Groovy dynamic property per object

Question

Using Groovy 1.8. I'm trying to create a dynamic class definition that will cache properties per object. I did use propertyMissing without adding the property to the object just fine. I just think caching the properties would be more efficient. Right?

Note that each instance must have its own different properties.

The code below works fine:

class C {}
def c = new C()
c.metaClass.prop = "a C property"
println c.prop

def x = new C()
x.prop

will output:

a C property
groovy.lang.MissingPropertyException: No such property: prop for class: C

If I need to this problematically:

class A {
    def propertyMissing(String name) {
        if(!this.hasProperty(name)) {
            println "create new propery $name"
            this.metaClass."$name" = "Dyna prop $name"
            println "created new propery $name"
        }
        this.metaClass."$name"
    }
}

a = new A()
println a.p1

For A, I get as far as "create new property", but the line this.metaClass."$name" = "Dyna prop $name" fails with: No such property: p1 for class at line 5

Whats wrong?

Answer 1

This code should do what you want:

class A {
  A() {
    def mc = new ExpandoMetaClass( A, false, true)
    mc.initialize()
    this.metaClass = mc
  }

  def propertyMissing( String name ) {
    println "create new propery $name"
    def result = "Dyna prop $name"
    this.metaClass."$name" = result
    println "created new propery $name"
    result
  }
}

a = new A()
println a.p1
println a.p1

That outputs:

create new propery p1
created new propery p1
Dyna prop p1
Dyna prop p1

Answer 2

Why don't you store your dynamic properties in a simple HashMap?

class Foo {
    def storage = [:]
    def propertyMissing(String name, value) { storage[name] = value }
    def propertyMissing(String name) { storage[name] }
}
def f = new Foo()
f.foo = "bar"

This is the standard example from: http://groovy.codehaus.org/Using+methodMissing+and+propertyMissing

Note, you don't have to check if the property is really missing ... if it won't be missing, the method would not be called in the first place.