[#GROOVY-5294] This reference is null during construction of object of abstract class implemented as closure map - jira.codehaus.org

Details

Type: Bug
Status: Closed
Priority: Minor
Resolution: Fixed
Affects Version/s: 1.8.6
Fix Version/s: 2.0-beta-3
Component/s: None
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None

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0

Description

The following code produces NPE when the second object is created:

abstract class C {
    public abstract String f(Object o);

    private void g(Object val) { 
        println f(val) // NPE when 'y' is created !!!
    }

    public C() {this('DEF')}

    public C(Object val) { g(val) }
}

class D extends C {
    public String f(Object o) { return o ? 'Not null' : 'Is null' }
}

def x = new D()

def y = [ 'f' : { it -> return '123' + it } ] as C

Somehow 'this' becomes null when 'f' is invoked in 'g'.

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Alexander Monakhov added a comment - 16/Feb/12 2:42 AM

I see. Thanks for the explanation. I've alerady refactored methods so they are not invoked in constructor.

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Alexander Monakhov added a comment - 16/Feb/12 2:42 AM I see. Thanks for the explanation. I've alerady refactored methods so they are not invoked in constructor.

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Cedric Champeau added a comment - 16/Feb/12 2:36 AM

I fear we cannot fix that in 1.8.x. Groovy 2.0 provides a new proxy generator which will be capable of handling this though. Let me explain, in 1.x, proxies are generated using a code template. In this case, the code looks like this:

class C_groovyProxy extends C {
    private closureMap
    C_groovyProxy(map) {
        super()
        this.closureMap = map
    }
    java.lang.String f(java.lang.Object p0) { this.@closureMap['f'] (p0)    }
}
new C_groovyProxy(map)

The problem is that we call super() before setting the closure delegate map. The super call will eventually reach the g method which will lookup the implementation of f into the map. At this point, the map is still null, leading to the error you've seen. We would need a way of setting the closure delegate map before calling super. This is not allowed in Java, nor in Groovy. However, we can do this in bytecode, which explains why a fix could be made available in 2.0. Meanwhile, if possible, I'd recommand not using calls to delegated methods in the construction of the object.

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Cedric Champeau added a comment - 16/Feb/12 2:36 AM I fear we cannot fix that in 1.8.x. Groovy 2.0 provides a new proxy generator which will be capable of handling this though. Let me explain, in 1.x, proxies are generated using a code template. In this case, the code looks like this: class C_groovyProxy extends C { private closureMap C_groovyProxy(map) { super () this .closureMap = map } java.lang. String f(java.lang. Object p0) { this .@closureMap['f'] (p0) } } new C_groovyProxy(map) The problem is that we call super() before setting the closure delegate map. The super call will eventually reach the g method which will lookup the implementation of f into the map. At this point, the map is still null, leading to the error you've seen. We would need a way of setting the closure delegate map before calling super . This is not allowed in Java, nor in Groovy. However, we can do this in bytecode, which explains why a fix could be made available in 2.0. Meanwhile, if possible, I'd recommand not using calls to delegated methods in the construction of the object.

This reference is null during construction of object of abstract class implemented as closure map

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