当ThreadLocal碰上线程池
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 : usertest818202673
2016-11-29 17:21:38.923 INFO 36672 --- [nio-8080-exec-3] com.zallds.xy.servlet.HelloworldServlet : usertest1582591702
2016-11-29 17:21:45.810 INFO 36672 --- [nio-8080-exec-4] com.zallds.xy.servlet.HelloworldServlet : usertest55755037
2016-11-29 17:21:46.773 INFO 36672 --- [nio-8080-exec-5] com.zallds.xy.servlet.HelloworldServlet : usertest1495466807
2016-11-29 17:21:47.345 INFO 36672 --- [nio-8080-exec-6] com.zallds.xy.servlet.HelloworldServlet : usertest1149360245
2016-11-29 17:21:47.613 INFO 36672 --- [nio-8080-exec-7] com.zallds.xy.servlet.HelloworldServlet : usertest518375339
2016-11-29 17:21:47.837 INFO 36672 --- [nio-8080-exec-8] com.zallds.xy.servlet.HelloworldServlet : usertest92458992
2016-11-29 17:21:48.012 INFO 36672 --- [nio-8080-exec-9] com.zallds.xy.servlet.HelloworldServlet : usertest944867034
2016-11-29 17:21:48.199 INFO 36672 --- [io-8080-exec-10] com.zallds.xy.servlet.HelloworldServlet : usertest1410972809
2016-11-29 17:21:48.378 INFO 36672 --- [nio-8080-exec-1] com.zallds.xy.servlet.HelloworldServlet : usertest805332046
2016-11-29 17:21:48.552 INFO 36672 --- [nio-8080-exec-2] com.zallds.xy.servlet.HelloworldServlet : usertest818202673
2016-11-29 17:21:48.730 INFO 36672 --- [nio-8080-exec-3] com.zallds.xy.servlet.HelloworldServlet : usertest1582591702
2016-11-29 17:21:48.903 INFO 36672 --- [nio-8080-exec-4] com.zallds.xy.servlet.HelloworldServlet : usertest55755037
2016-11-29 17:21:49.072 INFO 36672 --- [nio-8080-exec-5] com.zallds.xy.servlet.HelloworldServlet : usertest1495466807
2016-11-29 17:21:49.247 INFO 36672 --- [nio-8080-exec-6] com.zallds.xy.servlet.HelloworldServlet : usertest1149360245
2016-11-29 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 : usertest413138571
2016-11-29 17:30:42.932 INFO 36903 --- [nio-8080-exec-2] com.zallds.xy.servlet.HelloworldServlet : usertest1402191945
2016-11-29 17:30:43.124 INFO 36903 --- [nio-8080-exec-3] com.zallds.xy.servlet.HelloworldServlet : usertest1957579173
2016-11-29 17:30:43.313 INFO 36903 --- [nio-8080-exec-4] com.zallds.xy.servlet.HelloworldServlet : usertest1582591702
2016-11-29 17:30:43.501 INFO 36903 --- [nio-8080-exec-5] com.zallds.xy.servlet.HelloworldServlet : usertest1917479582
2016-11-29 17:30:43.679 INFO 36903 --- [nio-8080-exec-6] com.zallds.xy.servlet.HelloworldServlet : usertest772036767
2016-11-29 17:30:43.851 INFO 36903 --- [nio-8080-exec-7] com.zallds.xy.servlet.HelloworldServlet : usertest162020761
2016-11-29 17:30:44.024 INFO 36903 --- [nio-8080-exec-8] com.zallds.xy.servlet.HelloworldServlet : usertest682232950
2016-11-29 17:30:44.225 INFO 36903 --- [nio-8080-exec-9] com.zallds.xy.servlet.HelloworldServlet : usertest2140650341
2016-11-29 17:30:44.419 INFO 36903 --- [io-8080-exec-10] com.zallds.xy.servlet.HelloworldServlet : usertest1327601763
2016-11-29 17:30:44.593 INFO 36903 --- [nio-8080-exec-1] com.zallds.xy.servlet.HelloworldServlet : usertest647738411
2016-11-29 17:30:44.787 INFO 36903 --- [nio-8080-exec-2] com.zallds.xy.servlet.HelloworldServlet : usertest944867034
2016-11-29 17:30:45.045 INFO 36903 --- [nio-8080-exec-3] com.zallds.xy.servlet.HelloworldServlet : usertest1886154520
2016-11-29 17:30:45.317 INFO 36903 --- [nio-8080-exec-4] com.zallds.xy.servlet.HelloworldServlet : usertest1592904273
2016-11-29 17:30:46.380 INFO 36903 --- [nio-8080-exec-5] com.zallds.xy.servlet.HelloworldServlet : usertest1410972809
