How to find classes that implement an interface dynamically? [closed]

Question

How do I determine at run time the classes that implement an interface using the tooling API?

Answer 1

Edit:

We can now query the ApexTypeImplementator table to find all public and global classes that implement a given interface locally, as well as global classes in managed packages that implement a specified interface.

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Original Answer:

For the most usual cases, I use an SOSL to find them:

ApexClass[] classes = (ApexClass[])[FIND 'database.batchable' 
                                    IN ALL FIELDS
                                    RETURNING ApexClass(Id, Name)][0];

You can also do this dynamically based on a variable:

ApexClass[] classes = (ApexClass[])[FIND :interfaceName 
                                    IN ALL FIELDS
                                    RETURNING ApexClass(Id, Name)][0];

Keep in mind that this also "searches" comments, so you may need to do some post-retrieval parsing using a Pattern if this is a concern, or if the class as a default constructor, you can also:

Boolean classImplementsInterface = Type.forName(classes[0].Name).newInstance() 
                                   instanceOf Database.Batchable<SObject>;