15.3 Review of MOP Techniques

Programming Groovy 2 — by Venkat Subramaniam (114 / 152)

👈 15.2 Method Delegation: Putting It All Together | TOC | Chapter 16 Applying Compile-Time Metaprogramming 👉

We’ve seen a number of options to intercept, inject, and synthesize methods. In this section, we’ll figure out which option is right for us.

We discussed method interception in Chapter 12, ​Intercepting Methods Using MOP​, and Section 13.1, ​Injecting Methods Using Categories​. We can use ​GroovyInterceptable​, ​ExpandoMetaClass​, or categories.

If we have the privileges to modify the class source, we can implement ​GroovyInterceptable​ on the class we want to intercept method calls. The effort is as simple as implementing ​invokeMethod​ .

If we can’t modify the class or if the class is a Java class, then we can use ​ExpandoMetaClass​ or categories. ​ExpandoMetaClass​ clearly stands out in this case because a single ​invokeMethod​ can take care of intercepting any methods of our class. Categories, on the other hand, would require separate methods, one per intercepted method. Also, if we use categories we’re restricted by the ​use​ block.

We discussed method injection in Section 13.1, ​Injecting Methods Using Categories​. We can use categories or ​ExpandoMetaClass​.

Categories compete well with ​ExpandoMetaClasses​ for method injection. If we use categories, we can control the location where methods are injected. We can easily implement different versions of method injection by using different categories. We can easily nest and mix multiple categories, as well. The control that categories offer—that method injection takes effect only within the ​use​ blocks and is limited to the executing thread—may also be considered a restriction. If we want to use the injected methods at any location and also want to inject static methods and constructors, ​ExpandoMetaClass​ is a better choice. Beware, though, that ​ExpandoMetaClass​ is not the default ​MetaClass​ in Groovy.

Using the ​ExpandoMetaClass​, we can inject methods into specific instances of a class instead of affecting the entire class.

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