Groovy way to dynamically instantiate a class from String

Question

The answers of this question about the Groovy way to dynamically invoke a static method were very helpful but I'm having trouble with the following case:

I defined a simple Groovy class:

class Item {
  def id = 1
  def data = [ "a", "b" ]
}

I then defined a simple utility class that wants to dynamically load the Item class:

class Util {
  static def main(args) {
     def cls = "Item" as Class
     def instance = cls.newInstance()
     println instance.toString()
  }
}

Util.groovy is in the same folder as Item.groovy

When I try to run Util.groovy I get the following error:

Caught: org.codehaus.groovy.runtime.typehandling.GroovyCastException: 
Cannot cast object 'Item' with class 'java.lang.String' 
to class 'java.lang.Class' due to: 
java.lang.ClassNotFoundException: Item
        at Util.main(Util.groovy:3)

The only way that I could make it work was by using groovyc to precompile Item.groovy, but this misses the point of being Groovy :)

score 38 · Accepted Answer · edited Jan 09 '17 at 20:43

38

This works, using the underlying GroovyClassLoader:

def instance = this.class.classLoader.loadClass( 'Item', true, false )?.newInstance()

edited Jan 09 '17 at 20:43

khagler

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answered Oct 13 '11 at 18:40

tim_yates

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1

You should probably change `this.class` to `this.getClass()`. It's generally recommended to use the full method to avoid [property or map lookups (or invokeMethod overrides)](http://groovy.329449.n5.nabble.com/class-vs-getClass-td368617.html). – OverZealous Oct 13 '11 at 22:52
9

Why bother when we know this isn't a map? Embrace the dynamic! ;-) – tim_yates Oct 14 '11 at 05:29
5

You can also supply constructor arguments straight into newInstance() in 3 different forms: map, array, or comma list – Daniel Jan 06 '17 at 00:13

score 7 · Answer 2 · edited Feb 25 '16 at 22:45

7

I just had to do this and found an interesting way--so I thought I'd come back and mention it.

I had A problem with this because I wanted to pass a value to newInstance (use a non-default constructor) and all the solutions seemed to be a little bit of work (I'm lazy, okay?)

Anyway, suppose you want to create a new Integer(5)... try this:

c = "java.lang.Integer"
p = "5"
def result = Eval.me("return new ${c}(${p})")
assert(result == 5)

Worked really well although I'm sure it's about the slowest solution possible. Has the advantage that the method is applicable to many other situations.

edited Feb 25 '16 at 22:45

GreenGiant

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answered Jan 31 '13 at 00:26

Bill K

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IT SEEMS javascript plagia :) – Abdennour TOUMI Dec 02 '13 at 14:57
@Bill Interesting at intellectual level, but this is really not in groovy way, like Abdennour TOUMI said – Jacek Cz Aug 05 '15 at 10:36
I agree, just mentioned it for completeness and ease--sometimes you're just writing a one-off script and this is easy to remember and flexible. – Bill K Apr 24 '17 at 16:39

Groovy way to dynamically instantiate a class from String

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