2016-11-29 17:30:46.524 INFO 36903 --- [nio-8080-exec-6] com.zallds.xy.servlet.HelloworldServlet : usertest1705570689
2016-11-29 17:30:46.692 INFO 36903 --- [nio-8080-exec-7] com.zallds.xy.servlet.HelloworldServlet : usertest1105134375
2016-11-29 17:30:46.802 INFO 36903 --- [nio-8080-exec-8] com.zallds.xy.servlet.HelloworldServlet : usertest407377722ThreadLocal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 : usertest1425505918
2016-11-29 18:09:16.483 INFO 38092 --- [ Thread-4] com.zallds.xy.servlet.HelloworldServlet : 子线程中:user为null
2016-11-29 18:09:20.995 INFO 38092 --- [nio-8080-exec-3] com.zallds.xy.servlet.HelloworldServlet : usertest1280373552
2016-11-29 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 : usertest495550128
2016-11-29 19:07:01.518 INFO 39644 --- [ Thread-4] com.zallds.xy.servlet.HelloworldServlet : 子线程中:usertest495550128
2016-11-29 19:07:05.839 INFO 39644 --- [nio-8080-exec-3] com.zallds.xy.servlet.HelloworldServlet : usertest1851717404
2016-11-29 19:07:05.840 INFO 39644 --- [ Thread-5] com.zallds.xy.servlet.HelloworldServlet : 子线程中:usertest1851717404ThreadLocal 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 : usertest1417641261
2016-11-29 19:51:51.974 INFO 40937 --- [pool-1-thread-1] com.zallds.xy.servlet.HelloworldServlet : 子线程中:usertest1417641261
2016-11-29 19:51:55.746 INFO 40937 --- [nio-8080-exec-2] com.zallds.xy.servlet.HelloworldServlet : usertest1116537955
2016-11-29 19:51:55.746 INFO 40937 --- [pool-1-thread-1] com.zallds.xy.servlet.HelloworldServlet : 子线程中:usertest1417641261
2016-11-29 19:51:58.825 INFO 40937 --- [nio-8080-exec-3] com.zallds.xy.servlet.HelloworldServlet : usertest1489938856
2016-11-29 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 : usertest1565180744
2016-11-29 20:04:12.154 INFO 41258 --- [pool-1-thread-1] com.zallds.xy.servlet.HelloworldServlet : 子线程中:usertest1565180744
2016-11-29 20:04:14.142 INFO 41258 --- [nio-8080-exec-2] com.zallds.xy.servlet.HelloworldServlet : usertest481396704
2016-11-29 20:04:14.142 INFO 41258 --- [pool-1-thread-1] com.zallds.xy.servlet.HelloworldServlet : 子线程中:usertest481396704
2016-11-29 20:04:15.248 INFO 41258 --- [nio-8080-exec-3] com.zallds.xy.servlet.HelloworldServlet : usertest400717395
2016-11-29 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 : usertest114760676
2016-11-29 20:19:48.699 INFO 41671 --- [pool-1-thread-1] com.zallds.xy.servlet.HelloworldServlet : 子线程中:usertest114760676
2016-11-29 20:19:48.700 INFO 41671 --- [pool-1-thread-1] com.zallds.xy.servlet.HelloworldServlet : original:usertest114760676
2016-11-29 20:19:57.123 INFO 41671 --- [nio-8080-exec-2] com.zallds.xy.servlet.HelloworldServlet : usertest941302199
2016-11-29 20:19:57.123 INFO 41671 --- [pool-1-thread-1] com.zallds.xy.servlet.HelloworldServlet : 子线程中:usertest941302199
2016-11-29 20:19:57.123 INFO 41671 --- [pool-1-thread-1] com.zallds.xy.servlet.HelloworldServlet : original:usertest114760676
2016-11-29 20:20:04.385 INFO 41671 --- [nio-8080-exec-3] com.zallds.xy.servlet.HelloworldServlet : usertest1489938856
2016-11-29 20:20:04.385 INFO 41671 --- [pool-1-thread-1] com.zallds.xy.servlet.HelloworldServlet : 子线程中:usertest1489938856
2016-11-29 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。
http://shiro.apache.org/subject.html
作者： 99793933e682
原文地址： https://www.jianshu.com/p/85d96fe9358b
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