org.springframework.orm.hibernate4.support.OpenSessionInViewFilter

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/*
 * Copyright 2002-2014 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.orm.hibernate4.support;

import java.io.IOException;
import javax.servlet.FilterChain;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.hibernate.FlushMode;
import org.hibernate.HibernateException;
import org.hibernate.Session;
import org.hibernate.SessionFactory;

import org.springframework.dao.DataAccessResourceFailureException;
import org.springframework.orm.hibernate4.SessionFactoryUtils;
import org.springframework.orm.hibernate4.SessionHolder;
import org.springframework.transaction.support.TransactionSynchronizationManager;
import org.springframework.web.context.WebApplicationContext;
import org.springframework.web.context.request.async.WebAsyncManager;
import org.springframework.web.context.request.async.WebAsyncUtils;
import org.springframework.web.context.support.WebApplicationContextUtils;
import org.springframework.web.filter.OncePerRequestFilter;

/**
 * Servlet 2.3 Filter that binds a Hibernate Session to the thread for the entire
 * processing of the request. Intended for the "Open Session in View" pattern,
 * i.e. to allow for lazy loading in web views despite the original transactions
 * already being completed.
 *使用过滤器将请求的全部 hibernate session 绑定到线程。 用于“打开回话视图” 模式，
　　　　允许在web 视图中延迟加载，尽管基本的事务已经完成，（虽然事务已经提交完成，但任然允许延迟加载数据？）
 * <p>This filter makes Hibernate Sessions available via the current thread, which
 * will be autodetected by transaction managers. It is suitable for service layer
 * transactions via {@link org.springframework.orm.hibernate4.HibernateTransactionManager}
 * as well as for non-transactional execution (if configured appropriately).
 *这个过滤器使hibernaete session 可以通过当前线程，其将自动检测事务管理，适合于服务处通过HibernateTransactionManager 管理事务，以及非事务执行（如果适当配置）
　　　　
 * <p><b>NOTE</b>: This filter will by default <i>not</i> flush the Hibernate Session,
 * with the flush mode set to {@code FlushMode.NEVER}. It assumes to be used
 * in combination with service layer transactions that care for the flushing: The
 * active transaction manager will temporarily change the flush mode to
 * {@code FlushMode.AUTO} during a read-write transaction, with the flush
 * mode reset to {@code FlushMode.NEVER} at the end of each transaction.
 *
 * <p><b>WARNING:</b> Applying this filter to existing logic can cause issues that
 * have not appeared before, through the use of a single Hibernate Session for the
 * processing of an entire request. In particular, the reassociation of persistent
 * objects with a Hibernate Session has to occur at the very beginning of request
 * processing, to avoid clashes with already loaded instances of the same objects.
 *
 * <p>Looks up the SessionFactory in Spring's root web application context.
 * Supports a "sessionFactoryBeanName" filter init-param in {@code web.xml};
 * the default bean name is "sessionFactory".
 *
 * @author Juergen Hoeller
 * @since 3.1
 * @see #lookupSessionFactory
 * @see OpenSessionInViewInterceptor
 * @see OpenSessionInterceptor
 * @see org.springframework.orm.hibernate4.HibernateTransactionManager
 * @see org.springframework.transaction.support.TransactionSynchronizationManager
 * @see org.hibernate.SessionFactory#getCurrentSession()
 */
public class OpenSessionInViewFilter extends OncePerRequestFilter {

    public static final String DEFAULT_SESSION_FACTORY_BEAN_NAME = "sessionFactory";

    private String sessionFactoryBeanName = DEFAULT_SESSION_FACTORY_BEAN_NAME;


    /**
     * Set the bean name of the SessionFactory to fetch from Spring's
     * root application context. Default is "sessionFactory".
     * @see #DEFAULT_SESSION_FACTORY_BEAN_NAME
     */
    public void setSessionFactoryBeanName(String sessionFactoryBeanName) {
        this.sessionFactoryBeanName = sessionFactoryBeanName;
    }

    /**
     * Return the bean name of the SessionFactory to fetch from Spring's
     * root application context.
     */
    protected String getSessionFactoryBeanName() {
        return this.sessionFactoryBeanName;
    }

    /**
     * Returns "false" so that the filter may re-bind the opened Hibernate
     * {@code Session} to each asynchronously dispatched thread and postpone
     * closing it until the very last asynchronous dispatch.
　　　　　　返回false 是因为这个filter 可以开启session的异步只读操作模式，延迟关闭它直到异步加载完成。
     */
    @Override
    protected boolean shouldNotFilterAsyncDispatch() {
        return false;
    }

    /**
     * Returns "false" so that the filter may provide a Hibernate
     * {@code Session} to each error dispatches.
　　　　　　　返回false以便这个filter可以提供一个session的错误输出。
     */
    @Override
    protected boolean shouldNotFilterErrorDispatch() {
        return false;
    }

