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Design_Pattern - Interceptng Filter Pattern


The intercepting filter design pattern is used when we want to do some pre-processing / post-processing with request or response of the application. Filters are defined and applied on the request before passing the request to actual target application. Filters can do the authentication/ authorization/ logging or tracking of request and then pass the requests to corresponding handlers. Following are the entities of this type of design pattern.
  • Filter - Filter which will perform certain task prior or after execution of request by request handler.
  • Filter Chain - Filter Chain carries multiple filters and help to execute them in defined order on target.
  • Target - Target object is the request handler
  • Filter Manager - Filter Manager manages the filters and Filter Chain.
  • Client - Client is the object who sends request to the Target object.
Implementation

We are going to create a FilterChain, FilterManager, Target, and Client as various objects representing our entities. AuthenticationFilter and DebugFilter represent concrete filters.

InterceptingFilterDemo, our demo class, will use Client to demonstrate Intercepting Filter Design Pattern.

Step 1

Create Filter interface.

Filter.java
public interface Filter {
public void execute(String request);
}

Step 2

Create concrete filters.

AuthenticationFilter.java
public class AuthenticationFilter implements Filter {
public void execute(String request){
System.out.println("Authenticating request: " + request);
}
}

DebugFilter.java
public class DebugFilter implements Filter {
public void execute(String request){
System.out.println("request log: " + request);
}
}

Step 3

Create Target

Target.java
public class Target {
public void execute(String request){
System.out.println("Executing request: " + request);
}
}

Step 4

Create Filter Chain.

FilterChain.java
import java.util.ArrayList;
import java.util.List;
public class FilterChain {
private List<Filter> filters = new ArrayList<Filter>();
private Target target;
public void addFilter(Filter filter){
filters.add(filter);
}
public void execute(String request){
for (Filter filter : filters) {
filter.execute(request);
}
target.execute(request);
}
public void setTarget(Target target){
this.target = target;
}
}

Step 5

Create Filter Manager.

FilterManager.java
public class FilterManager {
FilterChain filterChain;
public FilterManager(Target target){
filterChain = new FilterChain();
filterChain.setTarget(target);
}
public void setFilter(Filter filter){
filterChain.addFilter(filter);
}
public void filterRequest(String request){
filterChain.execute(request);
}
}

Step 6

Create Client.

Client.java
public class Client {
FilterManager filterManager;
public void setFilterManager(FilterManager filterManager){
this.filterManager = filterManager;
}
public void sendRequest(String request){
filterManager.filterRequest(request);
}
}

Step 7

Use the Client to demonstrate Intercepting Filter Design Pattern.

InterceptingFilterDemo.java
public class InterceptingFilterDemo {
public static void main(String[] args) {
FilterManager filterManager = new FilterManager(new Target());
filterManager.setFilter(new AuthenticationFilter());
filterManager.setFilter(new DebugFilter());
Client client = new Client();
client.setFilterManager(filterManager);
client.sendRequest("HOME");
}
}

Step 8

Verify the output.
Authenticating request: HOME
request log: HOME
Executing request: HOME
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