Introduction to J2EE Patterns Online Training Classes

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Introduction to J2EE Patterns Online Training
Classes
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Design Patterns
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• A pattern is a proven solution to a problem in a
  context.
• Christopher Alexander says each pattern is a
  three-part rule which expresses a relation
  between a certain context, a problem, and a
  solution.
• Design patterns represent a solutions to problems
  that arise when developing software within a
  particular context.
• i.e Patterns problems.solution pairs in a
  context

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Categorizing Pattern
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• Patterns, then, represent expert solutions to
• recurring problems in a context and thus have
• been captured at many levels of abstraction
• and in numerous domains. Numerous
• categories are
• Design
• Architectural
• Analysis
• Creational
• Structural
• Behavioral

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MVC Design Pattern
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• Name (essence of the pattern)
• Model View Controller MVC
• Context (where does this problem occur)
• MVC is an architectural pattern that is used when
  developing interactive application such as a
  shopping cart on the Internet.
• Problem (definition of the reoccurring
  difficulty)
• User interfaces change often, especially on the
  internet where look-and-feel is a competitive
  issue. Also, the same information is presented
  in different ways. The core business logic and
  data is stable.

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MVC Pattern
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• Solution (how do you solve the problem)
• Use the software engineering principle of
  separation of concerns to divide the
  application into three areas
• Model encapsulates the core data and
  functionality
• View encapsulates the presentation of the data
  there can be many views of the common data
• Controller accepts input from the user and makes
  request from the model for the data to produce a
  new view.

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MVC Architecture
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• The Model represents the structure of the data in
  the application, as well as application-specific
  operations on those data.
• A View (of which there may be many) presents data
  in some form to a user, in the context of some
  application function.
• A Controller translates user actions (mouse
  motions, keystrokes, words spoken, etc.) and user
  input into application function calls on the
  model, and selects the appropriate View based on
  user preferences and Model state.

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Intercepting Filter
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• Context The presentation-tier request handling
  mechanism receives many different types of
  requests, which require varied types of
  processing before forwarding to handler.
• Problem Preprocessing and post-processing of a
  client Web request and response are required.
• Solution Create pluggable filters to process
  common services in a standard manner without
  requiring changes to core request processing
  code.
• The filters intercept incoming requests and
  outgoing responses, allowing preprocessing and
  post-processing.
• Such filters can be added or removed
  unobtrusively, without requiring changes to our
  existing code.

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Intercepting Filter
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• Used for common services such as security,
  logging, debugging etc.
• These filters are components that are independent
  of the main application code.
• Filters allow on the fly transformations of
  payload and header of both the request into a
  resource and the response from a resource.
• Filters do not generally create a response or
  respond to a request as servlets do , rather they
  are used to modify the request or the response

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Writing a Filter
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• Implement the javax.servlet.Filter interface
• Container will call the doFilter() method.
• The doFilter method will modify the request or
  response and then call the next filter in the
  filter chain.
• The FilterManager manages filter processing. It
  creates the FilterChain with the appropriate
  filters, in the correct order, and initiates
  processing.
• The FilterChain is an ordered collection of
  independent filters.

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Filter Configuration
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• Configuring filter in the deployment descriptor (
  web.xml )
• ltfiltergt
• ltfilter-namegtImage Filterlt/filter-namegt
• ltfilter-classgtcom.acme.ImageFilterlt/filter-cla
  ssgt
• lt/filtergt
• ltfilter-mappinggt
• ltfilter-namegtImage Filterlt/filter-namegt
• lturl-patterngt/lt/url-patterngt
• lt/filter-mappinggt

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Interceptor Filter Pattern
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Filter Sequence Diagram
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Front Controller
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• Context The presentation-tier request handling
  mechanism must control and coordinate processing
  of each user across multiple requests, in either
  a centralized or decentralized manner.
• Problem The system requires a centralized access
  point for presentation-tier request handling to
  support the integration of system services,
  content retrieval, view management, and
  navigation.

