java-design-patterns/chain-of-responsibility/README.md

---
title: Chain of responsibility
category: Behavioral
language: en
tag:
    - Gang of Four
    - Decoupling
---

## Also known as

* Chain of Command
* Chain of Objects
* Responsibility Chain

## Intent

Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it.

## Explanation

Real-world example

> The Orc King gives loud orders to his army. The closest one to react is the commander, then an officer, and then a soldier. The commander, officer, and soldier form a chain of responsibility.

In plain words

> It helps to build a chain of objects. A request enters from one end and keeps going from an object to another until it finds a suitable handler.

Wikipedia says

> In object-oriented design, the chain-of-responsibility pattern is a design pattern consisting of a source of command objects and a series of processing objects. Each processing object contains logic that defines the types of command objects that it can handle; the rest are passed to the next processing object in the chain.

**Programmatic Example**

Translating our example with the orcs from above. First, we have the `Request` class:

```java
import lombok.Getter;

@Getter
public class Request {

    private final RequestType requestType;
    private final String requestDescription;
    private boolean handled;

    public Request(final RequestType requestType, final String requestDescription) {
        this.requestType = Objects.requireNonNull(requestType);
        this.requestDescription = Objects.requireNonNull(requestDescription);
    }

    public void markHandled() {
        this.handled = true;
    }

    @Override
    public String toString() {
        return getRequestDescription();
    }
}

public enum RequestType {
    DEFEND_CASTLE, TORTURE_PRISONER, COLLECT_TAX
}
```

Next, we show the request handler hierarchy.

```java
public interface RequestHandler {

    boolean canHandleRequest(Request req);

    int getPriority();

    void handle(Request req);

    String name();
}

@Slf4j
public class OrcCommander implements RequestHandler {
    @Override
    public boolean canHandleRequest(Request req) {
        return req.getRequestType() == RequestType.DEFEND_CASTLE;
    }

    @Override
    public int getPriority() {
        return 2;
    }

    @Override
    public void handle(Request req) {
        req.markHandled();
        LOGGER.info("{} handling request \"{}\"", name(), req);
    }

    @Override
    public String name() {
        return "Orc commander";
    }
}

// OrcOfficer and OrcSoldier are defined similarly as OrcCommander

```

The Orc King gives the orders and forms the chain.

```java
public class OrcKing {

    private List<RequestHandler> handlers;

    public OrcKing() {
        buildChain();
    }

    private void buildChain() {
        handlers = Arrays.asList(new OrcCommander(), new OrcOfficer(), new OrcSoldier());
    }

    public void makeRequest(Request req) {
        handlers
                .stream()
                .sorted(Comparator.comparing(RequestHandler::getPriority))
                .filter(handler -> handler.canHandleRequest(req))
                .findFirst()
                .ifPresent(handler -> handler.handle(req));
    }
}
```

The chain of responsibility in action.

```java
var king=new OrcKing();
        king.makeRequest(new Request(RequestType.DEFEND_CASTLE,"defend castle"));
        king.makeRequest(new Request(RequestType.TORTURE_PRISONER,"torture prisoner"));
        king.makeRequest(new Request(RequestType.COLLECT_TAX,"collect tax"));
```

The console output.

```
Orc commander handling request "defend castle"
Orc officer handling request "torture prisoner"
Orc soldier handling request "collect tax"
```

## Class diagram

![alt text](./etc/chain-of-responsibility.urm.png "Chain of Responsibility class diagram")

## Applicability

Use Chain of Responsibility when

* More than one object may handle a request, and the handler isn't known a priori. The handler should be ascertained automatically.
* You want to issue a request to one of several objects without specifying the receiver explicitly.
* The set of objects that can handle a request should be specified dynamically.

## Known uses

* Event bubbling in GUI frameworks where an event might be handled at multiple levels of a UI component hierarchy
* Middleware frameworks where a request passes through a chain of processing objects
* Logging frameworks where messages can be passed through a series of loggers, each possibly handling them differently
* [java.util.logging.Logger#log()](http://docs.oracle.com/javase/8/docs/api/java/util/logging/Logger.html#log%28java.util.logging.Level,%20java.lang.String%29)
* [Apache Commons Chain](https://commons.apache.org/proper/commons-chain/index.html)
* [javax.servlet.Filter#doFilter()](http://docs.oracle.com/javaee/7/api/javax/servlet/Filter.html#doFilter-javax.servlet.ServletRequest-javax.servlet.ServletResponse-javax.servlet.FilterChain-)

## Consequences

Benefits:

* Reduced coupling. The sender of a request does not need to know the concrete handler that will process the request.
* Increased flexibility in assigning responsibilities to objects. You can add or change responsibilities for handling a request by changing the members and order of the chain.
* Allows you to set a default handler if no concrete handler can handle the request.

Trade-Offs:

* It can be challenging to debug and understand the flow, especially if the chain is long and complex.
* The request might end up unhandled if the chain doesn't include a catch-all handler.
* Performance concerns might arise due to potentially going through several handlers before finding the right one, or not finding it at all.

## Related Patterns

* [Command](https://java-design-patterns.com/patterns/command/): can be used to encapsulate a request as an object, which might be passed along the chain.
* [Composite](https://java-design-patterns.com/patterns/composite/): the Chain of Responsibility is often applied in conjunction with the Composite pattern.
* [Decorator](https://java-design-patterns.com/patterns/decorator/): decorators can be chained in a similar manner as responsibilities in the Chain of Responsibility pattern.

## Credits

* [Design Patterns: Elements of Reusable Object-Oriented Software](https://www.amazon.com/gp/product/0201633612/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0201633612&linkCode=as2&tag=javadesignpat-20&linkId=675d49790ce11db99d90bde47f1aeb59)
* [Head First Design Patterns: A Brain-Friendly Guide](https://www.amazon.com/gp/product/0596007124/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596007124&linkCode=as2&tag=javadesignpat-20&linkId=6b8b6eea86021af6c8e3cd3fc382cb5b)
* [Pattern-Oriented Software Architecture, Volume 1: A System of Patterns](https://amzn.to/3PAJUg5)
* [Refactoring to Patterns](https://amzn.to/3VOO4F5)
* [Pattern languages of program design 3](https://amzn.to/4a4NxTH)