顾名思义，Wrapper是对Servlet的包装，它持有一个对应Servlet的实例池。
/** * Stack containing the STM instances. */ protected Stack<Servlet> instancePool = null;
在Wrapper类里，有对Servlet的加载（load方法）、卸载（unload方法）、收回分配（deallocate方法）和分配（allocate方法）。
load方法
/**
* Load and initialize an instance of this servlet, if there is not already
* at least one initialized instance. This can be used, for example, to
* load servlets that are marked in the deployment descriptor to be loaded
* at server startup time.
* <p>
* <b>IMPLEMENTATION NOTE</b>: Servlets whose classnames begin with
* <code>org.apache.catalina.</code> (so-called "container" servlets)
* are loaded by the same classloader that loaded this class, rather than
* the classloader for the current web application.
* This gives such classes access to Catalina internals, which are
* prevented for classes loaded for web applications.
*
* @exception ServletException if the servlet init() method threw
* an exception
* @exception ServletException if some other loading problem occurs
*/
@Override
public synchronized void load() throws ServletException {
instance = loadServlet();
if (!instanceInitialized) {
initServlet(instance);
}
if (isJspServlet) {
StringBuilder oname = new StringBuilder(getDomain());
oname.append(":type=JspMonitor");
oname.append(getWebModuleKeyProperties());
oname.append(",name=");
oname.append(getName());
oname.append(getJ2EEKeyProperties());
try {
jspMonitorON = new ObjectName(oname.toString());
Registry.getRegistry(null, null).registerComponent(instance, jspMonitorON, null);
} catch (Exception ex) {
log.warn(sm.getString("standardWrapper.jspMonitorError", instance));
}
}
}
loadServlet方法里则是使用上级容器Context的InstanceManager，InstanceManger里会使用持有的ClassLoader对Servlet进行加载。
@Override
public Object newInstance(String className) throws IllegalAccessException,
InvocationTargetException, NamingException, InstantiationException,
ClassNotFoundException, IllegalArgumentException, NoSuchMethodException, SecurityException {
Class<?> clazz = loadClassMaybePrivileged(className, classLoader);
return newInstance(clazz.getConstructor().newInstance(), clazz);
}
@Override
public Object newInstance(final String className, final ClassLoader classLoader)
throws IllegalAccessException, NamingException, InvocationTargetException,
InstantiationException, ClassNotFoundException, IllegalArgumentException,
NoSuchMethodException, SecurityException {
Class<?> clazz = classLoader.loadClass(className);
return newInstance(clazz.getConstructor().newInstance(), clazz);
}
在加载后，则会调用Servlet的init方法。
unload方法
会将实例置为null，将初始化标示置为false，将实例池清空的同时调用Servlet的destory方法。
// Deregister the destroyed instance
instance = null;
instanceInitialized = false;
if (isJspServlet && jspMonitorON != null ) {
Registry.getRegistry(null, null).unregisterComponent(jspMonitorON);
}
if (singleThreadModel && (instancePool != null)) {
try {
while (!instancePool.isEmpty()) {
Servlet s = instancePool.pop();
if (Globals.IS_SECURITY_ENABLED) {
try {
SecurityUtil.doAsPrivilege("destroy", s);
} finally {
SecurityUtil.remove(s);
}
} else {
s.destroy();
}
// Annotation processing
if (!((Context) getParent()).getIgnoreAnnotations()) {
((StandardContext)getParent()).getInstanceManager().destroyInstance(s);
}
}
} catch (Throwable t) {
t = ExceptionUtils.unwrapInvocationTargetException(t);
ExceptionUtils.handleThrowable(t);
instancePool = null;
nInstances = 0;
unloading = false;
fireContainerEvent("unload", this);
throw new ServletException
(sm.getString("standardWrapper.destroyException",
getName()), t);
}
instancePool = null;
nInstances = 0;
}
singleThreadModel = false;
unloading = false;
allocate方法
根据Wrapper对应的Servlet是否被标示为singleThreadModel，分为两种处理情况。
- 不是singleThreadModel：在instance为null时，加载Servlet并初始化，否则直接放回instance；
- 是singleThreadModel：在实例池小于最大实例数（默认20）时，会从实例池中pop出一个实例，否则会向实例池中load Servlet。在load Servlet的过程中，如果发现实例池中实例的数目已经大于最大实例数，则会等待（等待unload或deallocate发生）。
boolean newInstance = false;
// If not SingleThreadedModel, return the same instance every time
if (!singleThreadModel) {
// Load and initialize our instance if necessary
if (instance == null || !instanceInitialized) {
synchronized (this) {
if (instance == null) {
try {
if (log.isDebugEnabled()) {
log.debug("Allocating non-STM instance");
}
// Note: We don't know if the Servlet implements
// SingleThreadModel until we have loaded it.
instance = loadServlet();
newInstance = true;
if (!singleThreadModel) {
// For non-STM, increment here to prevent a race
// condition with unload. Bug 43683, test case
// #3
countAllocated.incrementAndGet();
}
} catch (ServletException e) {
throw e;
} catch (Throwable e) {
ExceptionUtils.handleThrowable(e);
throw new ServletException(sm.getString("standardWrapper.allocate"), e);
}
}
if (!instanceInitialized) {
initServlet(instance);
}
}
}
if (singleThreadModel) {
if (newInstance) {
// Have to do this outside of the sync above to prevent a
// possible deadlock
synchronized (instancePool) {
instancePool.push(instance);
nInstances++;
}
}
} else {
if (log.isTraceEnabled()) {
log.trace(" Returning non-STM instance");
}
// For new instances, count will have been incremented at the
// time of creation
if (!newInstance) {
countAllocated.incrementAndGet();
}
return instance;
}
}
synchronized (instancePool) {
while (countAllocated.get() >= nInstances) {
// Allocate a new instance if possible, or else wait
if (nInstances < maxInstances) {
try {
instancePool.push(loadServlet());
nInstances++;
} catch (ServletException e) {
throw e;
} catch (Throwable e) {
ExceptionUtils.handleThrowable(e);
throw new ServletException(sm.getString("standardWrapper.allocate"), e);
}
} else {
try {
instancePool.wait();
} catch (InterruptedException e) {
// Ignore
}
}
}
if (log.isTraceEnabled()) {
log.trace(" Returning allocated STM instance");
}
countAllocated.incrementAndGet();
return instancePool.pop();
}
deallocate方法
/**
* Return this previously allocated servlet to the pool of available
* instances. If this servlet class does not implement SingleThreadModel,
* no action is actually required.
*
* @param servlet The servlet to be returned
*
* @exception ServletException if a deallocation error occurs
*/
@Override
public void deallocate(Servlet servlet) throws ServletException {
// If not SingleThreadModel, no action is required
if (!singleThreadModel) {
countAllocated.decrementAndGet();
return;
}
// Unlock and free this instance
synchronized (instancePool) {
countAllocated.decrementAndGet();
instancePool.push(servlet);
instancePool.notify();
}
}
来源：https://www.cnblogs.com/Logan12138/p/12004073.html