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Front Controller
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• Solution Use a controller as the initial point
  of contact for handling a request.
• The controller manages the handling of the
  request, including invoking security services
  such as authentication and authorization,
  delegating business processing, managing the
  choice of an appropriate view, handling errors,
  and managing the selection of content creation
  strategies.

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Front Controller
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• The controller provides a centralized entry point
  that controls and manages Web request handling.
• Helps to reduce the code in form of scriptlets in
  JSP page and promotes code reuse across requests.
• The Controller coordinates with a dispatcher
  component for view management and navigation.
• An application may use multiple controllers in a
  system, each mapping to a set of distinct
  services.

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Front Controller Pattern
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Controller Sequence Diagram
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View Helper
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• Context The system creates presentation content,
  which requires processing of dynamic business
  data.
• Problem Presentation tier changes occur often
  and are difficult to develop and maintain when
  business data access logic and presentation
  formatting logic are interwoven.
• This makes the system less flexible, less
  reusable, and generally less resilient to change
  and reduces modularity.

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View Helper
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• Solution A view contains formatting code,
  delegating its processing responsibilities to its
  helper classes, implemented as JavaBeans or
  custom tags.
• Helpers also store the view's intermediate data
  model and serve as business data adapters.
• Encapsulating business logic in a helper instead
  of a view makes our application more modular and
  facilitates component reuse.
• The overriding goal is partitioning of business
  logic outside of the view.

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View Helper Pattern
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View Helper Sequence Diagram
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Dispatcher View

• Context System controls flow of execution and
  access to presentation processing, which is
  responsible for generating dynamic content.
• Problem There is no centralized component for
  managing access control, content retrieval or
  view management, and there is duplicate control
  code scattered throughout various views.
• Additionally, business logic and presentation
  formatting logic are intermingled within these
  views, making the system less flexible, less
  reusable, less resilient to change and reducing
  modularity.

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Dispatcher View
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• Solution Combine a controller and dispatcher
  with views and helpers to handle client requests
  and prepare a dynamic presentation as the
  response.
• Controllers do not delegate content retrieval to
  helpers, because these activities are deferred to
  the time of view processing.
• A dispatcher is responsible for view management
  and navigation and can be encapsulated either
  within a controller, a view, or a separate
  component.

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Dispatcher View
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• Dispatcher pattern and the Service to Worker
  pattern describe a similar structure.
• Unlike the Service to Worker pattern, the
  Dispatcher View pattern suggests deferring
  content retrieval to the time of view processing.
• In the Dispatcher View pattern, the dispatcher
  typically plays a limited to moderate role in
  view management.
• Dispatcher View describes the combination of the
  Front Controller and View Helper patterns with a
  dispatcher component.

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Dispatcher View

• A limited role for the dispatcher occurs when no
  outside resources are utilized in order to choose
  the view. For example
• http//some.server.com/servlet/Controller?
  nextlogin.jsp
• The dispatcher playing a moderate role is the
  case where the client submits a request directly
  to a controller with a query parameter that
  describes an action to be completed
• http//some.server.com/servlet/Controller?
  actionlogin
• The dispatcher may access resources such as an
  XML configuration file that specifies the
  appropriate view to display.

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Dispatcher View Pattern
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Dispatcher View Sequence Diagram
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Service to Worker
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• Context The system controls flow of execution
  and access to business data, from which it
  creates presentation content.
• Problem There is no centralized component for
  managing access control, content retrieval, or
  view management, and there is duplicate control
  code scattered throughout various views.
• Additionally, business logic and presentation
  formatting logic are intermingled within thee
  views, making the system less flexible, less
  reusable, and less resilient to change.

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Service to Worker
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• Solution Combine a controller and dispatcher
  with views and helpers to handle client requests
  and prepare a dynamic presentation as the
  response.
• Controllers delegate content retrieval to
  helpers, which manage the population of the
  intermediate model for the view.
• A dispatcher is responsible for view management
  and navigation and can be encapsulated either
  within a controller or a separate component.

