Listener

Listener

节选部分源码、源码版本 Tomcat8.5

接口

public interface EventListener {}

常见Listener

• ServletContextListener：容器创建、销毁

public interface ServletContextListener extends EventListener {
    // 初始化
    void contextInitialized(ServletContextEvent sce);
    // 销毁
    void contextDestroyed(ServletContextEvent sce);
}

• HttpSessionListener：session的创建、销毁

public interface HttpSessionListener extends EventListener {
    // session 创建
    void sessionCreated(HttpSessionEvent se);
	// session 销毁
    void sessionDestroyed(HttpSessionEvent se);
}

• ServletRequestListener：request初始化、销毁

public interface ServletRequestListener extends EventListener {
    // request 销毁
    public void requestDestroyed (ServletRequestEvent sre);
    // request 初始化
    public void requestInitialized (ServletRequestEvent sre);
}

Listener初始化

// org.apache.catalina.core.StandardContext
@Override
protected synchronized void startInternal() throws LifecycleException {
    // 准备好环境
    if (ok) {
        if (!listenerStart()) { ok = false; }
    }
}

public boolean listenerStart() {
    // 获取所有的 Listener
    String listeners[] = findApplicationListeners();
    Object results[] = new Object[listeners.length];
    boolean ok = true;
    for (int i = 0; i < results.length; i++) {
        try {
            String listener = listeners[i];
            // 通过反射实例化 listener
            results[i] = getInstanceManager().newInstance(listener);
        }
    }

    // 对 listener 进行分类
    ArrayList<Object> eventListeners = new ArrayList<>();
    ArrayList<Object> lifecycleListeners = new ArrayList<>();
    for (int i = 0; i < results.length; i++) {
        if ((results[i] instanceof ServletContextAttributeListener)
            || (results[i] instanceof ServletRequestAttributeListener)
            || (results[i] instanceof ServletRequestListener)
            || (results[i] instanceof HttpSessionIdListener)
            || (results[i] instanceof HttpSessionAttributeListener)) {
            eventListeners.add(results[i]);
        }
        if ((results[i] instanceof ServletContextListener)
            || (results[i] instanceof HttpSessionListener)) {
            lifecycleListeners.add(results[i]);
        }
    }

    // 监听器实例可能已通过 ServletContextInitializers 和其他代码通过可插拔API
    // 直接添加到此Context。
    // 【不太理解】
    for (Object eventListener: getApplicationEventListeners()) {
        eventListeners.add(eventListener);
    }
    // 重新设置 listener
    setApplicationEventListeners(eventListeners.toArray());
    for (Object lifecycleListener: getApplicationLifecycleListeners()) {
        lifecycleListeners.add(lifecycleListener);
        if (lifecycleListener instanceof ServletContextListener) {
            noPluggabilityListeners.add(lifecycleListener);
        }
    }
    setApplicationLifecycleListeners(lifecycleListeners.toArray());

    // 确保 context 不为 null
    getServletContext();
    context.setNewServletContextListenerAllowed(false);
	// 获取 应用程序生命周期监听器
    Object instances[] = getApplicationLifecycleListeners();
    if (instances == null || instances.length == 0) {
        return ok;
    }

    ServletContextEvent event = new ServletContextEvent(getServletContext());
    ServletContextEvent tldEvent = null;
    if (noPluggabilityListeners.size() > 0) {
        noPluggabilityServletContext = new NoPluggabilityServletContext(getServletContext());
        tldEvent = new ServletContextEvent(noPluggabilityServletContext);
    }
    for (int i = 0; i < instances.length; i++) {
        if (!(instances[i] instanceof ServletContextListener))
            continue;
        ServletContextListener listener =
            (ServletContextListener) instances[i];
        try {
            // 触发事件 beforeContextInitialized
            fireContainerEvent("beforeContextInitialized", listener);
            // 调用 Listener.contextInitialized 方法
            if (noPluggabilityListeners.contains(listener)) {
                listener.contextInitialized(tldEvent);
            } else {
                listener.contextInitialized(event);
            }
            fireContainerEvent("afterContextInitialized", listener);
        } catch (Throwable t) {
            fireContainerEvent("afterContextInitialized", listener);
            ok = false;
        }
    }
    return (ok);
}

