidclearContext(){contextThreadLocal.remove();}publicvoidsetAttribute(Stringkey,Objectvalue){attributes.put(key,value);}publicObjectgetAttribute(Stringkey){returnattributes.get(key);}publicStringgetRequestId(){returnrequestId;}}classSolutionExample{publicstaticvoidmain(String[]args){// 模拟HTTP请求处理for(inti=0;i<3;i++){StringrequestId="REQ-"+i;// 开始请求，创建上下文RequestContext.createContext(requestId);try{// 处理请求RequestContextcontext=RequestContext.getCurrentContext();// 不同服务可以共享这个上下文OrderService2orderService=newOrderService2();PaymentService2paymentService=newPaymentService2();orderService.createOrder("ORDER-123");paymentService.processPayment("PAY-456");// 处理业务逻辑processBusinessLogic();}finally{// 必须清理，防止内存泄漏RequestContext.clearContext();}}}staticvoidprocessBusinessLogic(){RequestContextcontext=RequestContext.getCurrentContext();System.out.println("处理请求 "+context.getRequestId()+" 的业务逻辑");}}classOrderService2{publicvoidcreateOrder(StringorderId){RequestContextcontext=RequestContext.getCurrentContext();context.setAttribute("orderId",orderId);System.out.println("创建订单: "+orderId+"，请求ID: "+context.getRequestId());}}classPaymentService2{publicvoidprocessPayment(StringpaymentId){RequestContextcontext=RequestContext.getCurrentContext();StringorderId=(String)context.getAttribute("orderId");System.out.println("处理支付: "+paymentId+"，对应订单: "+orderId);}}关键总结
ThreadLocal 的共享问题：
- 同一个线程内的不同对象通过
static ThreadLocal访问的是同一个对象实例 - 这可能导致线程内多个组件间意外的状态共享
- 同一个线程内的不同对象通过
解决方案：
- 方案1：显式传递上下文对象作为方法参数
- 方案2：使用请求/会话级别的上下文，并在处理完成后及时清理
- 方案3：如果确实需要线程内共享，使用专门设计的共享对象，并确保线程安全
最佳实践：
// 正确使用 ThreadLocal 的模式try{threadLocal.set(value);// 执行业务逻辑}finally{threadLocal.remove();// 必须清理}
核心原则：ThreadLocal适用于存储线程隔离的上下文信息，而不是用于同一个线程内不同对象间共享和更新状态。如果需要线程内对象间共享状态，应考虑其他设计模式。
