源码分析之RequestContextHolder

先看源码

/*
 * Copyright 2002-2012 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.web.context.request;

import javax.faces.context.FacesContext;

import org.springframework.core.NamedInheritableThreadLocal;
import org.springframework.core.NamedThreadLocal;
import org.springframework.util.ClassUtils;

/**
 * Holder class to expose the web request in the form of a thread-bound
 * {@link RequestAttributes} object. The request will be inherited
 * by any child threads spawned by the current thread if the
 * {@code inheritable} flag is set to {@code true}.
 *
 * <p>Use {@link RequestContextListener} or
 * {@link org.springframework.web.filter.RequestContextFilter} to expose
 * the current web request. Note that
 * {@link org.springframework.web.servlet.DispatcherServlet} and
 * {@link org.springframework.web.portlet.DispatcherPortlet} already
 * expose the current request by default.
 *
 * @author Juergen Hoeller
 * @author Rod Johnson
 * @since 2.0
 * @see RequestContextListener
 * @see org.springframework.web.filter.RequestContextFilter
 * @see org.springframework.web.servlet.DispatcherServlet
 * @see org.springframework.web.portlet.DispatcherPortlet
 */
public abstract class RequestContextHolder  {

    private static final boolean jsfPresent =
            ClassUtils.isPresent("javax.faces.context.FacesContext", RequestContextHolder.class.getClassLoader());

    private static final ThreadLocal<RequestAttributes> requestAttributesHolder =
            new NamedThreadLocal<RequestAttributes>("Request attributes");

    private static final ThreadLocal<RequestAttributes> inheritableRequestAttributesHolder =
            new NamedInheritableThreadLocal<RequestAttributes>("Request context");

    /**
     * Reset the RequestAttributes for the current thread.
     */
    public static void resetRequestAttributes() {
        requestAttributesHolder.remove();
        inheritableRequestAttributesHolder.remove();
    }

    /**
     * Bind the given RequestAttributes to the current thread,
     * <i>not</i> exposing it as inheritable for child threads.
     * @param attributes the RequestAttributes to expose
     * @see #setRequestAttributes(RequestAttributes, boolean)
     */
    public static void setRequestAttributes(RequestAttributes attributes) {
        setRequestAttributes(attributes, false);
    }

    /**
     * Bind the given RequestAttributes to the current thread.
     * @param attributes the RequestAttributes to expose,
     * or {@code null} to reset the thread-bound context
     * @param inheritable whether to expose the RequestAttributes as inheritable
     * for child threads (using an {@link InheritableThreadLocal})
     */
    public static void setRequestAttributes(RequestAttributes attributes, boolean inheritable) {
        if (attributes == null) {
            resetRequestAttributes();
        }
        else {
            if (inheritable) {
                inheritableRequestAttributesHolder.set(attributes);
                requestAttributesHolder.remove();
            }
            else {
                requestAttributesHolder.set(attributes);
                inheritableRequestAttributesHolder.remove();
            }
        }
    }

    /**
     * Return the RequestAttributes currently bound to the thread.
     * @return the RequestAttributes currently bound to the thread,
     * or {@code null} if none bound
     */
    public static RequestAttributes getRequestAttributes() {
        RequestAttributes attributes = requestAttributesHolder.get();
        if (attributes == null) {
            attributes = inheritableRequestAttributesHolder.get();
        }
        return attributes;
    }

    /**
     * Return the RequestAttributes currently bound to the thread.
     * <p>Exposes the previously bound RequestAttributes instance, if any.
     * Falls back to the current JSF FacesContext, if any.
     * @return the RequestAttributes currently bound to the thread
     * @throws IllegalStateException if no RequestAttributes object
     * is bound to the current thread
     * @see #setRequestAttributes
     * @see ServletRequestAttributes
     * @see FacesRequestAttributes
     * @see javax.faces.context.FacesContext#getCurrentInstance()
     */
    public static RequestAttributes currentRequestAttributes() throws IllegalStateException {
        RequestAttributes attributes = getRequestAttributes();
        if (attributes == null) {
            if (jsfPresent) {
                attributes = FacesRequestAttributesFactory.getFacesRequestAttributes();
            }
            if (attributes == null) {
                throw new IllegalStateException("No thread-bound request found: " +
                        "Are you referring to request attributes outside of an actual web request, " +
                        "or processing a request outside of the originally receiving thread? " +
                        "If you are actually operating within a web request and still receive this message, " +
                        "your code is probably running outside of DispatcherServlet/DispatcherPortlet: " +
                        "In this case, use RequestContextListener or RequestContextFilter to expose the current request.");
            }
        }
        return attributes;
    }

    /**
     * Inner class to avoid hard-coded JSF dependency.
      */
    private static class FacesRequestAttributesFactory {

        public static RequestAttributes getFacesRequestAttributes() {
            FacesContext facesContext = FacesContext.getCurrentInstance();
            return (facesContext != null ? new FacesRequestAttributes(facesContext) : null);
        }
    }

}

里面定义了final修饰的ThreadLocal,对此可以再了解下《Java源码分析》：ThreadLocal /ThreadLocalMap

今天520,晚上有约,还是先干活,改天再来完善.