• Listener是如何添加到 列表的？

// org.apache.catalina.core.StandardContext
private String applicationListeners[] = new String[0];
private final Object applicationListenersLock = new Object();

@Override
public void addApplicationListener(String listener) {
    // 加锁
    synchronized (applicationListenersLock) {
        // 扩大一个位子
        String results[] = new String[applicationListeners.length + 1];
        for (int i = 0; i < applicationListeners.length; i++) {
            // 判断是否已经存在
            if (listener.equals(applicationListeners[i])) {
                return;
            }
            results[i] = applicationListeners[i];
        }
        // 转移到 results
        results[applicationListeners.length] = listener;
        // 重新赋值 给 applicationListeners
        applicationListeners = results;
    }
    // 触发 事件
    fireContainerEvent("addApplicationListener", listener);
}

销毁操作

public boolean listenerStop() {
    boolean ok = true;
    Object listeners[] = getApplicationLifecycleListeners();
    if (listeners != null && listeners.length > 0) {
        ServletContextEvent event = new ServletContextEvent(getServletContext());
        ServletContextEvent tldEvent = null;
        if (noPluggabilityServletContext != null) {
            tldEvent = new ServletContextEvent(noPluggabilityServletContext);
        }
        for (int i = 0; i < listeners.length; i++) {
            int j = (listeners.length - 1) - i;
            if (listeners[j] == null)
                continue;
            if (listeners[j] instanceof ServletContextListener) {
                ServletContextListener listener =
                    (ServletContextListener) listeners[j];
                try {
                    // 触发 contextDestroyed 方法
                    fireContainerEvent("beforeContextDestroyed", listener);
                    if (noPluggabilityListeners.contains(listener)) {
                        listener.contextDestroyed(tldEvent);
                    } else {
                        listener.contextDestroyed(event);
                    }
                    fireContainerEvent("afterContextDestroyed", listener);
                } catch (Throwable t) {
                    ok = false;
                }
            }
            try {
                if (getInstanceManager() != null) {
                    getInstanceManager().destroyInstance(listeners[j]);
                }
            } catch (Throwable t) {
                ok = false;
            }
        }
    }
    // 销毁 listener
    listeners = getApplicationEventListeners();
    if (listeners != null) {
        for (int i = 0; i < listeners.length; i++) {
            int j = (listeners.length - 1) - i;
            if (listeners[j] == null)
                continue;
            try {
                if (getInstanceManager() != null) {
                    getInstanceManager().destroyInstance(listeners[j]);
                }
            } catch (Throwable t) {
                ok = false;
            }
        }
    }
    // 最后的清理操作
    setApplicationEventListeners(null);
    setApplicationLifecycleListeners(null);
    noPluggabilityServletContext = null;
    noPluggabilityListeners.clear();
    return ok;
}

说明

• Listenrer 在 Filter 之前执行
• Filter 在 Interceptor 之前执行

整合Spring

<listener>
    <listener-class>
        org.springframework.web.context.ContextLoaderListener
    </listener-class>
</listener>

public class ContextLoaderListener extends ContextLoader implements ServletContextListener {

    @Override
    public void contextInitialized(ServletContextEvent event) {
        // 初始化 webApplication上下文环境
        initWebApplicationContext(event.getServletContext());
    }

    @Override
    public void contextDestroyed(ServletContextEvent event) {
        closeWebApplicationContext(event.getServletContext());
        ContextCleanupListener.cleanupAttributes(event.getServletContext());
    }
}

总结

• ServletContextListener ：初始化是在 Listener 初始化时执行的。
• ServletRequestListener：见 Pipeline 章节
• HttpSessionListener：Session章节