文章目录

• RequestContextHolder源码：
• 项目中使用RequestContextHolder
• • 在项目中获得完整请求路径
  • 关于ServletRequestAttributes源码 ：
  • 关于 HttpServletRequest和 HttpServletResponse

1.通过RequestContextHolder的静态方法可以随时随地取到当前请求的request对象
该类可以用于获得客户端请求的上下文

RequestContextHolder源码：

Holder class to expose the web request in the form of a thread-bound RequestAttributes object. The request will be inherited by any child threads spawned by the current thread if the inheritable flag is set to true.
Use RequestContextListener or org.springframework.web.filter.RequestContextFilter to expose the current web request. Note that org.springframework.web.servlet.DispatcherServlet already exposes the current request by default.
package org.springframework.web.context.request;
翻译：
Holder类以线程绑定的RequestAttributes对象的形式公开web请求。如果可继承标志设置为true，该请求将被当前线程的所有子线程继承。

使用RequestContextListener或org.springframework.web.filter.RequestContextFilter来暴露当前的web请求。注意，org.springframework.web.servlet.DispatcherServlet已经默认暴露了当前请求。

源码：

//


import javax.faces.context.FacesContext;
import org.springframework.core.NamedInheritableThreadLocal;
import org.springframework.core.NamedThreadLocal;
import org.springframework.lang.Nullable;
import org.springframework.util.ClassUtils;

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

    public RequestContextHolder() {
    }

    public static void resetRequestAttributes() {
        requestAttributesHolder.remove();
        inheritableRequestAttributesHolder.remove();
    }

    public static void setRequestAttributes(@Nullable RequestAttributes attributes) {
        setRequestAttributes(attributes, false);
    }

    public static void setRequestAttributes(@Nullable RequestAttributes attributes, boolean inheritable) {
        if (attributes == null) {
            resetRequestAttributes();
        } else if (inheritable) {
            inheritableRequestAttributesHolder.set(attributes);
            requestAttributesHolder.remove();
        } else {
            requestAttributesHolder.set(attributes);
            inheritableRequestAttributesHolder.remove();
        }

    }

    @Nullable
    public static RequestAttributes getRequestAttributes() {
        RequestAttributes attributes = (RequestAttributes)requestAttributesHolder.get();
        if (attributes == null) {
            attributes = (RequestAttributes)inheritableRequestAttributesHolder.get();
        }

        return attributes;
    }

    public static RequestAttributes currentRequestAttributes() throws IllegalStateException {
        RequestAttributes attributes = getRequestAttributes();
        if (attributes == null) {
            if (jsfPresent) {
                attributes = RequestContextHolder.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: In this case, use RequestContextListener or RequestContextFilter to expose the current request.");
            }
        }

        return attributes;
    }

    private static class FacesRequestAttributesFactory {
        private FacesRequestAttributesFactory() {
        }

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

个人理解用ThreadLocal保证了数据间的隔离性

项目中使用RequestContextHolder

用一个servlet工具类：

public class ServletUtils
{
    /**
     * 定义移动端请求的所有可能类型
     */
    private final static String[] agent = { "Android", "iPhone", "iPod", "iPad", "Windows Phone", "MQQBrowser" };

    /**
     * 获取String参数
     */
    public static String getParameter(String name)
    {
        return getRequest().getParameter(name);
    }

    public static ServletRequestAttributes getRequestAttributes()
    {

        RequestAttributes attributes = RequestContextHolder.getRequestAttributes();//这里用RequestContextHolder获得ServletRequestAttributes 
        return (ServletRequestAttributes) attributes;
    }
        /**
     * 获取request
     */
    public static HttpServletRequest getRequest()
    {
        return getRequestAttributes().getRequest();
    }

    /**
     * 获取response
     */
    public static HttpServletResponse getResponse()
    {
        return getRequestAttributes().getResponse();
    }

    /**
     * 获取session
     */
    public static HttpSession getSession()
    {
        return getRequest().getSession();
    }
    
 /**
     * 将字符串渲染到客户端
     * 
     * @param response 渲染对象
     * @param string 待渲染的字符串
     * @return null
     */
    public static String renderString(HttpServletResponse response, String string)
    {
        try
        {
            response.setContentType("application/json");
            response.setCharacterEncoding("utf-8");
            response.getWriter().print(string);
        }
        catch (IOException e)
        {
            e.printStackTrace();
        }
        return null;
        }

通过这个工具类获得cookies:
Cookie[] cookies = ServletUtils.getRequest().getCookies();

在项目中获得完整请求路径

 /**
     * 获取完整的请求路径，包括：域名，端口，上下文访问路径
     * 
     * @return 服务地址
     */
    public String getUrl()
    {
        HttpServletRequest request = ServletUtils.getRequest();
        return getDomain(request);
    }

    public static String getDomain(HttpServletRequest request)
    {
        StringBuffer url = request.getRequestURL();
        String contextPath = request.getServletContext().getContextPath();//getServletContext()方法：获取这个ServletRequest最后被分派到的servlet上下文。返回:这个ServletRequest最后被分派到的servlet上下文
        return url.delete(url.length() - request.getRequestURI().length(), url.length()).append(contextPath).toString();
    }