    @Override
    protected void doFilterInternal(
            HttpServletRequest request, HttpServletResponse response, FilterChain filterChain)
            throws ServletException, IOException {
　　　　　//通过指定的名称获取sessionFactory　
        SessionFactory sessionFactory = lookupSessionFactory(request);
        boolean participate = false;
　　　　//通过当前的request请求获得web 异步管理器，如果没有异步管理器则创建一个并与request关联。
        WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
　　　　　　　　
　　　　　　　　//返回一个请求的属性名称已用于识别其已经过滤。
        String key = getAlreadyFilteredAttributeName();

　　　　　　//检测这个sessionFactory是否绑定到当前线程。
        if (TransactionSynchronizationManager.hasResource(sessionFactory)) {
            // Do not modify the Session: just set the participate flag.
            participate = true;
        }
        else {
　　　　　　　　　　//这个过滤器是否是异步执行的，因为一个filter是可以调用多个线程的单个请求的。
            boolean isFirstRequest = !isAsyncDispatch(request);
            if (isFirstRequest || !applySessionBindingInterceptor(asyncManager, key)) {
                logger.debug("Opening Hibernate Session in OpenSessionInViewFilter");
                Session session = openSession(sessionFactory);
　　　　　　　　　　　　//包装一个session以使下一句将其绑定到thread.
                SessionHolder sessionHolder = new SessionHolder(session);
                TransactionSynchronizationManager.bindResource(sessionFactory, sessionHolder);
　　　　　　　　　//实例化一个opensessionInViewFilter类专用的异步请求拦截器，从其类解释来看很有必要。
                AsyncRequestInterceptor interceptor = new AsyncRequestInterceptor(sessionFactory, sessionHolder);
　　　　　　　　　　　　//将异步请求拦截器与reques关联
                asyncManager.registerCallableInterceptor(key, interceptor);
　　　　　　　　　　　　//延迟的请求
                asyncManager.registerDeferredResultInterceptor(key, interceptor);
            }
        }

        try {
            filterChain.doFilter(request, response);
        }

        finally {
            if (!participate) {
                SessionHolder sessionHolder =
                        (SessionHolder) TransactionSynchronizationManager.unbindResource(sessionFactory);
                if (!isAsyncStarted(request)) {
                    logger.debug("Closing Hibernate Session in OpenSessionInViewFilter");
                    SessionFactoryUtils.closeSession(sessionHolder.getSession());
                }
            }
        }
    }

    /**
     * Look up the SessionFactory that this filter should use,
     * taking the current HTTP request as argument.
     * <p>The default implementation delegates to the {@link #lookupSessionFactory()}
     * variant without arguments.
     * @param request the current request
     * @return the SessionFactory to use
     */
    protected SessionFactory lookupSessionFactory(HttpServletRequest request) {
        return lookupSessionFactory();
    }

    /**
     * Look up the SessionFactory that this filter should use.
     * <p>The default implementation looks for a bean with the specified name
     * in Spring's root application context.
     * @return the SessionFactory to use
     * @see #getSessionFactoryBeanName
     */
    protected SessionFactory lookupSessionFactory() {
        if (logger.isDebugEnabled()) {
            logger.debug("Using SessionFactory '" + getSessionFactoryBeanName() + "' for OpenSessionInViewFilter");
        }
        WebApplicationContext wac = WebApplicationContextUtils.getRequiredWebApplicationContext(getServletContext());
        return wac.getBean(getSessionFactoryBeanName(), SessionFactory.class);
    }

    /**
     * Open a Session for the SessionFactory that this filter uses.
     * <p>The default implementation delegates to the {@link SessionFactory#openSession}
     * method and sets the {@link Session}'s flush mode to "MANUAL".
　　　　　　　打开session因为filter要使用，同时将session的事务提交模式设置为手动提交（MANUAL）
     * @param sessionFactory the SessionFactory that this filter uses
     * @return the Session to use
     * @throws DataAccessResourceFailureException if the Session could not be created
     * @see org.hibernate.FlushMode#MANUAL
     */
    protected Session openSession(SessionFactory sessionFactory) throws DataAccessResourceFailureException {
        try {
            Session session = sessionFactory.openSession();
            session.setFlushMode(FlushMode.MANUAL);
            return session;
        }
        catch (HibernateException ex) {
            throw new DataAccessResourceFailureException("Could not open Hibernate Session", ex);
        }
    }
　　
　　　　　　
　　　　　/*检测前面获取的请求属性名是否已注册至异步管理器中，如果已经注册了，则通过绑定到当前线程的sessionFactory创建session*/　
    private boolean applySessionBindingInterceptor(WebAsyncManager asyncManager, String key) {
        if (asyncManager.getCallableInterceptor(key) == null) {
            return false;
        }
　　　　　　　　
        ((AsyncRequestInterceptor) asyncManager.getCallableInterceptor(key)).bindSession();
        return true;
    }

}

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