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Service to Worker
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• In the Service to Worker pattern, the dispatcher
  typically plays a moderate to large role in view
  management.
• In Service to Worker pattern, the dispatcher can
  be more sophisticated and may invoke a business
  service to determine the appropriate view to
  display.
• The shared structure of Service to Worker and
  Dispatcher View consists of a controller working
  with a dispatcher, views, and helpers.

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Service to Worker Pattern
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Service to Worker Sequence Diagram
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Business Delegate
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• Context A multi-tiered, distributed system
  requires remote method invocations to send and
  receive data across tiers. Clients are exposed to
  the complexity of dealing with distributed
  components.
• Problem Presentation-tier components interact
  directly with business services, exposing the
  underlying implementation details of the business
  service API to the presentation tier.
• The presentation-tier components are vulnerable
  to changes in implementation of the business
  services.
• There is a detrimental impact on network
  performance because presentation-tier components
  using the business service API make too many
  invocations over the network, with no client-side
  caching mechanism or aggregating service.
• Exposing the service APIs directly to the client
  forces the client to deal with the networking
  issues associated with the distributed nature of
  Enterprise JavaBeans (EJB) technology.

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Business Delegate
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• Solution Use a Business Delegate to reduce
  coupling between presentation-tier clients and
  business services.
• The Business Delegate hides the underlying
  implementation details of the business service,
  such as lookup and access details of the EJB
  architecture.
• Business Delegate reduces the coupling between
  presentation-tier clients and the system's
  business services, and other tiers.
• The Business Delegate hides underlying service,
  handles the exceptions from the business
  services, performs retry or recovery operations
  on failure.
• It can cache results and references to remote
  business services improving the performance.

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Business Delegate Pattern
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Business Delegate Sequence Diagram
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Delegate Sequence Diagram with ID
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Session Facade
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• Context Enterprise beans encapsulate business
  logic and business data and expose their
  interfaces, and thus the complexity of the
  distributed services, to the client tier.
• Problem
• Tight coupling, which leads to direct dependence
  between clients and business objects
• Too many method invocations between client and
  server, leading to network performance problems
• Lack of a uniform client access strategy,
  exposing business objects to misuse.

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Session Facade The Problem
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• The client must represent and implement the
  complex interactions regarding business object
  lookups and creations.
• The client grows larger and more complex to
  fulfill increasing requirements.
• The client becomes very susceptible to changes in
  the business object layer
• The client is unnecessarily exposed to the
  underlying complexity of the system.

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Session Facade
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• Solution Use a session bean as a facade to
  encapsulate the complexity of interactions
  between the business objects participating in a
  workflow.
• The Session Facade manages the business objects,
  and provides a uniform coarse-grained service
  access layer to clients.
• The Session Facade manages the interactions
  between the business data and business service
  objects participating in the workflow, and it
  encapsulates the business logic associated with
  the requirements.

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Session Facade
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• The session bean (representing the Session
  Facade) manages the relationships between
  business objects.
• The session bean also manages the life cycle of
  these participants by creating, locating (looking
  up), modifying, and deleting them as required by
  the workflow.
• Some relationships between business objects are
  transient, which means that the relationship is
  applicable to only that interaction or scenario,
  while other relationships may be more permanent.

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Session Facade Pattern
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Session Facade Sequence Diagram
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Service Locator
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• Context Service lookup and creation involves
  complex interfaces and network operations.
• Problem The J2EE clients must either locate the
  service component or create a new component in
  order to interact with the service components
  such as EJB and JMS.
•  The JNDI API enables clients to obtain an
  initial context object that holds the component
  name to object bindings and is valid for the
  client session. 
• Since locating a JNDI-administered service object
  is common to all clients that need to access that
  service object, the JNDI code is duplicated.
• Also the JNDI code is dependent on the vendor
  supplied context factory implementation.

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Service Locator
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• Solution Use a Service Locator object to
  abstract all JNDI usage and to hide the
  complexities of initial context creation, EJB
  home object lookup, and EJB object re-creation.
• Multiple clients can reuse the Service Locator
  object to reduce code complexity, provide a
  single point of control, and improve performance
  by providing a caching facility.
• The pattern provides a mechanism to abstract all
  dependencies and network details into the Service
  Locator.

