ThreadLocal与Inheritable ThreadLocal

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ThreadLocal使用
ThreadLocal可以让线程拥有本地变量，在web环境中，为了方便代码解耦，我们通常用它来保存上下文信息，然后用一个util类提供访问入口，从controller层到service层可以很方便的获取上下文。下面我们通过代码来研究一下ThreadLocal。
新建一个ThreadContext类，用于保存线程上下文信息
public class ThreadContext {    private static ThreadLocal<UserObj> userResource = new ThreadLocal<UserObj>();    public static UserObj getUser() {        return userResource.get();    }    public static void bindUser(UserObj user) {        userResource.set(user);    }    public static UserObj unbindUser() {        UserObj obj = userResource.get();        userResource.remove();        return obj;    }}
新建一个sessionFilter ，用来操作线程变量
    @Override    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {        HttpServletRequest request = (HttpServletRequest) servletRequest;        try {            // 假设这里是从cookie拿token信息, 调用服务/或者从缓存查询用户信息            // 为了避免后续逻辑中多次查询/请求缓存服务器, 这里拿到user后放到线程本地变量中            UserObj user = ThreadContext.getUser();            // 如果当前线程中没有绑定user对象,那么绑定一个新的user            if (user == null) {                ThreadContext.bindUser(new UserObj("usertest"));            }            filterChain.doFilter(servletRequest, servletResponse);        } finally {            // ThreadLocal的生命周期不等于一次request请求的生命周期            // 每个request请求的响应是tomcat从线程池中分配的线程, 线程会被下个请求复用.            // 所以请求结束后必须删除线程本地变量            // ThreadContext.unbindUser();        }    }
新建UserUtils工具类
/** * 配合SessionFilter使用,从上下文中取user信息 */public class UserUtils {    public static UserObj getCurrentUser() {        return ThreadContext.getUser();    }}
新建一个servlet测试
public class HelloworldServlet extends HttpServlet {    private static Logger logger = LoggerFactory.getLogger(HelloworldServlet.class);    @Override    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {        UserObj user = UserUtils.getCurrentUser();        logger.info(user.getName() + user.hashCode());        super.doGet(req, resp);    }    @Override    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {        super.doGet(req, resp);    }}
循环请求servlet，控制台显示结果如下。可以发现tomcat线程池的初始大小是10个，后面的请求复用了前面的线程，ThreadContext中的user对象的hashcode也一样。
2016-11-29 17:21:35.975  INFO 36672 --- [nio-8080-exec-2] com.zallds.xy.servlet.HelloworldServlet  : usertest8182026732016-11-29 17:21:38.923  INFO 36672 --- [nio-8080-exec-3] com.zallds.xy.servlet.HelloworldServlet  : usertest15825917022016-11-29 17:21:45.810  INFO 36672 --- [nio-8080-exec-4] com.zallds.xy.servlet.HelloworldServlet  : usertest557550372016-11-29 17:21:46.773  INFO 36672 --- [nio-8080-exec-5] com.zallds.xy.servlet.HelloworldServlet  : usertest14954668072016-11-29 17:21:47.345  INFO 36672 --- [nio-8080-exec-6] com.zallds.xy.servlet.HelloworldServlet  : usertest11493602452016-11-29 17:21:47.613  INFO 36672 --- [nio-8080-exec-7] com.zallds.xy.servlet.HelloworldServlet  : usertest5183753392016-11-29 17:21:47.837  INFO 36672 --- [nio-8080-exec-8] com.zallds.xy.servlet.HelloworldServlet  : usertest924589922016-11-29 17:21:48.012  INFO 36672 --- [nio-8080-exec-9] com.zallds.xy.servlet.HelloworldServlet  : usertest9448670342016-11-29 17:21:48.199  INFO 36672 --- [io-8080-exec-10] com.zallds.xy.servlet.HelloworldServlet  : usertest14109728092016-11-29 17:21:48.378  INFO 36672 --- [nio-8080-exec-1] com.zallds.xy.servlet.HelloworldServlet  : usertest8053320462016-11-29 17:21:48.552  INFO 36672 --- [nio-8080-exec-2] com.zallds.xy.servlet.HelloworldServlet  : usertest8182026732016-11-29 17:21:48.730  INFO 36672 --- [nio-8080-exec-3] com.zallds.xy.servlet.HelloworldServlet  : usertest15825917022016-11-29 17:21:48.903  INFO 36672 --- [nio-8080-exec-4] com.zallds.xy.servlet.HelloworldServlet  : usertest557550372016-11-29 17:21:49.072  INFO 36672 --- [nio-8080-exec-5] com.zallds.xy.servlet.HelloworldServlet  : usertest14954668072016-11-29 17:21:49.247  INFO 36672 --- [nio-8080-exec-6] com.zallds.xy.servlet.HelloworldServlet  : usertest11493602452016-11-29 17:21:49.402  INFO 36672 --- [nio-8080-exec-7] com.zallds.xy.servlet.HelloworldServlet  : usertest518375339
