Tomcat Servlet loaded spy

As the name suggests, the Wrapper packaging of the Servlet, which holds a pool of instances corresponding to the Servlet.

/**
   * Stack containing the STM instances.
  */
protected Stack<Servlet> instancePool = null;

In the Wrapper class, there are loads of the Servlet (load method), unloading (unload method), to recover the distribution (deallocate method) and allocation (allocate method).

load method

/**
     * 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 method is to use InstanceManager in the Context of the superior container, InstanceManger held in ClassLoader use of Servlet loaded.

@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);
    }

After loading, Servlet's init method is called.

 

unload method

Examples will be set to null, initialization mark set to false, the method invokes the Servlet instance pooling destory emptied simultaneously.

// 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 method

 Wrapper according to whether the corresponding Servlet is denoted singleThreadModel, the processing is divided into two cases.

• Not singleThreadModel: In the instance is null, the Servlet is loaded and initialized, otherwise directly back into the instance;
• It is singleThreadModel: In instances when the pool is less than the maximum number of instances (default 20), will be an instance from the pop instances in the pool, otherwise they will be an example to the pool load Servlet. During the load Servlet, if the number of instances in the pool have been found instance is greater than the maximum number of instances, it will wait (wait occurs unload or deallocate).

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 method

/**
     * 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();
        }

    }