DependencyFilter (Java Web Parts API Documentation)

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javawebparts.filter
Class DependencyFilter

java.lang.Object
  |
  +--javawebparts.filter.DependencyFilter

All Implemented Interfaces:: javax.servlet.Filter

public class DependencyFilter
extends java.lang.Object
implements javax.servlet.Filter

This filter is a very simple hybrid IoC (Inversion Of Control) implementation. It is hybrid in that the dependencies are not strictly speaking "injected", as is usually the case when discussing IoC. They are created as needed, and made available to client code in a simple manner, so while the dependencies aren't injected, it is still very useful. They are in a sense injected into either the incoming request or session objects though... I'm sure you can see why I call it "hybrid" IoC! :) It allows you to define objects via XML configuration file that will be created per-session or per-request, and initialzed as you determine is neccessary.

Init parameters are:

createSession - When this filter fires, it gets a reference to the current session. If none exists, by default it creates it. However, a developer may determine this should not happen. In that case, setting this parameter to false will stop a session from being created. However, it is important to realize that in such a case, a call to getDependency() will result in null being returned, and it becomes the developers' responsibility to deal with this. Required: No. Default: true.

configFile - Context-relative path to the config file that defines the dependencies. Reqired: Yes. Default: None.

Sample Configuration File:

<dependencies>
  <dependency scope="request" name="MyTestBean1" maxAge="10" createForPaths="/myPath.do">
        <className> javawebparts.sampleapp.DFTestBean</className>
        <callMethod name="setDOB" arguments="February,17,1973" />
        <initClass name="myInitClass" mathod="initBean" />
        <initProp name="firstName" value="Frank" />
        <initProp name="lastName" value="Zammetti" />
        <initList name="children">
            < listItem value="Andrew" />
            < listItem value="Michael" />
        </initList>
        <initList name="children" method="addChild">
            < listItem value="Ashley" />
        </initList>
        < initMap name="dimensions">
            < mapItem key="height" value="5ft, 9in" />
        </initMap>
        <initMap name="dimensions" method="addDimension">
            < mapItem key="weight" value="185lbs" />
        </initMap>
    </dependency>
</dependencies>

The simplest possible config file would be this:

<dependencies>
  <dependency scope="?AA?" name=" ?BB?">
        <className>?CC?< /className>
  </dependency>
</dependencies>

?AA? would be either "request" or "session", ?BB? would be the name of the object as it will be referenced from your code, and ?CC? is the class to create. The object will be created per-request or per-session as specified, and will not be initialized in any way. Note that if it is request-scoped, it will be created for EVERY request! You will be able to retrieve it by using:

?CC? obj = (?CC?)DependencyFilter.getDependency("?BB?", request);

?CC? is again the name of the class, ?BB? is the name and request is the current request object. You DO NOT need to specify whether the object is in request or session scope, it will be found either way.

More explanation on the config file:

scope attribute - The scope attribute of the <dependency> element determines where the created object will be placed, either "request" or "session". Obviously, if placed in request-scope, the object will not persist beyond the current request, whereas it will in session-scope. Required: Yes. Default: None.

name attribute - The name attribute of the <dependency> element defines what the name of the created object will be. This is the name you will reference when getting the object. It is a request or session attribute key, depending on the specified scope. Required: Yes. Default: None.
createForPaths attribute - The createForPaths attribute of the <dependency> element is an optional attribute which specifies what paths the object will be created for *IF* the specified scope is "request" (if scope if "session", the object will be created with the first path requested). This is a comma-separated list of paths, including wildcard support (in the same way as all the other filters in JWP). If you want the object to be created for all request paths, DO NOT include this attribute at all. Required: No. Default: All request paths.

maxAge attribute - The maxAge attribute of the <dependency> element is an optional attribute which specifies the maximum age, in seconds, for the dependency to be considered "fresh". In other words, if you want to maintain a bean in a users' session that contains a list of values read from a database, but you want the list to be refreshed every five minutes, set the maxAge attribute of the dependency to 300 (5 minutes * 60 seconds) and each request that the filter handles will check the age of the existing object instance in session and will recreate it if its age exceeds this value. To mark a dependency as never expiring, i.e., it will be created once per session only, set maxAge to 0, or simply leave out the maxAge attribute.

<className> - This is the class of the object to be created. This element IS required.

<initClass> - You can have a single one of these as a child of the <dependency> element. This defines a class (via the name attribute) and method (via the method attribute) that will be called after the object has been instantiated. The object itself will be passed as the only parameter to the method, and you can do whatever kind of initialization you want. This method SHOULD NOT return a value as it will be ignored. This element IS NOT required.

<callMethod> - You can have one or more of these as a child of the <dependency> element. This element specifies a method of the object being created that will be called AFTER all initialization has taken place. The name attribute specifies the name of the method that will be called and the arguments attribute is a list of arguments (comma-separated) that will be passed. NOTE THAT ONLY STRING ARGUMENTS ARE SUPPORTED! This element IS NOT required.