去掉注释// ThreadContext.unbindUser(); 重新请求，每次从ThreadLocal中拿到的user对象完全不一样了。
2016-11-29 17:30:37.150  INFO 36903 --- [nio-8080-exec-1] com.zallds.xy.servlet.HelloworldServlet  : usertest4131385712016-11-29 17:30:42.932  INFO 36903 --- [nio-8080-exec-2] com.zallds.xy.servlet.HelloworldServlet  : usertest14021919452016-11-29 17:30:43.124  INFO 36903 --- [nio-8080-exec-3] com.zallds.xy.servlet.HelloworldServlet  : usertest19575791732016-11-29 17:30:43.313  INFO 36903 --- [nio-8080-exec-4] com.zallds.xy.servlet.HelloworldServlet  : usertest15825917022016-11-29 17:30:43.501  INFO 36903 --- [nio-8080-exec-5] com.zallds.xy.servlet.HelloworldServlet  : usertest19174795822016-11-29 17:30:43.679  INFO 36903 --- [nio-8080-exec-6] com.zallds.xy.servlet.HelloworldServlet  : usertest7720367672016-11-29 17:30:43.851  INFO 36903 --- [nio-8080-exec-7] com.zallds.xy.servlet.HelloworldServlet  : usertest1620207612016-11-29 17:30:44.024  INFO 36903 --- [nio-8080-exec-8] com.zallds.xy.servlet.HelloworldServlet  : usertest6822329502016-11-29 17:30:44.225  INFO 36903 --- [nio-8080-exec-9] com.zallds.xy.servlet.HelloworldServlet  : usertest21406503412016-11-29 17:30:44.419  INFO 36903 --- [io-8080-exec-10] com.zallds.xy.servlet.HelloworldServlet  : usertest13276017632016-11-29 17:30:44.593  INFO 36903 --- [nio-8080-exec-1] com.zallds.xy.servlet.HelloworldServlet  : usertest6477384112016-11-29 17:30:44.787  INFO 36903 --- [nio-8080-exec-2] com.zallds.xy.servlet.HelloworldServlet  : usertest9448670342016-11-29 17:30:45.045  INFO 36903 --- [nio-8080-exec-3] com.zallds.xy.servlet.HelloworldServlet  : usertest18861545202016-11-29 17:30:45.317  INFO 36903 --- [nio-8080-exec-4] com.zallds.xy.servlet.HelloworldServlet  : usertest15929042732016-11-29 17:30:46.380  INFO 36903 --- [nio-8080-exec-5] com.zallds.xy.servlet.HelloworldServlet  : usertest14109728092016-11-29 17:30:46.524  INFO 36903 --- [nio-8080-exec-6] com.zallds.xy.servlet.HelloworldServlet  : usertest17055706892016-11-29 17:30:46.692  INFO 36903 --- [nio-8080-exec-7] com.zallds.xy.servlet.HelloworldServlet  : usertest11051343752016-11-29 17:30:46.802  INFO 36903 --- [nio-8080-exec-8] com.zallds.xy.servlet.HelloworldServlet  : usertest407377722
ThreadLocal子线程场景
需求新增， 需要在原有的业务逻辑中增加一个给用户发送邮件的操作。发送邮件我们采用异步处理，新建一个线程来执行。
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {        UserObj user = UserUtils.getCurrentUser();        logger.info(user.getName() + user.hashCode());        SendEmailTask emailThread = new SendEmailTask();        new Thread(emailThread).start();        super.doGet(req, resp);    }    class SendEmailTask implements Runnable {        @Override        public void run() {            UserObj user = UserUtils.getCurrentUser();            logger.info("子线程中:" + (user == null ? "user为null" : user.getName() + user.hashCode()));        }    }
主线程中创建异步线程，子线程中能拿到吗？通过测试发现是不能的
2016-11-29 18:09:16.482  INFO 38092 --- [nio-8080-exec-2] com.zallds.xy.servlet.HelloworldServlet  : usertest14255059182016-11-29 18:09:16.483  INFO 38092 --- [       Thread-4] com.zallds.xy.servlet.HelloworldServlet  : 子线程中:user为null2016-11-29 18:09:20.995  INFO 38092 --- [nio-8080-exec-3] com.zallds.xy.servlet.HelloworldServlet  : usertest12803735522016-11-29 18:09:20.996  INFO 38092 --- [       Thread-5] com.zallds.xy.servlet.HelloworldServlet  : 子线程中:user为null