关于ServletRequestAttributes源码 ：

/**
 * Servlet-based implementation of the {@link RequestAttributes} interface.
 *
 * <p>Accesses objects from servlet request and HTTP session scope,
 * with no distinction between "session" and "global session".
 *
 * @author Juergen Hoeller
 * @since 2.0
 * @see javax.servlet.ServletRequest#getAttribute
 * @see javax.servlet.http.HttpSession#getAttribute
 */
public class ServletRequestAttributes extends AbstractRequestAttributes {

	/**
	 * Constant identifying the {@link String} prefixed to the name of a
	 * destruction callback when it is stored in a {@link HttpSession}.
	 */
	public static final String DESTRUCTION_CALLBACK_NAME_PREFIX =
			ServletRequestAttributes.class.getName() + ".DESTRUCTION_CALLBACK.";

	protected static final Set<Class<?>> immutableValueTypes = new HashSet<>(16);

	static {
		immutableValueTypes.addAll(NumberUtils.STANDARD_NUMBER_TYPES);
		immutableValueTypes.add(Boolean.class);
		immutableValueTypes.add(Character.class);
		immutableValueTypes.add(String.class);
	}


	private final HttpServletRequest request;

	@Nullable
	private HttpServletResponse response;

	@Nullable
	private volatile HttpSession session;

	private final Map<String, Object> sessionAttributesToUpdate = new ConcurrentHashMap<>(1);


	/**
	 * Create a new ServletRequestAttributes instance for the given request.
	 * @param request current HTTP request
	 */
	public ServletRequestAttributes(HttpServletRequest request) {
		Assert.notNull(request, "Request must not be null");
		this.request = request;
	}

	/**
	 * Create a new ServletRequestAttributes instance for the given request.
	 * @param request current HTTP request
	 * @param response current HTTP response (for optional exposure)
	 */
	public ServletRequestAttributes(HttpServletRequest request, @Nullable HttpServletResponse response) {
		this(request);
		this.response = response;
	}


	/**
	 * Exposes the native {@link HttpServletRequest} that we're wrapping.
	 */
	public final HttpServletRequest getRequest() {
		return this.request;
	}

	/**
	 * Exposes the native {@link HttpServletResponse} that we're wrapping (if any).
	 */
	@Nullable
	public final HttpServletResponse getResponse() {
		return this.response;
	}

	/**
	 * Exposes the {@link HttpSession} that we're wrapping.
	 * @param allowCreate whether to allow creation of a new session if none exists yet
	 */
	@Nullable
	protected final HttpSession getSession(boolean allowCreate) {
		if (isRequestActive()) {
			HttpSession session = this.request.getSession(allowCreate);
			this.session = session;
			return session;
		}
		else {
			// Access through stored session reference, if any...
			HttpSession session = this.session;
			if (session == null) {
				if (allowCreate) {
					throw new IllegalStateException(
							"No session found and request already completed - cannot create new session!");
				}
				else {
					session = this.request.getSession(false);
					this.session = session;
				}
			}
			return session;
		}
	}

	private HttpSession obtainSession() {
		HttpSession session = getSession(true);
		Assert.state(session != null, "No HttpSession");
		return session;
	}


	@Override
	public Object getAttribute(String name, int scope) {
		if (scope == SCOPE_REQUEST) {
			if (!isRequestActive()) {
				throw new IllegalStateException(
						"Cannot ask for request attribute - request is not active anymore!");
			}
			return this.request.getAttribute(name);
		}
		else {
			HttpSession session = getSession(false);
			if (session != null) {
				try {
					Object value = session.getAttribute(name);
					if (value != null) {
						this.sessionAttributesToUpdate.put(name, value);
					}
					return value;
				}
				catch (IllegalStateException ex) {
					// Session invalidated - shouldn't usually happen.
				}
			}
			return null;
		}
	}

	@Override
	public void setAttribute(String name, Object value, int scope) {
		if (scope == SCOPE_REQUEST) {
			if (!isRequestActive()) {
				throw new IllegalStateException(
						"Cannot set request attribute - request is not active anymore!");
			}
			this.request.setAttribute(name, value);
		}
		else {
			HttpSession session = obtainSession();
			this.sessionAttributesToUpdate.remove(name);
			session.setAttribute(name, value);
		}
	}

	@Override
	public void removeAttribute(String name, int scope) {
		if (scope == SCOPE_REQUEST) {
			if (isRequestActive()) {
				removeRequestDestructionCallback(name);
				this.request.removeAttribute(name);
			}
		}
		else {
			HttpSession session = getSession(false);
			if (session != null) {
				this.sessionAttributesToUpdate.remove(name);
				try {
					session.removeAttribute(DESTRUCTION_CALLBACK_NAME_PREFIX + name);
					session.removeAttribute(name);
				}
				catch (IllegalStateException ex) {
					// Session invalidated - shouldn't usually happen.
				}
			}
		}
	}

