tomcat源码研究之 web.xml中load-on-startup标签的含义
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2022-06-06 22:02:26
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(1)
在servlet的配置当中，<load-on-startup>5</load-on-startup>的含义是：
标记容器是否在启动的时候就加载这个servlet。
当值为0或者大于0时，表示容器在应用启动时就加载这个servlet；
当是一个负数时或者没有指定时，则指示容器在该servlet被选择时才加载。
正数的值越小，启动该servlet的优先级越高。
web.xml的配置如下:
<servlet>
<servlet-name>initservlet</servlet-name>
<servlet-class>com.ccc.lxy.util.initServlet</servlet-class>
<load-on-startup>1</load-on-startup>
</servlet>
（2） 在我的tomcat源码研究中我已经介绍到，tomcat本身就是组件构成的容器，我们的应用程序程序将对应到Context组件
而Context组件实现是StandardContext这个类，而Servlet对应的组件是Wrapper，它的实现是StandardWrapper，
既然当load-on-startup值大于等于0时候 在启动时候 那么我看StandardContext 这个类启动时的情况
protected synchronized void startInternal() throws LifecycleException {
if(log.isDebugEnabled())
log.debug("Starting " + getBaseName());
// Send j2ee.state.starting notification
if (this.getObjectName() != null) {
Notification notification = new Notification("j2ee.state.starting",
this.getObjectName(), sequenceNumber.getAndIncrement());
broadcaster.sendNotification(notification);
}
setConfigured(false);
boolean ok = true;
// Currently this is effectively a NO-OP but needs to be called to
// ensure the NamingResources follows the correct lifecycle
if (namingResources != null) {
namingResources.start();
}
// Add missing components as necessary
if (webappResources == null) { // (1) Required by Loader
if (log.isDebugEnabled())
log.debug("Configuring default Resources");
try {
if ((getDocBase() != null) && (getDocBase().endsWith(".war")) &&
(!(new File(getBasePath())).isDirectory()))
setResources(new WARDirContext());
else
setResources(new FileDirContext());
} catch (IllegalArgumentException e) {
log.error("Error initializing resources: " + e.getMessage());
ok = false;
}
}
// 代码省略...
if (ok) {
if (!listenerStart()) {
log.error( "Error listenerStart");
ok = false;
}
}
try {
// Start manager
if ((manager != null) && (manager instanceof Lifecycle)) {
((Lifecycle) getManager()).start();
}
} catch(Exception e) {
log.error("Error manager.start()", e);
ok = false;
}
// Configure and call application filters
if (ok) {
if (!filterStart()) {
log.error("Error filterStart");
ok = false;
}
}
// Load and initialize all "load on startup" servlets
//加载servlet的loadOnStardup属性
if (ok) {
if (!loadOnStartup(findChildren())){
log.error("Error loadOnStartup");
ok = false;
}
}
// Start ContainerBackgroundProcessor thread
super.threadStart();
//代码省略
从上述代码中，我可知Context组件在启动时调用方法 loadOnStartup这个方法
public boolean loadOnStartup(Container children[]) {
// Collect "load on startup" servlets that need to be initialized
TreeMap<Integer, ArrayList<Wrapper>> map =
new TreeMap<Integer, ArrayList<Wrapper>>();
for (int i = 0; i < children.length; i++) {
Wrapper wrapper = (Wrapper) children[i];
int loadOnStartup = wrapper.getLoadOnStartup();
//从这儿可知loadonStartup 小于0时 跳过，
//也就是只有大于等于0时才进行的 初始化化加载了
if (loadOnStartup < 0)
continue;
Integer key = Integer.valueOf(loadOnStartup);
ArrayList<Wrapper> list = map.get(key);
if (list == null) {
list = new ArrayList<Wrapper>();
map.put(key, list);
}
list.add(wrapper);
}
//下面是具体初始化加载的loadonStartup>=0的Servlet
// Load the collected "load on startup" servlets
for (ArrayList<Wrapper> list : map.values()) {
for (Wrapper wrapper : list) {
try {
wrapper.load();
} catch (ServletException e) {
getLogger().error(sm.getString("standardWrapper.loadException",
getName()), StandardWrapper.getRootCause(e));
// NOTE: load errors (including a servlet that throws
// UnavailableException from tht init() method) are NOT
// fatal to application startup, excepted if failDeploymentIfServletLoadedOnStartupFails is specified
if(getComputedFailCtxIfServletStartFails()) {
return false;
}
}
}
}
return true;
}
从上面可知只有loadonStartup>=0时才调用wrapper的load
而且从上面可知将这些大于等于0的Servlet都放入一个treemap， treemap恰恰是一个排序map且升序map 所以loadonstartup 越小越先遍历到 越先执行load
而Wrapper的load就是StandardWrapper的load方法具体如下
public synchronized void load() throws ServletException {
//实例化一个Servlet
instance = loadServlet();
//实例化完成以后开始调用InitServlet方法
if (!instanceInitialized) {
initServlet(instance);
}
if (isJspServlet) {
StringBuilder oname =
new StringBuilder(MBeanUtils.getDomain(getParent()));
oname.append(":type=JspMonitor,name=");
oname.append(getName());
oname.append(getWebModuleKeyProperties());
try {
jspMonitorON = new ObjectName(oname.toString());
Registry.getRegistry(null, null)
.registerComponent(instance, jspMonitorON, null);
} catch( Exception ex ) {
log.info("Error registering JSP monitoring with jmx " +
instance);
}
}
}
具体调用InitServlet方法如下：
private synchronized void initServlet(Servlet servlet)
throws ServletException {
if (instanceInitialized && !singleThreadModel) return;
// Call the initialization method of this servlet
try {
instanceSupport.fireInstanceEvent(InstanceEvent.BEFORE_INIT_EVENT,
servlet);
//调用Servlet的init方法
if( Globals.IS_SECURITY_ENABLED) {
boolean success = false;
try {
//反射调用init方法
Object[] args = new Object[] { facade };
SecurityUtil.doAsPrivilege("init",
servlet,
classType,
args);
success = true;
} finally {
if (!success) {
// destroy() will not be called, thus clear the reference now
SecurityUtil.remove(servlet);
}
}
} else {
//{初始化servlet的init（ServletConfig config）方法然后 在这方法中init()方法了了}
servlet.init(facade);
}
instanceInitialized = true;
instanceSupport.fireInstanceEvent(InstanceEvent.AFTER_INIT_EVENT,
servlet);
} catch (UnavailableException f) {
instanceSupport.fireInstanceEvent(InstanceEvent.AFTER_INIT_EVENT,
servlet, f);
unavailable(f);
throw f;
} catch (ServletException f) {
instanceSupport.fireInstanceEvent(InstanceEvent.AFTER_INIT_EVENT,
servlet, f);
// If the servlet wanted to be unavailable it would have
// said so, so do not call unavailable(null).
throw f;
} catch (Throwable f) {
ExceptionUtils.handleThrowable(f);
getServletContext().log("StandardWrapper.Throwable", f );
instanceSupport.fireInstanceEvent(InstanceEvent.AFTER_INIT_EVENT,
servlet, f);
// If the servlet wanted to be unavailable it would have
// said so, so do not call unavailable(null).
throw new ServletException
(sm.getString("standardWrapper.initException", getName()), f);
}
}
从这里可知我们Servlet调用情况