子线程怎么拿到父线程的ThreadLocal数据？jdk给我们提供了解决办法，ThreadLocal有一个子类InheritableThreadLocal，创建ThreadLocal时候我们采用InheritableThreadLocal类可以实现子线程获取到父线程的本地变量。
private static ThreadLocal<UserObj> userResource = new InheritableThreadLocal<UserObj>();
然后子线程中就可以正常拿到user对象了
2016-11-29 19:07:01.518  INFO 39644 --- [nio-8080-exec-2] com.zallds.xy.servlet.HelloworldServlet  : usertest4955501282016-11-29 19:07:01.518  INFO 39644 --- [       Thread-4] com.zallds.xy.servlet.HelloworldServlet  : 子线程中:usertest4955501282016-11-29 19:07:05.839  INFO 39644 --- [nio-8080-exec-3] com.zallds.xy.servlet.HelloworldServlet  : usertest18517174042016-11-29 19:07:05.840  INFO 39644 --- [       Thread-5] com.zallds.xy.servlet.HelloworldServlet  : 子线程中:usertest1851717404
ThreadLocal 子线程传递－线程池场景
当我们执行异步任务时，大多会采用线程池的机制(如Executor)。这样就会存在一个问题，即使父线程已经结束，子线程依然存在并被池化。这样，线程池中的线程在下一次请求被执行的时候，ThreadLocal的get()方法返回的将不是当前线程中设定的变量，因为池中的“子线程”根本不是当前线程创建的，当前线程设定的ThreadLocal变量也就无法传递给线程池中的线程。
我们修改一下发送邮件的代码，改用线程池来实现。
2016-11-29 19:51:51.973  INFO 40937 --- [nio-8080-exec-1] com.zallds.xy.servlet.HelloworldServlet  : usertest14176412612016-11-29 19:51:51.974  INFO 40937 --- [pool-1-thread-1] com.zallds.xy.servlet.HelloworldServlet  : 子线程中:usertest14176412612016-11-29 19:51:55.746  INFO 40937 --- [nio-8080-exec-2] com.zallds.xy.servlet.HelloworldServlet  : usertest11165379552016-11-29 19:51:55.746  INFO 40937 --- [pool-1-thread-1] com.zallds.xy.servlet.HelloworldServlet  : 子线程中:usertest14176412612016-11-29 19:51:58.825  INFO 40937 --- [nio-8080-exec-3] com.zallds.xy.servlet.HelloworldServlet  : usertest14899388562016-11-29 19:51:58.826  INFO 40937 --- [pool-1-thread-1] com.zallds.xy.servlet.HelloworldServlet  : 子线程中:usertest1417641261
可以发现发送邮件的任务三次用的都是同一个线程[pool-1-thread-1]，第一次子线程和父线程中的user对象相同，后面的“子线程”（前面提到过，后面的已经不是子线程了）中的user对象都是和第一个父线程中的相同。
那么在线程池的场景下，怎么能让“子线程”正常拿到父线程传递过来的变量呢？如果我们能在创建task的时候主动传递过去就好了。按照这个想法我们来实施一下。
继续修改代码
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {        UserObj user = UserUtils.getCurrentUser();        logger.info(user.getName() + user.hashCode());        SendEmailTask emailThread = new SendEmailTask();        executor.execute(new UserRunnable(emailThread, user));        super.doGet(req, resp);    }    /**     * 做一个wrapper, 将目标任务做一层包装, 在run方法中传递线程本地变量     */    class UserRunnable implements Runnable {        /**         * 目标任务对象         */        Runnable runnable;        /**         * 要绑定的user对象         */        UserObj user;        public UserRunnable(Runnable runnable, UserObj user) {            this.runnable = runnable;            this.user = user;        }        @Override        public void run() {            ThreadContext.bindUser(user);            runnable.run();            ThreadContext.unbindUser();        }    }    class SendEmailTask implements Runnable {        @Override        public void run() {            UserObj user = UserUtils.getCurrentUser();            logger.info("子线程中:" + (user == null ? "user为null" : user.getName() + user.hashCode()));        }    }
重新请求，得到我们想要的结果
2016-11-29 20:04:12.153  INFO 41258 --- [nio-8080-exec-1] com.zallds.xy.servlet.HelloworldServlet  : usertest15651807442016-11-29 20:04:12.154  INFO 41258 --- [pool-1-thread-1] com.zallds.xy.servlet.HelloworldServlet  : 子线程中:usertest15651807442016-11-29 20:04:14.142  INFO 41258 --- [nio-8080-exec-2] com.zallds.xy.servlet.HelloworldServlet  : usertest4813967042016-11-29 20:04:14.142  INFO 41258 --- [pool-1-thread-1] com.zallds.xy.servlet.HelloworldServlet  : 子线程中:usertest4813967042016-11-29 20:04:15.248  INFO 41258 --- [nio-8080-exec-3] com.zallds.xy.servlet.HelloworldServlet  : usertest4007173952016-11-29 20:04:15.249  INFO 41258 --- [pool-1-thread-1] com.zallds.xy.servlet.HelloworldServlet  : 子线程中:usertest400717395