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Service Locator Pattern
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Service Locator Sequence Diagram
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Transfer Object Assembler
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• Context  The server-side business components are
  implemented using session beans, entity beans,
  DAOs etc.
• Application clients frequently need to access
  data that is composed from multiple objects.
• Problem Application clients typically require
  the data for the model or parts of the model to
  present to the user or to use for an intermediate
  processing step before providing some service.
• The model is a distributed collection of objects
  such as DAO, session beans etc put together in a
  structured manner.
• The client obtains the model by access
  individually each distributed object that defines
  the model.

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Transfer Object Assembler
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• A tight coupling between the client and the
  distributed components of the model over the
  network.
• Client accesses the numerous distributed
  components degrading the perfroamnce.
• The client must reconstruct the model after
  obtaining the model's parts from the distributed
  components.
• Changes to the model require changes to the
  client as client tightly coupled to the model.

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Transfer Object Assembler
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• Solution Use a Transfer Object Assembler to
  build the required model or sub-model.
• The Transfer Object Assembler uses Transfer
  Objects to retrieve data from various business
  objects and other objects that define the model
  or part of the model.
• Transfer Object Assembler constructs a immutable
  composite Transfer Object that represents data
  from different business components for the model
• When a model is composed of a combination of many
  components, the client must interact with
  numerous such business components to obtain all
  the data necessary to represent the model.
• Hence when model has multiple components, the
  clients become tightly coupled to the model
  implementation and make numerous remote method
  invocations to obtain data.

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Transfer Object Assembler Pattern
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Transfer Object Assembler Sequence
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Transfer Object
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• Context Application clients need to exchange
  data with enterprise beans.
• Problem Some methods exposed by the business
  components return data to the client.
• The client invokes a business object's get
  methods multiple times until it obtains all the
  attribute values.
• Every method call made to the business service
  object is potentially remote.

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Transfer Object Problem
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• In EJB application such remote invocations use
  the network layer regardless of the proximity of
  the client to the bean, creating a network
  overhead.
• As the usage of the remote methods increases,
  application performance can significantly
  degrade.
• Therefore, using multiple calls to get methods
  that return single attribute values is
  inefficient for obtaining data values from an
  enterprise bean.

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Transfer Object

• Solution Use a Transfer Object to encapsulate
  the business data.
• A single method call is used to send and retrieve
  the Transfer Object.
• When the client requests the enterprise bean for
  the business data, the enterprise bean can
  construct the Transfer Object, populate it with
  its attribute values, and pass it by value to the
  client.
• It  reduces the number of remote calls and avoids
  the associated overhead.
• Since the Transfer Object is passed by value to
  the client, all calls to the Transfer Object
  instance are local calls instead of remote method
  invocations.

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Transfer Object Pattern
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Transfer Object Sequence Diagram
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Updater Transfer Object Sequence
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Data Access Object
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• Context Access to data varies depending on the
  source of the data.
• Access to persistent storage, such as database,
  varies depending on type of storage and the
  vendor implementation.
• Problem Persistent storage is implemented with
  different mechanisms, with marked differences in
  the APIs to access the different persistent
  storage mechanisms.
• Access mechanisms, supported APIs, and features
  vary between different types of persistent
  stores.

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Data Access Object Problem
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• Applications that need to access data from a
  legacy or disparate system are often required to
  use APIs that may be proprietary, creating a
  direct dependency between application code and
  data access code.
• Including the connectivity and data access code
  within these components introduces a tight
  coupling between the components and the data
  source implementation making it difficult and
  tedious to migrate the application from one type
  of data source to another.
• When the data source changes, the components need
  to be changed to handle the new type of data
  source.

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Data Access Object

• Solution Use a Data Access Object (DAO) to
  abstract and encapsulate all access to the data
  source.
• The DAO manages the connection with the data
  source to obtain and store data.
• The DAO completely hides the data source
  implementation details from its clients.
• The DAO adapts to different storage schemes
  without affecting its clients or business
  components

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Data Access Object Pattern
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Data Access Object Sequence Diagram
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