Groovy call operator throws MissingMethodException when used on a class field

问题

I am trying to use call operator () overloading, but it does not work on class fields. What's wrong?

class Foo {
    void call(int x){
        println("x="+x)
    }
}

class MyCallable {
    Foo foo = new Foo()
}

Foo foo = new Foo() 
foo(5)  //works

MyCallable mc = new MyCallable()
mc.foo(2) //not works

But program terminated with exception:

Exception in thread "main" groovy.lang.MissingMethodException: No
signature of method: mpctests.MyCallable.foo() is applicable for
argument types: (java.lang.Integer) values: [2]

回答1:

You get MissingMethodException when you call mc.foo(5), because Groovy's invoke object method mechanism gets triggered. There is one thing worth explaining to get a better understanding about this situation. Your MyCallable class:

class MyCallable {
    Foo foo = new Foo()
}

gets compiled to something like this:

import groovy.lang.GroovyObject;
import groovy.lang.MetaClass;
import org.codehaus.groovy.runtime.ScriptBytecodeAdapter;
import org.codehaus.groovy.runtime.callsite.CallSite;

public class MyCallable implements GroovyObject {
    private Foo foo;

    public MyCallable() {
        CallSite[] var1 = $getCallSiteArray();
        Object var2 = var1[0].callConstructor(Foo.class);
        this.foo = (Foo)ScriptBytecodeAdapter.castToType(var2, Foo.class);
        MetaClass var3 = this.$getStaticMetaClass();
        this.metaClass = var3;
    }

    public Foo getFoo() {
        return this.foo;
    }

    public void setFoo(Foo var1) {
        this.foo = var1;
    }
}

Groovy also compiles every field access like mc.foo to a getter method call mc.getFoo(). So when you call mc.foo(5) it is clear for Groovy runtime that you expect to call a foo(5) method on mc object. And this method does not exist and MissingMethodException gets thrown.

However, it works if you create object def foo = new Foo() and then you call foo(5), because foo is an object and foo(5) is a strict instruction to invoke call(5) method on foo object (foo(5) is a shorthand version of foo.call(5)). The same situation would take place if you call mc() - Groovy would try to invoke mc.call() method. But when you say mc.foo(5) it's clear that you are trying to invoke foo(5) method.

---

If you want to use call operator on mc.foo field there are two options:

1. Use direct field access operator @

mc.@foo(5)

In this case you refer directly to foo field and you can use shorthand call operator.

2. Use with {} method

mc.with {
    foo(5)
}

In this case it is also a straightforward for Groovy runtime that you are accessing foo field and you can use call operator on it.

Alternatives

Using getter method:

mc.getFoo()(5)

Using method call() directly:

mc.foo.call(5) // equivalent of mc.getFoo().call(5)

来源：https://stackoverflow.com/questions/51727088/groovy-call-operator-throws-missingmethodexception-when-used-on-a-class-field