Factories in Java without a switch statement

问题:

I am trying to build a factory object, but having trouble working out a good way of doing it in Java.

The application I am writing is used for processing files in various formats, so there is a CodecInterface which applies to all classes which are used for reading and writing files. Let's assume it defines the following methods. Each of these files has a unique human-designated ID string which is used for id'ing the encoder\decoder.

String read();
void write(String data);
String getID();

The factory class would have a create method which is intended to create instances of these codec classes. I imagine the method signature would look something like this.

static CodecInterface CodecFactory.create(String filename, String codecid, String args);

The filename is the name of the file to read/write, and the codecid is the unique ID indicating what codec to use. The args parameter is a string of arguments passed to the decoder/encoder object being generated. The return of this should be an instance of the requested codec object.

All of the Factory examples I have seen typically have a switch statement inside of the create method which creates an object instance dependent on the ID. I want to avoid doing it this way as it doesn't seem like the 'right' way, and it also means that the list is more or less fixed unless you modify the create method. Ideally I would like to use something like a dictionary (indexed by the codec ID) which contains something which can be used to create an instance of the codec classes I want (I will call this mystery class ClassReference). Again to use some quasi-java code, here is what I was thinking as the body for the create method.

static Dictionary<String, ClassReference>;

static CodecInterface CodecFactory.create(String filename, String codecid, String args);
{
    ClassReference classreference;

    classreference = codeclibrary(codecid);

    return classreference.instanceOf(args);
}

The dictionary of ID's is easy enough, but I can't work out what ClassReference should be. Class Reference should allow me to create an instance of the desired class, as in the example above.

From looking around online, the class method, and instanceOf seem to be heading in the right direction, but I haven't found anything which puts the two together. As an added complication, the constructors for the objects being created will have arguments.

Any tips on what I should be looking at would be greatly appreciated.

Thanks in advance.

SOLUTION

Thanks everyone for your advice. I've ended up taking bits and pieces from all of your suggestions and came up with the following which seems to work as I wanted.

Note that I have omitted much of the sanity\error checking code to show off the important bits.

import java.lang.reflect.Constructor;
import java.util.HashMap;

public class CodecFactory
{
    private static HashMap<String, Class<? extends CodecInterface>> codecs;

    static
    {        
        codecs = new HashMap<String, Class<? extends CodecInterface>>();

        //Register built-in codecs here
        register("codecA", CodecA.class);
        register("codecB", CodecB.class);
        register("codecC", CodecC.class);
    }

    public static void register(String id, Class<? extends CodecInterface> codec)
    {
        Class<? extends CodecInterface> existing;

        existing = codecs.get(id);        
        if(existing == null)
        {
          codecs.put(id, codec);
        }
        else
        {
          //Duplicate ID error handling
        }
    }

    public static CodecInterface create(String codecid, String filename, String mode, String arguments)
    {
        Class<? extends CodecInterface> codecclass;
        CodecInterface codec;
        Constructor constructor;

        codec = null;

        codecclass = codecs.get(codecid);
        if(codecclass != null)
        {
          try
          {
            constructor = codecclass.getDeclaredConstructor(String.class, String.class, String.class, String.class);
            codec = (CodecInterface)(constructor.newInstance(codecid, filename, mode, arguments));
          }
          catch(Exception e)
          {
            //Error handling for constructor/instantiation
          }
        }

        return codec;
    }
}

回答1:

Try something like that:

public class CodecFactory {
    final private static Map<String, Class<? extends CodecInterface>> codecLibrary;

    static {
        codecLibrary = new HashMap<String, Class<? extends CodecInterface>>();
        codecLibrary.put("codec1", Codec1.class);
        //...
    }

    static CodecInterface create(String filename, String codecid, String args) throws InstantiationException, IllegalAccessException {
        Class<? extends CodecInterface> clazz;

        clazz = codecLibrary.get(codecid);

        CodecInterface codec = clazz.newInstance();

        codec.setArgs(args);
        codec.setFilename(filename);

        return codec;
    }
}

回答2:

There's a zillion options. For example, you might create a base factory class that also has static methods to manage registered factories (untested code typed here, sorry for errors):

public abstract class CodecFactory {

    private final String name;

    public CodecFactory (String name) {
        this.name = name;
    }

    public final String getName () { 
        return name; 
    }

    // Subclasses must implement this.
    public abstract Codec newInstance (String filename, String args);

    // --- Static factory stuff ---

    private static final Map<String,CodecFactory> factories = new HashMap<String,CodecFactory>();

    public static void registerFactory (CodecFactory f) {
        factories.put(f.getName(), f);
    }

    public static Codec newInstance (String filename, String codec, String args) {
        CodecFactory factory = factories.get(codec);
        if (factory != null)
            return factory.newInstance(filename, args);
        else
            throw new IllegalArgumentException("No such codec.");
    }

}

Then:

public class QuantumCodecFactory extends CodecFactory {

    public QuantumCodecFactory {
        super("quantum");
    }

    @Override public Codec newInstance (String filename, String args) {
        return new QuantumCodec(filename, args);
    }

}

Of course this means at some point you must:

CodecFactory.registerFactory(new QuantumCodecFactory());

Then usage is:

Codec codec = CodecFactory.newInstance(filename, "quantum", args);

Another option is to use reflection and maintain a Map<String,Class<? extends CodecInterface>>, using Class.newInstance() to instantiate. This is convenient to implement because it works on top of Java's Class, which already supports a factory-style model for instantiating objects. The caveats are, like above the classes must be explicitly registered, and also (unlike above) you can't implicitly enforce constructor parameter types at compile-time (although you could at least abstract it behind some method instead of calling Class.newInstance() directly from client code).