	@Override
	public String[] getAttributeNames(int scope) {
		if (scope == SCOPE_REQUEST) {
			if (!isRequestActive()) {
				throw new IllegalStateException(
						"Cannot ask for request attributes - request is not active anymore!");
			}
			return StringUtils.toStringArray(this.request.getAttributeNames());
		}
		else {
			HttpSession session = getSession(false);
			if (session != null) {
				try {
					return StringUtils.toStringArray(session.getAttributeNames());
				}
				catch (IllegalStateException ex) {
					// Session invalidated - shouldn't usually happen.
				}
			}
			return new String[0];
		}
	}

	@Override
	public void registerDestructionCallback(String name, Runnable callback, int scope) {
		if (scope == SCOPE_REQUEST) {
			registerRequestDestructionCallback(name, callback);
		}
		else {
			registerSessionDestructionCallback(name, callback);
		}
	}

	@Override
	public Object resolveReference(String key) {
		if (REFERENCE_REQUEST.equals(key)) {
			return this.request;
		}
		else if (REFERENCE_SESSION.equals(key)) {
			return getSession(true);
		}
		else {
			return null;
		}
	}

	@Override
	public String getSessionId() {
		return obtainSession().getId();
	}

	@Override
	public Object getSessionMutex() {
		return WebUtils.getSessionMutex(obtainSession());
	}


	/**
	 * Update all accessed session attributes through {@code session.setAttribute}
	 * calls, explicitly indicating to the container that they might have been modified.
	 */
	@Override
	protected void updateAccessedSessionAttributes() {
		if (!this.sessionAttributesToUpdate.isEmpty()) {
			// Update all affected session attributes.
			HttpSession session = getSession(false);
			if (session != null) {
				try {
					for (Map.Entry<String, Object> entry : this.sessionAttributesToUpdate.entrySet()) {
						String name = entry.getKey();
						Object newValue = entry.getValue();
						Object oldValue = session.getAttribute(name);
						if (oldValue == newValue && !isImmutableSessionAttribute(name, newValue)) {
							session.setAttribute(name, newValue);
						}
					}
				}
				catch (IllegalStateException ex) {
					// Session invalidated - shouldn't usually happen.
				}
			}
			this.sessionAttributesToUpdate.clear();
		}
	}

	/**
	 * Determine whether the given value is to be considered as an immutable session
	 * attribute, that is, doesn't have to be re-set via {@code session.setAttribute}
	 * since its value cannot meaningfully change internally.
	 * <p>The default implementation returns {@code true} for {@code String},
	 * {@code Character}, {@code Boolean} and standard {@code Number} values.
	 * @param name the name of the attribute
	 * @param value the corresponding value to check
	 * @return {@code true} if the value is to be considered as immutable for the
	 * purposes of session attribute management; {@code false} otherwise
	 * @see #updateAccessedSessionAttributes()
	 */
	protected boolean isImmutableSessionAttribute(String name, @Nullable Object value) {
		return (value == null || immutableValueTypes.contains(value.getClass()));
	}

	/**
	 * Register the given callback as to be executed after session termination.
	 * <p>Note: The callback object should be serializable in order to survive
	 * web app restarts.
	 * @param name the name of the attribute to register the callback for
	 * @param callback the callback to be executed for destruction
	 */
	protected void registerSessionDestructionCallback(String name, Runnable callback) {
		HttpSession session = obtainSession();
		session.setAttribute(DESTRUCTION_CALLBACK_NAME_PREFIX + name,
				new DestructionCallbackBindingListener(callback));
	}


	@Override
	public String toString() {
		return this.request.toString();
	}

}

关于 HttpServletRequest和 HttpServletResponse

关于servlet的介绍：
客户端的网络请求首先会被Http服务器接收（也叫Web服务器、web容器，其需要提供web应用运行所需的环境，接收客户端的Http请求）；

Web服务器根据请求的路径将请求转交给对应的Servlet容器（也称Servlet引擎，为Servlet的运行提供环境支持，可以理解为tomcat或其他服务器）；

Servlet容器根据对应的虚拟路径（@WebServlet中配置的）来加载Servlet，如果Serlvet没有被实例化则创建该Servlet的一个实例（调用init方法）；

Servlet容器根据用户的HTTP请求，创建一个ServletRequest对象（HTTP的请求信息被封装在其中）和一个可以对HTTP请求进行响应的ServletResponse对象（类似于寄信，并在信中说明回信的地址），然后调用HttpServlet中重写的service(ServletRequest req, ServletResponse res)方法，并在这个方法中，将ServletRequest、ServletResponse这两个对象向下转型，得到我们非常熟悉的HttpServletRequest和HttpServletResponse两个对象，然后将客户端的请求转发到HttpServlet中protected修饰的service(HttpServletRequest req, HttpServletResponse resp)