<initProp> - You can have one or more of these as a child of the <dependency> element. This will initialize a property of the object being created (as named by the name attribute) to the value specified by the value attribute. This element IS NOT required.

<initList> - You can have one or more of these as a child of the <dependency> element. This will initialize a List property of the object being created (as named by the name attribute). You can optionally specify a method attribute. If NOT present, then a Javabean spec-compliant method matching the name attribute must be present (i.e., if the property is named firstName, then setFirstName() must be an accessible method). If the method attribute is present, that method will be called instead. Within each <initList> element can be any number of child <listItem> elements. For each of these simply specify the value attribute as the value to be added to the list. This element IS NOT required.

<initMap> - You can have one or many of these as a child of the <dependency> element. This will initialize a Map property of the object being created (as named by the name attribute). You can optionally specify a method attribute. If NOT present, then a Javabean spec-compliant method matching the name attribute must be present (i.e., if the property is named firstName, the setFirstName() must be an accessible method). If the method attribute is present, that method will be called instead. Within each <initMap> element can be any number of child <mapItem> elements. For each of these simply specify the key attribute as the key to be added to the list and the value attribute as the value to be added to the list. This element IS NOT required.

Note that because Commons Beanutils is used to set the various properties, you can use whatever types it supports and conversions will be automatically handled.

The order things occur in when an object is created is:

(1) The object is created
(2) The simple properties (<initProp> elements) are done, if present
(3) The List properties (<initList> element) are done, if present
(4) The Map properties (<initMap> element) are done, if present
(5) The <initClass> element is done, if present
(6) The <callMethod> element(s) are done, if present

Author:: Frank W. Zammetti.

Field Summary

private boolean createSession
          Flag: should sessions be created by this filter if one does not exist already, or not.

private static java.util.HashMap dependencyConfigs
          Collection of DependencyConfig objects.

private static org.apache.commons.logging.Log log
          Log instance.

Constructor Summary

DependencyFilter()


Method Summary

void addDependency(DependencyConfig dc)
          This method is called during config file parsing by Digester to add the configured DepedencyConfig object to the collection.

void destroy()
          Destroy.

void doFilter(javax.servlet.ServletRequest request, javax.servlet.ServletResponse response, javax.servlet.FilterChain filterChain)
          Do filter's work.

static java.lang.Object getDependency(java.lang.String name, javax.servlet.ServletRequest request)
          This method is called by client code to get a reference to a particular created object.

void init(javax.servlet.FilterConfig filterConfig)
          Initialize this filter.

static void updateDependency(java.lang.String inName, java.lang.Object inObj, javax.servlet.ServletRequest inRequest)
          This method is called by client code to update a dependent object.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

log

private static org.apache.commons.logging.Log log

Log instance.

dependencyConfigs

private static java.util.HashMap dependencyConfigs

Collection of DependencyConfig objects.

createSession

private boolean createSession

Flag: should sessions be created by this filter if one does not exist already, or not.

Constructor Detail

DependencyFilter

public DependencyFilter()

Method Detail

destroy

public void destroy()

Destroy.

Specified by:: destroy in interface javax.servlet.Filter

init

public void init(javax.servlet.FilterConfig filterConfig)
          throws javax.servlet.ServletException

Initialize this filter. This amounts to reading in the specifid config file and creating a bunch of DependencyConfig objects from it and adding them to the collection for later use.

Specified by:: init in interface javax.servlet.Filter

Parameters:: filterConfig - The configuration information for this filter.
Throws:: javax.servlet.ServletException - ServletException.

addDependency

public void addDependency(DependencyConfig dc)

This method is called during config file parsing by Digester to add the configured DepedencyConfig object to the collection.

Parameters:: dc - The fully-configured DependencyConfig object.

doFilter

public void doFilter(javax.servlet.ServletRequest request,
                     javax.servlet.ServletResponse response,
                     javax.servlet.FilterChain filterChain)
              throws javax.servlet.ServletException,
                     java.io.IOException

Do filter's work.

Specified by:: doFilter in interface javax.servlet.Filter

Parameters:: request - The current request object.; response - The current response object.; filterChain - The current filter chain.
Throws:: javax.servlet.ServletException - ServletException.; java.io.IOException - IOException.

getDependency

public static java.lang.Object getDependency(java.lang.String name,
                                             javax.servlet.ServletRequest request)

This method is called by client code to get a reference to a particular created object. The caller does not need to specify or even necessarily care where the object is (request or session).

Parameters:: name - The name of the object as specified in the config file.; request - The current ServletRequest instance.
Returns:: Reference to the specified object, or null if not present, or if object was in session but session was null.

updateDependency

public static void updateDependency(java.lang.String inName,
                                    java.lang.Object inObj,
                                    javax.servlet.ServletRequest inRequest)

This method is called by client code to update a dependent object. In other words, if a caller gets a reference to a bean in session, makes some changes to it and wants to save it again, they can call this method. This way, the caller doesn't have to know or care whether the object was in request or session.

Parameters:: inName - The name of the object as specified in the config file.; inObj - The object to store.; inRequest - Current request being services.