Builder

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[[APIDesignPatterns:Builder|Builder pattern]] evolution characteristics are simpler to [[APIDesignPatterns:CumulativeFactory|cumulative factory]] pattern, yet in some situations can its use simplify internal implementation of the [[API]].
[[APIDesignPatterns:Builder|Builder pattern]] evolution characteristics are simpler to [[APIDesignPatterns:CumulativeFactory|cumulative factory]] pattern, yet in some situations can its use simplify internal implementation of the [[API]].
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[[Category:APIDesignPatterns]]
[[Category:APIDesignPatterns]]
[[Category:APIDesignPatterns:Creational]]
[[Category:APIDesignPatterns:Creational]]

Revision as of 13:01, 15 November 2008

Builder pattern is build around configuration cumulative factory pattern followed by final call to a factory method. It is very useful, especially in cases where the intermediate states created by the use of cumulative factory are incomplete and cannot be used by itself. If an additional action (write to disk, establish connnection, etc.) is needed, it makes sense to finish the API calls by final use of factory method.

Imagine that we have an API representing a server:

Code from ServerConnector.java:
See the whole file.

public final class ServerConnector {
    public String getName() {
        return name == null ? "noname" : name.getName();
    }
 
    public URL getURL() {
        return url == null ? null : url.getURL();
    }
 
    public void reset() {
        if (reset != null) {
            reset.reset();
        }
    }
 
    /** Additional method for API clients not available from first version.
     * @since 2.0
     */
    public void shutdown() {
        if (shutdown != null) {
            shutdown.shutdown();
        }
    }
}
 

And we need a way to establish connection to it. For that purpose we can define additional class that has no getters, no way to obtain its state via external APIs. Instead it offers users of its API only setters (or in this case cumulative factory methods) allowing the clients to change enough properties before final creation of the connector:

Code from ServerInfo.java:
See the whole file.

public class ServerInfo {
 
    public static ServerInfo empty() {
        return new ServerInfo(null, null, null, null);
    }
 
    public final ServerInfo nameProvider(NameProvider np) {
        this.name = np;
        return this;
    }
 
    public final ServerInfo urlProvider(URLProvider up) {
        this.url = up;
        return this;
    }
    public final ServerInfo reset(ResetHandler h) {
        this.reset = h;
        return this;
    }
 
    /** All one needs to do when there is a need to add new
     * style of creation is to add new method for a builder.
     * @param handler
     * @return
     * @since 2.0
     */
    public final ServerInfo shutdown(ShutdownHandler handler) {
        this.shutdown = handler;
        return this;
    }
 
    /** Creates the server connector based on current values in the 
     * info. Builder factory method.
     * @return server connector
     */
    public final ServerConnector connect() {
        return new ServerConnector(name, url, reset, shutdown);
    }
}
 

The use is similar as in case of cumulative factory, just the writer of the API can simplify internals of the ServerInfo class, as it is know to never connect directly, to the server, it is just a recipe for creation of the connection:

Code from BuilderFactoryTest.java:
See the whole file.

ServerInfo empty = ServerInfo.empty();
ServerInfo name = empty.nameProvider(prov);
ServerInfo urlAndName = name.urlProvider(prov);
ServerInfo all = urlAndName.reset(prov);
connection = all.connect();
 

The previous snippet is the verbose way to use the API, the shorter one liner looks like:

Code from BuilderFactoryTest.java:
See the whole file.

connection = ServerInfo.empty()
        .nameProvider(np).urlProvider(up).reset(res).connect();
 

Builder pattern evolution characteristics are simpler to cumulative factory pattern, yet in some situations can its use simplify internal implementation of the API.

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