到此为止，ThreadLocal常见的场景和对应解决方案应该可以满足了。接下来就是怎么在实际应用中运用了。
为了引出此文的初衷以及后面要讲的东西，针对最后一个解决方案，我们可以进一步完善一下。
    ThreadContext.bindUser(user);    runnable.run();    ThreadContext.unbindUser();
这个地方在bind的时候是直接覆盖，无法对线程之前的状态进行保存和恢复。要实现这一点，我们可以抽象一个ThreadState来保存线程的状态，在bind之前保存original，任务执行完以后进行restore。
public interface ThreadState {    void bind();    void restore();    void clear();}public class UserThreadState implements ThreadState {    private UserObj original;    private UserObj user;    public UserThreadState(UserObj user) {        this.user = user;    }    @Override    public void bind() {        this.original = ThreadContext.getUser();        ThreadContext.bindUser(this.user);    }    @Override    public void restore() {        ThreadContext.bindUser(this.original);    }    @Override    public void clear() {        ThreadContext.unbindUser();    }}protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {        UserObj user = UserUtils.getCurrentUser();        logger.info(user.getName() + user.hashCode());        SendEmailTask emailThread = new SendEmailTask();        executor.execute(new UserRunnable(emailThread, new UserThreadState(user)));        super.doGet(req, resp);    }    /**     * 做一个wrapper, 将目标任务做一层包装, 在run方法中传递线程本地变量     */    class UserRunnable implements Runnable {        /**         * 目标任务对象         */        Runnable runnable;        /**         * 要绑定的user对象         */        UserThreadState userThreadState;        public UserRunnable(Runnable runnable, UserThreadState userThreadState) {            this.runnable = runnable;            this.userThreadState = userThreadState;        }        @Override        public void run() {            userThreadState.bind();            runnable.run();            userThreadState.restore();            UserObj userOrig = UserUtils.getCurrentUser();            logger.info("original:" + userOrig.getName() + userOrig.hashCode());        }    }    class SendEmailTask implements Runnable {        @Override        public void run() {            UserObj user = UserUtils.getCurrentUser();            logger.info("子线程中:" + (user == null ? "user为null" : user.getName() + user.hashCode()));        }    }
实现效果是相同的，至于为什么三次的original对象都是一样的，通过前面的说明应该能够理解
2016-11-29 20:19:48.694  INFO 41671 --- [nio-8080-exec-1] com.zallds.xy.servlet.HelloworldServlet  : usertest1147606762016-11-29 20:19:48.699  INFO 41671 --- [pool-1-thread-1] com.zallds.xy.servlet.HelloworldServlet  : 子线程中:usertest1147606762016-11-29 20:19:48.700  INFO 41671 --- [pool-1-thread-1] com.zallds.xy.servlet.HelloworldServlet  : original:usertest1147606762016-11-29 20:19:57.123  INFO 41671 --- [nio-8080-exec-2] com.zallds.xy.servlet.HelloworldServlet  : usertest9413021992016-11-29 20:19:57.123  INFO 41671 --- [pool-1-thread-1] com.zallds.xy.servlet.HelloworldServlet  : 子线程中:usertest9413021992016-11-29 20:19:57.123  INFO 41671 --- [pool-1-thread-1] com.zallds.xy.servlet.HelloworldServlet  : original:usertest1147606762016-11-29 20:20:04.385  INFO 41671 --- [nio-8080-exec-3] com.zallds.xy.servlet.HelloworldServlet  : usertest14899388562016-11-29 20:20:04.385  INFO 41671 --- [pool-1-thread-1] com.zallds.xy.servlet.HelloworldServlet  : 子线程中:usertest14899388562016-11-29 20:20:04.385  INFO 41671 --- [pool-1-thread-1] com.zallds.xy.servlet.HelloworldServlet  : original:usertest114760676
由于在使用shiro框架的SecurityUtils.getSubject()过程中碰到问题，才有了本文的示例，例子中的部分代码参考了shiro框架的实现机制。后面会再研究一下shiro的subject相关设计。


作者：99793933e682
链接：http://www.jianshu.com/p/85d96fe9358b
來源：简书
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