项目上使用Shiro框架，在使用多线程进行业务处理的时候，发现用shiro获取到的session不对

源码追踪，查找原因

Shiro获取session

Session session = SecurityUtils.getSubject().getSession()

然后我们往下看 SecurityUtils 的 getSubject() 方法

    /**
     * Returns the currently accessible {@code Subject} available to the calling code depending on
     * runtime environment.
     * <p/>
     * This method is provided as a way of obtaining a {@code Subject} without having to resort to
     * implementation-specific methods.  It also allows the Shiro team to change the underlying implementation of
     * this method in the future depending on requirements/updates without affecting your code that uses it.
     *
     * @return the currently accessible {@code Subject} accessible to the calling code.
     * @throws IllegalStateException if no {@link Subject Subject} instance or
     *                               {@link SecurityManager SecurityManager} instance is available with which to obtain
     *                               a {@code Subject}, which which is considered an invalid application configuration
     *                               - a Subject should <em>always</em> be available to the caller.
     */
    public static Subject getSubject() {
        Subject subject = ThreadContext.getSubject();
        if (subject == null) {
            subject = (new Subject.Builder()).buildSubject();
            ThreadContext.bind(subject);
        }
        return subject;
    }

由上面的方法，我们可以看到subject是从 ThreadContext 中获取出来的，我们再来看下这个subject是怎么存的：

1. 从ThreadContext 的subject获取，可以看到用户信息其实是存在 InheritableThreadLocalMap 对象中的，而 InheritableThreadLocalMap 继承了 InheritableThreadLocal 对象。
2. InheritableThreadLocal 对象会在创建子线程时，将其在父线程中的值复制到子线程中去
3. 线程池默认会在第一次使用线程时，根据最小线程数创建出线程

public abstract class ThreadContext {

    /**
     * Private internal log instance.
     */
    private static final Logger log = LoggerFactory.getLogger(ThreadContext.class);

    public static final String SECURITY_MANAGER_KEY = ThreadContext.class.getName() + "_SECURITY_MANAGER_KEY";
    public static final String SUBJECT_KEY = ThreadContext.class.getName() + "_SUBJECT_KEY";

    private static final ThreadLocal<Map<Object, Object>> resources = new InheritableThreadLocalMap<Map<Object, Object>>();

    /**
     * 中间去掉一大波代码，只留下 getSubject 的追踪链
     */

    /**
     * Convenience method that simplifies retrieval of a thread-bound Subject.  If there is no
     * Subject bound to the thread, this method returns <tt>null</tt>.  It is merely a convenient wrapper
     * for the following:
     * <p/>
     * <code>return (Subject)get( SUBJECT_KEY );</code>
     * <p/>
     * This method only returns the bound value if it exists - it does not remove it
     * from the thread.  To remove it, one must call {@link #unbindSubject() unbindSubject()} instead.
     *
     * @return the Subject object bound to the thread, or <tt>null</tt> if there isn't one bound.
     * @since 0.2
     */
    public static Subject getSubject() {
        return (Subject) get(SUBJECT_KEY);
    }

    /**
     * Returns the object for the specified <code>key</code> that is bound to
     * the current thread.
     *
     * @param key the key that identifies the value to return
     * @return the object keyed by <code>key</code> or <code>null</code> if
     *         no value exists for the specified <code>key</code>
     */
    public static Object get(Object key) {
        if (log.isTraceEnabled()) {
            String msg = "get() - in thread [" + Thread.currentThread().getName() + "]";
            log.trace(msg);
        }

        Object value = getValue(key);
        if ((value != null) && log.isTraceEnabled()) {
            String msg = "Retrieved value of type [" + value.getClass().getName() + "] for key [" +
                    key + "] " + "bound to thread [" + Thread.currentThread().getName() + "]";
            log.trace(msg);
        }
        return value;
    }

    /**
     * Returns the value bound in the {@code ThreadContext} under the specified {@code key}, or {@code null} if there
     * is no value for that {@code key}.
     *
     * @param key the map key to use to lookup the value
     * @return the value bound in the {@code ThreadContext} under the specified {@code key}, or {@code null} if there
     *         is no value for that {@code key}.
     * @since 1.0
     */
    private static Object getValue(Object key) {
        Map<Object, Object> perThreadResources = resources.get();
        return perThreadResources != null ? perThreadResources.get(key) : null;
    }
    
    private static final class InheritableThreadLocalMap<T extends Map<Object, Object>> extends InheritableThreadLocal<Map<Object, Object>> {

        /**
         * This implementation was added to address a
         * <a href="http://jsecurity.markmail.org/search/?q=#query:+page:1+mid:xqi2yxurwmrpqrvj+state:results">
         * user-reported issue</a>.
         * @param parentValue the parent value, a HashMap as defined in the {@link #initialValue()} method.
         * @return the HashMap to be used by any parent-spawned child threads (a clone of the parent HashMap).
         */
        @SuppressWarnings({"unchecked"})
        protected Map<Object, Object> childValue(Map<Object, Object> parentValue) {
            if (parentValue != null) {
                return (Map<Object, Object>) ((HashMap<Object, Object>) parentValue).clone();
            } else {
                return null;
            }
        }
    }
}

结论总结

也就是说，子线程的session一直都是他的创建者的session，假如第一个用户A使用线程池，创建出10个线程，后续用户B、C、D，如果直接复用线程，拿到的都是用户A的session。