Using Groovy MetaClass to overwrite Methods

问题

I have a POJO that uses a service to do something:

public class PlainOldJavaObject {

    private IService service;

    public String publicMethod(String x) {
        return doCallService(x);
    }

    public String doCallService(String x) {
        if(service == null) {
            throw new RuntimeException("Service must not be null");
        }
        return service.callX(x);
    }

    public interface IService {
        String callX(Object o);
    }
}

And I have a Groovy test case:

class GTest extends GroovyTestCase {

    def testInjectedMockIFace() {
        def pojo = new PlainOldJavaObject( service: { callX: "very groovy" } as IService )
        assert "very groovy" == pojo.publicMethod("arg")
    }

    def testMetaClass() {
        def pojo = new PlainOldJavaObject()
        pojo.metaClass.doCallService = { String s ->
            "no service"
        }
        assert "no service" == pojo.publicMethod("arg")
    }
}

The first test method, testInjectedMockIFace works as expected: The POJO is created with a dynamic implementation of IService. When callX is invoked, it simply returns "very groovy". This way, the service is mocked out.

However I don't understand why the second method, testMetaClass does not work as expected but instead throws a NullPointerException when trying to invoke callX on the service object. I thought I had overwritten the doCallService method with this line:

pojo.metaClass.doCallService = { String s ->

What am I doing wrong?

Thanks!

回答1:

Your syntax is a tiny bit off. The problem is that pojo is a Java Object and does not have a metaClass. To intercept calls to PlainOldJavaObject's doCallService using ExpandoMetaClass:

Just replace:

    pojo.metaClass.doCallService = { String s ->
        "no service"
    }

With:

    PlainOldJavaObject.metaClass.doCallService = { String s ->
        "no service"
    }

回答2:

If your POJO really is a Java class, and not a Groovy class, then that is your problem. Java classes don't invoke methods via the metaClass. e.g., in Groovy:

pojo.publicMethod('arg')

is equivalent to this Java:

pojo.getMetaClass().invokeMethod('publicMethod','arg');

invokeMethod sends the call through the metaClass. But this method:

public String publicMethod(String x) {
    return doCallService(x);
}

is a Java method. It doesn't use invokeMethod to call doCallService. To get your code to work, PlainOldJavaObject needs to be a Groovy class so that all calls will go through the metaClass. Normal Java code doesn't use metaClasses.

In short: even Groovy can't override Java method calls, it can only override calls from Groovy or that otherwise dispatch through invokeMethod.

回答3:

What you have looks fine. I ran a slightly modified version on it on the groovy console webapp and it ran without issue. See for yourself using this code at http://groovyconsole.appspot.com/.

public interface IService {
    String callX(Object o);
}

public class PlainOldJavaObject {

    private IService service;

    public String publicMethod(String x) {
        return doCallService(x);
    }

    public String doCallService(String x) {
        if(service == null) {
            throw new RuntimeException("Service must not be null");
        }
        return service.callX(x);
    }
}

def pojo = new PlainOldJavaObject()
pojo.metaClass.doCallService = { String s ->
    "no service"
}
println pojo.publicMethod("arg")

What version of Groovy are you using. It could very well be a bug in Groovy in the metaclass implementation. The groovy language moves pretty quickly and the metaclass implementation changes from version to version.

Edit - Feedback from Comment:

The version of the groovy console webapp is 1.7-rc-1. So it looks like that version may work as you want it to. They are currently in RC2 so I expect it would be released soon. Not sure if what you are seeing is a bug or just a difference in how it works in the 1.6.x version.

来源：https://stackoverflow.com/questions/1927796/using-groovy-metaclass-to-overwrite-methods