For example:

public final class CodecFactory {

    private static final Map<String,Class<? extends Codec>> classes = new HashMap<String,Class<? extends Codec>>();

    public static void registerClass (String name, Class<? extends Codec> clz) {
        classes.put(name, clz);
    }

    public static Codec newInstance (String filename, String codec, String args) {
        Class<? extends Codec> clz = classes.get(codec);
        if (clz != null)
            return clz.getDeclaredConstructor(String.class, String.class).newInstance(filename, args);
        else 
            throw new IllegalArgumentException("No such codec.");
    }

}

Where every Codec is expected to have a constructor that takes (String filename, String args). Registration, then, is:

CodecFactory.registerClass("quantum", QuantumCodec.class);

Usage is the same as above:

Codec codec = CodecFactory.newInstance(filename, "quantum", args);

You could even leave out the map and just use Class.forName() -- this doesn't give you as much flexibility with codec names, but it essentially lets the class loader do all of the work for you and you don't need to explicitly register types ahead of time.

---

Edit: Re: Question in comments below. You could come up with a system that combined the two above examples to create a reusable, reflection based generic factory derived from CodecFactory, which still leaves you with the ability to create other more specialized factories, e.g.:

public class GenericCodecFactory extends CodecFactory {

    private final String name;
    private final Class<? extends Codec> clz;

    public GenericCodecFactory (String name, String clzname) {
        this.name = name;
        this.clz = Class.forName(clzname);
    }

    public GenericCodecFactory (String name, Class<? extends Codec> clz) {
        this.name = name;
        this.clz = clz;
    }

    // parameter type checking provided via calls to this method, reflection
    // is abstracted behind it.
    @Override public Codec newInstance (String filename, String args) {
        return clz.getDeclaredConstructor(String.class, String.class).newInstance(filename, args);
    } 

}

Then you could use that for whatever:

// you can use specialized factories
ClassFactory.registerFactory(new QuantumCodecFactory());
// you can use the generic factory that requires a class at compile-time
ClassFactory.registerFactory(new GenericCodecFactory("awesome", AwesomeCodec.class));
// you can use the generic factory that doesn't need to have class present at compile-time
ClassFactory.registerFactory(new GenericCodecFactory("ninja", "com.mystuff.codecs.NinjaCodec"));

As you can see, there's a ton of possibilities. Using Class.forName() in reflection-based factories is nice because the class doesn't need to be present at compile-time; so you can drop in codec classes on the class path and, say, specify a list of class names in a run-time configuration file (then you could have static ClassFactory.registerFactoriesListedInFile(String confgFilename) or something), or scan a "plugin" directory. You can even construct class names from simpler strings, if you're comfortable with that, e.g.:

public class GenericPackageCodecFactory extends GenericCodecFactory {
    public GenericPackageCodecFactory (String name) {
        super(name, "com.mystuff." + name + ".Codec");
    }
}

You could even use something like that as a fallback in ClassFactory if the codec name is not found, to get around having to explicitly register types.

The reason reflection keeps popping up, by the way, is that it's very flexible and the Class interface is essentially an all-encompassing class factory, so it frequently parallels what specific factory architectures are trying to accomplish.

Another option is to use the second example I mentioned above (with the Map<String,Class>) but make a version of registerFactory that takes a String class name instead of a Class, similar to the generic implementation I just mentioned. That's probably the least amount of code required to avoid having to create instances of CodecFactorys.

I can't possibly give examples for every combination of things you can do here, so here is a partial list of the tools you have available, which you should use as you see fit. Remember: Factories are a concept; it's up to you to use the tools you have to implement that concept in a clean way that meets your requirements.

• Reflection (Class<?> and Class.forName)
• Static initializer blocks (sometimes a good place to register a factory; requires class to be loaded, but Class.forName can trigger this).
• External configuration files
• Plugin frameworks like http://jpf.sourceforge.net/ or https://code.google.com/p/jspf/ or https://code.google.com/p/jin-plugin/ (good comparison of OSGi, JPF, JSPF can be found here; I've never heard of jin-plugin prior to looking through the answers in the link).
• Maps of registered factories and/or ability to use reflection to generate class names on the fly.
• Don't forget concurrent maps and/or synchronization primitives for multi-threaded support if necessary.
• Lots of other stuff.

Also: Don't go crazy implementing all of these possibilities if you don't have to; think about your requirements and decide on the minimum amount of work you need to do here to meet them. For example, if you need extensible plugins, JSPF alone might be enough to satisfy all of your requirements without you having to do any of this work (I haven't actually checked it out, so I'm not sure). If you don't need that kind of plugin "scanning" behavior, simple implementations like the examples above will do the trick.