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/* 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.camunda.bpm.application.impl;

import java.lang.ref.WeakReference;
import java.util.HashMap;
import java.util.Map;

import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;

import org.camunda.bpm.application.AbstractProcessApplication;
import org.camunda.bpm.application.ProcessApplicationInfo;
import org.camunda.bpm.application.ProcessApplicationReference;
import org.camunda.bpm.engine.delegate.JavaDelegate;
import org.camunda.bpm.engine.impl.util.ClassLoaderUtil;

/**
 * A {@link AbstractProcessApplication} Implementation to be used in a Servlet container environment.
 *
 * This class implements the {@link ServletContextListener} interface and can thus participate in the
 * deployment lifecycle of your web application.
 *
 * Usage
 * In a Servlet 3.0 container it is sufficient adding a custom subclass of
 * {@link ServletProcessApplication} annotated with {@literal @}ProcessApplication to your
 * application:
 * 
 * {@literal @}ProcessApplication("Loan Approval App")
 * public class LoanApprovalApplication extends ServletProcessApplication {
 * // empty implementation
 * }
 * 
 * This, in combination with a META-INF/processes.xml file is sufficient for making sure
 * that the process application class is picked up at runtime.
 * In a Servlet 2.5 container, the process application can be added as a web listener
 * to your project's web.xml
 *  * {@literal <}?xml version="1.0" encoding="UTF-8"?{@literal >}
 * {@literal <}web-app version="2.5" xmlns="http://java.sun.com/xml/ns/javaee"
 *       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 *       xsi:schemaLocation="http://java.sun.com/xml/ns/javaee    http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"{@literal >}
 *
 * {@literal <}listener{@literal >}
 *   {@literal <}listener-class{@literal >}org.my.project.MyProcessApplication{@literal <}/listener-class{@literal >}
 * {@literal <}/listener{@literal >}
 *{@literal <}/web-app{@literal >}
 * 
 * 
 *
 * Invocation Semantics
 * When the {@link #execute(java.util.concurrent.Callable)} method is invoked, the servlet process
 * application modifies the context classloader of the current Thread to the classloader that loaded
 * the application-provided subclass of this class. This allows
 * 

 * the process engine to resolve {@link JavaDelegate} implementations using the classloader
 * of the process application
 * In apache tomcat this allows you to resolve Naming Resources (JNDI) form the naming
 * context of the process application. JNDI name resolution is based on the TCCL in Apache Tomcat.
 * 
 * 
 *
 *
 *  *                        Set TCCL of Process Application
 *                                     |
 *                                     |  +--------------------+
 *                                     |  |Process Application |
 *                       invoke        v  |                    |
 *      ProcessEngine -----------------O--|--> Java Delegate   |
 *                                        |                    |
 *                                        |                    |
 *                                        +--------------------+
 *
 * 
 *
 * Process Application Reference
 * The process engine holds a {@link WeakReference} to the {@link ServletProcessApplication} and does not cache any
 * classes loaded using the Process Application classloader.
 *
 *
 * @author Daniel Meyer
 * @author Thorben Lindhauer
 *
 */
public class ServletProcessApplication extends AbstractProcessApplication implements ServletContextListener {

  protected String servletContextName;
  protected String servletContextPath;

  protected ProcessApplicationReferenceImpl reference;

  protected ClassLoader processApplicationClassloader;
  protected ServletContext servletContext;


  protected String autodetectProcessApplicationName() {
    String name = (servletContextName != null && !servletContextName.isEmpty()) ? servletContextName : servletContextPath;
    if(name.startsWith("/")) {
      name = name.substring(1);
    }
    return name;
  }

  public ProcessApplicationReference getReference() {
     if(reference == null) {
       reference = new ProcessApplicationReferenceImpl(this);
     }
     return reference;
  }

  public void contextInitialized(ServletContextEvent sce) {
    servletContext = sce.getServletContext();
    servletContextPath = servletContext.getContextPath();
    servletContextName = sce.getServletContext().getServletContextName();

    processApplicationClassloader = initProcessApplicationClassloader(sce);

    // perform lifecycle start
    deploy();
  }

  protected ClassLoader initProcessApplicationClassloader(ServletContextEvent sce) {

    if (isServlet30ApiPresent(sce) && getClass().equals(ServletProcessApplication.class)) {
      return ClassLoaderUtil.getServletContextClassloader(sce);

    } else {
      return ClassLoaderUtil.getClassloader(getClass());

    }

  }

  private boolean isServlet30ApiPresent(ServletContextEvent sce) {
    return sce.getServletContext().getMajorVersion() >= 3;
  }

  public ClassLoader getProcessApplicationClassloader() {
    return processApplicationClassloader;
  }

  public void contextDestroyed(ServletContextEvent sce) {

    // perform lifecycle stop
    undeploy();

    // clear the reference
    if(reference != null) {
      reference.clear();
    }
    reference = null;
  }

  public Map getProperties() {
    Map properties = new HashMap();

    // set the servlet context path as property
    properties.put(ProcessApplicationInfo.PROP_SERVLET_CONTEXT_PATH, servletContextPath);

    return properties;
  }

  public ServletContext getServletContext() {
    return servletContext;
  }

}